What is lithium metaborate？

Background and overview

Lithium borate system has received a great deal of attention because this system contains many important nonlinear optics crystals, such as 2Li₂O–3B₂O₃, Li₂O–B₂O₃, Li₂O–2B₂O₃, Li₂O–3B₂O₃^[1].Nonlinear optics crystals are important in information, energy resource, medical application, optoelectronic technology, etc. Borate alkali lithium metaborate is a white powder with the formula LiBO₂. Lithium metaborate is the lithium salt of boric acid. It is considered to be an isopoly of the Li₂O-B₂O₃ system.

Structure 

Lithium metaborate can be synthesized by reacting orthoboric acid with lithium carbonate. Its crystals belong to the monoclinic crystal system having space group P2₁/c. Recrystallization from water at about room temperature yields lithium metaborate- octahydrate, LiBO₂·8H₂O. And during the course of thermal dehydration of LiBO₂·8H₂O, several types of LiBO₂ hydrate and anhydrous products can be found:

A. Below 100 °C, LiBO₂·8H₂O is firstly dehydrated to a lower hydrate LiBO₂·2H₂O which is stable to 140 °C^[2].

B. When the temperature is higher than 190°C, the dehydrated amorphous hydrate could crystallize into other poorly crystalline phases designated as LiBO₂·xH₂O (x=0.3/0.5)^[3].

C. An anhydrous monoclinic α-LiBO₂ could be obtained when the baking temperature reaches 600 °C. Centrosymmetric monoclinic α-LiBO₂ contains endless chains of BO₃ triangles with Li–O bonds between chains. The Li atom forms three Li–O bonds with one chain and one bond with each of the two neighboring chains. The α-LiBO₂ lattice parameters are a=5.838 Å, b=4.348 Å, c=6.449 Å, and β=115.12°, space group is P2₁/n and the density is 2.180 g/cm³. 

D. At 950 °C, by applying 15 kbar of pressure to a LiCl flux containing α-LiBO₂， α-LiBO2 could be converted to metastable γ-LiBO² crystals^[4]. γ-LiBO2 has tetragonal symmetry with lattice parameter a=4.1961 Å, c=6.5112 Å, space group I-42d and a density of 2.882 g/cm³. At present, γ-LiBO2 is the densest phase in LiBO2 system.

Uses

Lithium metaborate could used in borate fusion sample preparation of various samples for analysis by XRF, AAS, ICP-MS, ICP-AES and ICP-OES. It could also used to determine the trace level of elements in contaminated soil. Useful fod dissolving acidic oxides such as SiO₂, and Fe₂O₃. A mixture of lithium metaborate and lithium tetraborate is useful for dissolving most of the inorganic oxides to enable spectrochemical analysis. Lithium metaborate is an effective flux for silicates and other rock-forming minerals^[5]. Kowalenko et al. found that lithium metaborate is a good alternative to sodium carbonate fusion for the measurement of total P and many other elements (except S) in soil and related materials in 2012^[6].

Since Lithium metaborate is widely used in chemical analysis, ceramics manufacturing and industrial production, the exploration of its physicochemical properties is still of great value.

References

[1] X.M. Shi, Q. Wang, C.X. Li, X.J. Niu, F.P. Wang, K.Q. Lu, Densities of Li₂O–B₂O₃ melts, Densities of Li₂O–B₂O₃ melts, 2006, 290,  637-641.
[2] E.I.Kamitsos, A.P.Patsis, M.A.Karakassides, G.D.Chryssikos, Infrared reflectance spectra of lithium borate glasses, Journal of Non-Crystalline Solids, 1990, 126, 52-67.
[3] L. Lei, D. He, K. He, J. Qin, S. Wang, Pressure-induced coordination changes in LiBO₂, Journal of Solid State Chemistry, 2009, 182, 3041-3048.
[4] Colin D. McMillen, Henry G. Giesber, Joseph W. Kolis, The hydrothermal synthesis, growth, and optical properties of γ-LiBO₂, Journal of Crystal Growth, 310, 2008, 299-305.
[5] C.O. Ingamells, Lithium metaborate flux in silicate analysis, Analytica Chimica Acta, 1970, 52, 323-334.  
[6] C. Grant Kowalenko, Dean Babuin, Use of Lithium Metaborate to Determine Total Phosphorus and Other Element Concentrations in Soil, Plant, and Related Materials, Communications in Soil Science and Plant Analysis, 2014, 45, 15-28.

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