Comparison of neutron diffraction and EPR results on the cubic crystal structures of piezoelectric Bi4Y3O12(Y = Ge,Si)

By means of neutron diffraction measurements the structure parameters of polycrystalline Bi₄Y₃O₁₂(Y = Ge, Si) crystallizing according to space group $\text{I}\text{4}\text{3d?T}{}^6{}_{\mathrm{d}}$ were determined at room temperature. With respect to the trigonal coordination of oxygen ions around Bi³⁺ the results are compared to EPR investigations on equivalent single-crystal specimens containing Gd³⁺ impurities on Bi sites. Corresponding approximations in the interpretation of the measurements are discussed. Although both methods yield similar systematic differences between the Ge- and Si-compounds, the influence of Gd on the structures appears to be substantial. Moreover point charge models should be improved with respect to details of the electron density distribution.