Features of the Structure and Thermal Properties of Borate Tungstates LnBWO₆ (Ln = La; La_0.999Nd_0.001 and La_0.99Gd_0.01) Synthesized by the Sol–Gel Method

Borate tungstates LnBWO₆ (Ln = La; La_0.999Nd_0.001 and La_0.99Gd_0.01) are synthesized by the Pechini method followed by annealing of the intermediates and examined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The crystallographic parameters of the synthesized LnBWO₆ are refined by powder X-ray diffraction in two crystal systems: monoclinic (space group P2₁) and orthorhombic (space group P222). Reversible first-order phase transitions in the synthesized LnBWO₆ are detected by the DSC method, and the temperatures and enthalpies of phase transformations are determined. The doping of LnBWO₆ with Nd³⁺ and Gd³⁺ ions is shown to decrease the temperature of the LT (low-temperature) → HT (high-temperature) phase transition of LaBWO₆. Two independent positions of Gd in the La_0.99Gd_0.01BWO₆ structure are determined from the experimental electron paramagnetic resonance (EPR) data.