“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.     

A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr1−xCaxTiO3 (x=0.04, 0.06, and 0.12)

Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr_1−xCa_xTiO₃ (SCT) are presented for x=0.04, 0.06 and 0.12 in the temperature range 8-473 K. It is proposed that the space group of SCT in the composition range 0.06?x?0.35 is Imma with a⁰b^-b^- tilt system and not I4/mcm with a⁰a⁰c^- tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I4/mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural E_g mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x=0.06 and x=0.12 but not of x=0.04 indicating the change in the local structure of the cubic phase of SCT for x?0.06. The presence of symmetry forbidden TO₂ mode in the Raman spectra of the cubic phase of SCT for x<0.06 and its absence for x?0.06 provides yet another characteristic feature distinguishing the I4/mcm and Imma space groups. The implications of the change in the tilt system from a⁰a⁰c^- to a⁰b^-b^- on the development of the polar order is also discussed.     

Structural, magnetic and electronic properties of LaNi0.5Fe0.5O3 in the temperature range 5-1000 K

The structure, magnetism, transport and thermal expansion of the perovskite oxide LaNi_0.5Fe_0.5O₃ were studied over a wide range of temperatures. Neutron time-of-flight data have shown that this compound undergoes a first-order phase transition between ∼275 and ∼310 K. The structure transforms from orthorhombic (Pbnm) at low temperatures to rhombohedral (R3¯c) above room temperature. This phase transition is the cause for the previously observed co-existence of phases at room temperature. The main structural modification associated with the phase transition is the change of tilting pattern of the octahedra from a⁺b⁻b⁻ at low temperatures to a⁻a⁻a⁻ at higher. Magnetic data strongly suggests that a spin-glass magnetic state exists in the sample below 83 K consistent with the absence of magnetic ordering peaks in the neutron data collected at 30 K. At high temperatures the sample behaves as a small polaron electronic conductor with two regions of slightly different activation energies of 0.07 and 0.05 eV above and below 553 K, respectively. The dilatometric data show an average thermal expansion coefficient of 14.7×10⁻⁶ K⁻¹ which makes this material compatible with frequently used electrolytes in solid oxide fuel cells.     

Temperature-induced phase transitions in BaTbO3

The crystal structures of BaTbO₃ have been investigated over a wide temperature range between 40 and 773 K using high-resolution time-of-flight neutron powder diffraction. Two-phase transitions were observed. Below about 280 K, BaTbO₃ adopts an orthorhombic perovskite structure (space group Ibmm), which is characterized by rotation of TbO₆ octahedra about the pseudocubic two-fold axis. Above 280 K, BaTbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm), in which the tilting of the octahedra is around the pseudocubic four-fold axis. As the temperature is further increased, BaTbO₃ adopts the primitive cubic aristotype at about 623 K. This later phase transformation is characterized by a gradual decrease of the rotation angle, indicating a continuous phase transition, which is described by a critical exponent β=0.35.     

Transmission electron microscopy study of Ruddlesden-Popper Can+1MnnO3n+1n=2 and 3 compounds

Two Ruddlesden-Popper compounds Ca_n+1Mn_nO_3n+1 with n=2 and 3 synthesized by a citrate gel technique have been studied by TEM. The structure of Ca₄Mn₃O₁₀ is consistent with the previously determined structure having the space group Pbca and a⁻ a⁻ c⁺/a⁻ a⁻ c⁺ tilt system. The presence of defects suggests the possible high-temperature phase transition from untilted I4/mmm to Pbca. The structure of Ca₃Mn₂O₇ was found to be different from the previously suggested I4/mmm symmetry. Ca₃Mn₂O₇ forms with an orthorhombic structure with either Cmcm or Cmc2₁ space group. A structural model for Cmc2₁ based on the tilting of almost-rigid octahedra with a⁺ c⁻ c⁻/a⁺ c⁻ c⁻ tilt system is proposed. The lamellar defects were shown to be twin variants of the Cmc2₁ structure with the (001)_t interfaces, which suggests the possible tilting phase transition from the ideal I4/mmm to Cmc2₁ following the maximal group-subgroup symmetry tree: I4/mmm→Fmmm→Bbmm(Cmcm)→Bb2₁m(Cmc2₁).     

Neutron diffraction study of phase transitions in perovskite-type strontium molybdate SrMoO3

The structure of a polycrystalline sample of SrMoO₃ has been investigated using powder neutron diffraction from 5 to 300 K, to reveal two structural phase transitions, the first from the cubic structure with a=3.97629(3) Å to a tetragonal structure in I4/mcm near 266 K and the second to an orthorhombic Imma phase below 125 K. The average Mo-O distance is essentially independent of temperature. The temperature dependence of the octahedral tilting appears typical of a tricritical phase transition.     

