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 triand 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 characteristics depending on the level of substance particle organisation and subsolidus crystallisation from the initial reagent mixture of lithium triand 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 remains 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) → (B3O7) transition with the (BO3) → (B4O9) 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.


INTRODUCTION
The Li2O -B2O3 system plays an important role in glass manufacturing technology. The compounds formed in this system are characterised by a number of unique properties [1,2]. In particular, lithium triborate (LiB3O5) is a highly in-demand material used to create effective nonlinear media for converting laser radiation. Lithium tetraborate (Li2B4O7) presents a promising material for nonlinear detectors [3,4]. It should be noted that the vast majority of experimental studies of this system were initially devoted to the production of single crystals [5]. However, the emphasis later shifted to the study of lithium-borate glasses. Recent publications confirm that lithium borate-based glasses doped with rare-earth elementsincluding praseodymium [6], samarium [7,8], dysprosium [9], europium [10], as well as their combinations [11]are among the most advanced thermoluminescent and scintillation materials. In connection with the manifestation of these properties, the topology and structure of lithium borates are of particular interest [12].
One the one hand, an important advantage of glassy matrices consists in the relative simplicity of the technology for their derviation. On the other hand, the synthesis of these materials is hampered due to the complex nature of the phase transformations accompanying their crystallisation. Taking all the above into consideration, a need arises for the construction of an accurate phase diagram of the Li2O -B2O3 system and ternary systems constituting it. Such studies, which were begun in the 1950s [13,14], are still ongoing [15][16][17]. However, a full thermodynamic description of the Li2O -B2O3 system is yet to be completed, including in the crystallisation region for the otherwise welldescribed compounds lithium triborate and lithium tetraborate.
As is known, the basic stage of thermodynamic research involves a consideration of the nature of phase formation during the interaction of system components. A mixture of lithium carbonate and boric acid is acknowledged to be the highestyielding precursor for the synthesis of lithium borates in the Li2O -B2O3 system [18]. Therefore, the present work was mainly focused on the synthesis of the most monophasic material of a given stoi-ХИМИЧЕСКИЕ НАУКИ / CHEMICAL SCIENCES chiometry for obtaining glass with the required properties.

EXPERIMENTAL PART
Li2CO3 and H3BO3 of extra pure grade were used following complete desiccation at 100 °C. Phase interactions were studied in the temperature range of 300-850 °C.
Two methods for preparing the initial mixture for the synthesis of lithium borates were tested: precipitation of components from solution and solid-phase sintering of the mechanically agitated dry reagents. The criteria for determining these methods included their rapidity and the absence of impurities in the system.
For the precipitation of target compounds from the liquid phase, a saturated solution of boric acid in distilled water was prepared. Then it was heated to 60 °C and a calculated stoichiometric amount of lithium carbonate was added according to the reactions: The completion of the reaction was monitored by the cessation of gas emission. The solution was evaporated followed by the drying of precipitate at 100 ºC and a sampling for analysis. For subsequent solid-phase sintering, the dried precipitate was powdered in a porcelain mortar and transferred to alundum crucibles.
The preparation of the mixture for solid-phase synthesis and sintering included: a) grinding of the starting reagents; b) taking sample weights corresponding to stoichiometry of the target phases; c) mixing to obtain visual uniformity. The resulting batch was also placed in crucibles.
The solid-phase synthesis of the target phases was carried out in air according to the technique for obtaining lithium triborate presented in [18]. In order to remove water, the mixture was heated from room temperature to 300 °C at a rate of 4 °C/min and left for 1 h. Then, after removing the crucibles from the furnace, the mixtures were remixed and heated at the same rate in the range of 400-850 °C (for LiB3O5, the maximum temperature was 800 °C). For every 50 °C step, curing and sampling from each crucible was carried out for an hour followed by further heating.
X-ray diffraction analysis of the samples was carried out using a Bruker D8 Advance diffractometer (CuKα-radiation, scintillation detector, Goebel mirror, Δ2Ɵ = 0.02 step mode). Data processing was performed using the DIFFRAC plus software package. XRD reflections were identified using the PDF-2 powder diffractometry database (ICDD, 2007) and EVA software (Bruker, 2007).

