Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure

We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li₇La₃Zr₂O₁₂. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li₇La₃Zr₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO₈ and octahedral ZrO₆. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8a site and the distorted octahedral 16f and 32g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li₇La₃Zr₂O₁₂ structure determined from the single-crystal data. The present tetragonal Li₇La₃Zr₂O₁₂ sample exhibits a bulk Li-ion conductivity of σ_b=1.63×10⁻⁶ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=5.59×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.54 eV in the temperature range of 300–560 K.     

A neutron diffraction study of the d and d lithium garnets Li3Nd3W2O12 and Li5La3Sb2O12

The garnets Li₃Nd₃W₂O₁₂ and Li₅La₃Sb₂O₁₂ have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950 °C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group with lattice parameters a=12.46869(9) Å (Li₃Nd₃W₂O₁₂) and a=12.8518(3) Å (Li₅La₃Sb₂O₁₂). Li₃Nd₃W₂O₁₂ contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li₅La₃Sb₂O₁₂ contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d⁰ and d¹⁰ phases shows that replacement of a d⁰ cation with d¹⁰ cation of the same charge leads to an increase in the lattice parameter due to polarisation effects.     

Synthesis, structural characterization and Li ion conductivity of a new vanado-molybdate phase, LiMg3VMo2O12

A new vanado-molybdate LiMg₃VMo₂O₁₂ has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li_2−zMg_2+zV_zMo_3−zO₁₂ was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li₂M₂(MoO₄)₃ (M=M²⁺, z=0). The structures comprise of two unique (Li/Mg)O₆ octahedra, (Li/Mg)O₆ trigonal prisms and two unique (Mo/V)O₄ tetrahedra. A well-defined 1:3 ratio of Li⁺:Mg²⁺ is observed in octahedral chains for LiMg₃VMo₂O₁₂. Li⁺ preferentially occupies trigonal prisms and Mg²⁺ favours octahedral sheets. Excess V⁵⁺ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li⁺ ion conductivity occurs at the phase transition and the high temperature structure is a good Li⁺ ion conductor, with σ=1×10⁻³-4×10⁻² S cm⁻¹ and E_a=0.6 to 0.8 eV.     

A comparison of the transport properties of lithium-stuffed garnets and the conventional phases Li3Ln3Te2O12

The structures of new phases Li₆CaLa₂Sb₂O₁₂ and Li_6.4Ca_1.4La₂Sb₂O₁₂ have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3?d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li_6.4Ca_1.4La₂Sb₂O₁₂ show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li₆CaLa₂Sb₂O₁₂ is several orders of magnitude smaller than related lithium-stuffed garnets with σ=10⁻⁷ S cm⁻¹ at 95 °C and an activation energy of 0.82(3) eV. The transport properties of the conventional garnets Li₃Gd₃Te₂O₁₂, Li₃Tb₃Te₂O₁₂, Li₃Er₃Te₂O₁₂ and Li₃Lu₃Te₂O₁₂ have been evaluated and consistently show much lower values of conductivity, σ≤4.4×10⁻⁶ S cm⁻¹ at 285 °C and activation energies in the range 0.77(4)≤E_a/eV≤1.21(3).     

A new ordered triple Hollandite: Ba1.33Sb2.66Al5.33O16

Ba_1.33Sb_2.66Al_5.33O₁₆ is a triple Hollandite, which crystallizes in a tetragonal cell (I4/m no. 87) with a=b=9.86090(5) Å and c=8.77612(6) Å. Its crystal structure was characterized using electron diffraction and X-ray powder diffraction; it is isotypic to K_1.33Mg_3.11Sb_4.89O₁₆, K_1.76Mg_3.25Sb_4.75O₁₆ and to K_1.8Li_2.45Sb_5.55O₁₆. In the rutile chains of Ba_1.33Sb_2.66Al_5.33O₁₆, the ordering of Al and Sb atoms into unmixed sites induces the tripling of the c parameter compared to a ‘single’ Hollandite structure. The Ba²⁺ cations are dispersed along c, in the largest tunnels on non-split and fully occupied sites. They lie into Ba-Ba pairs separated by vacancies. Their regular arrangement has been confirmed by high resolution electron microscopy. Electrochemical experiments have also been performed in Li-ion cell but no Li insertion was detected.     

