Structure of the La12GdEuB6Ge2O34 borogermanate as probed by NMR and IR spectroscopy

Abstract

The structure of fine crystalline borogermanate La12GdEuB6Ge2O34 has been studied by NMR and IR spectroscopy. It has been demonstrated that this compound is isostructural to the homonuclear Ln14B6Ge2O34 compounds (Ln = Pr-Gd) and crystallizes in space group P31. The rare-earth elements have been distributed over the LnOn polyhedra in La12GdEuB6Ge2O34 by analogy with the known structures. Lanthanum can occupy positions with CN 7–10, and the symmetry of these LnOn coordination polyhedra is not higher than C2v. In the La12GdEuB6Ge2O34 structure, the LnOn coordination polyhedra are formed by oxygen atoms of oxo groups and anions, some of the oxygen atoms being shared by LnOn polyhedra. The BO3 and GeO4 groups in the structure are also bridging, i.e., are involved in bonding of LnOn polyhedra. One of the B-O bonds in La12EuGd(BO3)6(GeO4)2O8 is elongated as compared with the B-O bond lengths in homonuclear compounds Pr14(BO3)6(GeO4)2O8 and Nd14(BO3)6(GeO4)2O8. In the La12GdEuB6Ge2O34 structure, germanium is located in isolated GeO4 tetrahedra with distorted Td symmetry. The local symmetry of lanthanum in fine crystalline La12GdEuB6Ge2O34 have been assessed using 139La NMR (B0 = 7.04 T, room temperature). For comparison, binary lanthanum compounds with a simpler structure— LaBO3, La(BO2)3, and La2GeO5—have been used. The spectra of all compounds are rather broad (ν1/2 = 180–240 kHz). The 139La NMR spectra of the LaBO3, La(BO2)3, and La12GdEu(BO3)6(GeO4)2O8 borates show a signal at (1080 ± 40) ppm, which is absent in the spectrum of La2GeO5. The shape of the 139La NMR spectra of La12GdEu(BO3)6(GeO4)2O8 and LaBO3 is characterized by the second-order quadrupole splitting with a downfield shoulder. The similarity of these spectra points to close 139La NMR chemical shifts of La12GdEu(BO3)6(GeO4)2O8 and LaBO3. No quadrupole splitting was observed in the spectra of La(BO2)3 and La2GeO5.