Pressure-induced zircon-type to scheelite-type phase transitions in YbPO4 and LuPO4

The tetragonal orthophosphates, YbPO₄ and LuPO₄, were studied by in situ X-ray diffraction (XRD) at pressures up to 52 and 43 GPa, respectively. A reversible phase transition from the zircon structure-type to the scheelite structure-type was found at ∼22 GPa for YbPO₄ and 19 GPa for LuPO₄. Coinciding with the transition from the zircon structure-type to the scheelite structure-type, there is a ∼10% reduction in volume and a significant increase in the bulk modulus for both compounds.     

Neutron powder diffraction study of phase transitions in Sr2SnO4

The phase transitions in Sr₂SnO₄ at high temperature have been studied using high resolution time-of-flight powder neutron diffraction. The room temperature structure of Sr₂SnO₄ is orthorhombic (Pccn), which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the tetragonal [100]_T- and [010]_T-axes with non-equal tilts. At the temperature of about 423 K, it transforms to another orthorhombic structure (Bmab) characterized by the SnO₆ octahedral tilt around the [110]_T-axis. At still higher temperatures (∼573 K) the structure was found to be tetragonal K₂NiF₄-type (I4/mmm).     

Synthesis, crystal structure, phase transition and thermal behaviour of a new dabcodiium hexaaquanickel(II) bis(sulphate), (C6H14N2)[Ni(H2O)6](SO4)2

A new dabcodiium-templated nickel sulphate, (C₆H₁₄N₂)[Ni(H₂O)₆](SO₄)₂, has been synthesised and characterised by single-crystal X-ray diffraction at 20 and −173 °C, differential scanning calorimetry (DSC), thermogravimetry (TG) and temperature-dependent X-ray powder diffraction (TDXD). The high temperature phase crystallises in the monoclinic space group P2₁/n with the unit-cell parameters: a = 7.0000(1), b = 12.3342(2), c = 9.9940(2) Å; β = 90.661(1)°, V = 862.82(3) Å³ and Z = 2. The low temperature phase crystallises in the monoclinic space group P2₁/a with the unit-cell parameters: a = 12.0216(1), b = 12.3559(1), c = 12.2193(1) Å; β = 109.989(1)°, V = 1705.69(2) Å³ and Z = 4. The crystal structure of the HT-phase consists of Ni²⁺ cations octahedrally coordinated by six water molecules, sulphate tetrahedra and disordered dabcodiium cations linked together by hydrogen bonds. It undergoes a reversible phase transition (PT) of the second order at −53.7/−54.6 °C on heating-cooling runs. Below the PT temperature, the structure is fully ordered. The thermal decomposition of the precursor proceeds through three stages giving rise to the nickel oxide.     

Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

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.     

“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.     

High-pressure phase transition in LiBH4

The high-pressure phase transition from ambient pressure α-LiBH₄ to high-pressure β-LiBH₄ was observed by Raman spectroscopy and X-ray diffraction between 0.8 and 1.1 GPa. The phase boundary between these two phases was mapped over a large range of temperatures using thermal conductivity studies and differential thermal analysis. The structure of the high-pressure phase could not be identified due to small number of experimentally observed reflections, but it was shown that it is different from previously reported theoretical predictions.     

Structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6

The evolution of the crystal structure of the double perovskite Sr₂YTaO₆ from room temperature to 1250 °C has been studied using powder neutron and synchrotron X-ray diffraction. At room temperature Sr₂YTaO₆ crystallises in a monoclinic superstructure with the space group P2₁/n. The tilting of the octahedra evident in the room temperature structure is progressively lost on heating, resulting in a sequence of phase transitions that ultimately yields the cubic structure in space group Fm3?m. The high temperature tetragonal and cubic phases are characterised by strongly anisotropic displacements of the anions. The amount of defects in the crystal structure of Sr₂YTaO₆ is found to be sensitive to the preparative method.     

