The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2– ions, and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.

Three new materials of composition Fe1−xMgxSb2O4 (x = 0.25, 0.50, 0.75) with the tetragonal schafarzikite structure have been synthesised. Magnetic susceptibility measurements suggest that Fe1−xMgxSb2O4 (x = 0.25, 0.50) are canted antiferromagnets whilst Fe0.25Mg0.75Sb2O4 is paramagnetic. The magnetic ordering temperatures decrease as the Mg2+ concentration increases. The materials form oxygen-excess phases when heated in oxygen-rich atmospheres at temperatures of ∼350 °C. 57Fe Mössbauer spectroscopy shows that the oxidation process involves the oxidation of Fe2+ to Fe3+. Powder neutron diffraction confirms the location of the excess oxygen within the structural channels and reveals a change in magnetic order at low temperatures from A-type (magnetic moments along 〈100〉) for Fe1−xMgxSb2O4 to C-type (magnetic moments along [001]) for the oxidised materials. The change is attributed to a weakening of the antiferromagnetic exchange interactions between edge-linked FeO6 octahedra for the Fe3+-containing materials.

The compound FePbBiO4 has been synthesised and has a tetragonal structure (P42/mbc, a = 8.48924(9) Å, c = 6.10597(8) Å) that is closely related to FeSb2O4. It provides the first example of a compound with this structure that (a) contains only trivalent ions (here Fe3+) within the chains of edge-linked octahedra and (b) accommodates Bi3+ in the walls of the channels formed between the chains of octahedra. FePbBiO4 orders antiferromagnetically with a Néel temperature of 24 K; neutron diffraction reveals A-type magnetic order with moments oriented perpendicular to [001]. The magnetic moment of Fe3+ at 4 K is 3.74(6) μB, slightly lower than that commonly observed for this cation in well-ordered magnetic oxides.

MnxCo1−xSb2O4 and FexCo1−xSb2O4 have been synthesised for 0 ≤ x ≤ 1 and their structures and magnetic properties examined. For all compounds, neutron powder diffraction (NPD) data reveal a canted AFM structure that changes gradually from C-type (x = 0) to A-type (x = 1). This transition corresponds to a gradual rotation of the moments through 90°, from ±[001] to ±[100]. It is primarily caused by a change in the relative magnitudes of the three types of magnetic exchange that exist between cations. Within a given chain, direct exchange promotes an antiferromagnetic ground state for the two cations and 90° superexchange that favours ferromagnetic order. Between chains, antiferromagnetic order is preferred. However, the observed magnetic moments (from NPD) are significantly lower than expected except for the end-members of the series; this suggests that incomplete magnetic order is present. Magnetic susceptibility data also suggest complex magnetic behaviour except for the end-member compounds. The complex magnetic features appear to originate from composition inhomogeneity, local magnetic order in the chains of octahedra being dependent on small clusters of the same transition metal ion and the delicate energy balance that clearly exists between the two ordered configurations in the mid-composition region where x is near to 0.5.

Versiliaite and apuanite are two minerals containing Fe2+ and Fe3+ in a low-dimensional structure exhibiting chains of edge-linked FeO6 octahedra. The chemistry of these minerals has not been fully examined because of their rarity. We demonstrate that chemical synthesis of these minerals is possible to allow measurement of their magnetic properties and a more complete description of their structural features using neutron powder diffraction. We also show that chemical manipulation is possible to provide isostructural phases with different chemical compositions.

Fluorination of the n = 2 Ruddlesden–Popper phase La2BaFe2O7 occurs at ∼300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F− ions in interstitial sites within the rocksalt regions and 2F− ions that have substituted for O2− ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ∼500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe3+ are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe3+ within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.

This experiment emphasizes the first example of two-phase sequential Rietveld refinements throughout a solid/gas chemical reaction monitored by Neutron Powder Diffraction (NPD) at high temperature. The reduction of the n = 1 Ruddlesden–Popper (RP) oxide Sr2MnO4 heated under a flow of 5% H2–He has been investigated throughout two heating/cooling cycles involving isothermal heating at 500 and 550 °C. Oxygen loss proceeds above T ∼ 470 °C and increases with temperature and time. When the oxygen deintercalated from the “MnO2” equatorial layers of the structure results in the Sr2MnO3.69(2) composition, the RP phase undergoes a first order I4/mmm → P21/c, tetragonal to monoclinic phase transition as observed from time-resolved in situ NPD. The phase transition proceeds at 500 °C but is incomplete; the weight ratio of the P21/c phase reaches ∼41% after 130 min of isothermal heating. The fraction of the monoclinic phase increases with increasing temperature and the phase transition is complete after 80 min of isothermal heating at 550 °C. The composition of the reduced material refined to Sr2MnO3.55(1) and does not vary on extended heating at 550 °C and subsequent cooling to room temperature (RT). The symmetry of Sr2MnO3.55(1) is monoclinic at 550 °C and therefore consistent with the RT structure determined previously for the Sr2MnO3.64 composition obtained from ex situ reduction. Consequently, the stresses due to phase changes on heating/cooling in reducing atmosphere may be minimized. The rate constants for the reduction of Sr2MnO4.00 determined from the evolution of weight ratio of the tetragonal and monoclinic phase in the time-resolved isothermal NPD data collected on the isotherms at 500 and 550 °C are k500 = 0.110 × 10–2 and k550 = 0.516 × 10–2 min–1 giving an activation energy of ∼163 kJ mol–1 for the oxygen deintercalation reaction.

