First-Principles Mode Gruneisen Parameters and Negative Thermal Expansion in α-ZrW2O8

Mode Grüneisen parameters were estimated for α-ZrW₂O₈ zone-center modes by means of density functional theory calculations and the temperature dependence of the coefficient of thermal expansion was obtained according to the Debye-Einstein model of the quasiharmonic approximation. The lowest energy optic modes were identified at 45 and 46cm^-1, and were shown to be the main modes responsible for negative thermal expansion at low temperature. Experimental evidence of the lowest energy, triply degenerated infrared active optic mode, was also found in the far infrared spectrum of α-ZrW₂O₈. Upon increasing temperature, other optic modes with E<25meV (particularly at 96, 100, 133, 161, and 164cm^-1) also contribute significantly to the coefficient of thermal expansion near room temperature. An analysis was made of selected zone-center modes in light of previously proposed models for explaining negative thermal expansion in open framework materials.