Physical properties of La2B2O7(B Zr, Sn, Hf and Ge) pyrochlore: First-principles calculations

Using first-principles theory and further verified by the experimental results, we have studied the electronic structure, mechanical and thermal properties of La2B2O7(BZr, Sn, Hf and Ge) pyrochlore. It is shown that the La2B2O7 pyrochlore are charge-transfer-type insulators with the strong ionic bonding. The calculated band gap of La2Zr2O7, La2Sn2O7, La2Hf2O7, and La2Ge2O7 are 4.01, 3.05, 3.13 and 2.42 eV, respectively. The elastic constants, hardness, shear modulus, bulk modulus, Young's modulus and Poisson's ratio of La2B2O7 compounds were investigated systematically. Anisotropic elastic and thermal properties of La2B2O7 compounds were discussed by using Zener's indexes (AZ) and universal elastic anisotropic index (AU). The sound velocities along [100], [110] and [111] directions were calculated for each La2B2O7 compounds. Both Clark's model and Cahill's model were used to evaluate the minimum thermal conductivity of all compounds. According to the Clark's model, when the temperature is much higher than Debye temperature, the minimum thermal conductivity of La2Zr2O7, La2Sn2O7,La2Hf2O7, and La2Ge2O7 are 1.27, 1.24, 1.09 and 1.41 W/m K, respectively. According to the calculated results of these crystals, the La2B2O7 compounds exhibit an extraordinary low thermal conductivity which can be used as thermal insulating materials.