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The response of the trimethylammonium–iodinechloride and diiodide (TMA–ICl/I2) crystal structures have been examined under high pressure using neutron powder diffraction. TMA–ICl exhibits impressive pressure-driven electronic flexibility, where the N⋯I–Cl interactions progressively encompass all the distances represented in analogous structures recorded in the Cambridge Structural Database. Comparison with the TMA–I2 complex reveals that this flexibility is owed to the electronegativity of the chlorine atom which induces increased distortion of the iodine electron cloud. This structural flexibility may be influential in the future design of functional molecular materials. [ABSTRACT FROM AUTHOR]

Experimental and computational studies of ammonium carbamate have been carried out, with the objective of studying the elastic anisotropy of the framework manifested in (i) the thermal expansion and (ii) the compressibility; furthermore, the relative thermodynamic stability of the two known polymorphs has been evaluated computationally. Using high-resolution neutron powder diffraction data, the crystal structure of α-ammonium carbamate (ND4αND2CO2) has been refined [space group Pbca, Z = 8, with a = 17.05189 (15), b = 6.43531 (7), c = 6.68093 (7) A ° and V = 733.126 (9) A ° 3 at 4.2 K] and the thermal expansivity of α-ammonium carbamate has been measured over the temperature range 4.2-180 K. The expansivity shows a high degree of anisotropy, with the b axis most expandable. The ab initio computational studies were carried out on the α- and β-polymorphs of ammonium carbamate using density functional theory. Fitting equations of state to the P(V) points of the simulations (run athermally) gave the following values: V0 = 744 (2) A³ and bulk modulus K0 = 16.5 (4) GPa for the α-polymorph, and V0 = 713.6 (5) A³ and K0 = 24.4 (4) GPa for the β-polymorph. The simulations show good agreement with the thermoelastic behaviour of α-ammonium carbamate. Both phases show a high-degree of anisotropy; in particular, α-ammonium carbamate shows unusual compressive behaviour, being determined to have negative linear compressibility (NLC) along its a axis above 5 GPa. The thermodynamically stable phase at ambient pressure is the α-polymorph, with a calculated enthalpy difference with respect to the β-polymorph of 0.399 kJ molβ1; a transition to the β-polymorph could occur at ~0.4 GPa. [ABSTRACT FROM AUTHOR]

Crystallographic distortions in the alternating aluminium and silicon tetrahedral framework of sodalite (Na8Al6Si6O24Cl2), and beryllium and silicon in helvine (Mn8Be6Si6O24S2), (framework designated SOD) are described in terms of a set of condensed normal mode amplitudes and phases derived from an ideal tetrahedron of a theoretical aristotype phase. For a sodalite-structured hettotype phase in space group $P{\overline 4} 3n$ , these normal modes transform as the irreducible representations A1, E(α) and T1(z) of point-group ${\overline 4} 3m$ , where to a good approximation A1 acts as a pure breathing mode, E(α) as a polyhedral distortive mode and T1(z) as a rigid unit rotation about the unique ${\overline 4}$ axis of the T-site under consideration. Parameterisation of the mode amplitudes in terms of low-order polynomials as a function of thermodynamic variable permits the crystal structure of sodalite-structured phases to be accurately interpolated at intermediate values of the thermodynamic variable. Published data for the high-temperature behaviour of sodalite have been re-analysed in terms of mode amplitudes which accurately reproduce the temperature dependence of the bond lengths, bond angles and the Al–O–Si inter-polyhedral angle. Full expressions for these derived structural parameters in terms of mode amplitudes and the lattice parameter are tabulated and agree with experimental results to within one estimated standard deviation of the experimental parameter. The potential for mode decomposition in lower symmetry SOD framework crystal structures is illustrated by deriving an aristotype structure for tugtupite (Na8Al2Be2Si8O24Cl2) at room temperature in space group $I{\overline 4}$. [ABSTRACT FROM AUTHOR]

