The molar heat capacity of the Lu2O3 ⋅ 2HfO2 solid solution has been determined at 4.5–347 K, and its thermodynamic functions have been calculated in the range 0–1300 K.

The enthalpies of formation of heavy rare-earth hafnates Tb2Hf2O7 (pyrochlore structure) and RE(2)O(3)(.)2HfO(2) (RE = Dy-Lu) (defect fluorite structure) have been determined by the method of oxide melt drop-solution calorimetry. The data obtained for Tb2Hf2O7 revealed that they are in trend with previously experimentally determined values for "light" RE hafnates. The formation enthalpies of fluorite structured compounds in general become more positive with the RE ionic radius contraction. The values of enthalpy of formation from oxides for the fluorite solid solutions obtained in the current study are sufficiently more negative than was predicted by the ab initio calculations. The calculation of the Gibbs energy dependencies as well as the contribution of both entropy and enthalpy factors allowed us to estimate the stability of the compounds in the high-temperature range.

The first measurements are made of the isobaric heat capacity of single-phase pyrochlore ytterbium titanate synthesized and studied via XRD, SEM, and EDX in the 2–1869 K range of temperatures. A magnetic transformation at <20 K and a lack of structural transformations throughout the region of Yb2Ti2O7 are confirmed. Thermodynamic functions (entropy, the increment of enthalpy, and the Gibbs free energy of the formation of Yb2Ti2O7 from elements and binary oxides at 298.15 K) are calculated, and the contribution to the heat capacity of the Schottky anomaly is evaluated.

Heat capacity of single-phase samarium-magnesium hexaaluminate SmMgAl11O19 was measured by relaxation, adiabatic and differential scanning calorimetry in the region 2–1864 K. Thermodynamic functions were calculated based on the smoothed values of heat capacity. Possible inaccuracies in the assessment of heat capacity and entropy using the Neumann–Kopp rule in the field of low and high temperatures are shown.

The enthalpies of formation of heavy rare-earth hafnates Tb 2 Hf 2 O 7 (pyrochlore structure) and RE 2 O 3 . 2HfO 2 (RE = Dy-Lu) (defect fluorite structure) have been determined by the method of oxide melt drop-solution calorimetry. The data obtained for Tb 2 Hf 2 O 7 revealed that they are in trend with previously experimentally determined values for “light” RE hafnates. The formation enthalpies of fluorite structured compounds in general become more positive with the RE ionic radius contraction. The values of enthalpy of formation from oxides for the fluorite solid solutions obtained in the current study are sufficiently more negative than was predicted by the ab initio calculations. The calculation of the Gibbs energy dependencies as well as the contribution of both entropy and enthalpy factors allowed us to estimate the stability of the compounds in the high-temperature range.

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 molar heat capacity of the solid solution Tm2O3‧2HfO2 has been determined for the first time by relaxation, adiabatic and differential scanning calorimetry, the temperature dependences of entropy and enthalpy increment in the temperature region 0–1800 K have been calculated, and the contribution to the heat capacity of the Schottky anomaly at 0–300 K has been evaluated.

The molar heat capacity of the solid solution Tm2O3‧2HfO2 has been determined for the first time by relaxation, adiabatic and differential scanning calorimetry, the temperature dependences of entropy and enthalpy increment in the temperature region 0–1800 K have been calculated, and the contribution to the heat capacity of the Schottky anomaly at 0–300 K has been evaluated.

The temperature stages of crystallization process for thulium titanate of pyrochlore structural type during the heating of hydroxide precursor obtained by reverse precipitation have been studied by DSC/TG, XRD, and electron microscopy. Tm2Ti2O7 has been measured in the temperature range 2–1870 K and thermodynamic functions at 0–1900 K and the Gibbs energy of formation from oxides and elements have been calculated based on smoothed heat capacity. The contribution of the Schottky anomaly at 20–320 K into heat capacity has been revealed.

The isobaric heat capacity of pyrochlore gadolinium titanate Gd2Ti2O7 is measured in the 2–1825 K range of temperatures. Thermodynamic functions (entropy, enthalpy change, reduced Gibbs energy) are calculated using consistent smoothed values of heat capacity. The Gibbs energy of the formation of Gd2Ti2O7 from oxides in the high temperature range is estimated.

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.

Lutetium stannate with a pyrochlore structure was synthesized using solid state reaction. The heat capacity of polycrystalline Lu2Sn2O7 in the temperature range of 7.99–1871 K was measured by adiabatic and differential scanning calorimetry methods. Entropy, enthalpy change, and derived Gibbs energy were calculated from the smoothed heat capacity data. The Gibbs free energy of Lutetium stannate from elements was estimated, using the ΔfS°(Т) values obtained in this work and the ΔfH°(Т) values known from the literature. The temperature dependence of the cubic crystal lattice parameter and the value of the coefficient of thermal expansion in the temperature range of 300–1273 K were determined by high-temperature X-ray diffraction.

Erbium titanate was synthesized by the coprecipitation of erbium and titanium hydroxides followed by high-temperature annealing. The temperature intervals of the formation sequence of the pyrochlore-type crystal structure were determined. The isobaric heat capacity of erbium titanate in the range 2–1870 K was measured by relaxation calorimetry, adiabatic calorimetry, and differential scanning calorimetry. The smoothed heat capacity values were used to calculate the entropy and enthalpy increment in the range 0–1900 K, evaluate the contribution of the Schottky anomaly at temperatures up to 300 K, and calculate the Gibbs free energy of erbium titanate formation at 298.15 K.

Pyrochlore-type neodymium and gadolinium zirconate hafnates have been prepared and identified. The heat capacities of the prepared samples have been measured by differential scanning calorimetry in the range 310–1800 K. Temperature-dependent cubic unit cell parameters have been determined and thermal expansion coefficients assessed in the range 298–1273 K using high-temperature X-ray diffraction. The thermal diffusivity of the samples was measured by the laser flash method, and the temperature-dependent thermal conductivity was calculated taking into account the porosity of the samples.

The synthesis and identification of lanthanum and samarium zirconate–hafnates of the pyrochlore structure type have been reported. The heat capacity of the samples in the temperature range 310–1380 K was measured by the differential scanning calorimetry method. The temperature dependences of the cubic lattice parameters were determined, and the thermal expansion coefficients were evaluated in the range 298–1273 K using high-temperature X-ray powder diffraction. The thermal diffusivity of the samples was measured by the laser flash method, and the temperature dependences of the thermal conductivity were calculated considering the porosity of the samples.

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.

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.

The molar heat capacity of Sm 3 TaO 7 was determined by relaxation, adiabatic and differential scanning calorimetry in the region of 2–1350 K and the thermodynamic functions were calculated. The total contribution of the Schottky anomaly to the heat capacities of samarium tantalate was evaluated. The temperature dependences of the parameters of the orthorhombic crystal lattice of the space group C 222 1 in the temperature range of 300–1204 K have been determined and the coefficients of thermal expansion have been estimated. The temperature dependence of heat capacity of samarium tantalate exhibits an anomaly in the region of 1116–1275 K associated with the structural phase transition. The space group of the high-temperature orthorhombic phase Sm 3 TaO 7 is identified as Cmcm .

The processes occurring during heating of a stoichiometric mixture of lanthanum, samarium, magnesium, and aluminum hydroxides synthesized by the reverse precipitation method have been studied by DTA/TG and X-ray powder diffraction methods. The conditions for the synthesis of single-phase LaMgAl11O19 and SmMgAl11O19 samples of the magnetoplumbite structure type have been determined, and the isobaric heat capacity has been measured in the temperature range 317–1817 K, showing the absence of structural transformations in this range.