Four antimony-containing pyrochlores with compositions Bi2−xFex(FeSb)O7, where x=0.1, 0.2, 0.3, and Nd1.8Fe0.2(FeSb)O7 have been synthesised, their structural properties examined, and compared to that of the pyrochlore phase Pr2(FeSb)O7 which we have structurally characterised for the first time. The Fe3+ ions in Pr2(FeSb)O7 are located only on the B-sites whereas in the bismuth- and neodymium-containing pyrochlore structures the Fe3+ ions are present on both the A- and B-sites giving rise to static displacive disorder in the A2O' chains. The presence of a smaller cation on the A-site in materials of composition Bi2−xFex(FeSb)O7 appears to lower the radius ratio (rA/rB) and leads to a stabilisation of the structure. Magnetic susceptibility measurements reveal no long range order, but Bi1.9Fe0.1(FeSb)O7 and Bi1.8Fe0.2(FeSb)O7 undergo transitions at ca. 12K to a spin-glass state; Nd1.8Fe0.2(FeSb)O7 and Pr2(FeSb)O7 are paramagnetic.graphical abstractIntersite cation mixing in A2(FeSb)O7 (A=Bi, Pr, Nd) pyrochlores.Highlights► The instability of the pyrochlore composition Bi2FeSbO7 is reported for the first time. ► Structure analysis confirms the presence of Fe on the Bi-site. ► The presence of Fe3+ on two distinct sites is supported by Mössbauer spectroscopy. ► For Nd2FeSbO7 and Pr2FeSbO7, Fe can also enter the Nd/Prsite. ► No magnetic order occurs in these materials.

The nuclear and magnetic structures of the synthetic schafarzikite related material CoSb2O4 have been determined from neutron powder diffraction data. The compound is tetragonal (P42/mbc) with refined lattice constants at 300 K of, a=8.49340(9) Å, c=5.92387(8) Å. The magnetic ordering is shown to be consistent with a C mode with moments aligned along [0 0 1]. Magnetic susceptibility measurements indicate a canted antiferromagnetic ground state, for which the ferromagnetic component shows unusually high coercivity. The thermal stability of CoSb2O4 in air is reported. The substitution of Pb2+ for Sb3+ has been investigated and found to cause oxidation of both Co2+ to Co3+ and Sb3+ to Sb5+.Graphical AbstractStructural changes on substitution of Pb2+ ions for Sb3+ ions in CoSb2O4.Highlights► The structural details of CoSb2O4 are reported for the first time. ► The magnetic structure of CoSb2O4 are reported for the first time. ► The ordered magnetic structure has a canted antiferromagnetic arrangement. ► The ferromagnetic component of the ordered arrangement has high coercivity. ► Pb2+ can substitute for Sb3+ and oxidation of both Co2+ and Sb3+ occurs.

The mineral schafarzikite, FeSb2O4 (space groupP42/mbc), has been synthesised and antimony substituted by lead to form a series of compounds of formulation FeSb2−xPbxO4 (x = 0.2–0.7). The replacement of Sb3+ by Pb2+ induces oxidation of Fe2+ to Fe3+ with a concomitant decrease in the a- and increase in the c-lattice parameter. SQUID magnetometry and neutron powder diffraction show that FeSb2O4 orders antiferromagnetically at 45 K and has a predominately A-type magnetic structure in which the moments are aligned parallel within the layers and antiparallel within the chains directed along the c-axis. The presence of a G-type component is also indicated in the neutron diffraction pattern with no evidence of an additional C-type component. The lead-doped variants exhibit magnetic ordering at ca. 50–70 K to a canted antiferromagnetic state with a C-type magnetic structure in which the moments are arranged ferrromagnetically within the chains and antiferromagnetically within the layers.

