Structure, elasticity, thermodynamics and high pressure behavior of ZnB4O7 and CdB4O7

Abstract

An accurate single crystal determination of the structure of α-ZnB4O7 is reported, and an improved description of the polyhedral network is presented. The experimental data are used to evaluate density functional theory calculations. Structural models based on the PBESOL exchange-correlation functional were in slightly better agreement with experimental data than those employing the PBE exchange-correlation functional. However, in both cases the agreement between the results of the experiments and the calculations was very satisfactory. The models were therefore used to predict the compression behavior (Bα-ZnB4O7=59.9(7) GPa), the elasticity tensor, bond populations, and a transition pressure of 3.7 GPa for the transition into the high pressure β-phase. The predicted bulk modulus of the high pressure polymorph is Bβ-ZnB4O7=210.4(4) GPa. The heat capacity of the α-phase has been determined with quasi-adiabatic microcalorimetric measurements and at low temperatures a Debye temperature of Θ = 787 K has been obtained. The results obtained for α- and β-ZnB4O7 are compared to those of the isostructural CdB4O7 compounds. The transition from the α- to a high pressure β-phase of CdB4O7 is predicted to occur at ≈1.9 GPa, but within the uncertainty of the model, the high pressure phase may also be isostructural to β-CaB4O7.