On the Behavior of beta-Cristobalite-like Boron Arsenate when Subjected to Uniaxial Loading in Its [001] Direction and the Implications on Its "Negative" Characteristics

Boron arsenate (BAsO4) is a beta-cristobalite-like material, which crystallizes in the I4 over bar space group. However, unlike beta-cristobalite itself, BAsO4 is well characterized and known to be stable and at ambient conditions. This work presents the results of extensive density functional theory (DFT)-based simulations aimed at elucidating the nanoscale deformations that such crystals undergo when subjected to uniaxial tensile and compressive loads along their crystallographic [001] c-direction. It is shown that such uniaxial loading results primarily in intra-tetrahedral deformations rather than their tilting/rotations (which causes the auxeticity in the (001) plane). Through their analysis in isolation from other modes of deformation, this study gives a much better insight into such intra-tetrahedral deformations and helps explain better the cause of the measured negative linear compressibility in the c-direction at elevated pressures, which recently has been attributed to a "demi wine-rack" mechanism caused by the same intra-tetrahedral deformations.