Ab initio structure determination of new compound Li4CaB2O6

A new compound, Li₄CaB₂O₆, has been synthesized by solid-state reaction and its structure has been determined from powder X-ray diffraction data by direct methods. The refinement was carried out using the Rietveld methods and the final refinement converged with R_p=10.4%, R_wp=14.2%, R_exp=4.97%. This compound belongs to the orthorhombic space group Pnnm, with lattice parameters a=9.24036(9) Å, b=8.09482(7) Å, and c=3.48162(4) Å. Fundamental building units are isolated [BO₃]³⁻ anionic groups, which are all parallel to the a-b plane stacked along the c-axis. The Ca atoms are six-coordinated by the O atoms to form octahedral coordination polyhedra, which are joined together through edges along the c-axis, forming infinitely long three-dimensional chains. The Li atoms have a four-fold and a five-fold coordination with O atoms that lead to complex Li-O-Li chains that also extend along the c-axis. The infrared spectrum of Li₄CaB₂O₆ was also studied, which is consistent with the crystallographic study.     

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La₂O₃-Ta₂O₅ system was considered with a focus on the La₃TaO₇ phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La_3−xSr_xTaO_7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La₃TaO₇ is a poor oxide ion conductor (σ_700 °C = 2 × 10⁻⁵S.cm⁻¹), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ_700 °C = 2 × 10⁻⁴S.cm⁻¹).     

Synthesis, crystal structure and magnetic characterization of Na2Cu5(Si2O7)2: An inorganic ferrimagnetic chain

The hydrothermal synthesis, crystal structure and magnetic properties of the new copper silicate Na₂Cu₅(Si₂O₇)₂, are reported. The crystal structure was determined through synchrotron powder diffraction data. The unit cell was indexed to a triclinic cell, space group P-1 (n° 2) with unit cell parameters a=5.71075(2) Å, b=7.68266(3) Å, c=7.96742(3) Å, α=64.2972(2)°, β=88.4860(2)° and γ=70.5958(2)° with Z=1. A structural model was obtained through a combination of a direct-space Monte-Carlo approach and Rietveld refinement. The crystal structure contains parallel chains consisting of zig-zag copper dimers and trimers. All silicon atoms are present as part of a [Si₂O₇]⁶⁻ anion that connects the chains; therefore the compound belongs to the sorosilicate mineral family. The magnetic susceptibility was measured and shows a behavior typical of one-dimensional ferrimagnetism, in agreement with the observed structure.     

Effect of sintering on the ionic conductivity of garnet-related structure Li5La3Nb2O12 and In- and K-doped Li5La3Nb2O12

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10⁻⁴ S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10⁻¹⁰-10⁻⁷ cm²/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, have the space group I2/m (tilt system a⁰b⁻b⁻) and Fm3¯m (tilt system a⁰a⁰a⁰), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a⁻a⁻a⁻) instead of the tetragonal I4/m phase (tilt system a⁰a⁰c⁻). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba₂MM′O₆-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d⁰-ion, determines the tetragonal symmetry in preference to the rhombohedral one.     

