Synthesis, thermal analysis, and band gap of ordered and disordered complex rock salt Li3TaO4

The temperature ranges of phase stability were examined for the disordered and ordered polymorphs of complex rock salt Li3TaO4. The disordered Li3TaO4, having an average structure of face-centered cubic unit cell, could be obtained by heating a mixture of Li2CO3 and Ta2O5 to 650–800 ​°C, but transformed irreversibly to the ordered and monoclinic β-Li3TaO4 at ≥ 820 ​°C. The thermal stability of β-Li3TaO4 depended heavily on the sealing of the container. When heated in an open environment, β-Li3TaO4 decomposed to LiTaO3 and Li2O at ≥ 1100 ​°C, but such a decomposition was significantly suppressed in a sealed container even at 1450 ​°C. Based on the differential scanning calorimetry, the enthalpy of formation of the disordered Li3TaO4 was estimated at −1680 ​kJ/mol, as compared with −2206 ​kJ/mol of β-Li3TaO4. The indirect band gaps of the disordered Li3TaO4 and β-Li3TaO4 were measured to be 4.65 and 4.4 ​eV, respectively.