Optical study of the piezochromic transition in by pressure spectroscopy

The aim of this work is to investigate the origin of the piezochromism and thermochromism exhibited by the copper oxide (Formula presented) These optical phenomena are associated with structural phase transition (PT) from the triclinic (Formula presented) (green) modification to the (Formula presented) (brownish-red) modification. The variation of the optical-absorption spectrum with pressure and temperature indicates that the piezochromic and thermochromic transitions can be reached from ambient conditions either by applying pressure at 2.5 kbar or by cooling at (Formula presented) We show that the change of color at the (Formula presented) is due to the broadening of the first (Formula presented) charge-transfer band, and the disappearance of an intense peak at 1.49 eV, related to the presence of pyramidal (Formula presented) complexes in (Formula presented) The measured oscillator strength suggests that this peak corresponds to the (Formula presented) crystal-field transition within (Formula presented) rather than to an (Formula presented) charge-transfer band. The correlation between optical and structural properties performed in this work confirms this interpretation, and also explains the strong dichroism exhibited by the crystal in the high-pressure (Formula presented) modification.