La(Li1/3Ti2/3)O3: a new 1:2 ordered perovskite

A new 1:2 ordered perovskite La(Li_1/3Ti_2/3)O₃ has been synthesized via solid-state techniques. At temperature >1185°C, Li and Ti are randomly distributed on the B-sites and the X-ray powder patterns can be indexed in a tilted (b⁻b⁻c⁺) Pbnm orthorhombic cell (a=a_c√2=5.545 Å, b=a_c√2=5.561 Å, c=2a_c=7.835 Å). However, for T?1175°C, a 1:2 layered ordering of Li and Ti along 〈111〉_c yields a structure with a P2₁/c monoclinic cell with a=a_c√6=9.604 Å, b=a_c√2=5.552 Å, c=a_c3√2=16.661 Å, β=125.12°. While this type of order is well known in the A²⁺(B²⁺_1/3B⁵⁺_2/3)O₃ family of niobates and tantalates, La(Li_1/3Ti_2/3)O₃ is the first example of a titanate perovskite with a 1:2 ordering of cations on the B-sites.     

LiB3O5 crystal structure at 20, 227 and 377 °C

Crystal structure of LiB₃O₅ (a framework of [B₃O₅]⁻ rings and Li atoms located in interspaces) was refined at high temperatures using single-crystal X-ray diffraction, MoKα-radiation, anharmonic approximation, orthorhombic; Pna2₁; Z=4; 20 °C (a=8.444, b=7.378, c=5.146 Å, 1411 F(hkl), R=0.022); 227 °C (a=8.616, b=7.433, c=5.063 Å, 1336 F(hkl), R=0.026), 377 °C (a=8.746, b=7.480, c=5.013 Å, 1193 F(hkl), R=0.035). A high mobility of Li atoms and their highly asymmetric vibrations are revealed. Ellipsoid of Li thermal vibrations is oviform. Li is shifted on heating to 0.26 Å mainly along a-axis causing high thermal expansion in this direction; Li temperature factors are multiplied by 4 on heating. Rigid boron-oxygen groups in LiB₃O₅ remain practically stable on heating similar to α-Na₂B₈O₁₃ and α-CsB₅O₈. At the same time these groups rotate relative to each other like hinges leading to extremely anisotropic thermal expansion (α_a=101, α_b=31, α_c=−71, α_v=60×10⁻⁶ °C⁻¹, 20-530 °C, HTXRPD data).     

Crystal structures and phase transition in the system SrTiO3-La2/3TiO3

The distribution of A-site cations in the perovskite system La_xSr_1−3x/2TiO₃ depends on the concentration of La³⁺ ions and associated vacancies. For small x (x?0.2), the substitutions are expected to be random. For x?0.55, the cations are ordered in such a way that successive layers of A-sites are occupied to greater and lesser degree, and this ordering drives a tetragonal distortion. For x from about 0.3 to 0.5, the X-ray patterns show diffuse peaks indicative of similar ordering, but this is not long-range order and no tetragonal distortion results. The lower temperature structures also exhibit out-of-phase tilting of the TiO₆ octahedra, setting in at temperatures varying linearly with composition from 105 K for x=0, to about 650 K at x=2/3.     

Synthesis, characterization and dielectric properties of new unidimensional quaternary tellurites: LaTeNbO6, La4Te6Nb2O23, and La4Te6Ta2O23

Three new tellurites, LaTeNbO₆ and La₄Te₆M₂O₂₃ (M=Nb or Ta) have been synthesized, as bulk phase powders and crystals, by using La₂O₃, Nb₂O₅ (or Ta₂O₅), and TeO₂ as reagents. The structures of LaTeNbO₆ and La₄Te₆Ta₂O₂₃ were determined by single crystal X-ray diffraction. LaTeNbO₆ consists of one-dimensional corner-linked chains of NbO₆ octahedra that are connected by TeO₃ polyhedra. La₄Te₆M₂O₂₃ (M=Nb or Ta) is composed of corner-linked chains of MO₆ octahedra that are also connected by TeO₄ and two TeO₃ polyhedra. In all of the reported materials, Te⁴⁺ is in an asymmetric coordination environment attributable to its stereo-active lone-pair. Infrared, thermogravimetric, and dielectric analyses are also presented. Crystallographic information: LaTeNbO₆, triclinic, space group P−1, a=6.7842(6) Å, b=7.4473(6) Å, c=10.7519(9) Å, α=79.6490(10)°, β=76.920(2)°, γ=89.923(2)°, Z=4; La₄Te₆Ta₂O₂₃, monoclinic, space group C2/c, a=23.4676(17) Å, b=12.1291(9) Å, c=7.6416(6) Å, β=101.2580(10)°, Z=4.     

