Structure, Elasticity, Thermodynamics and High Pressure Behavior of ZnB4O7 and CdB4O7.

Abstract An accurate single crystal determination of the structure of α -ZnB 4 O 7 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 ( B α - ZnB 4 O 7 = 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 B β - ZnB 4 O 7 = 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 -ZnB 4 O 7 are compared to those of the isostructural CdB 4 O 7 compounds. The transition from the α - to a high pressure β -phase of CdB 4 O 7 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 -CaB 4 O 7 .