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The structure of the complex formed between 1,4-dioxane and iodine monochloride has been studied as a function of pressure using neutron powder diffraction. Initial compression was accompanied by a decrease in the O⋯I halogen bond length together with an increase in the intramolecular I–Cl bond length. Two phase transitions were observed at ∼2.8 and ∼4.5 GPa. The transient intermediate phase coexists with the ambient pressure phase during the initial phase transition and with the final high-pressure phase at the second phase transition, before its disappearance. The driving force for the first phase transition is a shearing motion of the complex causing a reduction in the dipolar interaction of two I–Cl moieties. The formation of the highest pressure phase is accompanied by a net reduction of 2 C–H⋯Cl hydrogen bonds per formula unit. From these changes we conclude that Cl⋯Cl halogen bonds are favoured over C–H⋯Cl hydrogen bonds at high pressures. [ABSTRACT FROM AUTHOR]

The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution, the thermal evolution of the crystal structure is consistent with a first-order transition between 700 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent "avalanche transition" behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9. [ABSTRACT FROM AUTHOR]

The thermoelastic and high pressure crystallographic properties of KCaF 3 perovskite have been investigated by neutron powder diffraction techniques to a maximum pressure of 6.7 GPa. The effect of increasing pressure is to increase the structural distortion away from the ambient temperature and pressure crystal structure, whilst retaining the space group Pbnm to the highest pressure measured. Both primary order parameters increase with increasing pressure, with no evidence for the proximity of a perovskite - perovskite phase transition at slightly higher pressures. Using supplementary low temperature neutron diffraction data, and the published isobaric heat capacity, the thermoelastic properties of KCaF 3 are well described by a two-term Debye model with characteristic temperatures 180 K, and 482 K, and mean Grüneisen constant of 1.5. The isothermal bulk modulus and the individual polyhedral bulk moduli have been determined from Birch-Murnaghan third, and second order equations-of-state, respectively. The measured bulk modulus, 48.2 GPa, is consistent with the approximate linear trend with inverse molar volume that has been observed in other A I B II F 3 perovskite-structured compounds. [ABSTRACT FROM AUTHOR]

The temperature dependence of the crystal structure and the thermoelastic properties of SrCeO3 have been determined from Rietveld refinement of high resolution, neutron time-of-flight powder diffraction data collected in 5 K intervals between 1273 K and 1723 K. No evidence was found for critical behaviour in the amplitudes of the modes that soften in zone boundary phase transitions in perovskite-structured phases suggesting SrCeO3 may remain orthorhombic, space group Pbnm from 1.2 K up to the 1 atm melting point of 2266 K. The temperature variation of the crystal structure has been determined from mode decomposition techniques and the structural evolution has been inferred from the temperature-dependences of the spontaneous shear strain and the order parameter associated with the anti-phase tilt. Thermoelastic properties have been derived from the temperature variation of the unit cell, isobaric heat capacity, and atomic displacement parameters and shows good agreement with earlier work carried out on the lightly doped system SrCe0.95Yb0.05Oξ (ξ∼ 3). Temperature-dependent corrections for the bond valence parameters for strontium and cerium are reported. [ABSTRACT FROM AUTHOR]

The layered perovskite LaTaO4 has been prepared in its polar orthorhombic polymorphic form at ambient temperature. Although no structural phase transition is observed in the temperature interval 25° C ＜ T ＜ 500 °C, a very large axial thermal contraction effect is seen, which can be ascribed to an anomalous buckling of the perovskite octahedral layer. The non-polar monoclinic polymorph can be stabilised at ambient temperature by Nd-doping. A composition La0.90Nd0.10TaO4 shows a first-order monoclinic-orthorhombic (non-polar to polar) transition in the region 250° C ＜ T ＜ 350 °C. Dielectric responses are observed at both the above structural events but, despite the ‘topological ferroelectric’ nature of orthorhombic LaTaO4, we have not succeeded in obtaining ferroelectric P–E hysteresis behaviour. Structural relationships in the wider family of AnBnX3n+2 layered perovskites are discussed. [ABSTRACT FROM AUTHOR]

