Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba2RE2Si4O12F2 (RE = Er3+–Lu3+) and Ba2RE2Si4O13 (RE = La3+–Ho3+)

Structural variations across a series of barium rare earth (RE) tetrasilicates are studied. Two different formulas are observed, namely those of a new cyclo-silicate fluoride, BaRE2Si4O12F2 (RE = Er3+–Lu3+) and new compounds in the Ba2RE2Si4O13 (RE = La3+–Ho3+) family, covering the whole range of ionic radii for the rare earth ions. The Ba2RE2Si4O13 series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P{\overline 1} for La3+–Nd3+, and space group C2/c for Sm3+–Ho3+). 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.