Electronic, thermodynamics and mechanical properties of LaB6 from first-principles

Highlights: • Comparative first-principles investigation of c- and a-LaB6 was carried out. • For c-LaB6, an explanation of a feature at 50 K in the Cv(T) dependence was done. • The amorphous sample, a-LaB6, consists of icosahedron fragments. • The c- and a-LaB6 phases were found to be the metallic and semiconductor ones. • Hardness, elastic moduli, ideal stress for a-LaB6 are lower than for c-LaB6. - Abstract: Up to date, the electronic structure properties of amorphous lanthanum hexaboride, a-LaB₆, were not yet investigated, and the thermodynamic and mechanical properties of crystalline lanthanum hexaboride (c-LaB₆) were studied incompletely. The goal of this work was to fill these gaps in the study of lanthanum hexaborides. The electronic and phonon structures, thermodynamic and mechanical properties of both crystalline and amorphous lanthanum hexaborides (c-LaB₆, a-LaB₆, respectively) were investigated within the density functional theory. An amorphyzation of c-LaB₆ gives rise to the metal – semiconductor transition. The thermal conductivity decreases on going from c-LaB₆ to a-LaB₆. The elastic moduli, hardness, ideal tensile and shear strengths of a-LaB₆ are significantly lower compared to those of the crystalline counterpart, despite the formation of the icosahedron-like boron network in the amorphous phase. For c-LaB₆, the stable boron octahedrons are preserved after the failure under tensile and shear strains. The peculiarity in the temperature dependence of heat capacity, C_p(T), at 50 K is explained by the availability of a sharp peak at 100 cm⁻¹ in the phonon density of states of c-LaB₆. An analysis of the Fermi surface indicates that this peak is not related to the shape of the Fermi surface, and is caused by the vibration of lanthanum atoms. In the phonon spectrum of a-LaB₆, the peak at 100 cm⁻¹ is significantly broader than in the spectrum of c-LaB₆, for which reason the anomaly in the C_p(T) dependence of a-LaB₆ does not appear. The calculated characteristics are in good agreement with the available experimental data.