Flux Synthesis and Isostructural Relationship of Cubic Na1.5Pb0.75PSe4, Na0.5Pb1.75GeS4, and Li0.5Pb1.75GeS4

Na1.5Pb0.75PSe4 was synthesized by the reaction of Pb with a molten mixture of Na2Se/P2Se5/Se at 495°C. Na0.5Pb1.75GeS4 was synthesized by reacting Pb and Ge in molten Na2Sx at 530°C. Likewise, Li0.5Pb1.75GeS4 can be synthesized in a Li2Sx flux at 500°C. Na0.5Pb1.75GeS4 and Li0.5Pb1.75GeS4 are relatively air- and water-stable, while Na1.5Pb0.75PSe4 is only stable in air and water for less than 1 day. The structures of all three compounds were determined by single-crystal X-ray diffraction. The compounds crystallize in the cubic, noncentrosymmetric space group I 3d with a= 14.3479(2) ?, Z=16, R1=0.0226, and wR2=0.0517 for Na1.5Pb0.75PSe4, a=14.115(1) ?, Z=16, R1=0.0284, and wR2= 0.0644 for Na0.5Pb1.75GeS4, and a=14.0163(6) ?, Z=16, R1=0.0273, and wR2=0.0637 for Li0.5Pb1.75GeS4. The compounds adopt a structure that is similar to that of Ba3CdSn2S8 and feature [PSe4]3- or [GeS4]4- tetrahedral building blocks. In this three-dimensional structure, there are two types of metal sites. In each structure, these sites are occupied differently because of a disorder between the alkali and lead cations. All three compounds are semiconductors with band gaps around 2 eV. The observation of a large second harmonic generation (SHG) signal for Na0.5Pb1.75GeS4 indicates that it may be a potential nonlinear optical (NLO) material. Infrared and Raman spectroscopic characterization is also reported.