Boron arsenate and its pressure-dependent auxetic properties

The mechanical properties of single crystals boron arsenate, BAsO4, are simulated through density functional theory simulations over a wide range of pressures from −10 to +90 GPa. It was shown that the elastic constants, as well as the extent of auxeticity, are highly pressure dependent. In particular, the Poisson’s ratio in the (001) plane for stretching in the [100] crystallographic direction is only found to be negative up to pressures of ∼30 GPa, with maximum auxeticity being predicted to occur at ∼15 GPa. It was also shown that the extent of auxetic behavior may be explained through two mechanisms that act concurrently, namely (i) a mechanism which involves rotations of tetrahedra that project in the (001) plane in the form of “rotating semi-rigid squares” and (ii) a mechanism, which is being reported for the first time, whereby the tetrahedra deform and become flatter.