We present an optimized seeded chemical vapor transport method for the growth of Cu2OSeO3 that allows for chemical control in a system with many stable phases to selectively produce large phase-pure single crystals. This method is shown to consistently produce single crystals in the range of 120 to 180 mg. A Wulff construction model of a representative crystal shows that the minimum energy surface is {1 1 0}, followed by {1 0 0}. Analysis of the lowest index planes revealed that cleavage of Se–O bonds has a large energy cost, leading to an overall high surface energy. The seeded chemical vapor transport demonstrated here shows promise for large single crystal growth of other functional materials such as Weyl semimetals, frustrated magnets, and superconductors.

Insulating Nb3Cl8 is a layered chloride consisting of two-dimensional triangular layers of Seff = 1/2 Nb3Cl13 clusters at room temperature. Magnetic susceptibility measurement show a sharp, hysteretic drop to a temperature independent value below T = 90 K. Specific heat measurements show that the transition is first order, with ΔS ≈ 5 J K−1 mol−1 f.u.−1, and a low temperature T-linear contribution originating from defect spins. Neutron and X-ray diffraction show a lowering of symmetry from trigonal Pm1 to monoclinic C2/m symmetry, with a change in layer stacking from –AB–AB– to –AB′–BC′–CA′– and no observed magnetic order. This lowering of symmetry and rearrangement of successive layers evades geometric magnetic frustration to form a singlet ground state. It is the lowest temperature at which a change in stacking sequence is known to occur in a van der Waals solid, occurs in the absence of orbital degeneracies, and suggests that designer 2-D heterostructures may be able to undergo similar phase transitions.

A general design strategy is presented for tuning the convergence of direct and indirect bandgaps based on chemical adjustment of the s- and p-orbital character of the conduction band minimum. To demonstrate the viability of the design strategy, we successfully synthesized a family of double perovskites: Cs2AgSbCl6 with an indirect bandgap and Cs2AgInCl6 with a direct bandgap and the solid solutions between the two.

Three new compounds have been added to the alkali chalcogenide and oxychalcogenide families: Sr2O2Bi2Se3, Ba2O2Bi2Se3, and Sr2O2Sb2Se3 were synthesized by direct combination of SrO or BaO with Bi2Se3 or Sb2Se3. The structure, determined from laboratory X-ray powder diffraction data, consists of double chains of edge-sharing BiSe4O square pyramids. Temperature-dependent resistance data reveal all three compounds to be insulators, while heat capacity data of Ba2O2Bi2Se3 and Sr2O2Bi2Se3, in conjunction with the literature reports, reveal low energy phonon modes due to bismuth lone pair effects. We propose a specific materials design principle connecting electron count to structural dimensionality by comparison to related chalcogenides, including the BiS2 superconductors.

The synthesis and physical properties of the new distorted-Hollandite PbIr4Se8 are reported. Powder X-ray diffraction and transmission electron microscopy show that the structure consists of edge- and corner-sharing IrSe6 octahedra, with one-dimensional channels occupied by Pb. The structure contains Se-Se anion-anion bonding, leading to an electron count of Pb2+(Ir3+)4(Se2)2-(Se2−)6, confirmed by bond-valence sums and diamagnetic behavior. Structural and heat capacity measurements demonstrate disorder on the Pb site, due to the combination of lone-pair effects and the large size of the one-dimensional channels. Comparisons are made to known Hollandite and pseudo-Hollandite structures, which demonstrates that the anion-anion bonding in PbIr4Se8 distorts its structure, to accommodate the Ir3+ state. An electronic structure calculation indicates semiconductor character with a band gap of 0.76(11) eV.

