Study On Transition Metal Oxides As New Infrared Nonlinear Optical Materials:Synthesis, Structure And Properties

Second-order nonlinear optical (NLO) crystals have played an increasingly important role in laser science and technology. Even though intensive efforts in the field have been made for about40years, it is still very attractive to search for new nonlinear optical materials with various practical interests. These NLO materials have been widely used in laser frequency conversion, signal communication and so on. Up to now, inorganic crystals are still the main materials which satisfy the commercial applications. The NLO crystals used in the middle infrared (mid-IR) region, compared with the NLO crystals in the other wavelength regions, are still not good enough for the application of high-energy laser. The main defect is that their laser damage thresholds (LDT) are low. It has been gradually recognized that the intrinsic reason (narrow band gap) and extrinsic reason (defects of crystals) have caused the low LDT. To explore new IR NLO materials with large band gap and good comprehensive properties is still a challenging task in the field of NLO materials.Halides usually show a comparatively large band gap and wide transparency in IR region, with this reason, our group has been focused on the study of halide mid-IR NLO crystals. However, oxides as the most extensive inorganic compounds, also possess good comprehensive properties sometimes. Therefore, oxides as potential mid-IR NLO materials have re-entered people’s vision. This thesis choose oxides as the research target, to investigate the potential of them as mid-IR NLO materials. Meanwhile, single crystals of oxides somehow are difficult to obtain for the poor solvability in water or organic solvents. This thesis also make some efforts on the exploring of the hydrothermal and flux methods in new oxides single crystals synthesis.There are six chapters in this thesis as follows:In chapter one, some basic definitions and the main theory methods are introduced, and the research progress in the study of NLO materials is reviewed, especially the NLO materials in the IR region. The main idea of this thesis is then provided based on these.In chapter two, BaCuTe2O7has been investigated about its NLO property. And base on it, the synthesis, structure and property of a new potential mid-IR NLO oxide BaZnTe2O7is reported for the first time. The compound has a large band gap about3.34eV and it shows phase matchable SHG efficiency of about6times that of KH2PO4(KDP) base on powder SHG measurement. The BaZnTe2O7powders are transparent in the range of0.37～11μm and are thermally stable up to750℃. Band structures, densities of states, and optical properties are also calculated, which are close to the experimental values. Owing to these good properties, BaZnTe2O7appears to be a new potential NLO material in the mid-IR region.In chapter three, with guidance of the second-order Jahn-Teller (SOJT) distortion theory, d0transition metal cation Mo6+and lone-pair cation Te4+are choosed, Cs2Mo3TeO12is synthesized by both hydrothermal and solid state methods. It shows non-phase matchable SHG effeciency about11.2times as large as that of KDP and its band gap is about2.75eV. Its powders are transparent in the range of0.45～11μm and are thermally stable up almost to750℃. Band structures and densities of states calculations and analysis are also performed.In chapter four, the NLO properties of two tellurates are reported. Pb2TeO5as a potential mid-IR NLO material is reported. It shows phase matchable SHG efficiency about2.3times that of KDP, its powders are transparent in the range of0.41～14μm and it is thermally stable up to700℃. Single crystals of a new non-centrosymmetric tellurate K3V3Te2O12(OH)6is synthesized for the first time. The synthesis, crystal structure and NLO property are reported. Band structures and densities of states for both compounds are also calculated and analysed.In chapter five, the two stable valence of tellurium cation, Te4+and Te6+, are mixed into the d0transition metal oxides system for the first time. Single crystals of two new quaternary mixed-valence tellurium, K4V6[Te24+Te6+]O24å'ŒRb4V6[Te24+Te6+]O24, are synthesized by hydrothermal method for the first time. Their bulk powders are also synthesized by solid state reactions. Though they crystallized with centrosymmetric space group, their interesting structures are discussed and hydrothermal reactions for the single crystal synthesis of these composited oxides may serve as a reference for similar oxides in future.In chapter six, the single crystal synthesis by using hydroxide fluxes and structures of two new quaternary oxides, Sr3LiNbO6and Sr3LiTaO6, are reported. Sr(OH)2and LiOH·H2O are choosed as self-fluxes, KOH is choosed as a mineralizer for the hydroxide fluxes system. According to our experiment results, the hydroxide fluxes system have been demonstrated to be good solvents at relatively low temperatures, especially for single crystal synthesis of alkali-alkaline earth metal oxides with transition metals.