EPR Study of Strong Axial Cr³⁺-V_o Centers in WO₃

Abstract

The EPR spectra of Cr 3+ ion accompanied with a nearest neighbour oxygen vacancy (Cr 3+ -V o center) were observed over a temperature range from -150 to 200°C and showed `strong axial field lines'. The crystalline fields have been obtained as follows; | D |=84 GHz, | E |=2.6 GHz at 25°C, and | D |=120 GHz, | E |=23 GHz at -100°C. These fields are very large compared with those observed in many other crystals. The angular dependence of the EPR spectra has been interpreted by a rotation of oxygen octahedra about a quasi-cubic diagonal <111 > axis and the displacements of tungsten atoms. From the temperature dependence of the EPR spectra the softening of the rotational condensed phonon modes of the oxygen octahedra was observed. The relations between the EPR spectra and the structural phase transitions in WO 3 are discussed.