Rock Salt Superstructure Research Articles

Synthesis and crystal structures of two polymorphs of Li4–2xMg1+xTeO6

Two polymorphs of lithium magnesium tellurate Li4–2xMg1+xTeO6 have been prepared by solid-state reactions and their crystal structures characterised by powder X-ray and neutron diffraction. For x ​≈ ​0, a monoclinic C2/m phase is obtained, structurally similar to other O3 type honeycomb layered tellurate and antimonate compounds. The basic structure consists of [Mg2TeO6]3− honeycomb layers alternating with Li layers, with some anti-site disorder of Li and Mg between layers, analogous to the structure of Li4ZnTeO6. For 0 ​< ​x ​< ​~0.5 (specifically, x ​= ​0.33) an orthorhombic Fddd phase is obtained, with a rock-salt superstructure containing disordered Li/Mg cation sites surrounding ordered TeO6 octahedra, analogous to the structure of Li3Co2TaO6.

Mott insulator behavior in the yttrium-based antimoniate oxide Ba2YSbO6

A careful experimental and theoretical study of the double perovskite type material Ba2YSbO6 is reported. The results show that this yttrium-based antimoniate oxide crystallizes in a cubic structure, Fm3¯m (#225) space group, with evidence of the cationic ordering that characterizes a rocksalt superstructure, and tolerance factor slightly less than the unit (τ = 0.9791) due to the size differences between the Y-O6 and Sb-O6 octahedrons. The magnetic response shows weak ferromagnetic effects for the temperature values studied (50, 200 and 300 K), with evidence of an antiferromagnetic reentrance at T = 118 K under the application of low magnetic fields, which is attributed to the presence of polar ordering, accompanied by octahedral distortions caused by the movement of the cations Y3+ and Sb5+, whose effects are no longer observed in the presence of a fairly high external magnetic field (of the Tesla order). The results of the optical characterization by diffuse reflectance suggest the occurrence of a band gap Eg = 4.61 eV, which is characteristic of an insulating material. The electronic structure calculations corroborate the insulating nature of the Ba2YSBO6 complex perovskite. The results allow classifying this material as a Mott insulator, in which the occurrence of intra-site spin-exchange facilitates unpaired spins to the 4d-t2g Yttrium states, mediated by the 2p Oxygen orbitals and 5p Antimony orbitals, resulting in the ferromagnetic character of the insulating material.

Systematic color variation across the solid solution members, Li2Mn1-xTixO3 (0.0 ≤ x ≤ 1.0)

This study investigated the formation of a layered rock salt superstructure series for the oxides Li2(Mn1-xTix)O3 (0.0 ≤ x ≤ 1.0) and the effects of the progressive substitution of Ti4+ (d0) for Mn4+ (d3) on the structural and optical properties. The members where 0.0 < x ≤ 0.85 adopted the structure of Li2MnO3 (S.G. C2/m; a = 4.934 (1); b = 8.530 (2); c = 5.021 (1) Å; β = 109.46 (1)°) according to powder X-ray diffraction measurements and the characteristic Raman bands. By contrast, the members corresponding to the range of 0.85 < x < 1.0 resembled that of Li2TiO3 (S.G. C2/c; a = 5.0585 (2); b = 8.7780 (4); c = 9.7452 (4) Å; β = 100.183 (3)°). Starting from the brick-red color of Li2MnO3, the color changed to ochre and then to orange as the concentration of Ti4+ increased, and finally to colorless for Li2TiO3. The corresponding photoluminescence spectrum for the lowest (0.05) Mn4+ substitution in Li2TiO3 confirmed the presence of Mn4+ ions, where the bands at 684 and 698 nm were due to the transitions: 2E → 4A2 and 2T1→4A2. The photoluminescence intensities decreased systematically as the Mn4+ content increased starting from x = 0.95. The ionic conductivity measurements indicated the temperature dependence of the nominal conductivity values.

