Growth And Properties Of Several Bismuth Containing Functional Crystals

Bismuth is a special metal with a valence of +3. Bi³⁺ is highly polarizable and deformable due to the nonbonding electronic 6s pair. As a consequence, bismuth-containing compounds have many unusual and potentially useful properties, such as, ionic conductivity, ferroelectricity, ferromagnetism, ferroelasticity, superconductivity and catalytic etc. In this paper, three types of bismuth-containing crystals were chosen for the study, which were, 1. KBi(WO₄)₂, a double tungstate; 2. Na_1-xK_xBi(TiO₃)₂, a lead-free crystal; 3. Bi₂WO₆ andγ-Bi₂MoO₆, bismuth-containing layer-structured ferroelectrics. Accordingly, this paper was arranged in three parts:Part I: Potassium bismuth tungstate, KBi(WO₄)₂, has a potential use both as a luminescent and laser media. A single crystal with dimensions of 26×23×10mm³ and weight of 26.5g has been successfully grown by using the top-seeded solution growth technique and K₂W₂O₇ as solvent. Some properties of as-grown crystal were analysised or measured, which were crystallographic structure, density, hardness, chemical etching, growth habit, thermal properties, solubility curve, melting point, transparency curve, fluorescence spectra, infrared and Raman spectra etc. Different self-flux systems were chosen for the growth from the solution and the growth results were summarized in the phase diagram of K2O-WO3-Bi2O3. The defects of the as-grown crystals were inclusions and cracks. Experiments showed that the largest impact factor of the defects was the temperature gradient. Doping series with neodymium and chromium were grown and a concentration as high as 11.75mol% Nd³⁺ was obtained. The Nd-doped crystal showed a strong segregation effect.Part II: Sodium-potassium bismuth titanate, Na_1-xK_xBi(TiO₃)₂, is a kind of perovskite-type ferroelectric materials and has been considered to be a promising candidate for lead-free piezoelectric materials. It is well known that solid solutions with compositions close to the morphotropic phase boundary (MPB) exhibit promising elctrmechanical properties, yet the literatures show large discrepancies about the MPB of the system. In this paper, several system of (1-x)NaBi(TiO₃)₂-xKBi(TiO₃)₂ were prepared and the phase diagram, the MPB, the segregation effects were given. Single crystals with dimensions of several centmeters were grown by using the top-seeded Kyropoulos/solution growth technique. Some properties of as-grown crystals were analysised or measured, which were, density, hardness, transmission, piezoelectricity etc. However, all the two methods showed high volatilities and segregation effects. The growth process of this crystal is not perfect and need to improve further.Part III: Bismuth tungsten oxide, Bi₂WO₆,and bismuth molybdenum oxide,γ-Bi₂MoO₆, are simplest bismuth layer-structured ferroelectrics. Nanosized Bi₂WO₆ andγ-Bi₂MoO₆, have been successfully synthesized by a microwave hydrothermal approach, using Bi(NO3)3, NH3, H2WO4/MoO3 as the starting materials. The influences of the reaction conditions (including starting materials, concentration, reaction temperature, and holding time) on the phases and morphologies of the products were investigated by X-ray powder diffraction and transmission electron microscopy. The best conditions for the synthesis were provided. Transmission electron microscopic results revealed that the as-prepared particles were sheet-shaped. Comparison between the microwave hydrothermal approach, conventional hydrothermal and the solid-state reaction methods were made. Results showed that the microwave hydrothermal method was a satisfactory method for the synthesis of nanocrystals. The as-synthesized nanosheets of Bi₂WO₆ andγ-Bi₂MoO₆ showed a high photocatalytic activity for the degradation of methylene blue under visible light irradiation.The nonlinear optical crystal, CsLiB6O10, has an excellent perspective for the realization of high-power all-solid-state ultraviolet and deep ultraviolet lasers. Ion beam etching of the CsLiB6O10 crystals was studied under different conditions. Scanning atomic force microscopy was used to measure the surface roughness before and after etching to evaluate the results of the etching. Experiments showed that the surface condition would be degraded or had little change after the etching.