RE2[B2(SO4)6] (RE = Y, La–Nd, Sm, Eu, Tb–Lu): a silicate-analogous host structure with weak coordination behaviour

Abstract

The rare earth borosulfates RE₂[B₂(SO₄)₆] with RE = Y, La–Nd, Sm, Eu and Tb–Lu were synthesised under solvothermal conditions starting from the metal chlorides (Pr, Nd, Eu), the metal oxides (Y, La, Ce, Sm, Tb, Dy, Er, Tm, Lu), or the metal powders (Ho, Yb). They crystallize isotypically with Gd₂[B₂(SO₄)₆] in space group C2/c (Z = 4, a = 1346.9(3)–1379.24(17) pm, b = 1136.4(3)–1158.87(14) pm, c = 1079.9(3)–1139.54(14) pm, β = 93.369(8)–93.611(4)°). The anionic structure consists of an open-branched vierer single ring {oB, 1r}[B₂S₂O₁₂(SO₃)₄]⁶⁻, similar to the mineral eakerite (Ca₂Al₂Sn[Si₆O₁₈](OH)₂·2H₂O) which contains {oB, 1r}[Si₄O₁₂(SiO₃)₂]¹²⁻ moieties. The fluorescence spectroscopy of the samples with RE = Ce, Eu and Tb features emissions in the deep UV, the red, and the green part of the spectrum and furthermore revealed a weak coordination behaviour of the borosulfate anion. Thermal analysis of Eu₂[B₂(SO₄)₆] showed the highest thermal stability observed for borosulfates so far; respective trends within the borosulfate family are discussed. Additionally, the compounds were characterised by magnetic measurements, vibrational and ¹⁵¹Eu Mößbauer spectroscopy.