DFT calculations of the stability field and properties of a predicted lanthanum-scandium-aluminum garnet La 3 Sc 2 Al 3 O 12 and P-T phase diagram of Y 3 Sc 2 Al 3 O 12

In this study we present the results of our density functional theory calculations within the quasi-harmonic approximation, which predict formation and properties of a new compound of the garnet family, La 3 Sc 2 Al 3 O 12 (LSAG). LSAG was shown to be formed from 3LaAlO 3 + Sc 2 O 3 at ambient pressure and room temperature. The thermodynamic stability region of LSAG is limited by a temperature of 685 K, above which its decomposition into LaAlO 3 (LAP) and Sc 2 O 3 occurs. With increasing pressure, LSAG retains its stability up to 0.23 GPa, where the decomposition reaction LSAG <-> 3LAP + Sc 2 O 3 becomes thermodynamically favorable. In this work, not only the thermodynamic stability of LSAG was assessed, but also its thermal conductivity, elastic constants and band gap were calculated. At 300 K the thermal conductivity of LSAG is 13.204 Wm -1 K -1 . LSAG has the smallest elastic constants compared with YSAG and YAG, which indicates weaker atomic bonding in LSAG than in YSAG and YAG. The width of band gap of LSAG calculated using an hybrid functional is equal to 5.24 eV. In addition, the P-T phase diagram of the known Y 3 Sc 2 Al 3 O 12 (YSAG) was calculated for the first time. Unlike LSAG, YSAG is stable at fairly high temperatures, reaching values of at least 1500 K. With increasing temperature to 1500 K, the pressure of the phase transition increases slightly to 3.55 GPa, and the monovariant curve of the reaction YSAG <-> 3YAP+Sc 2 O 3 is represented by an almost straight line.