Synthesis and crystal structure of three MM′B9O15 borates (M=Ba, Sr and M′=Li; M=Ba and M′=Na)

The compounds BaLiB₉O₁₅ (BaLi), BaNaB₉O₁₅ (BaNa) and SrLiB₉O₁₅ (SrLi) have been synthesised, and their structure determined by single crystal X-ray diffraction methods. Each borate crystallises with six formula units in the R3c non-centrosymmetric space group. Cell parameters are: a=11.0223(19) Å, c=17.106(4) Å, and V=1799.8(6) ų for the (BaLi) compound; a=11.0933(15) Å, c=17.392(3) Å, and V=1853.5(5) ų for the (BaNa) compound; a=10.6129(9) Å, c=17.555(2) Å, and V=1712.4(3) ų for (SrLi) compound. The borate anion is characterised by a three-dimensional framework built up from [B₃O₇] rings; its shorthand notation is 3: ∞³ (2△+T), where △ is a BO₃ triangle and T is a BO₄ tetrahedron. In these three compounds, the alkaline earth (Ba, Sr) and the alkaline (Li, Na) ions occupy alternate sites in the channels along the c axis; however, their oxygenated environment is different. In (BaNa) and (SrLi), the [BaO₉] and [SrO₉] form tricapped trigonal prisms and [NaO₆] and [LiO₆] trigonal antiprisms whereas in (BaLi), [BaO₁₂] forms an irregular icosahedron and there are only three oxygen atoms in the neighboring of lithium ion.