A universal aristotype crystal structure for leucite/pollucite-structured phases (A2B2IIIC4IVO12, A2BIIC5IVO12) with space group I a 3 ¯ d has been determined by a non-linear optimisation of the fractional coordinates. The resultant T-site tetrahedron exhibits close to perfect 4 ¯ 3 m point group symmetry (quadratic elongation 1.0000124, tetrahedral angle variance 1.53 × 10−5 degrees2). For any distorted leucite/pollucite crystal structure, an associated aristotype phase can be derived from the optimised fractional coordinates, with the aristotype phase lattice parameter calculated from the volume of the hettotype T-site tetrahedron. Assuming the aristotype T-site tetrahedron to be ideal, the distortion of the hettotype tetrahedron from that of the aristotype is described as the linear sum of the amplitudes of an appropriate set of symmetry-adapted basis-vectors of the effective regular TO4 molecule consistent with the observed point group symmetry within the crystal structure. The methodology is illustrated by reference to the disordered T-site, leucite-structured phase K2MgSi5O12. [ABSTRACT FROM AUTHOR]

The synthesis of polycrystalline samples of B-site doped SrRu1−xMxO3 with x≤ 0.2 by solid state methods is described for a number of dopants (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) and the structures of these established using Synchrotron X-ray powder diffraction, and for SrRu0.8Cu0.2O3 high resolution neutron diffraction. With the exception of M = Cu, samples with x = 0.2 form an orthorhombic Pbnm type perovskite structure at room temperature and these exhibit a sequence of phase transitions upon heating associated with the gradual reduction in the cooperative tilting of the corner sharing octahedra. SrRu0.8Cu0.2O3 forms a unique monoclinic structure at low temperatures and this transforms to the cubic Pm3̅m structure via an I4/mcm intermediate upon heating. The magnetic and electronic properties of the samples have been studied. Doping results in a decrease in the Curie temperature and at x = 0.2 all the samples are insulators. This is a consequence of the partial oxidation of the Ru cation that narrows the Ru 4d bands coupled with the suppression of the itinerant nature of the Ru 4d electrons due to the random distribution of the dopant cations. Ru L3-edge X-ray absorption spectroscopy of the Cu doped samples reveal a gradual increase in the average Ru oxidation sate upon doping. Electrical resistivity measurements show that doping increases the resistivity of the samples, and the temperature dependence of the resistivities are consistent with Arrhenius-type charge conduction. [ABSTRACT FROM AUTHOR]

Abstract Non-linear optimisation of the anion coordinates in garnet-structured compounds (A 3 B 2 C 3 O 12) with space group I a 3 ¯ d has been used to define a universal prototypic crystal structure in which the tetrahedral site (C O 4) and the octahedral site (B O 6) exhibit close to the ideal point group symmetries of 4 ¯ 3 m and m 3 ¯ m respectively. Distortions in actual garnet-structured compounds are then described in terms of condensed normal modes of the two isolated, ideal polyhedra that are consistent with the site point group symmetry within the space group I a 3 ¯ d. The methodology is applied to the pressure-dependence of the crystal structure of Y 3 Al 5 O 12 from 0 GPa to 126 GPa that have been determined from first-principles calculations. The structural evolution of Y 3 Al 5 O 12 with pressure is dominated by changes in the AlivO 4 site which rotates and becomes significantly more regular as pressure increases. The coupled response of the octahedral site is more complex as it responds to the changes in the tetrahedral site. The evolution of the crystal structure of YAG with pressure is briefly compared to the experimentally determined pressure-dependence of the crystal structure of pyrope, Mg 3 Al 2 Si 3 O 12. Graphical abstract Image 1 Highlights • The crystal structure of garnet is derived in terms of condensed normal modes. • Normal modes consistent with site point group symmetry. • Tetrahedral site modes transform as the irreducible representations A 1 , E , T 1. • Octahedral site modes transform as the irreducible representations A 1g , T 2g , T 1g. • Application to the pressure dependence of the crystal structure of Y 3 Al 5 O 12. [ABSTRACT FROM AUTHOR]