RESULTS AND DISCUSSION
The main objective of this study consisted in obtaining monophasic lithium tri-and tetraborate for subsequent manufacture of glass products.
In the field of LiB3O5 and Li2B4O7, the studies of several authors were combined for the Li2O-B2O3 phase diagram (Fig. 1). The basis formed by the well-known study [14] was supplemented by data presented in [15][16][17] and reduced to a single expression of component concentrations in weight percent. The melting points of lithium borates were taken into account according to the results given in [16] and interpreted by the authors on the basis of the Raman spectra. According to these data, LiB3O5 and Li2B4O7 melts at 857 and 927 °C, respectively. Therefore, the coefficient of solid-phase synthesis (K = Тsynth / Тmelt) in present experiments was: К(LiB3O5) = 0.93, К(Li2B4O7) = 0.92. X-ray diffraction analysis demonstrated the sequence and nature of phase transformations to be almost identical during phase crystallisation from stoichiometric mixtures of both lithium triand tetraborate. In addition, no differences in the phase formation pattern were detected during solid-phase synthesis using both mechanical agitation and chemical precipitation sample preparation methods. Nevertheless, the crystallisation of each compound under consideration is characterised by its own specifics. Let us consider these in more detail.
As a result of the chemical interaction of lithium carbonate with boric acid in an aqueous solu-tion, mixtures of lithium borates with different component ratios were formed. According to the X-ray diffraction data, crystalline lithium tri-and tetraborate were present in the evaporated precipitate. However, further extraction and purification of the target compounds from the solution was a rather laborious process. In order to purify the phases by means their further crystallisation, the obtained precipitates were subjected to further solid-phase sintering.
A characteristic difference in the formation of lithium triborate compared with the tetraborate consists in the lower temperature region of the phase transformations resulting in target phase. The diffraction patterns of the stages in the solid-state synthesis illustrate the temperature ranges of the main phase transformations in the system. Thus, the maximum conversion of crystalline phases was recorded in the ranges of 500-600 and 600-700 °C in a lithium triborate (Fig. 2) and tetraborate (Fig. 3) mixtures, respectively. In both cases, a significant increase is observed in the amount of the target phase against the background of a sharp decrease in the proportion of the amorphous phase and intermediate compounds.
It is noteworthy that Li2B4O7, presented in a small quantity even in the final product of synthesis, comprised an essential component for the synthesis of lithium triborate. Using the terminology for ranking the organisation levels of the particles making up the substance [19] of the studied part of the Li2O -B2O3 system, the local phase portrait of the crystal structure transformation is as follows. The monoclinic lattice of the primary phases, represented by the remains of hydrated Li(OH)2B5O7 and the initial Li2CO3, is converted into the rhombic and tetragonal lithium tri-and tetraborate structures, respectively. The intermediate stage of this transformation appears as both the Li2B8O13, detected in a minimal amount only at a temperature of 650 ºC in the form of a monoclinic and rhombic variation, and the trigonal chain metaborate LiBO2 accompanying the crystallisation in terms of LiB3O5 and Li2B4O7 in the high-temperature range of 700-750 °C.
Taking into account the detailed description of the lithium borate structures provided in [20] and assuming the crystallographic polyhedron to be the unit of transformation at the structural level, the local phase portrait of the subsolidus crystallisation of lithium triborate and tetraborate was established to consist in the (BO3) 3-→ ( B3O7) 5and (BO3) 3-→ (B4O9) 6-ХИМИЧЕСКИЕ НАУКИ / CHEMICAL SCIENCES transitions, respectively. On this basis, the presence of lithium metaborate can be assumed to be an intermediate step facilitating the transition from the monoclinic island structure of the starting materials (lithium carbonate and boric acid) to the more complex lithium tri-and tetraborate structures, respectively.

CONCLUSIONS
1. The mechanical grinding of Li2CO3 and H3BO3 precursor stoichiometric mixtures followed by sintering was experimentally established to represent an efficient method for the synthesis of lithium tri-and tetraborate. The precipitation of batch components from solution provided no significant improvement in the results of the experiment.
2. The synthesis of crystalline lithium borates in the subsolidus region is characterised by a set of individual phase portraits. At the phase level, a higher temperature region of the maximum in phase transformation is observed for Li2B4O7 compared to LiB3O5 (600-700 °C and 500-600 °C, respectively) with a practically unchanged se-quence of phase transformations according to the scheme: starting materials → intermediate metastable phases → final borates. At the local level, the interaction of coordination polyhedra of the starting Li2CO3 with H3BO3 is followed by the formation of either LiB3O5 or Li2B4O7 due to the (BO3) 3-→ ( B3O7) 5and (BO3) 3-→ (B4O9) 6transitions, respectively. At the structural level, the occurring transformations are characterised by the transition of the monoclinic lattice of the Li2CO3 and H3BO3 primary phases into the LiB3O5 and Li2B4O7 rhombic and tetragonal structures, respectively. In this case, the stage of lithium metaborate formation appears to be a prior condition for the crystallisation of more complex structures.