Behaviour of thermal expansion of (1-x)Pb(Ni1/3Nb2/3)O3–xPbTiO3 solid solutions

The publication costs of this article were covered by the Estonian Academy of Sciences and the University of Tartu.The features of the dependence of the bulk coefficient of thermal expansion of the (1-x)Pb(Ni1/3Nb2/3)O3-xPbTiO3 solid solution studied in the series of x = 0-0.8 in the temperature range from 100 to 750 K are reported. The obtained data and observed anomalies of thermal expansion are compared with dielectric properties, heat capacity, and the phase diagram of concentration. The anomalous and lattice constituents of deformation and of the coefficient of thermal expansion are analysed and the data on deformation are used to estimate the Burns temperature and the mean square polarization of the samples found to decrease with the decrease of the PbTiO3 concentration.Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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|

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

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, 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

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

Crystal structure and phase transition mechanisms in CsFe2F6

For the first time, structural phase transitions induced by the temperature were found in AxMxIIM(1−x)IIIF3 fluorides with the defect pyrochlore structure (FdView the MathML sourcem, Z=8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100 K and 700 K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I41/amd (Z=4)→FdView the MathML sourcem (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500-560 K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations

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