The temperature dependence of the crystal structure and thermoelastic properties of RbCaF 3 perovskite are reported from a high-resolution neutron powder diffraction investigation in the temperature interval 2–300 K. Two phase transitions have been identified from these data, the first from space group P m 3 ¯ m to I 4/ mcm , at 200 K, has been well characterized in earlier studies, the second, from space group to I 4/ mcm to Pbnm has previously been less well understood. New results from the former transition show the order parameter evolves with a critical exponent of 0.284(5), and the phase is characterized by an Einstein temperature of 192(8) K, and a Debye temperature of 270(10) K. The temperature variation of the tetragonal spontaneous strain in this phase is consistent with an additional fourth order invariant in the Landau potential. The second phase transition is found to be strongly first order and a temperature dependent, and heating rate/cooling rate dependent phase coexistence is found at all temperatures below ~ 80 K on heating, and ~ 65 K on cooling. The evolution of the orthorhombic unit cell is atypical in the region of the phase transition, with a and b exhibiting strong negative linear thermal expansion, whilst the c axis behaves in the expected manner. The crystal structure of the Pbnm phase at 2 K is reported from an unconstrained Rietveld refinement for the first time. [ABSTRACT FROM AUTHOR]

Refinement of time-of-flight high-resolution neutron powder diffraction data for lueshite (Na, Ca)(Nb, Ta, Ti)O3, the natural analogue of synthetic NaNbO3, demonstrates that lueshite at room temperature (298 K) adopts an orthorhombic structure with a 2ap × 2ap × 4ap superlattice described by space group Pmmn [#59: a = 7.8032(4) Å; b = 7.8193(4) Å; c = 15.6156(9) Å]. This structure is analogous to that of phase S of synthetic NaNbO3 observed at 753–783 K (480–510 °C). In common with synthetic NaNbO3, lueshite exhibits a series of phase transitions with decreasing temperature from a cubic (Pm3¯m) aristotype through tetragonal (P4/mbm) and orthorhombic (Cmcm) structures. However, the further sequence of phase transitions differs in that for lueshite the series terminates with the room temperature S (Pmmn) phase, and the R (Pmmn or Pnma) and P (Pbcm) phases of NaNbO3 are not observed. The appearance of the S phase in lueshite at a lower temperature, relative to that of NaNbO3, is attributable to the effects of solid solution of Ti, Ta and Ca in lueshite. [ABSTRACT FROM AUTHOR]

The structure of the complex formed between trimethylamine and iodine monochloride had been redetermined by a combination of neutron powder diffraction and X-ray single crystal diffraction techniques. The structure determined using NPD shows that the geometry of the trimethylamine moiety resembles that in trimethylammonium cations rather than in the free trimethylamine molecule. The structure shows the presence of weak C–D…Cl hydrogen bonds, The I–Cl bond length is in agreement with that derived from an analysis of the pKb values of a series of nitrogen bases and previously determined structures containing N…ICl fragments. [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 thermoelastic and structural properties of SrZrO 3 perovskite in the Pnma ( Pbnm ) phase have been studied using neutron powder diffraction at 82 temperatures between 11 K and 406 K at ambient pressure, and at sixteen pressures between 0.07 and 6.7 GPa at ambient temperature. The bulk modulus, derived by fitting the equation of state to a second order Birch-Murnaghan equation-of-state, 157(5) GPa, is in excellent agreement with that deduced in a recent resonant ultrasound investigation. Experimental axial compressional moduli are in agreement with those calculated from the elastic stiffness coefficients derived by ab-initio calculation, although the experimental bulk modulus is significantly softer than that calculated. Following low temperature saturation for temperatures less than 40 K, the unit cell monotonically increases with a predicted high temperature limit in the volume expansivity of ∼2.65 × 10 −5 K −1 . Axial linear thermal expansion coefficients are found to be in the order α b < α c < α a for all temperatures greater than 20 K with the b axis indicating a weak, low temperature negative expansion coefficient at low temperatures. The thermoelastic properties of SrZrO 3 can be approximated by a two-term Debye model for the phonon density of states with Debye temperatures of 238(4) K and 713(6) K derived in a self-consistent manner by simultaneously fitting the isochoric heat capacity and the unit cell volume. Atomic displacement parameters have been fitted to a modified Debye model in which the zero-point term is an additional refinable variable and shows the cations and anions have well separated Debye temperatures, mirroring the need for two Debye-like distributions in the vibrational density of states. The temperature dependence of the crystal structure is presented in terms of the amplitudes of the seven symmetry-adapted basis vectors of the aristotype phase that are consistent with space group Pbnm , thus permitting a direct measure of the order parameter evolution in SrZrO 3 . The temperature variation of the in-phase tilt, which is lost at the phase transition at 973 K, is consistent with tricritical behaviour, in agreement with published results based on high temperature crystallographic data. [ABSTRACT FROM AUTHOR]