Crystal structure, phase relations and electrochemical properties of monoclinic Li2MnSiO4

Phase relations in the MnO-SiO₂-Li₄SiO₄ subsystem have been investigated by X-ray diffraction after solid-state reactions in hydrogen at 950-1150 °C. Both cation-deficient and cation-excess solid solutions Li_2+2xMn_1−xSiO₄ (−0.2?x?0.2) based on Li₂MnSiO₄ have been found. According to Rietveld analysis, Li₂MnSiO₄ (monoclinic, P2₁/n, a=6.3368(1), b=10.9146(2), c=5.0730(1) Å, β=90.987(1)°) is isostructural with γ_II-Li₂ZnSiO₄ and low-temperature Li₂MgSiO₄. All components are in tetrahedral environment, (MnSiO₄)²⁻ framework is built of four-, six- and eight-member rings of tetrahedra. Testing Li₂MnSiO₄ in an electrochemical cell showed that only 4% Li could be extracted between 3.5 and 5 V against Li metal. These results are discussed in comparison with those for recently reported orthorhombic layered Li₂MnSiO₄ and other tetrahedral Li₂MXO₄ phases.     

Synthesis and characterization of the novel Nb3O5F5 niobium oxyfluoride: the term n=3 of the NbnO2n−1Fn+2 series

The solid-state synthesis of the oxyfluoride Nb₃O₅F₅, its crystal structure determined from X-ray powder diffraction data as well as some physical characterizations, are reported. Nb₃O₅F₅ constitutes the term n=3 of the Nb_nO_2n−1F_n+2 series related to the Dion-Jacobson phases. It crystallizes, at room temperature, in the tetragonal system (space group I4/mmm (no. 139); Z=4; a=3.9135(1) Å, c=24.2111(2) Å, and V=370.80(3) Å³). The crystal structure appears to be an in-between of the three-dimensional network of NbO₂F and the two-dimensional packing of NbOF₃ (term n=1 of the Nb_nO_2n−1F_n+2 series). This layered structure consists of slabs made of three Nb(O,F)₆ corner-linked octahedra in thickness (n=3) shifted one from another by a ()/translation. Oxygen and fluorine atoms are randomly distributed over all the ligand sites.     

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7-H(D)2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)₂Ni₇ alloys. Substitution of La in La₂Ni₇ by Mg to form La_1.5Mg_0.5Ni₇ preserves the initial Ce₂Ni₇ type of the hexagonal P6₃/mmc structure and leads to contraction of the unit cell. The system La_1.5Mg_0.5Ni₇-H₂ (D₂) was studied using in situ synchrotron X-ray and neutron powder diffraction in H₂/D₂ gas and pressure-composition-temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi₄ slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La₂Ni₇, the unit cell of La_1.5Mg_0.5Ni₇D_9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δa/a=7.37%) and along [001] (Δc/c=9.67%). In contrast with La₂Ni₇D_6.5 where only LaNi₂ layers absorb hydrogen atoms, in La_1.5Mg_0.5Ni₇D_9.1 both LaNi₅ and LaMgNi₄ layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg)₂Ni₂, (La,Mg)Ni₃, Ni₄, tetragonal pyramidal La₂Ni₃ and trigonal bipyramidal (La,Mg)₃Ni₂ interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD₆ and a 16-vertex polyhedron LaD₁₆ around La. The interatomic distances are in the following ranges: La-D (2.28-2.71), Mg-D (2.02-2.08), Ni-D (1.48-1.86 Å). All D-D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the ΔH and ΔS of hydrogenation/decomposition; ΔH_H=−15.7±0.9 kJ (mol_H)⁻¹ and ΔS_H=−46.0±3.7 J (K mol_H)⁻¹ for H₂ absorption, and ΔH_H=16.8±0.4 kJ (mol_H)⁻¹ and ΔS_H=48.1±1.5 J (K mol_H)⁻¹ for H₂ desorption.     