LiSbO2 has been synthesized using a ceramic method involving evacuated quartz tubes to ensure stoichiometry. Its structure [monoclinic, P21/c; a = 4.8550(3) Å, b = 17.857(1) Å, c = 5.5771(3) Å; β = 90.061(6)°] has been determined using X-ray and neutron diffraction and refined on the basis of neutron data. The structure is significantly different from that of LiBiO2 and contains chains of corner-linked SbO3 trigonal pyramids, which provide a framework for the tetrahedral coordination of Li+ ions. A layer structure results in which the Li sites are located in planes perpendicular to [010]. LiSbO2 is stable in air up to ca. 400 °C, but at higher temperatures, oxidation to LiSbO3 occurs as a two-stage process, with evidence for a metastable, intermediate LiSbO2.5 phase presented. The Li+-ion conductivity, measured using alternating-current impedance spectroscopy, is similar to that of LiBiO2, with a value of ca. 10–6 S cm−1 at 300 °C.

Sr2Co2O5 with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) Å, b=15.6486(8) Å and c=5.5667(3) Å based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L–R–L–R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) μB directed along [1 0 0].Graphical abstractPossible ordering of the tetrahedral chains in Sr2Co2O5.Research highlights► Sr2Co2O5 has alternating (L and R) chains in a given tetrahedral layer. ► Order between layers is limited. ► NPD shows no evidence for the doubled unit cell-averaged structure. ► The length scale for a given technique is critical in considering space group.

The brownmillerite phase LaSrCoFeO5 has been prepared by partially reducing the parent perovskite material LaSrCoFeO6 in 10% H2/N2. The material crystallizes in the Icmm space group with the transition metal ions randomly distributed over the octahedral and tetrahedral sites of the structure. The material shows G-type antiferromagnetic order at room temperature which is consistent with the presence of high spin Co2+ and Fe3+. The perovskite phases LaSrCoFeO5F and LaSrCoFeO5.58 are synthesized by fluorination and room temperature air oxidation of LaSrCoFeO5, respectively. LaSrCoFeO5.56 could be prepared by quenching the sample from 1300 °C into liquid nitrogen. Neutron powder diffraction data, Mössbauer spectroscopy and magnetic susceptibility measurements suggest a G-type antiferromagnetism in these materials at room temperature due to the presence of Co3+ in the high spin state, which is not common in this type of materials.

Two bismuth oxide materials containing large lanthanide and rhenium cations, Bi25La3Re2O49 and Bi25Pr3Re2O49, have been synthesised in the stabilised face-centred cubic structure. They exhibit very high oxide ion conductivity but prolonged annealing at 600 °C causes a decrease in conductivity, which has been shown to relate to an order–disorder phase transformation. The details of this transition have been determined using neutron powder diffraction on annealed samples. Upon annealing, the samples undergo cation ordering to form a tetragonal phase (I4/m, a ≈ 8.7 Å, c ≈ 17.4 Å), which is structurally related to the unsubstituted phase, Bi28Re2O49. Neutron powder diffraction refinement reveals that the lanthanide cations enter only one of the three Bi sites in the ordered structure.

The K2NiF4 phases La1.5+xSr0.5−xCo0.5Ni0.5O4(+δ) (x = 0, 0.2) have been prepared by solid state reactions and structurally characterized by X-ray and neutron powder diffraction. The reduction behaviour, the magnetic properties and the electronic properties of these materials have also been examined. Oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La1.7Sr0.3Co0.5Ni0.5O4.08 with retention of the tetragonal symmetry. The excess oxygen occupies the ideal interstitial (0, 0.5, 0.25) sites of the tetragonal structure. The materials withstand reducing conditions (10% H2–N2) up to 800 °C via reduction of the B-site oxidation state to the divalent state (Ni2+/Co2+) and formation of oxide-ion vacancies within the equatorial planes of the structure. Formation of Ni1+ in these materials under reducing conditions is suggested to account for the oxygen stoichiometry and the magnetic behaviour of La1.5Sr0.5Co0.5Ni0.5O3.70. The electrical conductivity of the oxygen-rich semiconductor materials reaches a SOFC applicable limit (>100 S cm−1) at elevated temperatures.

This paper discusses the effects of cation substitutions on the structural (and linked electronic) transition which has been observed in Na0.63CoO2. The effects of the following substitutions are reported: Ca on the Na site; Fe and Ni on the Co site. Ca doping suppresses the transition and is suggested to interfere with the Na ordering and hence causes a variation in the electronic structure. Fe and Ni doped samples all show transitions, but the transition temperature decreases with the dopant cation concentration. This implies that the replacement of Co by Fe and Ni may enhance the instability in the low-temperature regime. The influence of the substitution is also reflected in the structure and magnetic behaviour of the doped samples.

