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

Abstract

The new compound HP-KB 3 O 5 was synthesized under highpressure/high-temperature conditions. It is the first compound exhibiting all three possible conjunctions simultaneously: corner-sharing BO 3 groups, corner-sharing BO 4

High-pressure Synthesis and Characterization of the Alkali Metal Borate HP-RbB3O5

“…The new compound HP-RbB 3 O 5 fulfills these expectations being isotypic to HP-KB 3 O 5 [19] and representing the fourteenth borate containing edge-sharing BO 4 tetrahedra. Furthermore, HP-RbB 3 O 5 is the fourth highpressure alkali metal borate in the recently synthesized series HP-LiB 3 O 5 [23], HP-Na 2 B 4 O 7 [24] and HP-KB 3 O 5 [19]. This paper reports about the synthesis, the single-crystal structure determination, and the vibrational spectroscopic investigations of HP-RbB 3 O 5 in comparison to the isotypic phase HP-KB 3 O 5 .…”

“…A detailed description can be found in ref. [19]. The isotypy to HP-KB 3 O 5 indicates that there are no large differences between the two structures.…”

“…Accordingly, high-pressure conditions favor the formation of tetrahedrally coordinated boron atoms, the edgesharing of BO 4 tetrahedra, an increased coordination number of the bridging oxygen atoms (O [3] ), and often an enhanced coordination of the metal cations as can be expected from the pressure coordination rule [22]. The new compound HP-RbB 3 O 5 fulfills these expectations being isotypic to HP-KB 3 O 5 [19] and representing the fourteenth borate containing edge-sharing BO 4 tetrahedra. Furthermore, HP-RbB 3 O 5 is the fourth highpressure alkali metal borate in the recently synthesized series HP-LiB 3 O 5 [23], HP-Na 2 B 4 O 7 [24] and HP-KB 3 O 5 [19].…”

“…For HP-KB 3 O 5 , the harmonic vibrational frequencies at the Γ point were calculated [19]. Based on these calculations, a more specific assignment of both, the IR and the Raman bands of HP-RbB 3 O 5 was possible.…”

“…Even the structural motif of two edge-sharing BO 4 tetrahedra forming a B 2 O 6 group is possible under high-pressure conditions, as first discovered in Dy 4 B 6 O 15 [11]. Meanwhile several other high-pressure phases are known to contain this B 2 O 6 group, e. g. RE 4 B 6 O 15 , (RE = Dy, Ho) [11,12], α-RE 2 B 4 O 9 (RE = Sm, Eu, Gd, Tb, Ho) [13,14] [19]. Besides these high-pressure phases, the recently discovered compound KZnB 3 O 6 [20,21] is the only normal-pressure phase exhibiting the structural element of two edge-sharing BO 4 tetrahedra.…”