The aim of this work is to investigate the origin of the piezochromism and thermochromism exhibited by the copper oxide ${\mathrm{CuMoO}}_4.$ 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 reached from ambient conditions either by applying pressure at 2.5 kbar or by cooling at $T=200\mathrm{}\hspace{0.5em}\mathrm{K}.$ We show that the change of color at the $\overrightarrow{\alpha }\gamma \hspace{0.5em}\mathrm{PT}$ is due to the broadening of the first ${\mathrm{O}}^{2-}\to {\mathrm{Cu}}^{2+}$ charge-transfer band, and the disappearance of an intense peak at 1.49 eV, related to the presence of pyramidal ${\mathrm{CuO}}_5$ complexes in $\alpha -{\mathrm{CuMoO}}_4.$ The measured oscillator strength suggests that this peak corresponds to the $\overrightarrow{e}{b}_1$ crystal-field transition within ${\mathrm{CuO}}_5$ rather than to an ${\mathrm{O}}^{2-}\to {\mathrm{Cu}}^{2+}$ 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 $\gamma -\mathrm{}{\mathrm{CuMoO}}_4$ modification.

• Received 24 January 2000

DOI:https://doi.org/10.1103/PhysRevB.61.16497