Transitions in network and molecular glasses at high pressure

The use of monochromatic, micro-focused, high energy x-ray beams have been used in combination with perforated diamond anvil cells to investigate transitions in several network glasses at high pressure. Of particular interest are glasses which readily form an open network structure e.g. SiO ₂, GeSe ₂, BeF ₂ and those which comprise of small molecular cages e.g. As ₄S ₃, As ₄O ₆. These materials have highly directional bonds and their normal pressure glassy forms tend to have a high degree of intermediate range order, signified by the existence of a first sharp diffraction peak. As the pressure is increased this peak decreases dramatically in intensity as the open regions of the network collapse. Simultaneously the (second) principal peak increases in intensity as the molecules in the glass move closer together resulting in increased extended range order. Gradual and abrupt amorphous-amorphous transitions have been observed in these binary systems, associated with both topological and local coordination number changes.