Structural phase transitions in BaV6O11

BaV₆O₁₁ was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. [V(1)O₆]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal [0 0 1] direction. The layers are connected in the direction of c via trigonal [V(3)O₅]-bipyramids and [V(2)O₆]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P6₃/mmc, at lower temperature the symmetry reduces to P6₃mc. For the refinements in P6₃mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the [V(1)O₆]-octahedra and [V(3)O₅]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.     

Crystal structures and phase transitions of Sr2CrSbO6

Sr₂CrSbO₆ was synthesized by the conventional solid-state reaction process. X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) has been used to reinvestigate the structure at room temperature and to study the phase transitions at high- and low-temperature. Rietveld analysis revealed that Sr₂CrSbO₆ crystallizes at room temperature in a monoclinic system having a space group I2/m, with a=5.5574(1) Å; b=5.5782(1) Å; c=7.8506(2) Å and β=90.06(2), no P2₁/n space group as was previously reported. The high-temperature study (300-870 K) has shown that the compound presents the following temperature induced phase-transition sequence: I2/m-I4/m-Fm-3m. The low-temperature study (100-300 K) demonstrated that the room-temperature I2/m monoclinic symmetry seems to be stable down to 100 K.     

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.     

NQR and phase transitions in mixed A2BX6 compounds

³⁵Cl NQR frequency and phase transition temperature were investigated in the mixed crystals (A_xK_1?x) SnCl₆ (A = Rb, NH₄) and K₂ (Re_x Sn_1?x) Cl₆. The frequency and the intensity of the NQR signal were measured in the temperature range 100 K to 300 K for different chemical compositions. Phase transition temperatures were determined from the modifications in the line pattern. The observed change of the transition temperature as a function of concentration x can be fitted by the relation T_c(x) = T_c(O) (1 + a x)/(1 + b x). This formula is explained in the context of the virtual crystal approximation and the Landau theory.     

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.     

NMR study of hindered rotation in solid (ND4)2GeF6

Nuclear spin-lattice relaxation times T₁ for deuterons and ¹⁹F nuclei in polycrystalline (ND₄)₂GeF₆ were measured by the pulse method at 8 MHz between 40 K and 300 K and between 4 K and 400 K, respectively. Correlation times and activation energies for the reorientational motions of ND₄⁺ and GeF₆^2? ions were calculated from the measured T₁ values.     

Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6, a NMR and DFT study

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH₄)₂NbOF₅ and (NH₄)₃NbOF₆ were studied by ¹⁹F, ¹H NMR and DFT calculations. Equatorial fluorine atoms (F_eq) in [NbOF₅]²⁻ and [NbOF₆]³⁻ are characterized by high ¹⁹F NMR chemical shifts while axial fluorine atoms (F_ax) have those essentially lower. The high-temperature ionic mobility in (NH₄)₂NbOF₅ does not stimulate the ligand exchange F_eq ↔ F_ax, whereas it is observed in (NH₄)₃NbOF₆ as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF₆]³⁻ into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH₄)₂NbOF₅ is accompanied by transition of anionic sublattice to a rigid state. The ¹⁹F and ¹H NMR data corroborate the independent motions of NH₄ groups and anionic polyhedra in (NH₄)₂NbOF₅ while they are coordinated in (NH₄)₃NbOF₆.     

Phase transitions in K3AlF6

Phase transitions in the elpasolite-type K₃AlF₆ complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures , and . The α-K₃AlF₆ phase is stable below T₁ and crystallizes in a monoclinic unit cell with a=18.8588(2)Å, b=34.0278(2)Å, c=18.9231(1)Å, β=90.453(1)° (a=2a_c−c_c, b=4b_c, c=a_c+2c_c; a_c, b_c, c_c—the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate β phase exists only in very narrow temperature interval between T₁ and T₂. The γ polymorph is stable in the T₂<T<T₃ temperature range and has an orthorhombic unit cell with a=36.1229(6)Å, b=17.1114(3)Å, c=12.0502(3)Å (a=3a_c−3c_c, b=2b_c, c=a_c+c_c) at 250 °C and space group Fddd. Above T₃ the cubic δ polymorph forms with a_c=8.5786(4)Å at 400 °C and space group . The similarity between the K₃AlF₆ and K₃MoO₃F₃ compounds is discussed.     