We have identified a new compound in the glycine-MgSO4-water ternary system, namely glycine magnesium sulfate trihydrate (or Gly·MgSO4·3H2O) {systematic name: catena-poly[[tetraaquamagnesium(II)]-μ-glycine-κ2 O: O′-[diaquabis(sulfato-κ O)magnesium(II)]-μ-glycine-κ2 O: O′]; [Mg(SO4)(C2D5NO2)(D2O)3] n}, which can be grown from a supersaturated solution at ∼350 K and which may also be formed by heating the previously known glycine magnesium sulfate pentahydrate (or Gly·MgSO4·5H2O) {systematic name: hexaaquamagnesium(II) tetraaquadiglycinemagnesium(II) disulfate; [Mg(D2O)6][Mg(C2D5NO2)2(D2O)4](SO4)2} above ∼330 K in air. X-ray powder diffraction analysis reveals that the trihydrate phase is monoclinic (space group P21/ n), with a unit-cell metric very similar to that of recently identified Gly·CoSO4·3H2O [Tepavitcharova et al. (2012). J. Mol. Struct. 1018, 113-121]. In order to obtain an accurate determination of all structural parameters, including the locations of H atoms, and to better understand the relationship between the pentahydrate and the trihydrate, neutron powder diffraction measurements of both (fully deuterated) phases were carried out at 10 K at the ISIS neutron spallation source, these being complemented with X-ray powder diffraction measurements and Raman spectroscopy. At 10 K, glycine magnesium sulfate pentahydrate, structurally described by the `double' formula [Gly( d5)·MgSO4·5D2O]2, is triclinic (space group P, Z = 1), and glycine magnesium sulfate trihydrate, which may be described by the formula Gly( d5)·MgSO4·3D2O, is monoclinic (space group P21/ n, Z = 4). In the pentahydrate, there are two symmetry-inequivalent MgO6 octahedra on sites of symmetry and two SO4 tetrahedra with site symmetry 1. The octahedra comprise one [tetraaquadiglcyinemagnesium]2+ ion (centred on Mg1) and one [hexaaquamagnesium]2+ ion (centred on Mg2), and the glycine zwitterion, NH3+CH2COO−, adopts a monodentate coordination to Mg2. In the trihydrate, there are two pairs of symmetry-inequivalent MgO6 octahedra on sites of symmetry and two pairs of SO4 tetrahedra with site symmetry 1; the glycine zwitterion adopts a binuclear-bidentate bridging function between Mg1 and Mg2, whilst the Mg2 octahedra form a corner-sharing arrangement with the sulfate tetrahedra. These bridged polyhedra thus constitute infinite polymeric chains extending along the b axis of the crystal. A range of O-H...O, N-H...O and C-H...O hydrogen bonds, including some three-centred interactions, complete the three-dimensional framework of each crystal. [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]

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]

