Soft-hard acid-base interactions: Probing coordination preferences of sulfur and selenium in mixed chalcophosphates in the family APbPS _4-xSe_x (A=K, Rb, Cs; x=0-4)

The synthesis and structures of the three new compounds, KPbPSL _1.84Se_2.16 (1), RbPbPSL_1.56Se_2.43 (2), and CsPbPS_3.46Se_0.54 (3), are reported. The solid state structures of 1-3 consist of two-dimensional layers of [PbP(S/Se) ₄] separated by alkali metal ions. The structure of 1 was solved in the orthorhombic space group Pna2h Compounds 2 and 3 possess the CsSmGeS ₄ structure type, crystallizing in the orthorhombic space group R2₁2₁2₁. All compounds were refined as racemic twins. All chalcogen sites around the tetrahedrally coordinated P atoms show mixed S/Se occupancy; however, there is a preference for Se binding to Pb ions and S binding to alkali ions. A ³¹P magic angle spinning NMR study on 1 suggests that, in mixed seleno-/thiophosphates, all of the anions [PS _xSe_4-x]^3- (x= 0,1,2,3,4) are present. The different amount of sulfur and selenium present in KPbPS_1.84Se _2.16 (1), RbPbPSL_1.56Se_2.43 (2), and CsPbPS_3.46Se_0.54 (3) is reflected in the solid state absorption spectra from which bandgaps of 2.2 eV were determined for 1 and 2, and a blue-shift to 2.5 eV was observed because of the higher sulfurcontent in 3. Thermogravimetric analysis experiments indicated that, upon heating, compound 1 decomposes forming PbSe and sulfur together with other unidentified products. A Raman spectrum of compound 1 showed more bands than are usually observed in seleno- or thiophosphate salts and is another indicator of the mixed seleno-Zthiophosphate anions found in 1.