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.