Asymmetrical LaO9 polyhedron inducing anisotropic positive thermal expansion and moderate lattice thermal conductivity in LaBO3 with isolated planar [BO3] group

Abstract

Thermal physical properties of inorganic borates are important in application of optical device working under high temperature. Herein, we investigate the temperature-dependent thermoelasticity, thermal expansion(α), and lattice thermal conductivity(κ) of simple orthohomibic LaBO3 with a two dimension like crystal structure though first principles for the first time. The calculated rate of temperature-dependent second elastic constant C11 is significantly larger than that of C22 and C33, and the theoretical slopes of corresponding bulk, shear, and Young's moduli in the range of 300–900 K are −0.0289 GPa•K−1, −0.0156 GPa•K−1, and −0.0398 GPa•K−1, respectively. The predicted α is positive and increases with the rise of temperature. The anisotropic ratio αc/αa or αc/αb is approximately 2 at 300 K, which might be attributed to the difference of elastic compliance S12 and S13. Besides, the anisotropic lattice thermal conductivity κa, κb and κc of LaBO3 is predicted to be 1.43, 1.40 and 0.80 W/m-K at 300 K via Debye-Callaway model. Most importantly, the atomic projected Grüneisen parameters γ reveal that asymmetrical LaO9 polyhedra in the crystal lattice dominate the thermal expansion and lattice thermal conductivity. Our results indicate that LaBO3 has good thermal physical properties working under high temperature.