Room temperature high resolution neutron powder diffraction data, measured in time-of-flight, from two independent samples of NaTaO 3 shows the presence of phase coexistence of two orthorhombic structures with space groups Pbnm , and Cmcm . The failure of earlier work to recognise the extent of the hysteresis associated with the high temperature (∼763 K on heating) Cmcm – Pbnm phase transition, that extends down to room temperature, and probably to 0 K, is due to data having been collected at too low a real-space resolution to characterise the diagnostic pseudocubic fundamental and superlattice reflection multiplicities. The phase fraction of the Cmcm phase increases with increasing temperature from 45 weight % at 298 K, to 74 weight % at 758 K. Throughout the whole temperature interval 298 K–758 K, the volume per formula unit of the Cmcm phase exceeds that of the Pbnm phase by an almost constant ∼0.01 Å 3 suggesting the addition of pressure would supress the volume fraction of the higher temperature phase. The crystal structure of both phases, determined from data collected at 298 K, are reported, with the atomic displacement parameters of the Cmcm phase being significantly larger than those associated with the Pbnm phase, probably reflecting a high degree of thermal and static disorder. [ABSTRACT FROM AUTHOR]

Abstract: The thermoelastic and structural properties of BaCeO3 perovskite in the Pbnm phase field have been studied using high resolution neutron diffractometry at 37 temperatures between 2K and 350K. From a simultaneous fit of the isochoric heat capacity and unit cell volume, the vibrational density of states in the Pbnm phase is shown to be approximated by a two-term modified Debye model with characteristic temperatures 176(1)K and 543(2)K. Vibrational Debye temperatures, determined from the temperature-variation of the atomic displacement parameters, suggest the cations to be more associated with the lower characteristic temperature, whilst those for the anions, are closer to the higher characteristic temperature. The weighted average of the Grüneisen constants associated with the two characteristic temperatures is in good agreement with the experimentally determined thermodynamic Grüneisen parameter. Structural parameters are presented as the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is outlined. The critical exponent associated with the temperature variation of the primary order parameter in the phase transition from Pbnm to Ibnn at 563K is consistent with this transition being tricritical in nature. [Copyright &y& Elsevier]

We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior of the multiferroic hexagonal polymorph of YMnO3 between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric-ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at 1258±14 K corresponding to a unit-cell tripling and a change in space group from centrosymmetric P63/mmc to polar P63cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition (analyzed in terms of symmetry-adapted displacement modes) clearly support previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K3 mode. We therefore verify previous suggestions that YMnO3 is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group P63/mcm does not occur. However, we do find evidence for an isosymmetric phase transition (i.e., P63cm to P63cm) at ≈920 K, which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization. [ABSTRACT FROM AUTHOR]

High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba2Bi3+Bi5+O6 (dibarium dibismuth hexaoxide) and Ba2BiSbO6 (dibarium bismuth antimony hexaoxide) in the temperature ranges 4.2–973 and 4.2–625 K, respectively. The charge-ordered bismuthate adopts four structures in the temperature range – monoclinic in P21/ n, monoclinic in I2/ m, rhombohedral in , and finally cubic in . The low-temperature monoclinic structure has been determined for the first time. The transitions from P21/ n to I2/ m, at 132 K, and to , at 820 K, are tricritical in nature; the transition from I2/ m to at ca 430 K is discontinuous. The behaviour of Ba2BiSbO6 is very similar, except that the transition temperatures are lower – 250 K for I2/ m to and 515 K for to – and the low-temperature structure is not formed at all. The to transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms. [ABSTRACT FROM AUTHOR]

