LiB₃O₅ (LBO) crystal, first invented in China, is a world-renowned nonlinear optical material and is expected to be applied in laser nuclear fusion, high energy physics and other fields. Since the beginning of the 21st century, countries including China, Russia, and the United States, have increased the investment for large-size and high-quality LBO crystals in order to realize frequency conversion of high-power lasers. However, many problems of LBO crystal growth remain unresolved.

Scientists at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science of Chinese Academy of Sciences developed a method to uncover the secrets in LBO crystal growth, according to a paper newly published in CrystEngComm.

After years of effort, Prof. WAN Songming’ group, collaborating with Prof. YOU Jinglin’ group at Shanghai University, established a method which combines high temperature Raman experimental technique with density functional theory, to analyze complicated melt structures. Based on this method, the joint team studied the phase transition, melt structure and growth mechanism of the LBO crystal to uncover the scientific essence behind several LBO crystal growth phenomena.

Researchers studied the structure transition of the LBO crystal during the melting process. They found a dynamics intermediate product Li₂B₄O₇ for the first time, except for the thermodynamic equilibrium product Li₃B₇O₁₂. Upon melting, the three-dimensional B₃O₃Ø₄ (Ø-bridged oxygen atom) frame of the LBO crystal first decomposes into B₄O₅Ø₄ and B₂O₃, and then converts into one-dimensional B₃O₄Ø₂ chains. The structural memory phenomena of the LBO melt are successfully explained. (CrystEngComm; J. Cryst. Growth)

After this, they studied the melt composition-structure relationship in the Li₂O-B₂O₃ pseudo-binary system, and established a complete structural spectrum of lithium borate melts. The results were used to explain the high viscosity and the viscosity anomaly of the Li₂O·4B₂O₃ melt (with the B₂O₃ flux to grow the LBO crystal), and to completely understand the micro-process of the LBO crystal growth. (Inorg. Chem.; CrystEngComm)

The Raman spectra of Li₂Mo₂O₇ and Li₂Mo₃O₁₀ melts were also studied in detail. According to the spectral changes of the Li₂Mo₃O₁₀ melt (another flux to grow the LBO crystal) during a LBO crystal melting process, researchers found a substitution reaction of Mo₃O_10²- groups with the LBO crystal, which produces MoO₃·B₃O₄Ø₂- intermediate of complex and Mo₂O₇^2- groups. The MoO₃·B₃O₄Ø₂- intermediate of complex is the key to understand the micro-process of the LBO crystal growth (Inorg. Chem.)

The above studies provided a method for analyzing complicated melt structures, and revealed the melt structure and growth mechanism of the LBO crystal, which promotes the development of science and technology of crystal growth by the flux method.

These works were supported by the National Natural Science Foundation of China.

Figure: A substitution reaction between a Mo₃O₁₀^2-group and a B₃O₄Ø₂-chain (Image by ZHANG Shujie)