固体C~6~0取向相变的理论研究

本文利用分子间相互作用势模型研究了固体C~6~0的各种可能的取向状态,根据不同温度下取向运动规律计算出了面心立方到简立方以及到取向玻璃态的两个取向相变的温度,并研究了压力对取向相变的影响,所得结果与实验结果符合     

Anomalous octahedral distortion and multiple phase transitions in the metal-ordered manganite YBaMn2O6

By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determined the crystal structures of a metal-ordered manganite YBaMn₂O₆ which undergoes successive phase transitions. A high-temperature metallic phase (T_c1=520 K<T) crystallizes in a triclinic P1 with the following unit cell: Z=2, a=5.4948(15) Å, b=5.4920(14) Å, c=7.7174(4) Å, α=89.804(20)°, β=90.173(20)°, γ=91.160(4)°. The MnO₆ octahedral tilting is approximately written as a⁰b⁻c⁻, leading to a significant structural anisotropy within the ab plane. The structure for T_c2<T<T_c1 is a monoclinic P2 (Z=2, a=5.5181(4) Å, b=5.5142(4) Å, c=7.6443(3) Å, β=90.267(4)°) with an a⁻b⁻c⁻ tilting. The structural features suggest a d_x²−y² orbital ordering (OO). Below T_c2=480 K, crystallographically inequivalent two octahedra show distinct volume difference, due to the Mn³⁺/Mn⁴⁺ charge ordering. The TEM study furthermore revealed a unique d_3x²−r²/d_3y²−r² OO with a modified CE structure. It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, the extremely high temperature at which charge degree of freedom freezes, T_c2, should be caused by the absence of the structural disorder and by heavily distorted MnO₆ octahedra.     

Crystal data and phase transitions of KLiWO4 and KLiMoO4

Crystal data and phase transitions of KLiWO₄ and KLiMoO₄ were investigated using a high-temperature X-ray powder diffractometer. Phase transitions were observed at 350°C for KLiWO₄ and at 360°C for KLiMoO₄. It was found that the room temperature phases were isostructural with stuffed derivatives of tridymite and the high-temperature phases were isostructural with stuffed derivatives of cristobalite.     

Etude radiocristallographique et calorimetrique des transitions de phase de Ag8GeTe6

Powder and single crystals of the Ag₈GeTe₆ phase have been prepared by direct synthesis and chemical vapor transport reaction (iodine), respectively. The low-temperature phase of this material has been investigated by differential scanning calorimetry analysis and X-ray diffraction. The phase transitions are interpreted as a result of low-temperature ordering of the Ag⁺ ions.     

Pressure-induced phase transitions in Al2(WO4)3

The high pressure behavior of aluminum tungstate [Al₂(WO₄)₃] has been investigated up to ∼18 GPa with the help of Raman scattering studies. Our results confirm the recent observations of two reversible phase transitions below 3 GPa. In addition, we find that this compound undergoes two more phase transitions at ∼5.3 and ∼6 GPa before transforming irreversibly to an amorphous phase at ∼14 GPa.     

Crystal structures and phase transitions of SrZr(PO4)2-BaZr(PO4)2 solid solutions

