A new square-planar zinc oxyhalide, Sr2ZnO2Cl2, was successfully synthesized using a high-pressure method. Absorption spectroscopy revealed an indirect band gap of 3.66 eV. Electronic structure calculations indicated a strong hybridization between Zn 3dx2−y2 and O 2p orbitals, which is distinct from tetrahedrally coordinated ZnO.

We have successfully synthesized Sr2MnO3F, a new layered perovskite oxyfluoride with a n = 1 Ruddlesden–Popper-type structure using a high-pressure, high-temperature method. Structural refinements against synchrotron X-ray diffraction data collected from manganese oxyfluoride demonstrated that it crystallizes in a tetragonal cell with the space group I4/mmm, in which the Mn cation is located at the octahedral center position. This is in stark contrast to the related oxyhalides that have square-pyramidal coordination such as Sr2MO3X (M = Fe, Co, Ni; X = F, Cl) and Sr2MnO3Cl. There was no evidence of O/F site order, but close inspection of the anion environment centered at the Mn cation on the basis of bond-valence-sum calculation suggested preferential occupation of the apical sites by the F ion with one oxide ion in a random manner. Magnetic susceptibility and heat capacity measurements revealed an antiferromagnetic ordering at 133 K (=TN), which is much higher than that of the chloride analogue with corrugated MnO2 planes (TN = 80 K).

Corundum-type Ti2O3 has been investigated over the last half century because it shows unusual insulator–metal (I-M) transition over a broad temperature range (420–550 K). In this work, we successfully synthesized Ti2O3 nanoparticles (20, 70, 300 nm in size) by the low-temperature reduction between precursors of rutile-type TiO2 and the reductant CaH2, in a non-topotactic manner. The reaction time required for obtaining the reduced phase increases with increasing the particle size. Synchrotron X-ray powder diffraction and electron microscopy studies reveal that the symmetry of all the present samples remains the same as that of bulk samples. However, the particle-size reduction results in three important features compared with bulk samples as follows, (i) color shift from dark brown to bluish black, (ii) anisotropic volume contraction involving the shrinkage of Ti–Ti bonds in the ab plane and along the c axis, (iii) reduction of the I-M transition temperature from 420 K to 350 K. These suggest that the a1g band broadening caused by the surface strain effects, which favors narrowing of the band gap, may play a critical role in the suppression of IM transition.

New members of Ruddlesden–Popper type layered oxychloride compounds, Sr2MO2Cl2 (M = Mn, Ni) and Ba2PdO2Cl2, were synthesized under high-pressure conditions. Synchrotron XRD analysis revealed that all the phases adopt the tetragonal space group I4/mmm, where two-dimensional sheets composed of corner-sharing MO4/PdO4 squares were separated by rock-salt SrCl/BaCl layers.

Controlling the distribution of mixed anions around a metal center is a long-standing subject in solid state chemistry. We successfully obtained single crystals of an iron-based layered perovskite compound, Sr2FeO3F, by utilizing a high-pressure and high-temperature technique. The phase prepared at 1300 °C and 3 GPa crystallized in tetragonal space group P4/nmm with O/F atoms at the apical sites being ordered. However, a temperature of 1800 °C and a pressure of 6 GPa resulted in partial O/F site disordering. The degree of anion disordering, which was 5%, showed that the anion-ordered arrangement was quite robust, in sharp contrast to that of Sr2BO3F (B = Co or Ni) with the fully disordered state. 57Fe Mössbauer spectroscopy measurements revealed no large difference in Néel temperatures between the two phases, but the phase prepared under the latter condition exhibited a quasi-continuous distribution of hyperfine fields caused by O/F site disordering. We discuss the mechanism of the anion order-to-disorder transition observed in related oxyfluoride perovskites.

