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

Abstract

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, BaRESiOF (RE = Er-Lu) and new compounds in the BaRESiO (RE = La-Ho) family, covering the whole range of ionic radii for the rare earth ions. The BaRESiO series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P{\overline 1} for La-Nd, and space group C2/c for Sm-Ho). 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.

Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]·H2O, a New Member of the OD-Family of Natural and Synthetic Layered Silicates: Topology-Symmetry Analysis and Structure Prediction

“…There are no silicate-germanates with Tb in the nature or synthetic compounds according to [1,2]. However, there are a lot of original synthetic Tb-silicates with different anionic radicals [2,3]: ortho-NaTb 9 (SiO 4 ) 6 O 2 [4], KTb 9 (SiO 4 ) 6 O 2 and Cd 2 Tb 8 (SiO 4 ) 6 O 2 [5] (structural type of apatite), Na 5 Tb 4 (OH)[SiO 4 ] 4 [6]; diortho-Tb 2 Si 2 O 7 [7], K 3 TbSi 2 O 7 [8], Tb 4 S 3 Si 2 O 7 [9]; triortho-K 3 TbSi 3 O 8 (OH) 2 [10]; tetraortho-groups Ba 2 Tb 2 Si 4 O 13 [11]; six-membered rings Na 3 Tb 3 Si 6 O 18 •H 2 O (synthetic gerenite) [12]; chains-unbranched Rb 2 TbGaSi 4 O 12 [13], wollastonite Na 2 Tb 1.08 Ca 2.92 Si 6 O 18 H 0.8 [14] and spiral Na 3 TbSi 3 O 9 •3H 2 O [15]; layers in Cs 3 TbSi 4 O 10 F 2 [16] [19]. For some of these compounds, the luminescence properties were investigated [6,10,[13][14][15]18,19].…”