PHASE-TRANSITION OF COPPER SELENIDE STUDIED BY POWDER X-RAY-DIFFRACTOMETRY

The temperature dependence of the lattice parameters of CuSe up to 420 K showed that a phase transition from hexagonal to orthorhombic obser ved at about 398 K with decreasing temperature is second order and ano ther transition from orthorhombic to hexagonal at 323 K is first order . The latter hexagonal phase has weak superlattice reflections in addi tion to main sublattice reflections. The intensities of weak superlatt ice reflections were increased by grinding the powder of CuSe at room temperature and they did not disappear over the temperature range of t he orthorhombic phase only upon heating. These superlattice reflection s pf this new but metastable phase can be indexed based on a monoclini c lattice.