Изобарная теплоемкость образца твердого раствора ЕГ2О3∙2HfOi, охарактеризованного методами РФА, электронной микроскопии и химического анализа, измерена методами релаксационной, адиабатической и дифференциальной сканирующей калориметрии в интервале 2.4–1807 K; рассчитаны термодинамические функции. Определен вклад аномалии Шоттки в области 2.4–300 K.

The temperature dependence of the heat capacity of Y2Ti2O7 and Eu2Ti2O7 with a pyrochlore structure in the temperature range of 7–1800 K has been studied. The existence of a small shallow anomaly of the heat capacity of europium titanate in the range of 10–60 K was confirmed. The thermodynamic properties (entropy, enthalpy change, and reduced Gibbs energy) were calculated. Based on the results of calculation of the Gibbs energy of formation of the titanates from oxides it was concluded that both titanates are thermodynamically stable in the high temperature region.

A sample of Yb2O3⋅2HfO2 solid solution is synthesized and characterized via X-ray diffraction, electron microscopy, and chemical analysis. Relaxation, adiabatic, and differential scanning calorimetry are used to measure the isobaric heat capacity of the sample in the range of 2.4–1807 K, and thermodynamic functions are calculated with allowance for the contribution from low-temperature transformation. The Schottky anomaly’s contribution to the heat capacity in the region of 2.4–300 K is determined.

The isobaric heat capacity of magnesium praseodymium hexaaluminate PrMgAl11O19 having the magnetoplumbite structure was measured by three calorimetric methods in the temperature range 2–1865 K. Reconciled and smoothed heat capacity data were used to calculate thermodynamic functions (entropy, enthalpy change, and reduced Gibbs energy) in this temperature range. A gentle heat capacity anomaly with a peak at about 8 K was found; its entropy and enthalpy were calculated. Magnetic properties of PrMgAl11O19 were studied by dynamic magnetic susceptibility measurements in the range 2–300 K. The results of magnetic measurements revealed an anomaly on the imaginary component of dynamic magnetic susceptibility, the temperature range of which matched that of the heat capacity anomaly.

Isobaric heat capacity measurements in the range 2.4–1807 K have been performed by relaxation calorimetry, adiabatic calorimetry, and differential scanning calorimetry on a Ho2O3‧2HfO2 solid solution sample prepared and characterized by X-ray powder diffraction, electron microscopy, and chemical analysis, and thermodynamic functions have been calculated. The Schottky anomaly contribution has been determined in the range 2.4–300 K.

Measuring data are presented for the molar heat capacity of pyrochlore gadolinium stannate and its thermodynamic functions are calculated in the 298.15–1350 K range of temperatures. The Gibbs energy of formation of gadolinium stannate from gadolinium(III) and tin(IV) oxides is estimated. High-temperature diffraction is used to determine the cubic lattice parameter of Gd 2 Sn 2 O 7 as a function of temperature, and the coefficients of thermal expansion in the range of 299–1273 K were calculated.

This is the first time that the heat capacity of polycrystalline gadolinium stannate Gd2Sn2O7 is experimentally studied by relaxation and adiabatic calorimetry in the range 2.35–350.35 K. The heat capacity values obtained by the two independent methods in the overlapping range are almost coincident and match the reported gadolinium stannate heat capacity below 7 K. The low-temperature heat capacity anomaly arising from the magnetic transition from the paramagnetic to antiferromagnetic state at TN = 1.015 K continues to 21 K. The measured heat capacity and $$C_{p}^{^\circ }$$ (Т) values are used to calculate temperature-dependent thermodynamic functions of Gd2Sn2O7 in the range 0–350 K. The enthalpy of formation ΔfH°(298.15 K), Gibbs free energy of formation ΔfG°(298.15 K), and characteristic Debye temperature ΘD of gadolinium stannate are determined.

The heat capacity of europium stannate with the pyrochlore structure was measured in the temperature range 3–1350 K using relaxation, adiabatic, and differential scanning calorimetry. The entropy, enthalpy change, and derived Gibbs energy of Eu 2 Sn 2 O 7 were calculated from the fitted heat capacity data. The temperature-dependent excess heat capacity related to Schottky’s anomaly was described in terms of the Westrum model. The Gibbs energies of formation Δ f G °(Eu 2 Sn 2 O 7 ) from elements and from constituent binary oxides were estimated from literature data and the values obtained in this work. An analysis of the magnetic properties of europium stannate showed the absence of magnetic phase transitions in the temperature range from 2 to 300 K. Thermal expansion coefficients were calculated from the unit cell parameters measured by high-temperature X-ray diffraction experiments in the range 300–1273 K.

Определены температурные зависимости молярной теплоемкости при 310–1360 K и параметра кристаллической решетки в интервале 298–1273 K гафната тербия со структурой пирохлора, охарактеризованного методами РФА, РЭМ и химического анализа.