Rb_(2)MGe_(3)S_(8)(M=Zn,Cd):[MGe_(3)S_(8)]^(2-)单元构型变换导致化合物从中心到非心的转变

Rb_(2)MGe_(3)S_(8)(M=Zn,Cd):Non-Centrosymmetry Transformation Led by Structure Change of[MGe_(3)S_(8)]^(2-)Unit

在本工作中,利用高温固相法成功合成了两个碱金属硫属化合物Rb_(2)MGe_(3)S_(8)[M=Zn(1),Cd(2)].化合物1,2都为二维层状结构且均具备大的光学带隙,化合物1和2的实验带隙分别为3.24和3.16 eV.化合物1属于P-1中心对称群,化合物2属于非心P2(1)2(1)2(1)空间群且具有一定的非线性光学(NLO)效应,在粒径为50~75μm时与KH2PO4(KDP)相当(@1064 nm).化合物2的激光损伤阈值为AgGaS_(2)的16.6倍.化合物1,2的分子式化学计量比相同但展示不同的对称性,通过分析它们的晶体结构发现,其基本构造单元[MGe_(3)S_(8)]^(2-)在两化合物中构型的不同导致化合物1、2对称性发生中心到非心的转变.为了深入了解化合物2的NLO效应的起源,基于密度泛函理论对其进行了电子能带结构和NLO极化率的理论计算.

Infrared nonlinear(IR NLO)optical crystals have an essential position in military and civilian fields because of their ability to convert lasers from near infrared(NIR)to mid/far infrared(MIR/FIR).In this work,two alkali-metal chalcogenides,Rb_(2)MGe_(3)S_(8)[M=Zn(1),Cd(2)],were successfully synthesized by high-temperature solid-state reactions.Both compounds feature a two-dimensional layered structure and have a large optical band-gap,the experimental band-gap of 1 and 2 are 3.24 eV and 3.16 eV,respectively.Compound 1 belongs to the centrosymmetric group P-1,while 2 belongs to the non-centrosymmetric space group P2(1)2(1)2(1)and exhibits obvious NLO effect,which is comparable to that of KH_(2)PO_(4)(KDP)(@1064 nm)at the particle size of 50~75μm.Particle-size dependent NLO response measurements indicated that 2 is non-phase-matchable.Compound 2 exhibits a high laser-induced damage threshold of 16.6×AGS at 1064 nm.Through the analysis of the crystal structures of these two compounds,the reason why their formulas have the same stoichiometric ratio but symmetries are different is the structure change of basic building unit[MGe_(3)S_(8)]^(2-)in 1 and 2.All M atoms in both compounds are coordinated by four S atoms to form MS_(4) tetrahedra.In each[CdGe_(3)S_(8)]^(2-)unit of 2,three S atoms bonded to the Cd atom are also bonded to all Ge atoms in that unit,that is to say,each CdS_(4) tetrahedron is connected to the other three GeS_(4) tetrahedra by sharing S vertices.Unlike the coordination manner in the[CdGe_(3)S_(8)]^(2-)unit of 2,there are only two S atoms bonded to both Zn and Ge atoms in[ZnGe_(3)S_(8)]^(2-)unit of 1.This structure change of[MGe_(3)S_(8)]^(2-)unit eventually led to the non-centrosymmetric transformation.What’s more,to get insight into the origin of NLO effect of 2,theoretical calculations of electronic band structure and NLO susceptibility were performed based on density functional theory.