Highhyphen;pressure Raman and xhyphen;ray studies of the alkali cyanides up to 27 GPa

摘要

We have investigated the phase diagrams of the alkali cyanides NaCN, KCN, RbCn, and CsCN in the pressure range up to 27 GPa, using the Raman and energyhyphen;dispersive xhyphen;ray techniques. We completed the information concerning the concept of a general phase diagram of the alkali cyanides with our measurements. In the case of KCN and RbCN we found at room temperature the expected high pressure phase. This phase IV, with a monoclinic structure (space groupCm) can be followed up to about 27 GPa. A different behavior was observed for NaCN. This crystal has in the pressure range between 10ndash;15 GPa a coexistence region between the antiferrohyphen;ordered orthorhombic phase B and the ferrohyphen;ordered monoclinic phase. However, this monoclinic phase IVa in NaCN has a different structure with space groupPm(C1s). Finally the combination of the xhyphen;ray and Raman data allowed the determination of the structure of CsCN at low temperature and at high pressures. The structure is rhombohedral with space groupR3m(phase VII).