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

• Published in: Journal of solid state chemistry Vol. 315; p. 123450
• Main Authors: Kim, Chaeeun, Pham, Thuy Linh, Lee, Jong-Sook, Kim, Young-Il
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-11-2022
• Subjects: Atomic ordering; Complex rock salt; Differential scanning calorimetry; Phase transition; Differential scanning calorimetry; Complex rock salt; Atomic ordering; Phase transition
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2022.123450

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.
TG-DSC profile recorded while heating the mixture of Li2CO3 and Ta2O5 at a rate of 10°/min, and an energy diagram for the Li3TaO4 phases. [Display omitted]
•Disorder-order transition of rock salt Li3TaO4 occurs at 800–820 ​°C.•Ordered phase β-Li3TaO4 is stable up to 1460 ​°C or higher, in the gas-tight environment.•Enthalpy of β-Li3TaO4 is lower than that of disordered Li3TaO4, by more than 500 ​kJ/mol.•β-Li3TaO4 has smaller band gap and larger lattice energy than the disordered one, due to the shorter Ta–O distances.