Songming Wan

Growth units and growth habit of α-BaB2O4 crystal

The structure of the melt near a crystal–melt interface is a fundamental problem in the dynamics of crystal growth. In this work, high-temperature Raman spectroscopy was applied to investigate in situ the structure of the melt near the α-BaB2O4 (α-BBO) crystal–melt interface. A structured melt was found in this region: (B3O6)3− groups form near the interface and vanish towards the bulk melt. The crystal growth habit was then explained by the periodic bond chain (PBC) theory. At the α-BBO crystal–melt interface, the growth units, namely the (B3O6)3− anion groups and Ba2+ cations, stack mainly along four types of PBCs. These four PBCs constitute three potential F faces: {10\bar{1} 2}, {01\bar{1} 4} and {10\bar{1} 10}. The predicted results are in good agreement with the observed growth habit of α-BBO crystal.

Raman spectroscopy and density functional theory analyses of the melt structure in a Li2B4O7 crystal growth system

Melt structure, a fundamental and challenging subject for borate crystal growth, has not been solved for many years. In this paper, a new method has been employed to study the Li2B4O7 melt structure. High-temperature Raman spectroscopy has been used to investigate the structural evolvement from a Li2B4O7 crystal to a Li2B4O7 melt. Based on the investigation, a model was proposed to describe the Li2B4O7 melt. The melt is made up of polymer-like boron–oxygen chains; the minimal repeated unit is the B4O6O22− (O = bridging oxygen) group which is formed by a B3O4O2− six-membered ring and a BOO2− triangle linked by a bridging oxygen atom. DFT calculations have verified the melt structure and provided accurate assignments for the vibrational bands present in the Li2B4O7 melt Raman spectrum.

Structural analyses of a K2O-rich KNbO3 melt and the mechanism of KNbO3 crystal growth

A melt structure is an intrinsic factor used to govern a variety of melt macro-properties and plays a fundamental role in understanding crystal growth mechanisms. In this paper, high-temperature Raman spectroscopy and a density functional theory (DFT) method were used to investigate the structure of a K2O-rich KNbO3 (KN) melt which was in equilibrium with a KN crystal. K+ ions and isolated NbO3− groups have been found to be the main structural units in the bulk melt. The NbO3− units connect with each other near the crystal–melt interface to form NbO2O2− (O = bridging oxygen) chains that further form NbO6− octahedra (the basic units in the KN crystal structure) on the crystal–melt interface. A boundary layer with the thickness of about 5 μm was observed around the interface. The DFT calculations verified the melt structures and provided accurate assignments for the vibrational bands present in the melt Raman spectra.

Investigation of a BiB3O6 crystal growth mechanism by high-temperature Raman spectroscopy

High-temperature Raman spectroscopy has been applied to investigate the melt structure near the BiB3O6 (BIBO) crystal–melt interface. Based on the experimental results, the crystal growth mechanism was proposed. (BOO)n (O = bridging oxygen atom) chains and free Bi3+ cations present in the BIBO bulk melt act as the crystal growth units. Bi-O bonds and [BO4]-tetrahedra were found near the BIBO crystal–melt interface. Two neighboring oxygen atoms in a (BOO)n chain or in two different (BOO)n chains are connected by the Bi3+ cations to form the structural prototype of BIBO crystal. Two adjacent (BOO)n chains are further connected to each other through the [BO4] tetrahedra to form the BIBO crystal structure. The predicted growth habit of the BIBO crystal is consistent with the observed one based on the mechanism.

Raman Spectral and Density Functional Theory Analyses of the CsB3O5 Melt Structure

Melt structures are essential to understand a variety of crystal growth phenomena of alkali-metal triborates, but have not been fully explored. In this work, Raman spectroscopy, coupled with the density functional theory (DFT) method, has been used to solve the CsB3O5 (CBO) melt structure. When the CBO crystal melts, the extra-ring B4-Ø bonds (the B-Ø bonds of BØ4 groups, Ø = bridging oxygen atom) that connect two B3O3Ø4 rings (the basic boron-oxygen unit in the CBO crystal structure) break. As a result, the three-dimensional boron-oxygen network collapses to unique polymer-like [B3O4Ø2]n chains. On the basis of the optimized [B3O4Ø2]n chain model, the CBO melt Raman spectrum was calculated by the DFT method for the first time and the calculated results confirm that the [B3O4Ø2]n chain is the primary species in the CBO melt. These results also demonstrate the capability of the combined Raman spectral and DFT method for analyzing borate melt structures.

In situ Raman investigation of a LiB3O5 melt toward understanding the structural memory phenomena

Lithium triborate (LiB3O5, LBO) is the most widely used nonlinear optical crystal. During the LBO crystal melting process, several interesting and important phenomena, called as structural memory, have been found. However, the essence of these phenomena remains unclear. In order to explain these phenomena, high-temperature Raman spectroscopy was used to investigate in situ the structures of a LiB3O5 melt at different temperatures. Two important structural transformations were found during the LBO crystal melting process: (1) when the crystal begins to melt, every two adjacent basic building units (B3O4O2) in the crystal structure transforms to a B4O5O4 group and a B2O3 molecule. (2) Above 860 °C, the B4O5O4 units react with the B2O3 molecules to form boron–oxygen chains whose minimal repeated unit is the B3O4O2 six-membered ring. These results prove that LBO is an incongruently melting compound and provide a rationale for understanding the special structural memory phenomena.

