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 phenomen a are associated with structural phase transition (PT) from the triclinic a lpha (green) modification to the gamma (brownish-red) modification. The var iation of the optical-absorption spectrum with pressure and temperature ind icates that the piezochromic and thermochromic transitions can be leached f rom ambient conditions either by applying pressure at 2.5 kbar or by coolin g 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, an d the disappearance of an intense peak at 1.49 eV, related to the presence of pyramidal CuO5 complexes in alpha-CuMoO4. The measured oscillator streng th suggests that this peak corresponds to the e-->b(1) crystal-field transi tion within CuO5 rather than to an O2---> Cu2+ charge-transfer band. The co rrelation between optical and structural properties performed in this work confirms this interpretation, and also explains the strong dichroism exhibi ted by the crystal in the high-pressure gamma- CuMoO4 modification.