Tb 2 Se 2 O 7 : Terbium(III) Oxide Oxoselenate(IV) according to Tb 2 O[SeO 3 ] 2 with a “Lone—Pair” Channel Structure

The colourless title compound Tb 2 Se 2 O 7 (≡ Tb 2 O [SeO 3 ] 2 , terbium(III) oxide oxoselenate(IV)) was obtained by reacting a mixture of Tb 4 O 7 , Tb and SeO 2 in a 3:2:14‐molar ratio at 850 °C for seven days in an evacuated silica vessel as needle‐shaped single crystals. It crystallizes tetragonally (space group: P4 2 /ncm, a = 1064.57(8), c = 521.56(4) pm, c/a = 0.490, Z = 4) and exhibits Tb 3+ cations in square‐antiprismatic coordination of eight oxygen atoms each. Six of the latter belong to six different isolated ψ 1 ‐tetrahedral [SeO 3 ] 2— anions whereas the remaining two are not bonded to selenium at all. Their coordination sphere actually consists of four Tb 3+ cations arranged as tetrahedron. These [OTb 4 ] 10+ tetrahedra form infinite one‐dimensional   ∞ 1 {[OTb 4/2 ] 4+ } chains along [001] by sharing trans‐ oriented edges which are packed such that a central strand is quadratically surrounded in (001) projection by four chains at a distance of √2 and another square of four more of them a factor of √2 (≡ the value of a) further apart and rotated by 45°. Charge balance and three‐dimensional interconnection of this particular rod‐packing occurs via discrete trigonal [SeO 3 ] 2— pyramids so that the whole structure gets strongly influenced by the stereochemical lone‐pair activity of these groups. Remarkably enough, the 4¯ symmetry of the   ∞ 1 {[OTb 4/2 ] 4+ } chains is perfectly repeated by the “lone‐pair” channels of the non‐ binding electrons at the Se 4+ cations.