Low‐temperature anomalies of cuprite observed by Raman spectroscopy and x‐ray powder diffraction

Anomalous low‐temperature behaviour of Cu 2 O 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‐resolution Guinier film camera. Changes in frequency of the Raman‐allowed Γ 25 + (LO) phonon band, and also of intensities of other Raman‐forbidden bands, indicate anomalies at temperatures of 90 and 190 K, respectively. The anomaly at 90 K is qualitatively interpreted by an order–disorder phase transition and that at 190 K by a discontinuous change of excitonic states. Close to these temperatures, the powder diffraction diagrams also revealed anomalous behaviour of the lattice constant. Upon cooling and heating, the thermal expansion coefficient becomes significantly negative at 200 K, and at about 80 K a small reproducible contraction/expansion of the cubic lattice is observed. Both effects add evidence for hitherto unknown instabilities of the Cu 2 O structure at low temperatures. © 1997 John Wiley & Sons, Ltd.