Synthesis and characterization of new rocksalt superstructure type layered oxides Li4.5M0.5TeO6 (M(III) = Cr, Mn, Al, Ga)

Oxides of stoichiometry Li4.5M0.5TeO6 (M(III)=Cr, Mn, Al, Ga) have been synthesized by the solid state method and are characterized for the first time. The Rietveld refinements of the PXRD measurements confirmed the structure to be similar to the Li3(Li1.5Fe0.5Te)O6 (S.G. C2/m; a∼5.183Å; b∼8.885Å; c∼5.168Å; β∼110.6°) oxide. The oxides undergo ion-exchange reactions with Ag+ ions. UV–vis diffuse reflectance measurements showed the variation in the band gaps in consistent with the colors observed from brown, brick red to white respectively for Cr3+, Mn3+ and Al3+ or Ga3+ containing oxides. The stabilization of Li4.5M0.5TeO6 (M(III)=Cr, Mn, Al, Ga) obtained by varying the alkali metal (Li+) ions together with other M3+ ions and Te6+ ions forming the edge shared octahedral layer in the rocksalt ordered structures yet again shows the rich compositional versatility of AMO2 type oxides.

Crystallographic, Electronic, and Magnetic Properties of Rock Salt Superstructure Sulfide Lu2CrS4 with Jahn–Teller Distortion

A new chromium(II) sulfide, Lu2CrS4, with a novel structure was prepared by a solid-state reaction. The powder X-ray diffraction pattern could be indexed as a tetragonal system, with a = 7.46373(2) Å, c = 22.6338(2) Å, and space group I4̅2d (No. 122). Rietveld analysis of the pattern provided the crystal structure consisting of CrS6 and LuS6 octahedra sharing edges and apexes and revealed a rock salt superstructure with new cation (vacancy) arrangements. The electrical resistivity indicates semiconducting behavior. The magnetic susceptibility and specific heat measurements showed that the Cr ions are in the high-spin d(4) configuration and that their magnetic moments ordered antiferromagnetically at 55 K. The basic antiferromagnetic structure was determined using powder neutron diffraction data at 10 K. The band structure calculations demonstrate that the densities of states of Cr 3d electrons split into two spin-up eg bands because of Jahn-Teller distortion.

Crystal growth of uranium-containing complex oxides: Ba 2Na 0.83U 1.17O 6, BaK 4U 3O 12 and Na 3Ca 1.5UO 6

Single crystals of complex uranium oxides, Ba 2Na 0.83U 1.17O 6, BaK 4U 3O 12 and Na 3Ca 1.5UO 6, were grown out of reactive carbonate melts. The crystal structures the three uranates were determined by single crystal X-ray diffraction. Ba 2Na 0.83U 1.17O 6 crystallizes in the F m 3 ¯ m space group, a = 8.6173(1), with a disordered cubic double perovskite structure. BaK 4U 3O 12 forms in the cubic I m 3 ¯ m space group, a = 8.7232(1), with the asymmetric unit consisting of three metal positions, which shows improvement of the structure refinement from the published results. Na 3Ca 1.5UO 6 crystallizes in the orthorhombic Fddd space group with a = 6.6573(2), b = 9.6111(3) and c = 20.1155(5) in a rock salt superstructure. In all three oxides the U(VI) cation is octahedrally coordinated. While the UO 6 octahedra in Ba 2Na 0.83U 1.17O 6 are undistorted due to oxygen disorder, the UO 6 octahedra in BaK 4U 3O 12 and Na 3Ca 1.5UO 6 feature the typical uranyl geometry with four long two short U–O bonds.

A review of cation-ordered rock salt superstructure oxides

The types of structures formed when two or more cations occupy the sodium sites in the NaCl unit cell in a non-random manner are reviewed with reference to the cation arrangement in layers of close-packed octahedra and the cation arrangement in the NaCl subcell. Factors influencing the stability of particular structure types, including bond valence and radius ratios, are discussed.

Continuous Order–Disorder Transition in Li3Ni2NbO6and Cr-Doped Li3Ni2NbO6Rock Salt Structures

Li3Ni2NbO6has a partially ordered rock salt superstructure in which Nb occupies fully one set of octahedral sites and Li/Ni are distributed nonrandomly over three other sets of octahedral sites (Matheret al.,J. Mater. Chem.5, 1177 (1995)). Over the temperature range 25–1100°C the Li/Ni site occupancy distribution is largely unchanged. Above 1100°C, Nb increasingly occupies the Li/Ni sites until, above 1300°C, the cations are completely disordered. The transition appears to be fully continuous, with no evidence of a transition enthalpy by DTA. Cr3+ions are able to substitute partially into both ordered and disordered structures by a range of mechanisms. The temperature range of the order–disorder transition decreases with Cr content, as shown in the phase diagram of the vacancy solid solutions, Li3−xNi2−xCrxNbO6(0 <x< 0.18). The materials are modest semiconductors.

