Transition-metal substitution in PbAlBO4: Synthesis,structural and spectroscopic studies of manganese containing phases

Mullite-type metal borates PbAlBO₄ with planar BO₃ groups are potential candidates for nonlinear optical properties. We successively substituted aluminum with manganese in the composition PbAl_1?xMn_xBO₄, which would help tuning the crystal chemistry and related physical properties. The end member PbAlBO₄ was synthesized by conventional solid state reaction; the other members of the PbAl_1?xMn_xBO₄ series were prepared by glycerin method. The fundamental parameter approach was applied in the Rietveld refinements to describe the X-ray powder diffraction profiles. The cell parameters and interatomic bond distances were studied with respect to the chemical composition x. The MO₆ octahedral distortion linearly increased with the averaged MO bond distance, which is correlated with the increasing amount of manganese atoms in the structure. The change of the average crystal size and microstrain were explained in terms of Al/Mn ratio in the solid solution. While the empirical bond-valence sums (BVS) of one oxygen atom, bridging the edge-sharing MO₆ octahedron, were found to be under-bonded, the other one was over-bonded. The stereoactivity of the 6s² lone electron pair of the Pb²⁺ cation was characterized in terms of the absolute value of eccentricity parameter (|Φ_i|). Both bond valence parameter (r_0i) and BVS of Pb showed a linear correlation with |Φ_i|. Deconvolution of the absorption feature of the infrared powder spectra between 1000 cm^?1 and 1400 cm^?1 required four fitted bands, which were assigned to BO stretching vibrations of the trigonal planar BO₃ groups. With increasing Al³⁺ dilution in PbAl_1?xMn_xBO₄ almost all observed modes shifted toward lower wavenumbers confirming the solid solution. The absorptions caused by the vibrations of the heavy atom Pb were observed in the far-IR region.