Investigation on the Structure of a LiB3O5–Li2Mo3O10 High-Temperature Solution for Understanding the Li2Mo3O10 Flux Behavior

LiB3O5 is the most widely used nonlinear optical crystal. Li2Mo3O10 (a nominal composition) is a typical flux used to produce large-sized and high-quality LiB3O5 crystals. The structure of the LiB3O5-Li2Mo3O10 high-temperature solution is essential to understanding the flux behavior of Li2Mo3O10 but still remains unclear. In this work, high-temperature Raman spectroscopy combined with density functional theory (DFT) was applied to study the LiB3O5-Li2Mo3O10 solution structure. Raman spectra of a LiB3O5-Li4Mo5O17-Li2Mo4O13 polycrystalline mixture were recorded at different temperatures until the mixture melted completely. The solution structure was deduced from the spectral changes and verified by DFT calculations. When the mixture began to melt, its molybdate component first changed into the Li2Mo3O10 melt; meanwhile, the complicated molybdate groups existing in the crystalline state transformed into Mo3O102- groups, which are formed by three corner-sharing MoO3Ø-/MoO2Ø2 (Ø = bridging oxygen atom) tetrahedra. When LiB3O5 dissolved in the Li2Mo3O10 melt, the crystal structure collapsed into polymeric chains of [B3O4Ø2-]n. Its basic structural unit, the B3O4Ø2- ring, coordinated with the Mo3O102- group to form a MoO3·B3O4Ø2- complex and a Mo2O72- group. On the basis of the LiB3O5-Li2Mo3O10 solution structure, we discuss the LiB3O5 crystal growth mechanism and the compositional dependence of the solution viscosity.

Structural investigation of Li2O–B2O3–MoO3 glasses and high-temperature solutions: toward understanding the mechanism of flux-induced growth of lithium triborate crystal

MoO3 is an important flux for lithium triborate (LiB3O5) crystal growth from high-temperature solutions. Although it has been widely used, the mechanism of the MoO3 flux-induced growth of LiB3O5 crystals is still not very clear. In this paper, we present a spectroscopic investigation of the Li2O–B2O3–MoO3 ternary glasses/solutions, which were prepared from high-temperature MoO3-based solutions for LiB3O5 crystal growth. By combining all the experimental data of Raman and MAS NMR, the types of structural species and the interactions between flux and solute are discussed to understand the MoO3 flux-reduced mechanism of LiB3O5 crystallization from the high-temperature solution. Considering the activities of lithium cations, an isomerization reaction is proposed to describe the structural evolution of the BO4 tetrahedron into the BO3 triangle in boroxol due to the MoO3 flux. The transition between the boron oxide species is essential for the LiB3O5 crystal growth. On cooling, the formed boroxol rings are polymerized by the re-formation of BO4 tetrahedrons again, and gather together to form the LiB3O5 crystal phase. Finally, the MoO3 flux-induced LiB3O5 crystallization may be elucidated with the decrease of the concentration of the BO4 tetrahedron in high-temperature solutions.

High-temperature Raman spectroscopy of microstructure around the growing β-BaB2O4 crystal in the BaO–B2O3–Na2O system

High-temperature Raman spectroscopy has been applied to study in situ the microstructure of the solution near the β-BaB2O4 crystal–solution interface in the BaO–B2O3–Na2O growth system. A boundary layer near the crystal–solution interface was observed. In accordance with the high-temperature Raman spectroscopy and first principles calculations, a boron–oxygen structural model is proposed to explain the microstructure of the solution and growth habit. The results show that the growth solution contains a special group, [BO2OBOOB=O]3− (O = bridging oxygen), which transformed to the growth unit [B3O6]3− near the interface.

Raman and Density Functional Theory Studies of Li2Mo4O13 Structures in Crystalline and Molten States

The Li2Mo4O13 melt structure and its Raman spectral characteristics are the key for establishing the composition-structure relationship of lithium molybdate melts. In this work, Raman spectroscopy, factor group analysis, and density functional theory (DFT) were applied to investigate the structural and spectral details of the H-Li2Mo4O13 crystal and a Li2Mo4O13 melt. Factor group analysis shows that the crystal has 171 vibrational modes (84Ag + 87Au), including three acoustic modes (3Au), six librational modes (2Ag + 4Au), 21 translational modes (7Ag + 14Au), and 141 internal modes (75Ag + 66Au). All of the Ag modes are Raman-active and were assigned by the DFT method. The Li2Mo4O13 melt structure was deduced from the H-Li2Mo4O13 crystal structure and demonstrated by the DFT method. The results show that the Li2Mo4O13 melt is made up of Li+ ions and Mo4O132- groups, each of which is formed by four corner-sharing MoO3Ø/MoO2Ø2 tetrahedra (Ø = bridging oxygen). The melt has three acoustic modes (3A) and 54 optical modes (54A). All of the optical modes are Raman-active and were accurately assigned by the DFT method.