The influence of temperature on the structure of Bi9ReO17 has been investigated using differential thermal analysis, variable temperature X-ray diffraction and neutron powder diffraction. The material undergoes an order–disorder transition at ∼1000 K on heating, to form a fluorite-related phase. The local environments of the cations in fully ordered Bi9ReO17 have been investigated by Bi LIII- and Re LIII-edge extended X-ray absorption fine structure (EXAFS) measurements to complement the neutron powder diffraction information. Whereas rhenium displays regular tetrahedral coordination, all bismuth sites show coordination geometries which reflect the importance of a stereochemically active lone pair of electrons. Because of the wide range of Bi–O distances, EXAFS data are similar to those observed for disordered structures, and are dominated by the shorter Bi–O bonds. Ionic conductivity measurements indicate that ordered Bi9ReO17 exhibits reasonably high oxide ion conductivity, corresponding to 2.9×10−5 Ω−1 cm-1 at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1×10−4 Ω−1 cm−1 at 673 K).Graphical abstractThe structure of Bi9ReO17 is discussed and related to the ionic conductivity of the ordered and disordered forms.

The n=2 Ruddlesden–Popper phases LaSr2CoMnO7 and La1.2Sr1.8CoMnO7 have been synthesized by a sol–gel method. The O6-type phases LaSr2CoMnO6 and La1.2Sr1.8CoMnO6 were produced by reduction of the O7 phases under a hydrogen atmosphere. The materials crystallize in the tetragonal I4/mmm space group with no evidence of long-range cation order in the neutron and electron diffraction data. Oxygen vacancies in the reduced materials are located primarily at the common apex of the double perovskite layers giving rise to square pyramidal coordination around cobalt and manganese ions. The oxidation states Co3+/Mn4+ and Co2+/Mn3+ predominate in the as-prepared and reduced materials, respectively. The materials are spin glasses at low temperature and the dominant magnetic interactions change from ferro- to antiferromagnetic following reduction.The n=2 Ruddlesden–Popper phases LaSr2CoMnO7, La1.2Sr1.8CoMnO7, LaSr2CoMnO6 and La1.2Sr1.8CoMnO6 are synthesized and characterized.

The structure and ionic conductivity of fluorite-related Bi20Ca7NbO39.5, Bi10.75Ca4.375GaO22 and the high temperature form of Bi9ReO17, formed by quenching from 800 °C, were studied by neutron powder diffraction, X-ray powder diffraction and impedance spectroscopy. All materials formed distorted δ-Bi2O3–related monoclinic superstructures of a fluorite-related hexagonal subcell. The supercell, with P21/m symmetry, is derived from the cubic fluorite subcell axes (af, bf, cf) using the transformation matrix: −1110.50.501−11. Both Bi20Ca7NbO39.5 and Bi10.75Ca4.375GaO22 are shown to display good oxide ion conductivity (2.52 × 10− 5 Ω− 1 cm− 1 and 1.02 × 10− 5 Ω− 1 cm− 1 at 673 K with activation energies of 1.12 eV and 1.25 eV, respectively); quenched Bi9ReO17 has enhanced oxide ion conductivity (1.44 × 10− 3 Ω− 1 cm− 1 at 673 K) with a lower activation energy of 0.76 eV.

The influence of cooling rate on the composition, structure and magnetic behaviour of NaxCoO2 (x ∼ 0.65) is reported. Slow cooled and quenched samples are shown to have different compositions: x = 0.63 and x = 0.68, respectively. On slow cooling, exsolution of sodium occurs and appears as a separate phase, Na2CO3. Whereas the slow-cooled sample has the expected hexagonal symmetry, P63/mmc, a distortion to orthorhombic symmetry (Cmcm, a = 2.83333(4) Å, b = 4.92297(7) Å, c = 10.8948(2) Å) is observed for the quenched material. High resolution neutron powder diffraction reveals displacement of sodium ions from their ideal positions for both materials. The different structure and composition are reflected in the Raman spectra and magnetic behaviour of the phases.

The M4+-containing K2NiF4-type phases La0.8Sr1.2Co0.5Fe0.5O4 and La0.8Sr1.2Co0.5Mn0.5O4 have been synthesized by a sol–gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M4+ to M3+ and of Co3+ to Co2+. The vacancies are confined to the equatorial planes of the K2NiF4-type structure. A partial reduction of Mn3+ to Mn2+ also occurs to achieve the oxygen stoichiometry in La0.8Sr1.2Co0.5Mn0.5O3.6. La0.8Sr1.2Co0.5Fe0.5O3.65 contains Co2+ and Fe3+ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La0.8Sr1.2Co0.5Fe0.5O4, La0.8Sr1.2Co0.5Mn0.5O4 and La0.8Sr1.2Co0.5Mn0.5O3.6 induce spin glass properties in these phases.La0.8Sr1.2Co0.5Fe0.5O4 and La0.8Sr1.2Co0.5Mn0.5O4 have been synthesized by a sol–gel procedure and characterized by diffraction techniques, thermal analysis and Mössbauer spectroscopy. Oxide ion vacancies, created via reduction, are located in the equatorial planes of the K2NiF4-type structure. La0.8Sr1.2Co0.5Fe0.5O3.65 shows antiferromagnetic, noncollinear magnetic order, whereas competing ferromagnetic and antiferromagnetic interactions in La0.8Sr1.2Co0.5Fe0.5O4, La0.8Sr1.2Co0.5Mn0.5O4 and La0.8Sr1.2Co0.5Mn0.5O3.6 induce spin glass behaviour.

