Flux synthesis and isostructural relationship of cubic Na(1.5)Pb(0.75)PSe₄, Na(0.5) Pb(1.75)GeS₄, and Li(0.5)Pb(1.75)GeS₄

Na(1.5)Pb90.75)PSe₄ was synthesized by the reaction of Pb with a molten mixture of Na₂Se/P₂Se₃/Se at 495°C. Na(0.5)Pb(1.75)GeS₄ was synthesized by reacting Pb and Ge in molten Na₂S(x) at 530°C. Likewise, L(0.5)Pb(1.75)GeS₄ can be synthesized in a Li₂S(x) flux at 500°C. Na(0.5)Pb(1.75)GeS₄ and Li(0.5)Pb(1.75)GeS₄ are relatively air- and water-stable, while Na(1.5)Pb(0.75)PSe₄ 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 I43d with a = 14.3479(2) Å, Z = 16, R1 = 0.0226, and wR2 = 0.0517 for Na(1.5)Pb(0.75)PSe₄, a = 14.115(1) Å, Z= 16, RI = 0.0284, and wR2 = 0.0644 for Na(0.5)Pb(1.75)GeS₄, and a = 14.0163(6) Å, Z = 16, RI = 0.0273, and wR2 = 0.0637 for Li(0.5)Pb(1.75)GeS₄. 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 Na(0.5)Pb(1.75)GeS₄ indicates that it may be a potential nonlinear optical (NLO) material. Infrared and Raman spectroscopic characterization is also reported. (C) 2000 Academic Press.