A first-principles investigation of the linear thermal expansion coefficients of BeF 2 : giant thermal expansion.

We present the results of a theoretical investigation of the linear thermal expansion coefficients (TECs) of BeF 2 , within a direct Grüneisen formalism where symmetry-preserving deformations are employed. The required physical quantities such as the optimized crystal structures, elastic constants, mode Grüneisen parameters, and phonon density of states are calculated from first-principles. BeF 2 shows an extensive polymorphism at low pressures, and the lowest energy phases [α-cristobalite with space group (SG) P 4 1 2 1 2 and its similar phase with SG P 4 3 2 1 2] are considered in addition to the experimentally observed α-quartz phase. For benchmarking purposes, similar calculations are performed for the rutile phase of ZnF 2 , where the volumetric TEC ( α v ), derived from the calculated linear TECs along the a ( α a ) and c ( α c ) directions, is in very good agreement with experimental data and previous theoretical results. For the considered phases of BeF 2 , we do not find any negative thermal expansion (NTE). However we observe diverse thermal properties for the distinct phases. The linear TECs are very large, especially α c of the α-cristobalite phase and its similar phase, leading to giant α v (∼175 × 10 -6 K -1 at 300 K). The giant α v arises from large Grüneisen parameters of low-frequency phonon modes, and the C 13 elastic constant that is negatively signed and large in magnitude for the α-cristobalite phase. The elastic constants, high-frequency dielectric constants, Born effective charge tensors, and thermal properties of the above phases of BeF 2 are reported for the first time and hence serve as predictions.