title: The first boroselenates as new silicate analogues.
abstract: The first boroselenates were obtained as single crystals by the reaction of selenic acid with boron acid and the corresponding alkali carbonates. The structure determinations showed a far-reaching analogy to very recently described borosulfates and the borophosphates, that is, tetrahedral BO4 and SeO4 units linked by common corners. In each case, the BO4 tetrahedra are surrounded by SeO4 tetrahedra. As a function of the B/Se ratio, this results in chains (1:3; Cs3 [B(SeO4)3], Rb3 [B(SeO4)3]), isolated pentamers (1:4; HK4 [(B(SeO4)4]), or pentamers with additional isolated SeO4 tetrahedra (1:5; (H3 O)Na6 [B(SeO4 )4 ](SeO4). Compound Rb3 [B(SeO4)3] (orthorhombic, Ibca, Z=8, a=7.508(2), b=15.249(3), c=23.454(5) Å) is isotypic to Rb3 [B(SO4)3]) and Ba3 [B(PO4)3]. Compound Cs3 [B(SeO4)3] (monoclinic, P21 /c, Z=4, a=11.3552(4), b=7.9893(3), c=15.7692(6) Å, β=101.013(1)°) represents a distorted variant of Rb3 [B(SeO4)3]. The isolated pentamers in HK4 [(B(SeO4)4]) (triclinic, P$\bar 1$, Z=6, a=7.5303(1), b=7.5380(1), c=42.3659(4) Å, α=88.740(1), β=89.971(1), γ=89.971(1)°) were also found in K5 [(B(SO4)4] and Na5 [(B(SO4)4]. Compound (H3 O)Na6 [B(SeO4)4](SeO4) (tetragonal, I$\bar 4$, a=9.9796(1), c=18.2614(2) Å) is a super structure of the borophosphates Sr6 [B(PO4)4](PO4) and Pb6 [B(PO4)4](PO4). Because the tetrahedra are only connected through apices, there are topological analogies to silicates. Therefore, boroselenates may have a similar variability of crystal structures, such as borosulfates and borophosphates.
authors.affiliation: Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität, Albertstrasse 21, 79104 Freiburg (Germany).
authors.forename: Michael
authors.initials: M
authors.lastname: Daub
authors.forename: Harald
authors.initials: H
authors.lastname: Hillebrecht
keywords: alkali metals; boron; boroselenates; selenium; structure determination