LiZn2Mo3O8 is an electrically insulating geometrically frustrated antiferromagnet in which inorganic Mo3O13 clusters each behaves as a single S = 1/2 unit, with the clusters arranged on a two-dimensional triangular lattice. Prior results have shown that LiZn2Mo3O8 does not exhibit static magnetic order down to at least T = 0.05 K, and instead possesses a valence bond ground state. Here, we show that LiZn2Mo3O8 can be hole doped by oxidation with I2 and subsequent removal of Zn2+ cations to access the entire range of electron count, from one to zero unpaired electrons per site on the triangular lattice. Contrary to expectations, no metallic state is induced; instead, the primary effect is to suppress the number of sites contributing to the condensed valence-bond state. Further, diffraction and pair-distribution function analysis show no evidence for local Jahn–Teller distortions or other deviations from the parent trigonal symmetry as a function of doping or temperature. Taken together, the data and density functional theory calculations indicate that removal of electrons from the magnetic layers favors Anderson localization of the resulting hole and an increase in the electrical band-gap over the formation of a metallic and superconducting state. These results put strong constraints on the chemical conditions necessary to realize metallic states from parent insulating geometrically frustrated antiferromagnets.

Photocurrent measurements on devices containing perovskite (CH3NH3)PbI3 show two distinct spectral responses when deposited in a mesoporous oxide matrix, compared with one response for planar perovskite alone. With a TiO2 matrix, the shorter wavelength response has an inverted temperature response with increasing performance on cooling.

We report the structures and physical properties of two new iridates, NaIrO3 and Sr3CaIr2O9, both of which contain continuous two-dimensional honeycomb connectivity. NaIrO3 is produced by room temperature oxidative deintercalation of sodium from Na2IrO3, and contains edge-sharing IrO6 octahedra that form a planar honeycomb lattice. Sr3CaIr2O9, produced via conventional solid-state synthesis, hosts a buckled honeycomb lattice with novel corner-sharing connectivity between IrO6 octahedra. Both of these new compounds are comprised of Ir5+ (5d4) and exhibit negligible magnetic susceptibility. They are thus platforms to investigate the origin of the nonmagnetic behavior exhibited by Ir5+ oxides, and provide the first examples of a J = 0 state on a honeycomb lattice.

The synthesis and physical properties of the K1–xIr4O8 (0 ≤ x ≤ 0.7) solid solution are reported. The structure of KIr4O8, solved with single-crystal X-ray diffraction at T = 110 K, is found to be tetragonal, space group I4/m, with a = 10.0492(3) Å and c = 3.14959(13) Å. A highly anisotropic displacement parameter is found for the potassium cation. Density functional theory calculations suggest that this anisotropy is due to a competition between atomic size and bond valence. KIr4O8 has a significant electronic contribution to the specific heat, γ = 13.9 mJ mol-Ir–1 K–2, indicating an effective carrier mass of m*/me ≈ 10. Further, there is a magnetic-field-dependent upturn in the specific heat at T < 3 K, suggestive of a magnetically sensitive phase transition below T < 1.8 K. Resistivity and magnetization measurements show that both end-members of the solid solution, KIr4O8 and K1–xIr4O8 (x ≈ 0.7), are metallic, with no significant trends in the temperature-independent contributions to the magnetization. These results are interpreted and discussed in the context of the importance of the variability of the oxidation state of iridium. The differences in physical properties between members of the K1–xIr4O8 (0 ≤ x ≤ 0.7) series are small and appear to be insensitive to the iridium oxidation state.

High-temperature superconductivity has a range of applications from sensors to energy distribution. Recent reports of this phenomenon in compounds containing electronically active BiS2 layers have the potential to open a new chapter in the field of superconductivity. Here we report the identification and basic properties of two new ternary Bi–O–S compounds, Bi2OS2 and Bi3O2S3. The former is non-superconducting; the latter likely explains the superconductivity at Tc = 4.5 K previously reported in “Bi4O4S3”. The superconductivity of Bi3O2S3 is found to be sensitive to the number of Bi2OS2-like stacking faults; fewer faults correlate with increases in the Meissner shielding fractions and Tc. Elucidation of the electronic consequences of these stacking faults may be key to the understanding of electronic conductivity and superconductivity which occurs in a nominally valence-precise compound.

Here, we study the nature of metal–metal bonding in the ThCr2Si2 structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi2Se2. For low extents of oxidation, alkali ions are removed exclusively to form K1–xNi2Se2. For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K1–xNi2–ySe2. The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr2Si2 structure type. Furthermore, the fully oxidized compounds, K0.25Ni1.5Se2, spontaneously convert from the tetrahedral [NiSe4]-containing ThCr2Si2 structure to a vacancy-ordered NiAs structure with [NiSe6] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials.

