Solid solubility and phase transitions in the system LaNb 1−xTa xo 4

Abstract

The solid solubility between LaNbO 4 and LaTaO 4 was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb 1− x Ta x O 4 where 0.4⩽ x⩽0.8. Single-phase LaNb 1− x Ta x O 4 (0⩽ x⩽0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb 1− x Ta x O 4 (0⩽ x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb 1− x Ta x O 4 (0.8⩽ x⩽1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO 4 could also be transformed to monoclinic LaTaO 4 at ambient temperature by applying a uniaxial pressure of 150–170 MPa, reflecting the lower molar volume of monoclinic LaTaO 4.