Phase transitions in the perovskite methylammonium lead bromide, CH3ND3PbBr3

The structure of phase IV of methylammonium lead bromide, CH₃ND₃PbBr₃, is shown from Rietveld refinement of neutron powder diffraction data to be centrosymmetric, with space group Pnma: Z=4; a=7.9434(4) Å, b=11.8499(5) Å, c=8.5918(4) Å at 11 K; R_wp=2.34% R_p=1.81%. This corresponds to one of the pure tilt transitions, a^-b⁺a⁻, commonly observed in perovskites. Additional distortions not required by pure tilting are found in the PbBr₆ octahedra, and it appears that the structure optimizes the hydrogen bonding between the methylammonium cation and the framework. It is likely that the lowest temperature phase of the corresponding iodide also has this structure. The structure is compared to the available data for that of other Pnma perovskites. A brief comparison to the higher temperature phases in which the methylammonium ion is disordered is given.     

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.     

Phase transition behaviour in the A-site deficient perovskite oxide La1/3NbO3

The crystal structure of the layered perovskite La_1/3NbO₃ has been studied between room temperature and 500 °C using synchrotron X-ray powder diffraction methods. The structure shows ordering of the La cations at all temperatures. At room temperature La_1/3NbO₃ is orthorhombic with the NbO₆ octahedra showing out-of-phase tilting about the a-axis. This tilting diminishes as the temperature increases, so that above 200 °C the structure is tetragonal. The transition to the tetragonal structure is found to be continuous and analysis of the spontaneous strains shows it to be tricritical in nature.     

Pressure-induced phase transition and octahedral tilt system change of Ba2BiSbO6

High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are reported for the ordered double perovskite Ba₂BiSbO₆. Near 4 GPa, the oxide undergoes a pressure-induced phase transition. The symmetry of the material changes during the phase transition from space group to space group I2/m, which is consistent with a change in the octahedral tilting distortion from an a⁻a⁻a⁻ type to a⁰b⁻b⁻ type using the Glazer notation. A fit of the volume-pressure data using the Birch-Murnagaham equation of state yielded a bulk modulus of 144(8) GPa for the rhombohedral phase.     

Phase transition in the Ruddlesden-Popper layered perovskite Li2SrTa2O7

The crystal structure of the Ruddlesden-Popper layered perovskite Li₂SrTa₂O₇ has been characterized at various temperatures between −185 and 300 °C by several techniques: X-ray and neutron powder diffraction, single crystal diffraction, transmission electron microscopy and Raman spectroscopy. The low temperature structure has been confirmed to be orthorhombic Cmcm with a small octahedra antiphase tilting (ΦΦ0) (ΦΦ0) inside the perovskite blocks. With temperature, the tilting progressively vanishes leading around 230 °C to a tetragonal symmetry (S.G. I4/mmm). This reversible phase transition, followed by X-ray and neutron thermodiffraction and thermal Raman measurements, is considered as of second order. An attribution of the Raman bands based on normal mode analysis is proposed.     

Crystal structure of Ln1/3NbO3 (Ln=Nd, Pr) and phase transition in Nd1/3NbO3

The crystal structure of the A-site deficient perovskite Ln_1/3NbO₃ (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO₆ octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd_1/3NbO₃ has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.     

The Jahn-Teller distortion and cation ordering in the perovskite Sr2MnSbO6

The perovskite Sr₂MnSbO₆ has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I4/m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn³⁺, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn³⁺ and Sb⁵⁺ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb⁵⁺ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.     

Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4−xSrxNaSb3O12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba_4−xSr_xNaSb₃O₁₂ has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba₄NaSb₃O₁₂ has a cubic structure in space group with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O₆ octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2₁/n as seen for Sr₄NaSb₃O₁₂ with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2.     

Crystal structure and phase transitions in perovskite-like C(NH2)3SnCl3

X-ray single-crystal diffraction, high-temperature powder diffraction and differential thermal analysis at ambient and high pressure have been employed to study the crystal structure and phase transitions of guanidinium trichlorostannate, C(NH₂)₃SnCl₃. At 295 K the crystal structure is orthorhombic, space group Pbca, Z=8, a=7.7506(2) Å, b=12.0958(4) Å and c=17.8049(6) Å, solved from single-crystal data. It is perovskite-like with distorted corner-linked SnCl₆ octahedra and with ordered guanidinium cations in the distorted cuboctahedral voids. At 400 K the structure shows a first-order order-disorder phase transition. The space group is changed to Pnma with Z=4, a=12.1552(2) Å, b=8.8590(2) Å and c=8.0175(1) Å, solved from powder diffraction data and showing disordering of the guanidinium cations. At 419 K, the structure shows yet another first-order order-disorder transformation with disordering of the SnCl₃⁻ part. The space group symmetry is maintained as Pnma, with a=12.1786(2) Å, b=8.8642(2) Å and c=8.0821(2) Å. The thermodynamic parameters of these transitions and the p-T phase diagram have been determined and described.     

