Investigations on the local structures of Cr3+(3d3) and Nd3+(4f3) ions at the trigonal Bi3+ sites in Bi4Ge3O12

The local structures of Cr3+(3d3) and Nd3+(4f3) ions at the trigonal Bi3+ sites in Bi4Ge3O12 (BGO) are theoretically studied by using the perturbation formulas of the EPR parameters for a 3d3 or 4f3 ion in trigonally-distorted octahedra. Based on the studies, we find that the impurity Cr3+ does not occupy exactly the host Bi3+ site but undergoes an off-center displacement ΔZCr (≈0.17 Å) towards the center of the oxygen octahedron along the C3 axis, whereas the Nd3+ suffers another outward shift ΔZNd (≈−0.08 Å) away from the center of the octahedron. The calculated EPR parameters based on the above displacements show reasonable agreement with the observed values.