The synthesis of polycrystalline samples of B-site doped SrRu1−xMxO3 with x≤ 0.2 by solid state methods is described for a number of dopants (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) and the structures of these established using Synchrotron X-ray powder diffraction, and for SrRu0.8Cu0.2O3 high resolution neutron diffraction. With the exception of M = Cu, samples with x = 0.2 form an orthorhombic Pbnm type perovskite structure at room temperature and these exhibit a sequence of phase transitions upon heating associated with the gradual reduction in the cooperative tilting of the corner sharing octahedra. SrRu0.8Cu0.2O3 forms a unique monoclinic structure at low temperatures and this transforms to the cubic Pm3̅m structure via an I4/mcm intermediate upon heating. The magnetic and electronic properties of the samples have been studied. Doping results in a decrease in the Curie temperature and at x = 0.2 all the samples are insulators. This is a consequence of the partial oxidation of the Ru cation that narrows the Ru 4d bands coupled with the suppression of the itinerant nature of the Ru 4d electrons due to the random distribution of the dopant cations. Ru L3-edge X-ray absorption spectroscopy of the Cu doped samples reveal a gradual increase in the average Ru oxidation sate upon doping. Electrical resistivity measurements show that doping increases the resistivity of the samples, and the temperature dependence of the resistivities are consistent with Arrhenius-type charge conduction. [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]

The temperature dependence of the unit-cell dimensions and the crystal structures of the fluorperovskite neighborite, NaMgF3 (analyzed in the Pbnm setting of the space group), have been determined at 88 temperatures between 9 and 440 K from high-resolution, time-of-flight, powder neutron diffraction data. Lattice parameters exhibit saturation at low temperatures, before developing linear thermal expansion coefficients at due temperatures above ~350 K. The temperature dependence of each axis has been analyzed, and fitted, using a two-term expression related to an Einstein internal energy function. The unit-cell parameters a and c behave in a conventional manner, however, an unexpected, and previously unobserved, region of negative linear thermal expansion has been found for the b axis in the temperature interval 20 ≤ T ≤ 90 K. Estimated, high-temperature axial thermal expansion coefficients derived from the lattice parameter fitting are in good agreement with those experimentally determined from an earlier synchrotron study, and indicate that high-temperature saturation has been achieved in neighborite by 440 K. The unit-cell volume varies smoothly and monotonically over the whole temperature interval, and the two-term Debye model of Barron has been successfully used to fit these data with characteristic temperatures of 369(2) and 1055(14) K. For the temperature interval 313.15-443.15 K, the thermodynamic Grüneisen constant has been determined using the Debye parameterization of the unit-cell volume, coupled with literature values of the isobaric molar heat capacity. The evolution of the crystal structure as a function of temperature is presented, and explained, in terms of the temperature dependence of the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase that are consistent with the orthorhombic space group. The calculated temperature variations of the bond lengths are in excellent agreement with those experimentally determined. The primary order parameters for centrosymmetric, zone-boundary phase transitions in perovskite-structured compounds, i.e., the amplitudes of the basis vectors that transform as the irreducible representations R+4 (anti-phase tilt) and M+3 (in-phase tilt), have been fitted to a Landau free energy expansion that incorporates low-temperature saturation. Within the temperature range studied, the temperature dependence of the displacement corresponding to the anti-phase tilt is consistent with tricritical behavior. Experimental evidence is presented for a quadratic coupling of the in-phase tilt to the anti-phase tilt for temperatures greater than ~135 K, suggesting critical behavior at the orthorhombic-cubic transition is purely related to an instability at the R point of the pseudocubic Brillouin zone. [ABSTRACT FROM AUTHOR]

MgTi2O5 (karrooite) exhibits cation exchange between the two non-equivalent octahedral M1 and M2 sites. The temperature dependence of Ti-Mg disorder has been determined using in situ time-of-flight powder neutron diffraction to establish the cation population of each site. The equilibrium Ti-Mg exchange commences above 700.800 °C, and continues up to 1300 °C. At ∼1350 °C MgTi2O5 appears to undergo a reversible, displacive phase transition, although the structure can still be refined in space group 63. This transition shows discontinuities in the degree of order, the c cell parameter, and in the M-O bond lengths, quadratic elongation, and bond angle variance for the M1 octahedron, which becomes pseudo-tetrahedral. By contrast, the M2 octahedron shows no significant change. An Arrhenius plot exhibits linear behavior from 750.1300 °C, gives an exchange enthalpy of ordering of 33.6 kJ/mol, and a value of 10.7 kJ/mol for the entropy of disordering. A value of 5.92 cm³/mol is deduced for the ΔV of the high-temperature reaction of geikielite (MgTiO3) + rutile (TiO2) to form fully disordered MgTi2O5 at an extrapolated temperature of 1860 °C. [ABSTRACT FROM AUTHOR]

