The results of the synthesis of complex YErYbNbO7 paraniobate using different methods are presented. The chemical composition of the final synthesis products was determined. The peculiarities of the thermal behavior of the precursors are revealed. Crystallographic parameters of single phase samples were calculated and crystallite sizes were determined.

This article discusses the specifics of high concentration rare earth element determination and critically evaluates the need for deconvolution when analysing heat-resistant materials using a handheld X-ray spectrometer.

This work describes a two-stage technique of X-ray fluorescence (XRF) analysis of rare earth niobates. A comparison between the two approaches revealed that the Fundamental Parameters Method (FPM) can be employed for a rapid preliminary assessment of the composition of the resulting material and the construction of calibration curves can be used to determine the contents of the major elements with precision. The results of the relative standard deviation (RSD) for FPM were no more than 7%, while the approach to construct calibration curves had an RSD of no more than 1%. Calibration samples were prepared using the same synthesis method as the study samples to construct the calibration curves. The possibility of constructing calibration dependencies using mixtures of oxides was assessed, but this approach could not provide the desired accuracy. The obtained results have been shown to have a good correlation with inductively coupled plasma optical emission spectrometry. The developed technique enables the determination of the major components in niobates containing two and three rare earth elements, which are used as optical materials and medium-entropy ceramics.

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 two-stage technique for X-ray fluorescence analysis of ceramic samples of composition Y 3– x Yb x NbO 7 (where x = 0 – 3) has been developed. At the first stage, using the method of fundamental parameters (FPM), a rapid semi-quantitative analysis of ceramic samples and products of intermediate synthesis was carried out to determine their preliminary composition. At the second stage, the quantitative composition of the samples was determined using the constructed calibration dependencies. To construct calibration dependencies a series of reference samples containing 3.16 – 56.55% Y, 8.78 – 71.0% Yb, and 12.83 – 19.70% Nb was synthesized using a method similar to that used for preparation of the ceramic samples under study. Analytical lines of elements free from spectral overlaps and XRF conditions (current and voltage of an X-ray tube, exposure time, method of taking into account the background near the analytical line) were selected. The relative standard deviation of the results of Y, Yb, and Nb determination in ceramic samples did not exceed 0.66%, the relative error was no more than 1.63%. The results obtained were compared with the calculated content of analytes in the samples of stoichiometric composition and with the results of ICP-AES analysis of real ceramic samples. The developed technique provides determination of the main components of ceramic samples and can be used for analytical control of synthesis of rare earth paraniobates.

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.

The results of a thermodynamic study of a group of inorganic compounds of the general formula Ln2M2O7, where M = Sn, Ti, are presented. Single-phase samples were obtained by reverse precipitation followed by high-temperature synthesis. Crystallographic parameters were calculated and their dependence on the radius of the element is shown. The temperature dependences of the heat capacity are experimentally studied in a wide temperature range. The thermodynamic properties of samarium compounds with a pyrochlore-type structure are compared.

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.

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 LaMgAl 11 O 19 and SmMgAl 11 O 19 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.

The paper discusses synthetic approaches to the preparation of compounds with a pyrochlore structure using the example of compounds of composition Ln2Sn2O7 (Ln = La–Lu). Optimal temperature-time schemes for synthesis of single-phase samples were established and crystallographic parameters of compounds were determined.Morphology of samples obtained at different conditions was determined. Heat capacity measured and calculated thermodynamic functions in a wide temperature range (2.5–1300 K).

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.

The heat capacity of polycrystalline neodymium orthoniobate NdNbO4 in the range 2–1370 K is studied by relaxation, adiabatic, and differential scanning calorimetry. The existence of a magnetic transformation with a Néel temperature below 2 K in the low-temperature region is verified. The reversible phase transition at 987 K is shown to be a second-order transition. The contribution of the Schottky anomaly (Schottky contribution) to neodymium orthoniobate heat capacity is estimated. The gained data are used to calculate temperature-dependent standard thermodynamic functions in the range from 0 to 1400 K. Their values at 298.15 K are $$C_{p}^{^\circ }$$ (298.15 K) = 121.4 ± 0.4 J/(K mol), S°(298.15 K) = 134.7 ± 0.5 J/(K mol), H°(298.15 K) – H°(0 K) = 21.24 ± 0.17 kJ/mol, Ф°(298.15 K) = 63.43 ± 0.21 J/(K mol).

