LOW-TEMPERATURE ANOMALIES OF CUPRITE OBSERVED BY RAMAN-SPECTROSCOPY AND X-RAY-POWDER DIFFRACTION

Anomalous low-temperature behaviour of Cu2O is observed by both Raman- spectroscopy using laser excitation frequencies near the resonance of the 1S 'blue exciton' and x-ray powder diffractometry using a high-res olution Guinier film camera, Changes in frequency of the Raman-allowed Gamma(25)(+)(LO) phonon band, and also of intensities of other Raman- forbidden bands, indicate anomalies at temperatures of 90 and 190 It, respectively, The anomaly at 90 It is qualitatively interpreted by an order-disorder phase transition and that at 190 K by a discontinuous c hange of excitonic states, Close to these temperatures, the powder dif fraction diagrams also revealed anomalous behaviour of the lattice con stant, Upon cooling and heating, the thermal expansion coefficient bec omes significantly negative at 200 K, and at about 80 K a small reprod ucible contraction/expansion of the cubic lattice is observed,Both eff ects add evidence for hitherto unknown instabilities of the Cu2O struc ture at low temperatures. (C) 1997 John Wiley & Sons, Ltd.