Hydrothermal crystal growth of 2-D and 3-D barium rare earth germanates: BaREGeO4(OH) and BaRE10(GeO4)4O8 (RE = Ho, Er)

Two new structural types of BaREGeO₄(OH) and BaRE₁₀(GeO₄)₄O₈ (RE = Ho³⁺,Er³⁺) single crystals were synthesized via high-temperature and high-pressure hydrothermal synthesis. The BaREGeO₄(OH) compounds were found to crystallize in the orthorhombic space group Pbca. BaHoGeO₄(OH) is used as a representative of the family with cell parameters of a = 5.7175(2) Å, b = 10.1556(5) Å, c = 10.6189(9) Å and V = 964.97(8) ų. The BaREGeO₄(OH) structure contains a one-dimensional chain of rare-earth polyhedra linked through edge sharing of oxygen atoms. High density BaRE₁₀(GeO₄)₄O₈ crystals crystallize in the monoclinic space group C2/m and feature a sheet like arrangement of rare-earth oxide polyhedra with Keggin-like features. BaHo₁₀(GeO₄)₄O₈ is used as a representative of this structure type with cell parameters of a = 12.4533(8) Å, b = 7.2008(5) Å, c = 12.0034(8) Å, β = 100.183(2)° and V = 1059.43(12) ų. Barium polyhedra and isolated GeO₄ units aid in connecting the rare earth oxide framework to extend it in three-dimensional (3-D) space. Characterization by single crystal X-ray diffraction and Raman and photoluminescence spectroscopies is reported.