Abstract: Three high temperature structural phase transitions have been identified in the perovskite-structured phase SrSnO3 using differential scanning calorimetry and dilatometry, and have subsequently been structurally characterised using high-resolution neutron powder diffraction. Between 298 and 905K, SrSnO3 is orthorhombic, space group Pmcn before undergoing a continuous phase transition to a second orthorhombic phase in space group Incn. At 1062K there is a first order phase transition to a tetragonal phase in space group I4/mcm before finally transforming to the aristotype phase at 1295K. Using the magnitude of the anti-phase tilt as a measure of the order parameter for the transition from I4/mcm to suggests this transition is tricritical in nature. Crystal structures are reported for the three hettotype phases. [Copyright &y& Elsevier]

Presents a study which investigated the phase transitions in deuterated lawsonite with neutron diffraction. Methods; Results; Discussion.

The nuclear and magnetic structures of the perovskite-structured compound KMnF 3 have been investigated using high resolution neutron powder diffraction techniques for the first time. Data have been collected in 1 K intervals from 105 K to 10 K to characterize previously identified structural phase transitions from Phase II (space group I 4/ mcm) – (88 K) Phase III (space group Cmcm) – (82 K) Phase IV (space group Pnma) using the diffractometer HRPD. In the temperature interval 91 K ≥ T ≥ 75 K, KMnF 3 exhibits a temperature-dependent phase coexistence between phases with space groups I 4/ mcm and Pnma , contrary to the earlier interpretation, with no evidence found for a discrete phase with space group Cmcm. A recent revision of the space group of phase IV from Pnma to B 2 1 / m is critiqued and shown to be inconsistent with the neutron diffraction data. The magnetically ordered phases of KMnF 3 , collinear antiferromagnet for 81.5 K ≤ T ≤ 88.3 K, and canted antiferromagnet with a ferromagnetic component for T ≤ 81.5 K, have been investigated in detail in the temperature interval 90 K–83 K, in 0.2 K steps using the cold neutron diffractometer WISH, supplemented by lower temperature measurements using HRPD. At a temperature of ∼86.8 K, magnetic scattering is observed consistent with the development of a G -type spin configuration in the majority tetragonal phase with a moment direction parallel to the c -axis, propagation vector k = 0 , magnetic space group I 4/ mcm. Below 75 K, in the pure orthorhombic phase, KMnF 3 is found to inherit the G -type ordering parallel to c in magnetic space group Pn ′ ma ' which in addition permits A -type antiferromagnetic ordering, and ferromagnetic ordering along the a- and b -axes respectively. • Phase III of KMnF 3 shown to be phase coexistence of phases I and IV. • Collinear antiferromagnetic structure reported in Shubnikov group I 4/ mcm. • Canted antiferromagnetisc structure reported in Shubnikov group Pn ′ ma ′. • Recent revision of space group of phase IV from Pnma to B 2 1 / m is shown to be incorrect. [ABSTRACT FROM AUTHOR]

Time-of-flight room temperature high resolution neutron powder diffraction data of NaTaO3 show a phase coexistence of two orthorhombic structures (space groups Cmcm and Pbnm) which are obtained by a phase transition at ≈763 K and which coexist down to room temperature and probably to 0 K. [ABSTRACT FROM AUTHOR]

Abstract: The thermal evolution and structural properties of fluorite-related δ-Bi2O3-type Bi9ReO17 were studied with variable temperature neutron powder diffraction, synchrotron X-ray powder diffraction and electron diffraction. The thermodynamically stable room-temperature crystal structure is monoclinic P21/c, a=9.89917(5), b=19.70356(10), c=11.61597(6)Å, β=125.302(2)° (R p=3.51%, wR p=3.60%) and features clusters of ReO4 tetrahedra embedded in a distorted Bi–O fluorite-like network. This phase is stable up to 725°C whereupon it transforms to a disordered δ-Bi2O3-like phase, which was modeled with δ-Bi2O3 in cubic Fm3¯m with a=5.7809(1)Å (R p =2.49%, wR p=2.44%) at 750°C. Quenching from above 725°C leads to a different phase, the structure of which has not been solved but appears on the basis of spectroscopic evidence to contain both ReO4 tetrahedra and ReO6 octahedra. [Copyright &y& Elsevier]

