Design And Synthesis Of New Chalcogenide Compounds For Nonlinear Optical Studies

Chalcogenides with non-centrosymmetric configuration have attracted great attention as efficient mid-infrared nonlinear optical(NLO)materials,due to the wide infrared transmission window and the ease of polarization.Design of noval chalcogenides with large second harmonic generation(SHG)and high laser-induced damage thresholds(LIDT)is one of the curcial issues for the further development of efficient NLO materials.The chalcogenide-coordinated tetrahedrons exhibit strong microscopic dipoles and tend to construct in non-centrosymmetric configurations,which is favored for large SHG.The framwrok of our new compounds is constructed by matching different functional asymmetric tetrahedral units.This,together with the introduction of highly electropositive metal cations or thihalogen anions,can realize the synergistic improvement of SHG and LIDT.The new coumpounds consist of strongly polarizable units such as[Ga Q₄],[Ge Q₄]and[Cd Q₄](Q=S,Se)as the framework,and the metal ions in the first and second families(Na⁺,K⁺,Rb⁺,Sr²⁺,Ba²⁺)as the balance cations.The highly electronpositive cations is beneficial to modify the configuration of the framework,expand the band gap,and increase the LIDT.The futher additon of thihalogen anions leads to more distortion of the framework,and then the increase of polarization and the enlargement of the band gap.The main achievements of this thesis are shown in bellow:By elemental substitution strategy,we successfully synthesized six new NLO chalcogenides with non-centrosymmetric configurations:Sr Li₂Ge Se₄,Sr Li₂Sn Se₄,Sr Na₂Ge Se₄,Sr Na₂Sn Se₄,L-Sr Cu₂Ge Se₄ and R-Sr Cu₂Ge Se₄.These compounds are designed by taking chalcopyrite-like Ag Ga S₂ as the prototype.The alkali and alkaline earth metals are introduced through the high temperature solid state method using alkali metal polysulphides as the reaction-flux.For Sr Na₂Ge Se₄ and Sr Na₂Sn Se₄,a symmetry configuration with the cubic phase F-43m is observed due to the distortion of Sr²⁺and Na⁺cations.While for Sr Li₂Ge Se₄ and Sr Li₂Sn Se₄,the crystal structure(I-42m)is similar to that of Ag Ga S₂.In addition,the introduction of Sr²⁺not only increases the content of ion-bondings and broadens the band gap,but also improves the polarization by extruding the[Li Se₄]layers.By introducing another unit[Cd Q₄]in Sr Na₂Ge S₄,we successfully synthesized six new NLO chalcogenides:Sr₃Li Cd_0.5Ge₂S₈,Sr₃Li Cd_0.5Sn₂S₈,Sr₂Na Cd_1.5Ge₂S₈,Sr₂Na Cd_1.5Sn₂S₈,Sr₂KCd_1.5Ge₂S₈ and Sr₂Na Cd_1.5Sn₂S₈.The reason to introduce the Cd element with d¹⁰ electron orbital is due to the stable bonding ability of Cd-S,and its Jahn-Teller distortion effect.These compounds possess the advantages of the tetrahedral frameworks and the high electronpositivity cations,showing good NLO performance with large SHG and high LIDT.Among them,Sr₂Na Cd_1.5Ge₂S₈ shows a high SHG coefficient of 0.9×Ag Ga S₂,a large band gap width of 3.37 e V and a wide infrared transmission window(3-24?m).With phase-matchable behavior,it is a promising candidate to be used as the infrared NLO materials.Two new non-centrosymmetric chalcohalides La₆Cd_0.75Ga₂Q_11.5Cl_2.5(Q=S and Se)and one centrosymmetric compound[K₄Ba₂Cl][Ga₆S₁₂]with thihalogen anions are synthesized by high temperature reaction using halogen-salts as the reaction-flux.The introduction of halogen into the chalcogenides is beneficial for the synergistic improvement of LIDTs and SHG,because the anions with different electronegativity and bonding-tendency increase the distortion of the framework and the polarity.The La₆Cd_0.75Ga₂S_11.5Cl_2.5 shows large LIDTs(18.6?Ag Ga S₂)and strong SHG response(0.8?Ag Ga S₂).