Crystal structure of NaRbB4O5(OH)(4)center dot 4H(2)O: comparison with other tetraborate hydrates

The crystal structure and the thermal behavior of a new tetraborate, NaRbB4 O5(OH)(4) . 4H(2)O, are presented. It crystallizes in the monoclinic system , space group P2(1)/c with unit cell dimensions a = 8.1138(5) Angstrom, b = 11.471(1) Angstrom, c = 12.8233(9) Angstrom, beta = 91.91(4)degrees, V = 1 192.8(1) Angstrom (3); Z = 4. The structure was determined from 3474 unique reflections and refined until R = 0.045 and R-w = 0.12. It is composed of isolated [B4O5(OH)(4)](2-) polyanions separated by free water molecules and Na,Rb cations. A structural relationship is found between NaRbB4O5(OH)(4). 4H(2)O and Cs2B4O5(OH)(4). 3H(2)O. It is related to the following replacem ents at caesium sites: Cs(1)(+) --> Na+ + H2O and Cs(2)(+) --> Rb+. The deh ydration of NaRbB4O5(OH)(4). 4H(2)O occurs in two close steps. The resultin g amorphous phase undergoes a recrystallization to give two successive meta stable phases before melting. Upon cooling, a new form of NdRbB4O7 appears. (C) 2001 Elsevier Science B;V, All rights reserved.