P-T PHASE DIAGRAM OF CUMOO4

摘要

Three distinct phases of CuMoO4 were observed and characterized by crystal structure analysis from single crystal and powder X-ray diffraction data. The phase stable at room temperature and ambient pressure is alpha-CuMoO4. A phase transition from alpha-CuMoO4 to gamma-CuMoO4 is observed in single crystal experiments during cooling at 190 K for ambient pressure as well as for increasing pressure at 0.2 GPa and room temperature. At temperatures above 500 K and with pressure applied both alpha- and gamma-CuMoO4 transform to CuMoO4-III of distorted wolframite structure. If pressure is released after cooling, CuMoO4-III remains stable to the lowest temperatures examined, but an irreversible transition from CuMoO4-III to alpha-CuMoO4 with a transition enthalpy of -15.3 kJ/mol occurs at T = 752 K if heated without pressure applied. In this contribution the proposed p-T phase diagram of CuMoO4 is determined with energy dispersive synchrotron radiation from powder sample from ambient pressure up to 7.0 GPa and in the temperature range between 180 and 1293 K. The high-pressure modifications of CuMoO4 are also obtained under ambient conditions, if some of the Mo ions are replaced by W. A detailed discussion of the structural relationship between alpha- and gamma-CuMoO4 is given. (C) 1997 Academic Press. [References: 19]