Energetics, electric-field gradients and optical properties of YH3 (YD3) by first-principles calculations

The exact crystal structure of the switchable mirror material YD3 is still not fully understood. Presently, three structure models with P3-bar c1, P63cm, and P63 symmetry are under discussion. While the first two structures are supported by neutron powder diffraction experiments, the latter one was derived by ab-initio methods. In this paper the phase stability of these structures is established by geometry optimization and accurate total energy calculations from first-principles. The P63 structure is obtained with lowest energy followed by the P63cm structure and the latter followed by the P3-bar c1 structure. However, these results are not decisive enough for definitive structure assignments because of the very small energy differences involved. Comparison of experimental and calculated electric-field gradients (EFGs) reveal best agreement for the P63cm structure. From band-structure calculations it is found that within standard density-functional theory the P3-bar c1 and P63cm structures are obtained as metals whereas for the P63 structure a small fundamental gap of ca. 1 eV results. Calculations within the screened-exchange local-density approximation (sX-LDA) lead to fundamental band gaps of 1.8-2.1 eV for all three structures