La3Ru8B6 and Y3Os8B6, new members of a homologous series R(A)nM3n−1B2n

Two new compounds, La₃Ru₈B₆ and Y₃Os₈B₆, were synthesized by arc melting the elements. Their structural characterization was carried out at room temperature on as-cast samples by using X-ray diffractometry. According to X-ray single-crystal diffraction results these borides crystallize in Fmmm space group (no. 69), Z=4, a=5.5607(1) Å, b=9.8035(3) Å, c=17.5524(4) Å, ρ=8.956 Mg/m³, μ=25.23 mm⁻¹ for La₃Ru₈B₆ and a=5.4792(2) Å, b=9.5139(4) Å, c=17.6972(8) Å, ρ=13.343 Mg/m³, μ=128.23 mm⁻¹ for Y₃Os₈B₆. The crystal structure of La₃Ru₈B₆ was confirmed from Rietveld refinement of X-ray powder diffraction data. Both La₃Ru₈B₆ and Y₃Os₈B₆ compounds are isotypic with the Ca₃Rh₈B₆ compound and their structures are built up from CeCo₃B₂-type and CeAl₂Ga₂-type structural fragments taken in ratio 2:1. They are the members of structural series R(A)_nM_3n−1B_2n with n=3 (R is the rare earth metal, A the alkaline earth metal, and M the transition metal). Structural and atomic parameters were also obtained for La_0.94Ru₃B₂ compound from Rietveld refinement (CeCo₃B₂-type structure, P6/mmm space group (no. 191), a=5.5835(9) Å, c=3.0278(6) Å).     

From a 3D protonic conductor VO(H2PO4)2 to a 2D cationic conductor Li4VO(PO4)2 through lithium exchange

A new phase, Li₄VO(PO₄)₂ was synthesized by a lithium ion exchange reaction from protonic phase, VO(H₂PO₄)₂. The structure was determined from neutron and synchrotron powder diffraction data. The exchange of lithium causes a stress, leading to a change in the dimensionality of the structure from 3D to 2D by the displacement of oxygen atoms. Thus, Li₄VO(PO₄)₂ crystallizes in P4/n space group with lattice parameters a=8.8204(1) Å and c=8.7614(2) Å. It consists of double layers [V₂P₄O₁₈]_∞ formed by successive chains of VO₆ octahedra and VO₅ pyramids with isolated PO₄ tetrahedra. The lithium ions located in between the layers promote mobility. Furthermore, the ionic conductivity of 10⁻⁴ S/cm at 550 °C for Li₄VO(PO₄)₂ confirms the mobility of lithium ions in the layers. On the other hand, VO(H₂PO₄)₂ exhibits a conductivity of 10⁻⁴ S/cm at room temperature due to the presence of protons in tunnels.     

“Unusual” phase transitions in CeAlO3

High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO₃ over a wide temperature range between 4.2 and 1423 K. Confirming the recent result of X-ray powder diffraction, the room temperature structure is tetragonal with the space group I4/mcm (tilt system (a⁰a⁰c⁻)). The tetragonal structure persists down to 4.2 K. However, above room temperature CeAlO₃ undergoes three phase transitions: first to the orthorhombic Imma structure (tilt system (a⁰b⁻b⁻)) at, e.g., 373 K, then to the rhombohedral structure (tilt system (a⁻a⁻a⁻)) at, e.g., 473 K, and finally, to the primitive cubic structure which exists above 1373 K. The sequence of phases, , which occurs in CeAlO₃ is a rare one in oxide perovskites.