New structural phase transitions in PbMBO$_{4}$ complex oxides: Raman spectroscopy and x-ray diffraction studies*

Complex oxides with the mullite crystal structure belong to the most important phase in both traditional and advanced ceramics. Mullites are built of infinite chains of edge-sharing MO$_{6}$ octahedra, bridged by various oxide groups. Interest in metal borates stems from their useful nonlinear optical properties. New complex oxides in the mullite family PbMBO$_{4}$ (M= Fe, Mn, Al) were synthesized and characterized. Using Raman spectroscopy and synchrotron x-ray diffraction at elevated pressure we demonstrate new structural phase transitions.