New rubidium pentaborate Rb[B5O7(OH)2] · 0.5H2O with a 5: [4Δ + 1T] anionic block and its relation to larderellite (NH4)[B5O7(OH)2] · H2O on the basis of the OD theory

RbB11O16(OH)2: First Hydroxyl Rubidium Borate with the B‐O 2D Layer Featuring Large Birefringence in the Rb‐B‐O‐H System

“…To the best of our knowledge, nine hydroxyl rubidium borates compounds have been reported except the title compound . The formula, space group and B−O anionic framework are summarized in Table S4 in the Supporting Information.…”

“…In general, the incorporation of the (OH) group in borate can avoid the appearance of terminal oxygen atoms and decrease the dimension of the B−O framework. According to the references, it can be known that the isolated anionic groups of those hydroxyl rubidium borates except Rb 3 [B 3 O 4 (OH) 4 ]⋅2H 2 O do not have terminal oxygen atoms (the terminal oxygen atoms are linked with H), so the anionic group cannot combine with adjacent ones. For Rb 3 [B 3 O 4 (OH) 4 ]⋅2H 2 O, its anionic group is [B 3 O 4 (OH) 4 ] 3− with a terminal oxygen atom, however, this terminal oxygen atom is further connected with two hydrogen atoms of water to form hydrogen bond, avoiding [B 3 O 4 (OH) 4 ] 3− combining with other anionic groups.…”