Effect of acids on crystallization of lithium borate films

Abstract

Lithium borate films in amorphous and crystalline (Li₂B₄O₇) states were prepared from alkoxide solutions hydrolyzed in the presence or absence of hydrochloric acid or acetic acid. The acids suppressed crystallization of the amorphous films into Li₂B₄O₇, with acetic acid being the stronger suppressor. In order to determine the suppressing mechanism, we investigated the distribution of boron-containing species in the solutions by¹¹B-NMR. Boron in the non-acidified solution existed as both B(OH) ⁻₄ and polyborate species, including B₄O₅(OH) ²⁻₄ , which is easily transformed into the crystal nucleous of Li₂B₄O₇. In the solutions containing hydrochloric acid or acetic acid, however, boron was present mainly as B(OH)₃ and the formation of the polyborate species was suppressed, leading in turn to the suppression of crystallization into Li₂B₄O₇. X-ray fluorescence analysis showed that acetic acid produced a larger amount of residual carbon in the films than did hydrochloric acid. We concluded that the residual carbon also inhibited crystallization of the films into Li₂B₄O₇.