Conventional and inverse barocaloric effects in ferroelectric NH4HSO4

In this study, the conventional and inverse barocaloric effects (BCE) in ferroelectric NH4HSO4 are reported. Maximum extensive and intensive BCE near orderedisorder phase transition can be achieved at low pressure p < 0.1 GPa. Large thermal expansion of the crystal lattice plays a very important role in the developing conventional BCE and conversation between BCE of different sign in the narrow temperature range

T − p phase diagrams and barocaloric effect in materials with successive phase transitions

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Abstract An analysis of the extensive and intensive barocaloric effect (BCE) at the successive structural phase transitions in some complex fluorides and oxyfluorides was performed. A high sensitivity of these compounds to a change of the chemical pressure allows one to vary successions and parameters of the transformations (temperature, entropy, baric coefficient) in wide ranges and obtain the optimal values of BCE. A comparison of different types of the schematic T − p phase diagrams with the complicated T(p) dependences observed experimentally shows that in some ranges of temperature and pressure BCE in compounds undergoing successive transformations can be increased due to summation of caloric effects associated with the distinct phase transitions. The maximum values of the extensive and intensive BCE in complex fluorides and oxyfluorides can be realized at rather low pressure, 0.1 – 0.3 GPa. In narrow temperature range around triple points the conversion from the conventional BCE to the inverse one is observed which is followed by gigantic change of both |∆SBCE| and |∆TAD|

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

Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

Sequences of phase transitions (PTs) occurring in (AI)2BIMIIIF6 elpasolites have been studied via X-ray diffraction, differential scanning calorimetry (DSC) and adiabatic calorimetry at low temperature (LT). The substitution of NH4+ ions for alkaline ions in sites with either 6 or 12 coordination number considerably affects both transition temperature and entropy change associated with the transition(s). Two main mechanisms are responsible for the PTs: displacive transitions, as for the Rb2KMIIIF6 series, in which the transitions are associated with small octahedral tilts and are followed by a rather small entropy change; order-disorder transitions of rigid octahedra, as for (NH4)2NH4MIIIF6. In this latter case, the fluoride atoms located at the apices of the octahedra are disordered onto several possible positions in the cubic..

The p-T phase diagram of ammonium hexafluoroaluminate

The heat capacity and the effect of hydrostatic pressure on structural phase transitions in ammonium hexafluoroaluminate, (NH4)3AlF6, have been studied. Two heat capacity anomalies have been found with maxima at T 1 = 218.5 ± 0.5 K and T 2 = 179 ± 2 K. Respective entropy changes of phase transitions are ΔS1 = 15.3 ± 0.5 J mol-1 K-1 and ΔS2 = 2.5 ± 0.5 J mol-1 K-1. It is shown that the p-T phase diagram of this compound is rather complex and contains a triple point (ptp = 0.12 GPa, Ttp = 221 K) and high-pressure phase. The mechanism of transformations and generalized p-T phase diagram for compounds of the (NH4)3MIIIF6 family are discussed within the framework of the rotational order-disorder model

Heat capacity and T-p phase diagram of Cs2NH4GaF6 elpasolite

Cs2NH4GaF6 prepared by solid-state reaction exhibits at room temperature a cubic FmView the MathML sourcem elpasolite structure with a0=9.167 Å. Calorimetric measurements have shown the presence of a first order phase transition at T0=161 K. Pressure dependence of the phase transition temperature has been studied by DTA under pressure. The data are analyzed in the framework of orientational disordering of [NH4]+ and [GaF6]3− ions

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

A study of the phase diagrams of (NH4)3Ga1-xScxF6 ammonium cryolites

This paper reports on the results of analyzing p-T and x-T phase diagrams and calorimetric properties of solid solutions in (NH4)3Ga1 - xScxF6 cryolites with scandium concentrations x = 0.0, 0.1, 0.35, 0.4, 0.6, 0.8, and 1.0. The thermodynamic parameters of the phase transitions observed in the studied compounds are determined. The generalized phase diagram and successive structural transformations in a series of (NH4)3Me3+F6 ammonium cryolites are discussed

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