A structural transition at 158 °C in Na0.63CoO2 is associated with a resistivity anomaly. Raman spectroscopy suggests a change in Na ordering at the transition and, for the first time, we have identified the nature of this rearrangement using variable temperature neutron powder diffraction. On heating, some Na ions migrate to the thermodynamically less stable (Na1) site from Na2, consistent with loss of an incommensurate superstructure. It is suggested that the change in Na order is linked to a change in the electronic band structure, which is reflected in the resistivity anomaly.

The K2NiF4 phases LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, and their reduced forms LaSrCo0.5Fe0.5O3.75 and La1.2Sr0.8Co0.5Fe0.5O3.85, have been successfully prepared by solid-state reactions, followed by reduction in 10% H2/N2 in order to produce oxygen-deficient materials. All materials crystallize in a tetragonal K2NiF4 structure (space group I4/mmm) with Co and Fe randomly distributed over the B-sites of the structure. Mössbauer spectra have confirmed the trivalent state of Fe in these materials. In the reduced materials, oxide ion vacancies are confined to the equatorial planes of the K2NiF4 structure and the Co is present almost entirely as Co2+ ions; low-temperature neutron powder diffraction data reveal that these reduced phases are antiferromagnetically ordered with a tetragonal noncollinear arrangement of the moments. The Co3+ ions, present in stoichiometric LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, inhibit magnetic order and are assumed to be in the low-spin state.The reduced K2NiF4 phases LaSrCo0.5Fe0.5O3.75 and La1.2Sr0.8Co0.5Fe0.5O3.85 accommodate disordered oxide ion vacancies confined to equatorial planes of the structure. Magnetic exchange results in AFM order at low temperature, which can be represented by a tetragonal noncollinear model for the moments. Co3+ ions, present in stoichiometric LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, inhibit magnetic order and are assumed to be in the low-spin state.

The n=3 Aurivillius material Bi2Sr2Nb2.5Fe0.5O12 is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe3+ and Nb5+ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr2+ and Bi3+ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie–Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi3−xSrxNb2FeO12, Bi2Sr2−xLaxNb2FeO12 and Bi2Sr2Nb3−xFexO12 were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi2Sr2Nb3−xFexO12 system.We report here the synthesis and characterisation of the Aurivillius material Bi2Sr2Nb2.5Fe0.5O12. Combined Rietveld refinements using NPD and XRPD data have been used to investigate the structure and suggest that the material shows significant cation ordering as well as some local structural distortions. Bi2Sr2Nb2.5Fe0.5O12 is paramagnetic in the temperature range studied.

Structural characterisation of Bi2NbO5F was performed using X-ray and neutron powder diffraction and electron diffraction. Structural refinements show that the material adopts a distorted structure with significant rotation of the octahedra around two axes. The physical properties of the material have been investigated and show no evidence of polar symmetry. The structure is therefore best described by space group Pbca (a = 5.428(1) Å, b = 5.426(1) Å, c = 16.656(1) Å). Bond valence sum calculations suggest that ordering of the anions in the structure is favourable with the fluorine atoms preferring the apices of the NbX6 (X = O/F) octahedra.

A new Bi–Re–O phase, Bi28Re2O49, has been synthesized and characterized. Its structure, determined by neutron powder diffraction, is a superstructure of the cubic fluorite unit cell: tetragonal, I4/m, a = 8.7216(1) Å, c = 17.4177(2) Å. The structure comprises an ordered framework of linked BiO4e trigonal bipyramids and square pyramids (e = lone pair of electrons), with discrete Re oxoanions at the origin and body-centre of the unit cell. The infra-red spectrum and Re K-edge X-ray absorption spectrum were consistent with the presence of tetrahedral ReO4− and octahedral ReO65− species in the ratio of 3 : 1. This correctly provides charge balance in the overall structure. The phase is found to display high oxide ion conductivity, which may relate to the presence of the two different oxoanion species, and possible migration of O2− ions between them.

