Investigations of the local structures and spin Hamiltonian parameters for the trigonal Rh4+ and Ir4+ centers in rhombohedral BaTiO3

The local structures and spin Hamiltonian parameters (g factors and the hyperfine structure constants) of the Rh4+(4d5) and Ir4+(5d5) centers in rhombohedral BaTiO3 are theoretically investigated from the formulas of these parameters for a nd5 (n = 4 and 5) ion with low spin (S = 1/2) in a trigonally distorted octahedron. From the calculations, the impurity ions are found not to occupy exactly the host Ti4+ site in BaTiO3 but to suffer a slight inward shift (∼0.13 Å) towards the center of the oxygen octahedron along the C3 axis, yielding much smaller trigonal distortion as compared with that of the host Ti4+ site. The theoretical spin Hamiltonian parameters based on the above impurity axial shifts are in good agreement with the observed values.