Solid-State Synthesis,Crystal Structure And Physical Properties Of Europium-Based Chalcogenides With Noncentrosymmetric Structures

Since the 1960s,nonlinear optical(NLO)crystal materials have become an important medium for obtaining laser sources due to their excellent capability for frequency conversion.Middle and far-infrared(MFIR)NLO materials with superior performance are key to extend the laser wavelength to the MFIR region,and have important technical requirements and applications in many civilian and military fields.So far,commercial MFIR NLO crystals mainly include AgGaS2(AGS),AgGaSe2(AGSe)and ZnGeP2(ZGP).Although these materials have large NLO coefficients and good growth habits,their inherent low laser damage threshold and double photon absorption limits their application in high-power mid-infrared lasers.Therefore,designing and exploring new IR NLO materials has become a research hotspot in the current NLO and laser fields.Chalcogenides are one of the main candidates for new MFIR NLO materials due to their wide infrared transmission ranges,large optical band gaps,and large second-harmonic generation(SHG)effects.Based on this background,this thesis introduced Eu,a rare earth element with high coordination number and special physical properties,into chalcogenides to explore new IR NLO crystal materials.The followings are the main research contents and results in the thesis(1)Two new quaternary sulfides Eu3Ti0.85SiS7 and Eu3Ti0.75GeS7 were synthesized by a high-temperature solid-state method.They are crystallized in the chiral space group P63 of the hexagonal crystal system.Their crystal structures can be described as a three-dimensional framework composed of EuS7 monocapped triangular prisms,Si/Ge atoms occupy the tetrahedral cavities,and the {[TiS3]2-}? helical axis occupies the channel extending along the c-axis.The Eu and Ti elements in this structure both exhibit mixed valences,resulting in their narrow band gaps,which is different from other compounds with simnon-centrosymmetric space group,it exhibits no obvious SHG response,which is also obviously different from the good NLO behaviors realized by their family compounds.According to ilar compositions and structures.Although Eu3Ti0.85SiS7 crystallizes in a the structure and absorption spectrum analysis,the NLO activity absence of Eu3Ti0.85SiS7 may be caused by the strong absorption of the crystal in the ultraviolet-visible-infrared spectrum.The result of variable temperature magnetic susceptibility in the range of 2?300 K shows that Eu3Ti0.85SiS7 has antiferromagnetical-like behavior and no magnetic order phenomenon.(2)Eu8Sn4Se20,one of the(M?2M?Q5)n(n=1,2,4)compounds,was obtained by a high-temperature solid-state method,and it is crystallized in the orthorhombic chiral space group P21212.Its three-dimensional structure consists of four structural units:SnSe4 tetrahedron,distorted SnSe6 octahedron,(Se3)2-chain,and EuSe9 tricapped triangular prism.Its optical band gap is 1.30 eV,and it exhibits antiferromagnetic-like behavior with no magnetic order.Under 2.1?m laser irradiation,Eu8Sn4Se20 powder crystal sample has a weak SHG response,close to that of ?-SiO2.Theoretical calculations show that the reverse arrangement of the Sn/Se structural units in the unit cell leads to its weak SHG response.Although the effect is weak,it has a good contribution to the development of the rare earth-based chalcogenide infrared NLO material system,which provides new inspiration for further study.(3)The ternary thiophosphate EuPS3 was obtained by a high-temperature solid-state method,and its crystal is crystallized in the noncentrosymmetric space group Pn of the monoclinic system.The structure contains two basic structural units:EuS8 bicapped triangular prism and[P2S6]4-ethane-like polyhedron.Among them,[P2S6]4-units are isolated from each other,and each of them is embedded in the three-dimensional cavity formed by EuS8 polyhedra units.The EuPS3 crystal is yellow and the optical band gap is 2.54 eV.Experimental and theoretical results show that EuPS3 has a large SHG about 0.9 times of AGS,and can achieve phase matchability.Moreover,it also has a wide infrared transmission range from 0.49 to 15.40?m.The birefringence obtained by experimental and theoretical calculations is within the range of 0.03-0.1,which also proves the phase matchable possibility.Therefore,EuPS3 is a new promising MFIR NLO material.