High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba2Bi3+Bi5+O6 (dibarium dibismuth hexaoxide) and Ba2BiSbO6 (dibarium bismuth antimony hexaoxide) in the temperature ranges 4.2–973 and 4.2–625 K, respectively. The charge-ordered bismuthate adopts four structures in the temperature range – monoclinic in P21/ n, monoclinic in I2/ m, rhombohedral in , and finally cubic in . The low-temperature monoclinic structure has been determined for the first time. The transitions from P21/ n to I2/ m, at 132 K, and to , at 820 K, are tricritical in nature; the transition from I2/ m to at ca 430 K is discontinuous. The behaviour of Ba2BiSbO6 is very similar, except that the transition temperatures are lower – 250 K for I2/ m to and 515 K for to – and the low-temperature structure is not formed at all. The to transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms. [ABSTRACT FROM AUTHOR]

Temperature dependent high resolution neutron diffraction studies demonstrate that the sequence and temperatures of the crystallographic phase transitions in NaTaO 3 are not impacted by doping with 1% K to form Na 0.99 K 0.01 TaO 3 . Rietveld analysis of the neutron diffraction data shows the structural transitions to be: P b n m ↔ 723       K C m c m ↔ 803   K P 4 / n b m ↔ 893   K P m 3 ¯ m . The two orthorhombic polymorphs, Pbnm and Cmcm , differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a continuous phase transition resulting in their co-existence between RT and 723 K. Chemical doping, does however, dramatically impact on the amount of the metastable Cmcm phase observed at room temperature. [ABSTRACT FROM AUTHOR]

The importance of sample history on the Pbnm-Cmcm phase coexistence in NaTaO 3 has been investigated using high resolution neutron and synchrotron X-ray diffraction methods. These results reveal that the phase coexistence, associated with the first order Pbnm-Cmcm phase transition, is not significantly affected by the annealing conditions used to prepare the sample. Rather it is found that compositional inhomogeneities or defects play a significant role in stabilising the high temperature Cmcm phase at room temperature. [ABSTRACT FROM AUTHOR]

The layered perovskite LaTaO4 has been prepared in its polar orthorhombic polymorphic form at ambient temperature. Although no structural phase transition is observed in the temperature interval 25° C ＜ T ＜ 500 °C, a very large axial thermal contraction effect is seen, which can be ascribed to an anomalous buckling of the perovskite octahedral layer. The non-polar monoclinic polymorph can be stabilised at ambient temperature by Nd-doping. A composition La0.90Nd0.10TaO4 shows a first-order monoclinic-orthorhombic (non-polar to polar) transition in the region 250° C ＜ T ＜ 350 °C. Dielectric responses are observed at both the above structural events but, despite the ‘topological ferroelectric’ nature of orthorhombic LaTaO4, we have not succeeded in obtaining ferroelectric P–E hysteresis behaviour. Structural relationships in the wider family of AnBnX3n+2 layered perovskites are discussed. [ABSTRACT FROM AUTHOR]

The mixed-valence manganite Sr0.65Pr0.35Mn03 has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the Mn06 octahedra about the c axis and a large Jahn-Teller-type distortion due to the presence of Mn3+. The sample exhibits a reversible phase transition from the cubic Pm3―m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm3―m ↔ I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn-Teller-type distortion of the Mn06 octahedra, exhibits an unusual et0.5 &agr; (Tc -- T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature TN of 250 K. The Mn magnetic moment at 7 K is 2.99(2) &mgr;B B/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting. [ABSTRACT FROM AUTHOR]

Abstract: 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 Nd1/3NbO3 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. [Copyright &y& Elsevier]