Lanthanum-magnesium-nickel tellurite: thermodynamic and electrophysical characteristics

Abstract

Lanthanum-magnesium-nickel tellurite with composition of La2MgNiTeO7 was synthesized from La2O3, NiO, TeO2 oxides and MgCO3 with the help of the ceramic technology. The temperature dependences of the isobaric heat capacity of tellurite La2MgNiTeO7 were studied on an IT-S-400 calorimeter using an experimental method of dynamic calorimetry in the range of 298.15–673 K. The operation of the calorimeter was checked by measuring the standard heat capacity of α-Al2O3. The specific heat capacities were measured, and then the molar heat capacities of the synthesized tellurite were calculated using them. In the study of the dependence of the heat capacity of tellurite La2MgNiTeO7 on temperature at 423 K, a sharp anomalous λ-shaped jump was found, probably related to a second-order phase transition. This transition can be associated with cationic redistribution, changes in the coefficient of thermal expansion and magnetic moment, as well as changes in dielectric constant and electrical resistivity. The equation of the temperature dependence of the heat capacity of the compound is derived on the basis of the experimental data, taking into account the phase transition temperature of the second kind. The temperature dependences of the heat capacity Cp0(T) and thermodynamic functions, namely, the entropy S0(T), the enthalpy H0(T) – H0(298.15) and the reduced thermodynamic potential Фхх(Т) were calculated based on the experimental data on heat capacities and the calculated standard entropy value S0(298.15) in the interval 298.15–673 K. For the first time, the temperature dependences of the dielectric constant and electrical resistance of tellurite La2MgNiTeO7 in the temperature range of 293–483 K were studied on the LCR-800 instrument. There are maxima and minima on the curves of lgƐ∼f(T) and lgR∼f(T), which confirm the λ-shaped effect on the Cp0∼f(T) curve of a successful compound, related to the second-order phase transition. The data obtained show that the tellurite studied has semiconductor properties.