Temperature dependence of the optical properties of CuMoO4

CuMoO4 crystals reversibly change their color from green to brown upon heating, accompanied by a loss in transmittance. UV/VIS spectroscopic analysis revealed that these changes are due to particular electronic properties of the crystal instead of its chemical decomposition or structural change. Investigations were carried out in the temperature range 23-400 degrees C. The intensive green color of the crystal at room temperature is caused by a small transmission window between two absorption bands, the band gap of the crystal in the blue and the 3d9--4p absorption of the Cu2+ ions in the red. With increasing temperature the band gap shifts towards longer wavelengths, and the crystal changes both color and transmittance. Spectroscopic features of the crystal are discussed together with the temperature dependence of its electrical resistance. Resistance measurements were performed simultaneously to the optical measurements.