Solid solubility and phase transitions in the system LaNb1−xTaxo4

The solid solubility between LaNbO4 and LaTaO4 was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb1−xTaxO4 where 0.4⩽x⩽0.8. Single-phase LaNb1−xTaxO4 (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 LaNb1−xTaxO4 (0⩽x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb1−xTaxO4 (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 LaTaO4 could also be transformed to monoclinic LaTaO4 at ambient temperature by applying a uniaxial pressure of 150–170 MPa, reflecting the lower molar volume of monoclinic LaTaO4.