Ionicity and phase transitions at negative pressures

The Gibbs-free-energy difference $\Delta G_t$ which determines the phase transition from fourfold to sixfold coordination in $A^N{B}^{8-N}$ compounds is presented for three compounds (CdO, MgO, and AgBr) which are stable at STP in the more-dense (NaCl) structure. The additional data follow a linear trend in spectroscopic ionicity predicted previously from high-pressure data on compounds stable in fourfold coordinated structures. These data resolve ambiguities concerning the proper definition of ionic character in partially covalent compounds with the NaCl structure.