A new Aurivillius phase (generic formula M2An−1BnO3n+3) has been synthesized with n = 3 and containing manganese, Bi2Sr1.4La0.6Nb2MnO12. The structure has been investigated by X-ray and neutron powder diffraction and found to be tetragonal (I4/mmm) at temperatures down to 2 K, with a = 3.89970(7) Å, c = 32.8073(9) Å at 2 K. There is significant cation disorder between Bi3+ (predominantly on the M sites) and Sr2+ and La3+ which prefer the A sites: 19(2)% of Bi3+ occupy the A sites. This disorder, leading to occupancy of M sites by Sr2+, is thought to relieve strain due to size-mismatch between the fluorite-like and perovskite-like blocks. A high level of order exists between Mn and Nb on the B sites, with Mn located predominantly (76.1(6)%) in the central B site whilst Nb preferentially occupies the lower symmetry, outer B site, where it undergoes an out-of-centre displacement towards the fluorite-like blocks. Magnetic measurements indicate that this material displays spin-glass behaviour on cooling. Synthesis of the Mn4+ analogue Bi2Sr2Nb2MnO12 was unsuccessful, possibly due to the small size of the Mn4+ cation.

A new compound Bi8O11(SO4), synthesised using ceramic techniques, has been studied using variable temperature X-ray powder diffraction, neutron powder diffraction and impedance spectroscopy. The material was found to undergo a phase change, on heating, from a complex structure stable at ambient temperatures to a high temperature form with tetragonal symmetry (P41212; a = 11.78840(4) Å, c = 22.7642(1) Å). A gradual phase transition was determined to occur between ∼545 K and ∼600 K. High-resolution neutron powder diffraction data collected at 623 K were used to provide structural information on the high temperature phase. Disorder of the sulfate groups, presumably due to dynamic rotation, renders it impossible to locate the oxygen atoms bonded to S, but the distribution of the sulfate groups within the structure has been established. The bismuth oxide framework also displays oxygen sublattice disorder, and the material exhibits relatively high oxide ion conductivity.

The phases Bi14MO24 (M=Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be ∼306 K (M=W) and ∼295 K (M=Mo), whereas a gradual transition between 275 and 200 K was observed for M=Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: am=√2at, bm=ct, cm=at, β∼135°. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO4 tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell.

The oxygen-deficient Ruddlesden–Popper (RP) phase Sr3Mn2O6 crystallizes with an ordered array of oxygen vacancies to afford a structure in which the Mn3+ ions exist in a square-pyramidal environment. The MnO5 polyhedra are linked through their corners to form a structure that is related to that observed for the single-layered material, Sr2MnO3.5. The nuclear and magnetic structures of a polycrystalline sample of Sr3Mn2O6 have been determined using Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn3+ double-layered phase crystallizes in a superstructure of the simple RP subcell: tetragonal, P4/mbm, a=10.8686(2) Å and c=20.2051(3) Å.Magnetic susceptibility studies suggest a transition at ∼250 K to a canted antiferromagnetic ordered structure. The magnetic unit-cell consists of ferromagnetic clusters of corner-sharing MnO5 units, which are antiferromagnetically aligned to other clusters within the layers.

The synthesis, structures and magnetic properties of two fluorine-intercalated layered manganates, LaSrMnO4F and La1.2Sr1.8Mn2O7F, are reported. Rietveld structure refinement based on time of flight neutron powder diffraction data indicates that both compounds are tetragonal (space group P4/nmm) with cell parameters a = 3.77696(7) Å, c = 14.1026(8) Å (LaSrMnO4F) and a = 3.8103(1) Å, c = 21.7220(2) Å (La1.2Sr1.8Mn2O7F). Fluorine occupies interstitial sites in only one of the two rock salt layers available in the unit cell, leading to novel staged intercalation structures. The interstitial fluorine increases the spacing between the two adjacent rock salt layers and thereby causes a significant reduction in the Mn–O apical bonds directed towards the intercalated regions. The Mn valences of both compounds are close to 4+, and no long range magnetic order was observed in the title compounds down to 5 K.

The crystal and magnetic structures of the two related phases, Sr2MnGaO5 and Ca2MnAlO5 are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr2MnGaO5 being best described by the space group Icmm (a = 5.4888(2) Å, b = 16.2256(6) Å, c = 5.35450(2) Å at 2 K) while that of Ca2MnAlO5 is best described by Ibm2 (a = 5.46258(9) Å, b = 14.9532(3) Å, c = 5.23135(8) Å at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture of the magnetic order within these phases.

A new layered manganese oxide, Sr2MnGaO5, has been synthesised and Rietveld analysis of X-ray powder diffraction data has shown it to be an oxygen deficient perovskite with the brownmillerite structure: a = 5.5033(6) Å, b = 16.234(2) Å, c = 5.3717(6) Å. Incomplete order of the oxygen vacancies is best described using the space group Icmm. The oxygen content can readily be varied to form Sr2MnGaO5 + δ (0 ≤ δ ≤ 0.5) to provide Mn oxidation states between Mn(III) and Mn(IV).

LaSrMnO4F has been synthesised and shown to have a staged structure in which the insertion of F atoms into the parent LaSrMnO4 structure has occurred only in alternate (La,Sr)O rocksalt blocks.