Neutron powder diffraction on monoclinic Li2CrCl4

Abstract The crystal structures of the room-temperature polymorphs of Li2CrCl4 and Li-deficient Li1.5Cr1.25Cl4, and of CrCl2 were determined by neutron powder diffraction studies using Rietveld's method. The refinements converged to final R I = 7.8%, 5.1% and 7.5%, respectively. Li2CrCl4 (and Li1.5Cr1.25Cl4) crystallize in a hitherto unknown deficient rocksalt superstructure with the space group C2/m (Z = 2). The vibrational modes of the Raman allowed lattice vibrations are given. The monoclinic distortion of the chromium compound is caused by the Jahn-Teller behavior of Cr2+. Li2CrCl4 mC14, which is metastable at ambient temperature as shown from cohesive-energy (MAPLE) calculations, decomposes below 585 K to LiCl and CrCl2 via Li deficient Li2−2x Cr1+x Cl4.

Synthesis and structures of the partially ordered rock salt phases, Li3M2XO6: MMg, Co, Ni; XNb, Ta, Sb

The crystal structures of four rock salt superstructure phases, Li3M2XO6(M2X Ni2Nb, Ni2Ta, Co2Ta and Mg2Nb) have been refined by X-ray and/or neutron powder diffraction; three members of this family were synthesized for the first time, M2X Ni2Nb, Ni2Sb and Co2Ta. All have similar rock salt superstructures but with partial order only, over three sets of octahedral sites, of the cations M2X. Differences in the distributions of site occupancies are discussed.

Li3Ni2TaO6: A novel rock salt superstructure phase with partial cation order

The new phase Li3Ni2TaO6 has a novel rock salt superstructure, a= 8.4259(3)A, b= 5.9073(3)A, c= 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO6 octahedra which edge-share with Li/NiO6 octahedra Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively The higher Li : Ni ratio of the first set, 0.73 : 0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li3Ni2TaO6 is a very modest semiconductor with conductivity of ca. 4 × 10–6Ω–1 cm–1 at 300 °C and activation energy 0.77 eV, the disorder in the Li+ site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

Crystallographic and magnetic structure of Li 2MnO 3

The structure of Li 2MnO 3 was refined on a single crystal: space group C2 m , a = 493.7(1), b = 853.2(1), c = 503.0(2) pm , β = 109.46(3)°, Z =4, R = 0.020, R w = 0.027 for 619 independent reflections with I > 5 σ. Distortions in the ordered rocksalt superstructure are induced by the cis positions of Mn 4− in the O 2 coordination octahedra. Powders of Li 2MnO 3 are slightly disordered with 12 ± 3% Mn on the Li sites. They are antiferromagnetic with T N = 36.5 K; the cell is doubled along c, corresponding to opposite moments on alternating Mn-containing layers along c. A comparison of structural data of rocksalt-related A 2 BO 3 compounds ( A = Li, Na; B = tetravalent metal) shows that many previously reported cells could be indexed in smaller monoclinic or hexagonal cells.

Order-disorder phenomena in oxides with rock salt structures: the system Li2TiO3-Mg0

Li2TiO3, which belongs to the family of rock salt superstructure phases, undergoes an order-disorder phase transition at 1213° C. It forms solid solutions with MgO and the temperature of the order-disorder change falls increasingly rapidly with increasing MgO content; the ordered solid solutions could not be prepared beyond ∼40 mol % MgO. Up to ∼20% MgO, the transition may be observed as a DTA heat effect and is probably first order. At ≳25% MgO the transition is second order; no DTA heat effect occurs and, by X-ray diffraction, the order-disorder change occurs, reversibly and continuously, over a range of several hundred ° C. For 25% MgO, the disorder, δ, is given by δ=exp[−m(Tc−T)] whereTc is the critical temperature, above which only short range order exists, andm is a constant. The order, δ, is determined from the relative intensity of the (0 0 2) superstructure reflection. The mechanisms of the order-disorder processes are discussed and a method of studying the kinetics is indicated. It is likely that many other oxide phases which have rock salt superstructures will exhibit order-disorder phenomena at high temperatures.