Stable Oxoborate with Edge‐Sharing BO₄ Tetrahedra Synthesized under Ambient Pressure

Abstract

Druck ist nicht alles: Kantenverknüpfte BO4‐Tetraeder treten nur selten in Boratstrukturen auf, und dann nur, wenn diese unter hohem Druck erzeugt werden. Jetzt wurde dieses Strukturmotiv auch in einem bei Umgebungsdruck erhaltenen Borat entdeckt. Während die Hochdruckphasen nur metastabil sind, ist die Struktur der Titelverbindung bis zu ihrem Schmelzpunkt beständig. Die Verbindung kann außerdem mobile Ionen in ihre Tunnelstruktur aufnehmen.

HP‐KB3O5 Highlights the Structural Diversity of Borates: Corner‐Sharing BO3/BO4 Groups in Combination with Edge‐Sharing BO4 Tetrahedra

“…The ribbons are interconnected by trigonal-planar BO 3 groups to form channels, in which the tenfold coordinated potassium ions are situated. The recently published KZnB 3 O 6 [13,14] shows remarkable similarities to our compound and is built up from [B 6 O 12 ] 6-groups (two rings of two BO 3 and one BO 4 unit, linked by edge-sharing BO 4 tetrahedra), which are connected by ZnO 4 tetrahedra to form a three-dimensional network. Figure 2 depicts a comparison of the crystal structures of HP-KB 3 O 5 and KZnB 3 O 6 .…”

“…It is for the first time that oxygen ions, involved in the edge-sharing of BO 4 tetrahedra, are observed to bridge three BO 4 tetrahedra (O [3] ). In KZnB 3 O 6 [13,14] (Figure 2, right), there are also edge-sharing BO 4 tetrahedra and BO 3 groups, but no corner-sharing BO 4 groups. Here, the edge-sharing BO 4 tetrahedra (light polyhedra) connect to four BO 3 units; two ZnO 4 groups are situated at the common edge of the B 2 O 6 unit (highlighted with arrows).…”

“…Figure 2 depicts a comparison of the crystal structures of HP-KB 3 O 5 and KZnB 3 O 6 . [13,14] In HP-KB 3 O 5 ( Figure 2, left), the edge-sharing BO 4 tetrahedra (light polyhedra) connect to two BO 3 and four BO 4 groups through common corners. Two of the BO 4 groups are situated at the common edge of the B 2 O 6 unit.…”

“…[12] To our astonishment, the structural feature of edge-sharing BO 4 tetrahedra was recently found in the compound KZnB 3 O 6 , synthesized under ambient pressure conditions. [13,14] Thus, the structural motif of edge-sharing BO 4 tetrahedra is no longer a domain of highpressure chemistry, but is still favored under these conditions, as there are eleven different compounds with three structure types prepared under high-pressure conditions and only one compound synthesized under ambient-pressure conditions. However, the synthesis of KZnB 3 O 6 [13,14] leads to the question, in principle, whether it is possible to synthesize a borate exhibiting all the above-mentioned structural motifs (corner-sharing BO 3 /BO 4 groups and edge-sharing BO 4 tetrahedra) simultaneously.…”

“…[13,14] Thus, the structural motif of edge-sharing BO 4 tetrahedra is no longer a domain of highpressure chemistry, but is still favored under these conditions, as there are eleven different compounds with three structure types prepared under high-pressure conditions and only one compound synthesized under ambient-pressure conditions. However, the synthesis of KZnB 3 O 6 [13,14] leads to the question, in principle, whether it is possible to synthesize a borate exhibiting all the above-mentioned structural motifs (corner-sharing BO 3 /BO 4 groups and edge-sharing BO 4 tetrahedra) simultaneously. With HP-KB 3 O 5 (HP = high pressure), we synthesized a new borate, in which the two well-known fundamental structural motifs, alias BO 3 and BO 4 groups, and all noted conjunctions, namely corner-sharing of BO 3 /BO 4 groups and corner-as well as edge-sharing of BO 4 tetrahedra, are discovered in one crystal structure for the first time.…”