A new transition metal hydroxide chloride containing kagomé layers of magnetic ions, CdCu3(OH)6Cl2, has been synthesized and structurally characterized. The actual low symmetry P21/n structure can be derived from the ideal trigonal one with a change in cation distribution and coherent distortions of the anion framework. The result is a fundamentally different Cu2+ kagomé framework than found in the related Herbertsmithite and Kapellasite minerals. Magnetization measurements show no transition to long range magnetic order above T=2 K, despite strong antiferromagnetic interactions with a Weiss temperature of θw=−150 K. Furthermore, we show that the structure of CdCu3(OH)6Cl2 and related hydroxide chlorides can be rationalized on the basis of [(OH)3Cl]4− pseudopolyatomic anions that pack and rotate, in much the same way as do traditional polyatomic anions. This opens the door to rational design of new and useful hydroxide chloride materials.Graphical AbstractThe [(OH)3Cl]4− pseudopolyatomic anion and the kagomé lattice of CdCu3[(OH)3Cl]2.Highlights► A new understanding of hydroxide chlorides, based on the polyatomic anion [(OH)3Cl]4−. ► Synthesis and structure of a new layered hydroxide chloride, CdCu3(OH)6Cl2, are reported. ► A new compound is reported with kagomé layers of Cu2+.

•First single crystal growth of FeSc2S4 by the traveling solvent technique.•Physical property measurements of crystals are similar to high quality powders.•Stoichiometric crystals and powder show plateau in low-T susceptibility.Here we report successful growth of mm scale single crystals of stoichiometric FeSc2S4. Single crystal X-ray diffraction yields a cubic structure, spacegroup Fd3̅m, with a=10.5097(2) Å at T=110(2) K consistent with previous literature on polycrystalline samples. Models fit to the data reveal no detectable antisite mixing or deviations from the ideal stoichiometry. Heat capacity and dc magnetization measurements on the single crystals match those of high quality powder specimens. The novel traveling solvent crystal growth method presented in this work opens the door to studies requiring sizable single crystals of the candidate spin-orbital liquid FeSc2S4.

Insulating Nb3Cl8 is a layered chloride consisting of two-dimensional triangular layers of Seff = 1/2 Nb3Cl13 clusters at room temperature. Magnetic susceptibility measurement show a sharp, hysteretic drop to a temperature independent value below T = 90 K. Specific heat measurements show that the transition is first order, with ΔS ≈ 5 J K−1 mol−1 f.u.−1, and a low temperature T-linear contribution originating from defect spins. Neutron and X-ray diffraction show a lowering of symmetry from trigonal Pm1 to monoclinic C2/m symmetry, with a change in layer stacking from –AB–AB– to –AB′–BC′–CA′– and no observed magnetic order. This lowering of symmetry and rearrangement of successive layers evades geometric magnetic frustration to form a singlet ground state. It is the lowest temperature at which a change in stacking sequence is known to occur in a van der Waals solid, occurs in the absence of orbital degeneracies, and suggests that designer 2-D heterostructures may be able to undergo similar phase transitions.

Photocurrent measurements on devices containing perovskite (CH3NH3)PbI3 show two distinct spectral responses when deposited in a mesoporous oxide matrix, compared with one response for planar perovskite alone. With a TiO2 matrix, the shorter wavelength response has an inverted temperature response with increasing performance on cooling.

We report the structures and physical properties of two new iridates, NaIrO3 and Sr3CaIr2O9, both of which contain continuous two-dimensional honeycomb connectivity. NaIrO3 is produced by room temperature oxidative deintercalation of sodium from Na2IrO3, and contains edge-sharing IrO6 octahedra that form a planar honeycomb lattice. Sr3CaIr2O9, produced via conventional solid-state synthesis, hosts a buckled honeycomb lattice with novel corner-sharing connectivity between IrO6 octahedra. Both of these new compounds are comprised of Ir5+ (5d4) and exhibit negligible magnetic susceptibility. They are thus platforms to investigate the origin of the nonmagnetic behavior exhibited by Ir5+ oxides, and provide the first examples of a J = 0 state on a honeycomb lattice.