A complete series of solid solutions was prepared in the SrZr(PO₄)₂-BaZr(PO₄)₂ system and examined by conventional X-ray powder diffraction (XRPD). The crystals of Sr_xBa_1−xZr(PO₄)₂ with x?0.1 were isomorphous with yavapaiite (KFe(SO₄)₂, space group C2/m). The solid solution with 0.2?x?0.7 has been composed of a new phase, showing a superstructure along the a-axis (c-axis of the yavapaiite substructure). The crystals with 0.8?x?0.9 were composed of both the new phase and the triclinic phase, the latter being isostructural with SrZr(PO₄)₂ (x=1). The crystal structure of the new phase has been determined using direct methods, and it has been further refined by the Rietveld method. The crystal of Sr_0.7Ba_0.3Zr(PO₄)₂ (x=0.7) is monoclinic (space group P2/c, Z=4 and D_x/Mg m⁻³=3.73) with a=1.53370(8) nm, b=0.52991(3) nm, c=0.84132(4) nm, β=92.278(1)° and V=0.68321(6) nm³. Final reliability indices are R_wp=7.32%, R_p=5.60% and R_B=3.22%. The powder specimen was also examined by high-temperature XRPD and differential thermal analysis (DTA) to reveal the occurrence of two phase transitions during heating; the space group changed from P2/c to C2/m at ∼400 K, followed by the monoclinic-to-hexagonal (or trigonal) transition at 1060 K. The P2/c-to-C2/m transition has been, for the first time, described in the yavapaiite-type compounds.     

Structure cristalline et moleculaire de KSbF3NO3.

The molecular addition compound KSbF₃NO₃ has been isolated from the SbF₃-KNO₃ system. Its structure has been determined from X-ray single crystal data in the space group Pbca. The final R factor is 0.044. The compound consists of ¦SbF₃NO₃¦^x?_x anions, which form double layers of linked SbF₃(NO₃)₃ monocapped octahedra, perpendicular to the z axis. The infrared and Raman spectra are correlated with the structural results.     

Dilatometric, dielectric and NMR studies of structural phase transitions of the (CH3NH3)3Bi2Cl9 (MACB) crystals

The phase transitions and molecular motions of the methylammonium cations were investigated in the (CH₃NH₃)₃Bi₂Cl₉ (MACB) crystal by dilatometric and dielectric measurements, and by the measurements of the ¹H spin-lattice relaxation times and second moment of the ¹H NMR absorptions over a wide temperature range. Structural phase transitions, weakly first order at 247 K (III ↔ II) and continuous at 352 K (II ↔ I), were detected by the dilatometric technique. The ¹H NMR measurements revealed the presence of the uniaxial reorientations of the three non-equivalent methylammonium cations in the lowest temperature phase (III).     

Vibrational and calorimetric study of phase transitions in n-C6F14 and n-C8F18 crystals

Heat capacities of n-perfluorohexane and n-perfluorooctane were measured from 4.2 K to room temperature. One solid—solid transition is found for n-C₆H₁₄,two for n-C₈F₁₈. Spectra show that the molecules have the same conformations in the two forms of n-C₆F₁₄. Several isomers are present in the high-temperature forms of n-C₈F₁₈;thus transitions in this compound have order—disorder character.     

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.     

Multinuclear NMR studies on substituted derivatives of Rh6(CO)16 in solution

Multinuclear NMR data (¹³C, ³¹P, ¹³C–{³¹P}, ¹³C–{¹⁰³Rh} and ³¹P–{¹⁰³Rh}) for a series of mono- and di-substituted derivatives of Rh₆(CO)₁₆ containing neutral two electron donor ligands [Rh₆(CO)₁₅L, (L=NCMe, py, cyclooctene, PPh₃, P(OPh)₃,1/2(μ₂,η¹:η¹-dppe)); Rh₆(CO)₁₄(LL), (LL=cis-CH₂=CMe-CMe=CH₂, dppm, dppe, (P(OPh)₃)₂)] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the ¹³CO NMR spectra of Rh₆(CO)₁₅(PPh₃) and Rh₆(CO)₁₄(dppm) clusters have been made on the basis ¹³C–{¹⁰³Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of ³J(P–C) for terminal carbonyl ligands is discussed and the values of ³J(P–C) are found to be mainly dependent on the bond angles in the P–Rh–Rh–C fragment; these data enable the fine structure of the complex multiplets in the ¹³C–{¹H} and ³¹P–{¹H} NMR spectra of Rh₆(CO)₁₄ (dppm) to be simulated. Variable temperature ¹³C–{¹H} NMR measurements on Rh₆(CO)₁₅(PPh₃) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh₃ on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ₃-CO's.