Phase transitions in A4Li(HSO4)3(SO4); A = Rb, K: Single crystal X-ray diffraction studies

The crystal structure of ferroelastic Rb₄Li(HSO₄)₃(SO₄) has been determined at two temperatures, which indicates a structural phase transition, tetragonal P4₃ witha = 7.629(1) ?,c = 29.497(2) ? at 293 K and monoclinic P2₁ witha = 7.583(3) ?,b = 29.230(19) ?,c = 7.536(5) ?,β = 90.14(1)° at 90 K. The crystal structure of K₄Li(HSO₄)₃(SO₄)₄ has also been determined at two temperatures, tetragonalP4₁ witha = 7.405(1) ?,c = 28.712(6) ? at 293 K and tetragonalP4₁ witha = 7.371(5) ?,c = 28.522(5) ? at 100 K. The overall coordination features in both the structures have been analysed in terms of bond valence sum calculations. Dedicated to Professor C N R Rao on his 70th birthday     

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.     

Crystal structure, phase transitions and ferroelastic properties of [(CH3)2NH2]3[Bi2Cl9]

A sequence of structural phase transitions in [(CH₃)₂NH₂]₃[Bi₂Cl₉] (DMACB) is established on the basis of differential scanning calorimetry (DSC) and dilatometric studies. Four phase transitions are found: at 367/369, 340/341, 323/325 and 285/292 K (on cooling/heating). The crystal structure of DMACB is determined at 350 K. It crystallizes in monoclinic space group P2₁/n: a=8.062(2), b=21.810(4), c=14.072(3) Å, β=92.63(3)°, Z=4, R₁=0.0575, wR₂=0.1486. The crystal is built of the double chain anions (“pleated ribbon structure”) and the dimethylammonium cations. Dielectric studies in the frequency range 75 kHz-900 MHz indicate relatively fast reorientation of the dimethylammonium cations over the I, II, III and IV phases. Infrared spectra are recorded in the temperature range 40-300 K and analyzed in region assigned to the symmetric and asymmetric NC₂ stretching vibrations. Optical observations show the existence of the ferroelastic domain structure over all phases below 367 K. The possible mechanisms of phase transitions are discussed on the basis of presented results.     

Synthesis, structures and phase transitions in the double perovskites Sr2−xCaxCrNbO6

The synthesis and crystal structures of nine members of the rock-salt ordered double perovskites Sr_2−xCa_xCrNbO₆ is presented. The crystal structures of the end members of the series Sr₂CrNbO₆ and Ca₂CrNbO₆ were refined using powder neutron diffraction data and are cubic in and monoclinic in P2₁/n, respectively, in both cases there being considerable anti-site Cr-Nb mixing. Variable temperature and/or composition studies suggest a direct first-order P2₁/n to transition, a suggestion supported by selected area electron diffraction studies.     

Phase transitions in Ca3(BN2)2 and Sr3(BN2)2

α-Ca₃(BN₂)₂ crystallizes in the cubic system (space group: ) with one type of calcium ions disordered over of equivalent (8c) positions. An ordered low-temperature phase (β-Ca₃(BN₂)₂) was prepared and found to crystallize in the orthorhombic system (space group: Cmca) with lattice parameters: , , and . Structure refinements on the basis of X-ray powder data have revealed that orthorhombic β-Ca₃(BN₂)₂ corresponds to an ordered super-structure of cubic α-Ca₃(BN₂)₂. The space group Cmca assigned for β-Ca₃(BN₂)₂ is derived from by a group-subgroup relationship.DSC measurements and temperature-dependent in situ X-ray powder diffraction studies showed reversible phase transitions between β- and α-Ca₃(BN₂)₂ with transition temperatures between 215 and 240 °C.The structure Sr₃(BN₂)₂ was reported isotypic with α-Ca₃(BN₂)₂ () with one type of strontium ions being disordered over of equivalent (2c) positions. In addition, a primitive () structure has been reported for Sr₃(BN₂)₂. Phase stability studies on Sr₃(BN₂)₂ revealed a phase transition between a primitive and a body-centred lattice around 820 °C. The experiments showed that both previously published structures are correct and can be assigned as α-Sr₃(BN₂)₂ (, high-temperature phase), and β-Sr₃(BN₂)₂ (, low-temperature phase).A comparison of Ca₃(BN₂)₂ and Sr₃(BN₂)₂ phases reveals that the different types of cation disordering present in both of the cubic α-phases () have a directing influence on the formation of two distinct (orthorhombic and cubic) low-temperature phases.     