Structural characterisation of Bi2NbO5F was performed using X-ray and neutron powder diffraction and electron diffraction. Structural refinements show that the material adopts a distorted structure with significant rotation of the octahedra around two axes. The physical properties of the material have been investigated and show no evidence of polar symmetry. The structure is therefore best described by space group Pbca (a = 5.428(1) Å, b = 5.426(1) Å, c = 16.656(1) Å). Bond valence sum calculations suggest that ordering of the anions in the structure is favourable with the fluorine atoms preferring the apices of the NbX6 (X = O/F) octahedra.

Three new materials of composition Fe1−xMgxSb2O4 (x = 0.25, 0.50, 0.75) with the tetragonal schafarzikite structure have been synthesised. Magnetic susceptibility measurements suggest that Fe1−xMgxSb2O4 (x = 0.25, 0.50) are canted antiferromagnets whilst Fe0.25Mg0.75Sb2O4 is paramagnetic. The magnetic ordering temperatures decrease as the Mg2+ concentration increases. The materials form oxygen-excess phases when heated in oxygen-rich atmospheres at temperatures of ∼350 °C. 57Fe Mössbauer spectroscopy shows that the oxidation process involves the oxidation of Fe2+ to Fe3+. Powder neutron diffraction confirms the location of the excess oxygen within the structural channels and reveals a change in magnetic order at low temperatures from A-type (magnetic moments along 〈100〉) for Fe1−xMgxSb2O4 to C-type (magnetic moments along [001]) for the oxidised materials. The change is attributed to a weakening of the antiferromagnetic exchange interactions between edge-linked FeO6 octahedra for the Fe3+-containing materials.

A structural transition at 158 °C in Na0.63CoO2 is associated with a resistivity anomaly. Raman spectroscopy suggests a change in Na ordering at the transition and, for the first time, we have identified the nature of this rearrangement using variable temperature neutron powder diffraction. On heating, some Na ions migrate to the thermodynamically less stable (Na1) site from Na2, consistent with loss of an incommensurate superstructure. It is suggested that the change in Na order is linked to a change in the electronic band structure, which is reflected in the resistivity anomaly.

The influence of cooling rate on the composition, structure and magnetic behaviour of NaxCoO2 (x ∼ 0.65) is reported. Slow cooled and quenched samples are shown to have different compositions: x = 0.63 and x = 0.68, respectively. On slow cooling, exsolution of sodium occurs and appears as a separate phase, Na2CO3. Whereas the slow-cooled sample has the expected hexagonal symmetry, P63/mmc, a distortion to orthorhombic symmetry (Cmcm, a = 2.83333(4) Å, b = 4.92297(7) Å, c = 10.8948(2) Å) is observed for the quenched material. High resolution neutron powder diffraction reveals displacement of sodium ions from their ideal positions for both materials. The different structure and composition are reflected in the Raman spectra and magnetic behaviour of the phases.

The K2NiF4 phases La1.5+xSr0.5−xCo0.5Ni0.5O4(+δ) (x = 0, 0.2) have been prepared by solid state reactions and structurally characterized by X-ray and neutron powder diffraction. The reduction behaviour, the magnetic properties and the electronic properties of these materials have also been examined. Oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La1.7Sr0.3Co0.5Ni0.5O4.08 with retention of the tetragonal symmetry. The excess oxygen occupies the ideal interstitial (0, 0.5, 0.25) sites of the tetragonal structure. The materials withstand reducing conditions (10% H2–N2) up to 800 °C via reduction of the B-site oxidation state to the divalent state (Ni2+/Co2+) and formation of oxide-ion vacancies within the equatorial planes of the structure. Formation of Ni1+ in these materials under reducing conditions is suggested to account for the oxygen stoichiometry and the magnetic behaviour of La1.5Sr0.5Co0.5Ni0.5O3.70. The electrical conductivity of the oxygen-rich semiconductor materials reaches a SOFC applicable limit (>100 S cm−1) at elevated temperatures.

Two bismuth oxide materials containing large lanthanide and rhenium cations, Bi25La3Re2O49 and Bi25Pr3Re2O49, have been synthesised in the stabilised face-centred cubic structure. They exhibit very high oxide ion conductivity but prolonged annealing at 600 °C causes a decrease in conductivity, which has been shown to relate to an order–disorder phase transformation. The details of this transition have been determined using neutron powder diffraction on annealed samples. Upon annealing, the samples undergo cation ordering to form a tetragonal phase (I4/m, a ≈ 8.7 Å, c ≈ 17.4 Å), which is structurally related to the unsubstituted phase, Bi28Re2O49. Neutron powder diffraction refinement reveals that the lanthanide cations enter only one of the three Bi sites in the ordered structure.

