Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba_2RE_2Si_4O_(12)F_2 (RE = Er~(3+)-Lu~(3+)) and Ba_2RE_2Si_4O_(13) (RE = La~(3+)-Ho~(3+))

摘要

Structural variations across a series of barium rare earth (RE) tetrasilicates are studied. Two different formulas are observed, namely those of a new cyclosilicate fluoride, BaRE_2Si_4O_(12)F_2 (RE = Er~(3+)-Lu~(3+)) and new compounds in the Ba_2RE_2Si_4O_(13) (RE = La~(3+)-Ho~(3+)) family, covering the whole range of ionic radii for the rare earth ions. The Ba_2RE_2Si_4O_(13) series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P1 for La~(3+)-Nd~(3+), and space group C2/c for Sm~(3+)-Ho~(3+)). Two of the structure types identified are based on dinuclear rare earth units that differ in their crystal chemistries, particularly with respect to the role of fluorine as a structural director. The broad study of rare earth ions provides greater insight into understanding structural variations within silicate frameworks and the nature of f-block incorporation in oxyanion frameworks. The single crystals are grown from high-temperature (ca 953 K) hydrothermal fluids, demonstrating the versatility of the technique to access new phases containing recalcitrant rare earth oxides, enabling the study of structural trends.