Extended layered oxyhalide compounds, Sr2NiO3X (X = F, Cl), with the square pyramidal coordination around the trivalent nickel ions in the low spin state (S = 1/2), are successfully synthesized by a high-pressure and high-temperature reaction. Both these compounds crystallize in the n = 1 Ruddlesden–Popper type structure, but the difference of halogen anions incorporated dictate the anion-site ordering patterns and the magnetic ground states. Sr2NiO3F adopts the tetragonal cell in the space group I4/mmm (a = b = 3.79125(2) Å and c = 13.13754(9) Å), with O/F anions being disordered at the apical sites, while the crystal structure of Sr2NiO3Cl is described in the tetragonal space group P4/nmm (a = b = 3.85566(1) Å and c = 14.43240(6) Å) with O/Cl anions being fully ordered at the apical sites. Additionally, Sr2NiO3Cl undergoes a long-range antiferromagnetic order below TN = 33 K, while the fluorine counterpart does not exhibit a long-range ordering but spin glass transition at TSG = 11 K. In light of the positive Weiss temperatures for both X = F and Cl, the unpaired electron likely occupies a d(xy) orbital. Namely, the superexchange interaction mediated by d(xy)-Opπ-d(xy) in the NiO2 basal plane is antiferromagnetic, while the direct exchange interaction between d(xy)-d(xy) along the diagonal directions is ferromagnetic. The origin of spin glass behavior observed in X = F is probably due to randomness of the direct d(xy)-d(xy) bonds caused by off-centering nickel ions and O/F site disordering.

The crystal structure of the layered cobalt oxyfluoride Sr2CoO3F synthesized under high-pressure and high-temperature conditions has been determined from neutron powder diffraction and synchrotron powder diffraction data collected at temperatures ranging from 320 to 3 K. This material adopts the tetragonal space group I4/mmm over the measured temperature range and the crystal structure is analogous to n = 1 Ruddlesden–Popper type layered perovskite. In contrast to related oxyhalide compounds, the present material exhibits the unique coordination environment around the Co metal center: coexistence of square pyramidal coordination around Co and anion disorder between O and F at the apical sites. Magnetic susceptibility and electrical resistivity measurements reveal that Sr2CoO3F is an antiferromagnetic insulator with the Néel temperature TN = 323(2) K. The magnetic structure that has been determined by neutron diffraction adopts a G-type antiferromagnetic order with the propagation vector k = (1/2 1/2 0) with an ordered cobalt moment μ = 3.18(5) μB at 3 K, consistent with the high spin electron configuration for the Co3+ ions. The antiferromagnetic and electrically insulating states remain robust even against 15%-O substation for F at the apical sites. However, applying pressure exhibits the onset of the metallic state, probably coming from change in the electronic state of square-pyramidal coordinated cobalt.

Topotactic reaction of the n = 2 Ruddlesden–Popper phase Sr3Fe2O7−δ (δ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr3Fe2O5+xF2–x (x ≈ 0.44), compared with Sr3Fe2O6F0.87 prepared by the reaction of Sr3Fe2O6 and F2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and 57Fe Mössbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group I4/mmm) with the lattice constants a = 3.87264(6) Å and c = 21.3465(6) Å at room temperature. The fluoride ions preferentially occupy the terminal apical anion sites with oxide ions in a disordered manner, which results in square pyramidal coordination around iron. The present compound also shows an antiferromagnetic order with a Néel temperature (TN) of 390 K, in sharply contrast to Sr3Fe2O6F0.87, which has a TN value that is lower than room temperature.Keywords: iron oxyfluoride; low-temperature fluorination; Ruddlesden−Popper phase; Sr3Fe2O5.44F1.56; topotactic reaction;

The first Ruddlesden–Popper type layered cobalt oxyfluoride, Sr2CoO3F, has been synthesized under a pressure of 6 GPa at 1700 °C and shown to adopt a K2NiF4-type structure with distorted square pyramidal coordination around Co and with O/F disorder at the apical sites.

A new square-planar zinc oxyhalide, Sr2ZnO2Cl2, was successfully synthesized using a high-pressure method. Absorption spectroscopy revealed an indirect band gap of 3.66 eV. Electronic structure calculations indicated a strong hybridization between Zn 3dx2−y2 and O 2p orbitals, which is distinct from tetrahedrally coordinated ZnO.

Correction for ‘New layered cobalt oxyfluoride, Sr2CoO3F’ by Yoshihiro Tsujimoto et al., Chem. Commun., 2011, 47, 3263–3265.

New members of Ruddlesden–Popper type layered oxychloride compounds, Sr2MO2Cl2 (M = Mn, Ni) and Ba2PdO2Cl2, were synthesized under high-pressure conditions. Synchrotron XRD analysis revealed that all the phases adopt the tetragonal space group I4/mmm, where two-dimensional sheets composed of corner-sharing MO4/PdO4 squares were separated by rock-salt SrCl/BaCl layers.

The first Ruddlesden–Popper type layered cobalt oxyfluoride, Sr2CoO3F, has been synthesized under a pressure of 6 GPa at 1700 °C and shown to adopt a K2NiF4-type structure with distorted square pyramidal coordination around Co and with O/F disorder at the apical sites.