Synthesis and Crystal Structure of a New Mixed NH4/Cs Borate (NH4)0.858Cs0.142[B5O7(OH)2]·0.5H2O

New mixed ammonium-cesium borate (NH4)0.858Cs0.142[B5O7(OH)2]·0.5H2O is prepared by soft hydrothermal synthesis in an evacuated quartz ampule and is structurally characterized (a = 7.6647(3), b = 9.2272(4), c = 12.0245(5) Å, α = 99.264(3), β = 106.204(4), γ = 91.511(3)°; space group P1̄ ; Z = 4). The compound belongs to a wide family of M[B5O7(OH)2]·0.5H2O (M = NH4, K, Rb, Cs, Ag) borates. The crystal structure of these compounds is formed by larderellite-like (5B:4ΔT:‹2ΔT›–‹2ΔT›)∞ boron-oxygen chains combined via large monovalent cations. The structural relation of M[B5O7(OH)2]·0.5H2O and M[B5O7(OH)2]·H2O families is discussed. The main differences between the structures are determined by different topologies of relative positions of their chains.