TETRAHEDRALLY COORDINATED CR-5+ IN SRTIO3

This paper reports on the electron paramagnetic resonance angular variation of the S = 1/2 Cr5+ center with orthorhombic symmetry, obtained after quenching Al and Cr-doped SrTiO3 from 1400-degrees-C. At 4.2 K, the main axes of the g-tensor in monodomain {001} samples are found to be along [001] and rotated by +/- alpha = 2 psi = 4-degrees from <110> axes, where psi is the intrinsic structural phase transition (SPT) order parameter. From this and the size of the g-values as well as stereometrical considerations, an assignment of the small Cr5+ ion to a tetrahedral site in the perovskite structure is made. Above the tetragonal-to-cubic SPT at T(c) = 105 K, a dynamic reorientation among the six equivalent sites is observed, and below T(c), a deviation from alpha(T) = 2 psi(T) between 70 K < T < T(c).