Powder Diffraction at High Pressure

Correspondingly, low-symmetry atomic or molecular arrangements comprising different types of chemical bonds normally exhibit pronounced anisotropy of the compressibility. Structural reorganizations due to pressure-induced phase transitions are associated with discontinuous volume decreases and normally increasing coordination numbers. These structural changes not only modify the coordination environment in the crystal structure but frequently also the electronic properties of the solid. 

Since the 1990s the combination of diamond anvil cell (DAC) techniques with imaging plate detectors has turned the investigation of compressibility and 

Pressure volume relation of three main group elements. Aluminium and sodium exhibit continuous compression behaviour in the selected pres sure range. The discontinuous volume changes of silicon are caused by structural phase transitions. 

In the following sections we assume that the application of pressure is non-directed, i.e. hydrostatic or quasi-hydrostatic (see below). The usual unit of pressure is the Giga-Pascal GPa (lPa=lNm ), but older literature often refers to kbar (1 kbar = 0.1 GPa). 

---