— Lanthanum stannate with the pyrochlore structure and a crystallite size in the range 100–400 nm has been prepared by solid-state synthesis. We have optimized synthesis parameters for obtaining a ceramic material. The heat capacity of La 2 Sn 2 O 7 has been determined for the first time using adiabatic and differential scanning calorimetry techniques in the range 19–1300 K, and the stannate has been shown to undergo no structural transitions in this range. We have calculated temperature-dependent standard thermodynamic functions of La 2 Sn 2 O 7 and evaluated the standard Gibbs energy of formation of this compound from its constituent elements at 298.15 K.

The heat capacity of polycrystalline dysprosium orthoniobate was studied over a wide temperature range by three calorimetric methods: relaxation calorimetry (2–50 K), adiabatic calorimetry (9–350 K), and differential scanning calorimetry (308–1300 K). Below 9 K a descending low-temperature anomaly branch was detected, whose peak is beyond the measurement limits. At 1086 K a reversible phase transition occurs. The behavior of heat capacity and $$C_{p}^{^\circ }(T)$$ in the phase-transition region implies that this is a second-order phase transition. The data gained were used to calculate temperature-dependent standard thermodynamic functions of DyNbO4 over the whole measurement range.

A praseodymium stannate sample, Pr 2 Sn 2 O 7 , was synthesized by ceramic route (solid-phase reaction of oxides) at high temperature and characterized by X-ray diffraction analysis, X-ray fluorescence analysis, and scanning electron microscopy. The heat capacity measurements by adiabatic and differential scanning calorimetry in the temperature range 7.9–1345 K made it possible to calculate the temperature dependences of the thermodynamic functions of Pr 2 Sn 2 O 7 . The magnetization of praseodymium stannate was studied in the temperature range 2–300 K. The enthalpy of formation and the Gibbs free energy of formation of Pr 2 Sn 2 O 7 were estimated. The shape of the anomalous Schottky heat capacity curve was determined.

The phase formation in Na3REE(PO4)2–Na3REE(VO4)2 (REE = Gd, Y) systems in the 873–1573 K temperature range was investigated by XRD and DSC methods. Formation of two solid solutions based on both compounds was revealed. Endothermic effects corresponding to structural phase transformations and melting of samples were registered.

Monoclinic M-type europium orthotantalate was obtained by co-precipitation and annealing at ~1673 K and characterized by X-ray powder diffraction, scanning electron microscopy, and chemical analysis. The temperature dependence of molar heat capacity of M-EuTaO4 was measured by adiabatic calorimetry at 8–350 K; the thermodynamic functions (entropy, enthalpy change, and reduced Gibbs free energy) were calculated. The general form of contribution of the Schottky anomaly to the molar heat capacity was determined.

Solid solution samples of yttrium orthophosphate-yttrium orthovanadate of general formula Y(VO4)(1-x)(PO4)(x) (x = 0.25; 0.50; 0.75) were obtained using solid state synthesis. The samples crystallize in zircon structure (I4(1)/amd sp. gr.) as the pure end-members of the system do. Unit cell parameters calculated using XRD data were found to be in good agreement with those calculated from the Vegard law. SEM-EDS spectra of samples are given. Heat capacities of the samples (x = 0.50; 0.75) were measured by means of adiabatic calorimetry at low temperatures (6-344 K). Thermodynamic functions in the studied temperature range were calculated and compared to those of pure yttrium orthophosphate and orthovanadate. According to the data obtained solid solution Y(VO4)(1-x)(PO4)(x) can be considered as ideal, thus making it possible to calculate thermodynamic functions for any other substitution x by summation of obtained values for closest compounds with corresponding coefficients or pure substances.