Correlating cation coordination, stiffness, phase transition pressures, and smart materials behavior in metal phosphates

Abstract

In this study we present x-ray diffraction data on metal phosphates containing either high or low coordination number zinc or calcium. The experiments reveal that low-coordination zinc phosphate crystals are relatively soft at ambient conditions but stiffen dramatically with pressure, $p$, thereby exhibiting smart materials behavior. In comparison, high-coordination zinc and calcium phosphates have higher initial bulk moduli, $K(0)$, and stiffen much less rapidly with increasing $p$. The investigated metal phosphate crystals all amorphize under compression when $K$ reaches a value near $210\ifmmode\pm\else\textpm\fi{}40\text{ }\text{GPa}$: the precise value depending on the chemical details. Our interpretation of this result is that elastic properties and structural instabilities are related to the motion of rigid phosphate units, which becomes more hindered as the material density increases, and that stiffening can occur independent of a change of cation coordination. These ideas are supported by ab initio simulations of $\ensuremath{\alpha}$ zinc phosphate. The efficacy of low-coordination zinc phosphates as antiwear agents is discussed in the context of these results.