The structure of CeAlO3 by Rietveld refinement of X-ray powder diffraction data

The room temperature structure of perovskite CeAlO₃ has been reinvestigated by X-ray powder diffraction. The Rietveld refinement has confirmed the tetragonal symmetry; but revealed a super cell, a=5.32489(6) Å and c=7.58976(10) Å, with the space group I4/mcm. In CeAlO₃, the distortion from the ideal cubic perovskite is caused by the cooperative tilting of the AlO₆ octahedra around the primitive cubic [001]_p-axis.     

Low-temperature single crystal X-ray diffraction and high-pressure Raman studies on [(CH3)2NH2]2[SbCl5]

The structure of bis(dimethylammonium) pentachloroantimonate(III), [(CH₃)₂NH₂]₂[SbCl₅], BDP, was studied at 15 K and ambient pressure by single-crystal X-ray diffraction as well as at ambient temperature and high pressures up to 4.87(5) GPa by Raman spectroscopy. BDP crystallizes in the orthorhombic Pnma space group with a=8.4069(4), b=11.7973(7), c=14.8496(7) Å, and Z=4; R₁=0.0381, wR₂=0.0764. The structure consists of distorted [SbCl₆]³⁻ octahedra forming zig-zag [{SbCl₅}_n]²ⁿ⁻ chains that are cross-linked by dimethylammonium [(CH₃)₂NH₂]⁺ cations. The organic and inorganic substructures are bound together by the N-H…Cl hydrogen bonds. The distortions of [SbCl₆]³⁻ units increase, partly due to the influence of the hydrogen bonds which became stronger, with decreasing temperature. The preliminary room temperature, high-pressure X-ray diffraction experiments suggest that BDP undergoes a first-order phase transition below ca. 0.44(5) GPa that destroys single-crystal samples. The transition is accompanied by changes in the intensities and positions of the Raman lines below 400 cm⁻¹.     

Crystal structures of disordered A2Mn3+M5+O6 (A=Sr,Ca; M=Sb,Nb, Ru) perovskites

Polycrystalline samples of A₂MnMO₆ (A=Sr, Ca; M=Nb, Sb, Ru) were prepared by conventional solid state synthesis and their crystal structures were determined using neutron powder diffraction data. All six compounds can be classified as distorted, disordered perovskites. The Mn³⁺/M⁵⁺ distribution is disordered in all six compounds. The strontium containing compounds, Sr₂MnMO₆ (M=Nb, Sb, Ru), undergo out of phase rotations of the octahedra about the c-axis (tilt system a⁰a⁰c⁻) leading to tetragonal I4/mcm space group symmetry. The calcium containing compounds, Ca₂MnMO₆ (M=Nb, Ru, Sb), have orthorhombic Pnma space group symmetry, as a result of a GdFeO₃-type octahedral tilting distortion (tilt system a⁻b⁺a⁻). A cooperative Jahn–Teller distortion is observed in Sr₂MnSbO₆ and Sr₂MnRuO₆, but it is much smaller than the distortion observed in LnMnO₃ (Ln=lanthanide ion) perovskites. It is possible that Jahn–Teller distortions of the MnO₆ octahedra take place on a short-range length scale in the other four compounds, but there is little or no evidence for cooperative ordering of the local distortions. These findings demonstrate a link between orbital ordering, cation ordering and octahedral tilting.     

Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd3NbO7 and La3NbO7

La₃NbO₇ and Nd₃NbO₇ are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb⁵⁺ ions inside the NbO₆ octahedra, combined with shifts of one third of the Ln³⁺ (Ln³⁺=La³⁺ and Nd³⁺) ions at the center of the LnO₈ polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.     

Pressure-induced development of bonding in NiAs type compounds and polymorphism of NiP

A reversible, displacive, pressure-induced structural phase transition has been found to occur in nickel monophosphide NiP at approximately 3.5 GPa by means of in situ synchrotron single-crystal X-ray diffraction. The new phase, with Pearson symbol oC56, assumes an orthorhombic structure with Cmc2₁ space group and unit cell parameters a=23.801(2) Å, b=5.9238(6) Å, and c=4.8479(4) Å at 5.79 GPa. The high-pressure phase is a superstructure of the ambient, oP16 phase with multiplicity of 3.5. The phosphorous sublattice gradually converts from the net of isolated P₂ dimers found in the ambient NiP, towards zig-zag polymeric P_∞ chains found in MnP-type structures. The transformation involves development of triatomic phosphorous clusters and interconnected Ni slabs with diamondoid topology. The high-pressure phase, which represents intermediate polymerization step, is a commensurately modulated superstructure of the NiAs aristotype. The phase transformation in NiP bears resemblance to the effect of successive substitution of Si or Ge in place of P found in the series of stoichiometric inhomogeneous linear structures in ternary NiP_1−xSi_x and NiP_1−xGe_x systems.