Study On The Chemical Bonding Behavior Of Crystal Growth Processes Of Functional Materials

Crystal is an important functional material with unique properties and crystal growth is a universal phenomenon in the field of materials science. Hence, the comprehensive understanding and predicting of the relations among crystal structure, crystal growth and crystal properties are very significant for effective controlling crystal growth and tuning physicochemical properties of desired materials. The crystallization is a rebuilding process of various chemical bonds, and thus the bond valence model and the chemical bond theory are used to describe the crystal growth process. Potassium dihydrogen phosphate family was selected as the prototype due to their excellent nonlinear optical property. The main results in this PhD dissertation are summarized herein:For the purpose of scaling the chemical bond strength and establishing the relation between bond strength and crystal structure, the bond parameters of the constituent chemical bonds were respectively calculated with bond valence model and the chemical bond theory. The linear fitting of the bond strength versus bond parameters indicated that the bond strength is proportional to the valence charge density along the selected chemical bond. Meanwhile, the bond strength is also influenced by the ionicity and covalency of the chemical bonds. Simply, the valence charge density along chemical bond could be selected to scale the chemical bond strength, and the further investigation demonstrated that the crystal growth could be studied using valence charge density.Since the morphology has an important effect on properties and utilization of crystals, the effective predicting and controlling of growth morphology are important. In this work, the microscopic mechanism of crystallization was investigated from the chemical bond viewpoint, the calculation model of the growth morphology was proposed by considering the breaking and rebuilding of the constituent chemical bonds. The influence of growth condition on crystal growth was also examined by analyzing the transformation of growth unit and the variation of crystal surface structure, which built a bridge between crystal structure, growth condition and crystal morphology. To validate the calculation method, potassium dihydrogen phosphate and ammonium dihydrogen phosphate were selected as the examples and the evolution of growth morphologies with pH value and impurity were also investigated, the calculation was in accordance with the experimental results. For the purpose of adjusting the physical properties, the mixed crystals of potassium dihydrogen phosphate and ammonium dihydrogen phosphate with different molar ratios were grown. The lattice parameters, phase transition temperature and the configuration of ammonium ions with respect to composition were also measured. The internal stress of the mixed crystal was studied by analyzing the structural distortion of the unit cell from the chemical bond viewpoint. The calculation result indicated that the internal stress rapidly accumulates along c axis with the approaching of ammonium and potassium concentration in the mixed crystals, which led to the evolution of crystal morphologies from normal to twin, dendrite and needles. The intrinsic motivation of the quality reduction of the mixed crystals was disclosed.A novel rapid crystal growth system was constructed on the basis of the high-speed acquisition card and the Labview virtual instruments. All growth parameters were real-time monitored and controlled using microcomputer to provide a stable growth environment. Furthermore, a simple equation was also derived from the solution properties to design the cooling curves for different growth parameters. By employing the present growth apparatus and accurate controlling the growth parameters, high-quality potassium dihydrogen phosphate single crystals were successfully prepared with fast growth rate from the analytical grade materials. In addition, the growth system could be effectively changed to prepare various functional crystals due to its excellent compatibility and expansibility. The successful growth of single crystals implied that the investigation of crystal growth from chemical bond viewpoint is reasonable, which provided a novel method to analyze crystalline materials.