Lanthanide borogermanates LnBGeO5: Synthesis and structural study by X-ray diffractometry and vibrational spectroscopy

LnBGeO5 borogermanates have been synthesized by solid-state reaction and investigated by powder X-ray diffractometry, infrared, and Raman spectroscopy. A hexagonal, stillwellite-like phase is obtained for Ln = La, Pr, and Nd (low-temperature phase). Smaller cations (Nd to Er and Y) lead to a monoclinic phase, Z = 4, with a = 9.80-10.03, b = 7.42-7.60, c = 4.79-4.96 Å, and β = 91°14′-91°54′, depending on the ionic radius of the rare earth. For the smallest lanthanides (Tm, Yb, Lu), no monoclinic phase is obtained: Ln2Ge2O7 is eventually formed, with a total loss of B2O3. The vibrational spectrum of the monoclinic phase points to some analogies with the stillwellite structure, namely a tetrahedral coordination of boron, and an ordered distribution of the BO4 and GeO4 tetrahedra. © 1988.