Optical study of the piezochromic transition in CuMoO4 by pressure spectroscopy

The aim of this work is to investigate the origin of the piezochromism and thermochromism exhibited by the copper oxide CuMoO4. These optical phenomena are associated with structural phase transition (PT) from the triclinic alpha (green) modification to the gamma (brownish-red) modification. The variation of the optical-absorption spectrum with pressure and temperature indicates that the piezochromic and thermochromic transitions can be leached from ambient conditions either by applying pressure at 2.5 kbar or by cooling at T = 200 K. We show that the change of color at the alpha --> gamma PT is due to the broadening of the first O2- --> Cu2+ charge-transfer band, and the disappearance of an intense peak at 1.49 eV, related to the presence of pyramidal CuO5 complexes in alpha-CuMoO4. The measured oscillator strength suggests that this peak corresponds to the e-->b(1) crystal-field transition within CuO5 rather than to an O2---> Cu2+ 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 gamma- CuMoO4 modification.