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Phase-change materials are functionally important materials that can be thermally interconverted between metallic (crystalline) and semiconducting (amorphous) phases on a very short time scale.Although the interconversion appears to involve a change in local atomic coordination numbers, the electronic basis for this process is still unclear. Here, we demonstrate that in a nearly vacancy-free binary GeSb system where we can drive the phase change both thermally and, as we discover, by pressure, the transformation into the amorphous phase is electronic in origin. Correlations between conductivity, total system energy, and local atomic coordination revealed by experiments and long time ab initio simulations show that the structural reorganization into the amorphous state is driven by opening of an energy gap in the electronic density of states. The electronic driving force behind the phase change has the potential to change the interconversion paradigm in this material class.ab initio simulations ͉ structural phase change ͉ electronic phase transitions ͉ ac conductivity

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 ± 40 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 zinc α-phosphate. The efficacy of low-coordination zinc phosphates as antiwear agents is discussed in the context of these results.

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