Dehydration Process of TlB5O6(OH)4·2H2O and Crystal Structure of TlB5O8

Abstract

The dehydration of TlB5O6(OH)4·2H2O leads to a loss of water in one step, forming anhydrous TlB5O8, which is amorphous to X-ray diffraction. This can be explained by structural considerations; a decrease in oxygenated coordination of thallium ions occurs during the heating. TlB5O8crystallizes at about 450°C and melts at 660°C. By slow cooling from the melt, single crystals were obtained. TlB5O8is orthorhombic, space groupPbca, with unit cell parametersa=7.557(3) Å,b=11.925(6) Å,c=14.734(19) Å;Z=8. The structure was solved from 1113 reflections untilR=0.023 andRw=0.026. The structure consists of two separate interlocking three-dimensional networks of pentaborate groups, which form large empty tunnels. Nine oxygen atoms belonging to four different pentaborate groups surround each thallium ion. Except for the presence of pentaborate groups with almost the same geometry, there is no relation between the structure of TlB5O8and that of its hydrated precursor, TlB5O6(OH)4·2H2O; the shorthand notation of polyborate anions in these compounds is 5:∞3(4Δ+T) and 5:4Δ+T, respectively.