Aspects of lithium tri- and tetraborate synthesis in the subsolidus region

The present work is focused at studying the transformation patterns for the crystalline structure of phases formed during the synthesis of polycrystalline lithium tri- and tetraborate. In the field of lithium triborate (LiB3O5) and tetraborate (Li2B4O7) of the Li2O – B2O3 system, LiB3O5 and Li2B4O7 polycrystalline powders were synthesised. In terms of precursors, lithium carbonate (Li2CO3) and boric acid (H3BO3) were selected. Two synthesis methods were tested including precipitation from solution and solid-phase synthesis. As a result, the direct sintering of a mechanically-grinded stoichiometric precursor mixture is shown to be the optimal method for crystallising LiB3O5 and Li2B4O7. The crystallisation patterns of lithium borates were studied in a temperature range of 500-850 °C with sampling carried out every 50 °C. Individual phase portraits were established presenting a set of process character-istics depending on the level of substance particle organisation and subsolidus crystallisation from the initial reagent mixture of lithium tri- and tetraborate at the phase, local and structural levels. In a lithium triborate stoichiometric mixture, the maximum conversion of crystalline phases is observed in the region of 500–600 °C, while, for lithium tetraborate, the temperature maximum is in the range of 600–700 °C. The sequence of phase transformations re-mains almost unchanged and occurs according to the following scheme: starting reagents > intermediate metastable phases > final borates. The local level of phase portraits characterises the interaction of coordination polyhedra forming the crystal lattice of the studied phases. Solid-phase synthesis of crystalline LiB3O5 from the Li2CO3 and H3BO3 takes place as a result of (BO3)3- → (B3O7)5- transition with the (BO3)3- → (B4O9)6- scheme realised in obtaining Li2B4O7. At the crystal structure level, such transitions correspond to transformations of the monoclinic lattice of the Li2CO3 and H3BO3 primary phases into the LiB3O5 and Li2B4O7 rhombic and tetragonal structure, respectively. In this case, an intermediate step of this transformation consists in formation of the LiBO2 trigonal chain metaborate and metastable Li2B8O13.

borate synthesis, lithium triborate, lithium tetraborate, Li2O – B2O3 system, solid-phase synthesis, phase portrait of crystallization