Orientational disorder and phase transitions in crystals of (NH4)2NbOF5

Structural phase transitions in a crystal of (NH4)2NbOF5 are the consequence of dynamic changes in its structural units as the temperature decreases. Using X-ray diffraction, it is possible to identify O and F atoms in the disordered structure of (NH4)2NbOF5 as a result of its dynamic nature

Optical and calorimetric studies of K2TaF7

Optical and calorimetric experiments on K2TaF7 are performed in a wide temperature range. No features were found in the behavior of the birefringence Δnb(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔCp(T) except for those associated with the Pnma↔P21/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pretransition phenomena in Δnb(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS=22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase

Heat capacity, thermal expansion and sensitivity to hydrostatic pressure of (NH4)3SiF7 at successive structural phase transitions

Heat capacity, thermal dilatation, permittivity and sensitivity to external pressure of (NH4)3SiF7 were studied. Due to the absence of cubic phases Pm3m and Pa3, a strong decrease in the total entropy change dSi = 19 J/molK associated with four successive transformations P4=mbm - Pbam - P21=c11 - P1 - P121/c1 was found in silicate in comparison with other double uoride salts (NH4)3MeF7 (Me: Ge, Ti, Sn). Using analysis of the excess heat capacity in the framework of the thermodynamic theory, the entropies associated with each individual phase transition were determined. In accordance with the entropic parameters, the complete ordering of the structural elements occurs in the monoclinic phase P21=c11. Further change in symmetry is associated with small entropy changes which prove insignicant displacement of structural units. A T-p phase diagram was constructed and good agreement was found between measured and calculated baric coefficients

Anomalous behaviour of thermodynamic properties at successive phase transitions in (NH4)3GeF7

Heat capacity, thermal dilatation, susceptibility to hydrostatic pressure and dielectric properties associated with succession of three phase transitions below room temperature in double fluoride salt (NH4)3GeF7 were studied. A possible transformation into the parent Pm-3m cubic phase was not observed up to the decomposition of compound. Nonferroelectric nature of structural distortions was confirmed. The DTA under pressure studies revealed a high temperature stability of two phases: P4/mbm and Pbam. The entropies of the phase transitions agree well with the model of structural distortions. Analysis of the thermal properties associated with the individual phase transitions in the framework of thermodynamic equations has shown a high reliability of the data obtained

Bonding and Structure of Oxofluoroniobate-Based Glasses

Glasses based on the oxofluoroniobate anion have been characterized by vibrational and solid-state NMR spectroscopy. The mechanism of glass formation in the systems K₂NbOF₅–MF₃ (M = Al, In) has been suggested. A glass network is built from the chains of corner-sharing octahedra through −Nb–F­(O)–M– and −Nb–F–Nb– bridges. Isolated NbOF₅^2– octahedra are also present, which is consistent with the glass composition. The high ionic mobility of NbOF₅^2– due to its fast reorientations results in equalization of the Nb–O and Nb–F distances, which is reflected in the appearance of the IR band at 700–800 cm^–1 not observed in the Raman spectrum. Its assignment to bridging −Nb–O–Nb– species accepted in the literature was not proven

Thermal, structural, optical, dielectric and barocaloric properties at ferroelastic phase transition in trigonal (NH4)2SnF6: A new look at the old compound

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Structural phase transition P-3m1 ↔ P-1 of the first order and nonferroelectric nature was found in (NH4)2SnF6 at about 110 K in the process of thermal, optical, dielectric and X-ray studies. Order-disorder transformation is accompanied by large changes of entropy (ΔS0 = 16.4 J/mol•K) and volume (ΔV0/V ≈ 1 %). The structural model associated with three orientations of NH4 tetrahedra in P-3m1 phase and their complete ordering in P-1 phase was suggested with the entropy change (ΔS0 = 18.3 J/mol•K) close to the experimental value. A good agreement between baric coefficients measured in a direct way (dT0/dp = -157 K/GPa) and evaluated using entropy and volume changes at phase transition (- 170 K/GPa) was found. Barocaloric effects in (NH4)2SnF6 are comparable with those of known solid state refrigerants and are characterised by a low-pressure need for realising their maximum values

Thermal, structural, optical, dielectric and barocaloric properties at ferroelastic phase transition in trigonal (NH4)2SnF6: A new look at the old compound

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Structural phase transition P-3m1 ↔ P-1 of the first order and nonferroelectric nature was found in (NH4)2SnF6 at about 110 K in the process of thermal, optical, dielectric and X-ray studies. Order-disorder transformation is accompanied by large changes of entropy (ΔS0 = 16.4 J/mol•K) and volume (ΔV0/V ≈ 1 %). The structural model associated with three orientations of NH4 tetrahedra in P-3m1 phase and their complete ordering in P-1 phase was suggested with the entropy change (ΔS0 = 18.3 J/mol•K) close to the experimental value. A good agreement between baric coefficients measured in a direct way (dT0/dp = -157 K/GPa) and evaluated using entropy and volume changes at phase transition (- 170 K/GPa) was found. Barocaloric effects in (NH4)2SnF6 are comparable with those of known solid state refrigerants and are characterised by a low-pressure need for realising their maximum values

Synthesis, Structural, Magnetic, and Electronic Properties of Cubic CsMnMoO₃F₃ Oxyfluoride

A powder sample of CsMnMoO₃F₃ oxyfluoride has been prepared by solid state synthesis. The pyrochlore-related crystal structure of CsMnMoO₃F₃ has been refined by the Rietveld method at <i>T</i> = 298 K (space group <i>Fd-</i>3<i>m</i>, <i>a</i> = 10.59141(4) Å, <i>V</i> = 1188.123(8) ų; <i>R</i>_B = 3.44%). The stability of the cubic phase has been obtained over the temperature range <i>T</i> = 110–293 K by heat capacity measurements. Magnetic properties have been measured over the range of <i>T</i> = 2–300 K. The electronic structure of CsMnMoO₃F₃ has been evaluated by X-ray photoelectron spectroscopy. Chemical bonding effects have been discussed for all metal ions using binding energy difference parameters and wide comparison with related oxides and fluorides. The competition between O^2– and F^– ions for metal valence electrons has been found