The thermal evolution of the crystal structure of the prototypical orthorhombic perovskite LaFeO 3 has been studied in detail by powder neutron diffraction in the temperature range 25< T <1285 K. A conventional bond length/bond angle analysis, combined with an analysis in terms of symmetry-adapted modes, allows key aspects of the thermal behavior to be understood. In particular, the largest-amplitude symmetry modes ( viz. in-phase and out-of-phase octahedral tilts, and A-site cation displacements) are shown to display relatively ‘normal’ behavior, increasing with decreasing temperature, which contrasts with the anomalous behavior previously shown by the derivative Bi 0.5 La 0.5 FeO 3 . However, an unexpected behavior is seen in the nature of the intra -octahedral distortion, which is used to rationalize the unique occurrence of a temperature dependent crossover of the a and c unit cell metrics in this compound. [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]

In this article we comment on the results published by Thompson et al. (, J. Solid State Chem. 219 (2014) 173–178) on the crystal structure of SrFeO 2 F, who claim the compound to crystallize in the cubic space group Pm -3 m . We give a more detailed explanation of the determination of our previously reported structural model with Imma symmetry (Clemens et al., J. Solid State Chem. 206 (2013) 158–169), with addition of variable temperature XRD measurements with high counting time to provide unambiguous evidence for the Imma model being correct for our sample. [ABSTRACT FROM AUTHOR]

The compounds 15R-BaFeO2F and 15R-BaFeO2.27F0.5 have been synthesised by the low temperature fluorination of 15R-BaFeO3−dF0.2 using polyvinylidenedifluoride (PVDF) as a fluorination agent. The materials have been structurally characterised by Rietveld analysis of the X-ray- and HRPD-powder neutron diffraction data. A detailed analysis of bond valence sums suggests that the oxide and fluoride ions order on the different anion sites. A reinvestigation of our recently published structure (Clemens et al., 2013) [34] of 6H-BaFeO2F is also reported and incorporation of fluoride in h-type layers is also confirmed in this compound. The magnetic moments for 15R-BaFeO2F and 15R-BaFeO2.25F0.5 align in the a/b-plane with antiferromagnetic alignment of the moments between adjacent layers, and are flipped by 90° as compared to the precursor compound. 15R-BaFeO2F exhibits very robust antiferromagnetism with a Néel temperature between 300 and 400°C. [ABSTRACT FROM AUTHOR]

We have studied the crystal structure of mixed-valence Sr0.7Ce0.3MnO3 from 4.2 to 973 K using high-resolution neutron powder diffraction. The crystal structure is tetragonal in space group I4/mcm at 4.2-923 K and cubic in Pm3m at T ≥ 948 K. Lattice parameters and Mn-O bond distances, obtained by Rietveld refinement, have been used to derive the spontaneous strains and MnO6 octahedral distortion, which are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3m reference structure with two instabilities (R+4 and &Tao;+3). Two phase transitions were proposed: an octahedral tilting transition at Tc,ø ∼ 938 K (Pm3m &xharr; I4/mcm, R+4), and an isosymmetric, electronically driven (Jahn-Teller-like) transition at Tc,JT ∼ 770 K (I4/mcm, R+4 &xharr; I4/mcm, (R+4 and &Tao;+3)). The nature of the tilting transition appears to be tricritical, while that of the Jahn-Teller-like transition is second order. In addition to the contributions from octahedral tilting and Jahn-Teller-like distortions, there is an excess octahedral distortion at temperatures below 250 K; this is speculated to be associated with an anomaly observed over the temperature range of 275-300 K in the heat-capacity measurements. [ABSTRACT FROM AUTHOR]