MnxCo1−xSb2O4 and FexCo1−xSb2O4 have been synthesised for 0 ≤ x ≤ 1 and their structures and magnetic properties examined. For all compounds, neutron powder diffraction (NPD) data reveal a canted AFM structure that changes gradually from C-type (x = 0) to A-type (x = 1). This transition corresponds to a gradual rotation of the moments through 90°, from ±[001] to ±[100]. It is primarily caused by a change in the relative magnitudes of the three types of magnetic exchange that exist between cations. Within a given chain, direct exchange promotes an antiferromagnetic ground state for the two cations and 90° superexchange that favours ferromagnetic order. Between chains, antiferromagnetic order is preferred. However, the observed magnetic moments (from NPD) are significantly lower than expected except for the end-members of the series; this suggests that incomplete magnetic order is present. Magnetic susceptibility data also suggest complex magnetic behaviour except for the end-member compounds. The complex magnetic features appear to originate from composition inhomogeneity, local magnetic order in the chains of octahedra being dependent on small clusters of the same transition metal ion and the delicate energy balance that clearly exists between the two ordered configurations in the mid-composition region where x is near to 0.5.

The compound FePbBiO4 has been synthesised and has a tetragonal structure (P42/mbc, a = 8.48924(9) Å, c = 6.10597(8) Å) that is closely related to FeSb2O4. It provides the first example of a compound with this structure that (a) contains only trivalent ions (here Fe3+) within the chains of edge-linked octahedra and (b) accommodates Bi3+ in the walls of the channels formed between the chains of octahedra. FePbBiO4 orders antiferromagnetically with a Néel temperature of 24 K; neutron diffraction reveals A-type magnetic order with moments oriented perpendicular to [001]. The magnetic moment of Fe3+ at 4 K is 3.74(6) μB, slightly lower than that commonly observed for this cation in well-ordered magnetic oxides.

Versiliaite and apuanite are two minerals containing Fe2+ and Fe3+ in a low-dimensional structure exhibiting chains of edge-linked FeO6 octahedra. The chemistry of these minerals has not been fully examined because of their rarity. We demonstrate that chemical synthesis of these minerals is possible to allow measurement of their magnetic properties and a more complete description of their structural features using neutron powder diffraction. We also show that chemical manipulation is possible to provide isostructural phases with different chemical compositions.

This paper discusses the effects of cation substitutions on the structural (and linked electronic) transition which has been observed in Na0.63CoO2. The effects of the following substitutions are reported: Ca on the Na site; Fe and Ni on the Co site. Ca doping suppresses the transition and is suggested to interfere with the Na ordering and hence causes a variation in the electronic structure. Fe and Ni doped samples all show transitions, but the transition temperature decreases with the dopant cation concentration. This implies that the replacement of Co by Fe and Ni may enhance the instability in the low-temperature regime. The influence of the substitution is also reflected in the structure and magnetic behaviour of the doped samples.

Fluorination of the n = 2 Ruddlesden–Popper phase La2BaFe2O7 occurs at ∼300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F− ions in interstitial sites within the rocksalt regions and 2F− ions that have substituted for O2− ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ∼500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe3+ are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe3+ within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.

The brownmillerite phase LaSrCoFeO5 has been prepared by partially reducing the parent perovskite material LaSrCoFeO6 in 10% H2/N2. The material crystallizes in the Icmm space group with the transition metal ions randomly distributed over the octahedral and tetrahedral sites of the structure. The material shows G-type antiferromagnetic order at room temperature which is consistent with the presence of high spin Co2+ and Fe3+. The perovskite phases LaSrCoFeO5F and LaSrCoFeO5.58 are synthesized by fluorination and room temperature air oxidation of LaSrCoFeO5, respectively. LaSrCoFeO5.56 could be prepared by quenching the sample from 1300 °C into liquid nitrogen. Neutron powder diffraction data, Mössbauer spectroscopy and magnetic susceptibility measurements suggest a G-type antiferromagnetism in these materials at room temperature due to the presence of Co3+ in the high spin state, which is not common in this type of materials.

The mineral schafarzikite, FeSb2O4 (space groupP42/mbc), has been synthesised and antimony substituted by lead to form a series of compounds of formulation FeSb2−xPbxO4 (x = 0.2–0.7). The replacement of Sb3+ by Pb2+ induces oxidation of Fe2+ to Fe3+ with a concomitant decrease in the a- and increase in the c-lattice parameter. SQUID magnetometry and neutron powder diffraction show that FeSb2O4 orders antiferromagnetically at 45 K and has a predominately A-type magnetic structure in which the moments are aligned parallel within the layers and antiparallel within the chains directed along the c-axis. The presence of a G-type component is also indicated in the neutron diffraction pattern with no evidence of an additional C-type component. The lead-doped variants exhibit magnetic ordering at ca. 50–70 K to a canted antiferromagnetic state with a C-type magnetic structure in which the moments are arranged ferrromagnetically within the chains and antiferromagnetically within the layers.