MGe(PO4)2 (M=Ca, Sr, Ba): Crystal structure, phase transitions and thermal expansion

Three earth alkali-germanium monophosphates M^IIGe(PO₄)₂ (M=Ca, Sr, Ba) were prepared by solid state reaction and their structures, previously unknown, studied by Rietveld analysis. BaGe(PO₄)₂ and high-temperature β-SrGe(PO₄)₂ (space group C2/m, Z=2) are fully isotypic with yavapaiite, whereas CaGe(PO₄)₂ and low-temperature α-SrGe(PO₄)₂ (C2/c, Z=4) are distorted derivatives. The phase transition between the two forms is observed for the first time. The thermal expansion, resulting from several structural mechanisms, is very anisotropic.     

NMR study of the order-disorder phase transitions of KHCO3 and KDCO3 single crystals

KHCO₃ and its deuterated analogue KDCO₃ are typical materials that undergo order-disorder phase transitions at 318 and 353 K, respectively. The spin-lattice relaxation times, T₁, spin-spin relaxation times, T₂, and the number of resonance lines for the ¹H, ²D, and ³⁹K nuclei of these crystals were investigated using NMR spectrometer. These materials are known to exhibit anomalous decreases in T₁ near T_C, which have been attributed to a structural phase transition. Additionally, changes in the symmetry of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers in these materials are associated with large changes in T₁, T₂, and the number of resonance lines. Here we found that the resonance lines for ¹H, ²D, and ³⁹K nuclei decrease in number as the temperature is increased up to T_C, indicating that the orientations of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers and the environments of the K ions change at T_C. Moreover, based on number of resonance lines, the results further indicate that the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers reorientate to approximately parallel to the directions of the hydrogen bonds (or deuteron bonds) and the direction of the a-axis. The transitions at 318 and 345 K of the two crystals are of the order-disorder type. The present results therefore indicate that the orientations of the (HCO₃)₂²⁻ and (DCO₃)₂²⁻ dimers and the environment of the K ion play a significant role in these phase transitions.     

High-resolution neutron powder diffraction study on the phase transitions in BaPbO3

Phase transitions that occurred in perovskite BaPbO₃ have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO₆ octahedra around the two-fold axis (tilt system a⁰b⁻b⁻). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO₃ undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO₆ octahedra being about the four-fold axis of the cubic aristotype (tilt system a⁰a⁰c⁻). With further increasing the temperature, BaPbO₃ experiences a continuous phase transition to a simple cubic structure (space group Pm3¯m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→I2/m phase transition above room temperature has not been observed.     

Quantum chemical study and low-temperature calorimetry of phase transition in V4S9Br4

The phase transition in a mixed-valence tetranuclear compound V₄S₉Br₄ was studied by DFT calculations and low-temperature high-precision calorimetry. According to DFT data, the phase transition of the low-temperature modification may be accompanied by C_4v→C_2v symmetry lowering and electron spin pairing. A weak exchange-correlation interaction discovered between μ₄-S²⁻ and S²⁻ ions in V₄S₉Br₄ may play an important part in multicentered interactions. Experimental calorimetric data show that the phase transition is a displacive phase transformation.     

Structural and electronic phase transitions in Sr1−xCexMnO3 perovskites

The structures of eight members of the series Sr_1−xCe_xMnO₃ with 0.075?x?0.4 have been established using synchrotron X-ray powder diffraction. These exhibit the sequence of structures