Correlation between structures and thermal properties of hydrated thallium(I) diborates; H2O-Tl2B4O7 phase diagram

Abstract

Two synthetic hydrated thallium(l) diborates have been found in the liquid~.~-solid equilibria of the I00 ~ isotherm of the ternary system H20---B203--TI20; they were characterized via the powder diagrams, but classical chemical analysis does not lead to the correct degree of hydration. Through TG of the powders, a complex process is found with no explanation. Structural resolution and TG of the monocrystals allow a.correct explanation of the thermal dehydration: these thallium(I) diborates are two distinct compounds, TlzB407 9 3H20 and TI2B407 -1.5H20 , which have their own process of dehydration; they contain infinite chains of polyanions and their structural formulae are TI2[B406(OH)2 ].2H20 and TL[BsO~2(OH)4]" H20; the latter polyanion may be considered as the dimer of the first.Tlae H20~T12B407 phase diagram was established by thermal analysis and solubility experiments, both under pressure; it allows the prediction that another hydrated thallium(1) diborate, T12B407. H20, exists, with possible structural formula TIr[B12OIa(OH)6 ]. Actually, only monocrystals of T14[BsO12(OH)4]-H20 have been obtained hydrothermally from TI2[B406(OH)2] "2H20.During establishment of the 100 ~ isotherm of the ternary system H20~B203-T120 [1], the existence of two thallium(l) diborate hydrates became obvious: one with congruent solubility and the other with incongruent solubility and having a very narrow crystallization zone (Fig. 1). Chemical analysis of the solid phases separated from their respective solutions does not allow the exact determination of the degree of hydration; on the other hand, these products were clearly identified by their very different powder diagrams. It therefore became necessary to resort to thermogravimetry (TG) so as to propose plausible chemical formulae.