Chemistry in molten alkali metal polyselenophosphate fluxes. Influence of flux composition on dimensionality. Layers and chains in APbPSe(4), A(4)Pb(PSe4)(2) (A=Rb, Cs), and K4Eu(PSe4)(2)

The reaction of Pb and Eu with a molten mixture of A(2)Se/P2Se5/Se produced the quaternary compounds APbPSe(4), A(4)Pb(PSe4)(2) (A = Rb,Cs), and K4Eu(PSe4)(2). The red crystals of APbPSe(4) are stable in air and water. The orange crystals of A(4)Pb(PSe4)(2) and K4Eu(PSe4)(2) disintegrate in water and over a long exposure to air. CsPbPSe4 crystallizes in the orthorhombic space group Pnma (No. 62) with a = 18.607(4) Angstrom, b = 7.096(4) Angstrom, c = 6.612(4) Angstrom, and Z = 4. Rb4Pb(PSe4)(2) crystallizes in the orthorhombic space group Ibam (No. 72) with a = 19.134(9) Angstrom, b = 9.369(3) Angstrom, c = 10.488(3) Angstrom, and Z = 4. The isomorphous K4Eu(PSe4)(2) has a = 19.020(4) Angstrom, b = 9.131(1) Angstrom, c = 10.198(2) Angstrom, and Z = 4. The APbPSe(4) have a layered structure with [PbPSe4](n-)(n) layers separated by A(+) ions. The coordination geometry around Pb is trigonal prismatic. The layers are composed of chains of edge sharing trigonal prisms running along the b-direction. [PSe4](3-) tetrahedra link these chains along the c-direction by sharing edges and corners with the trigonal prisms. A(4)M(PSe4)(2) (M = Pb, Eu) has an one-dimensional structure in which [M(PSe4)(2)](n-)(n) chains are separated by A(+) ions. The coordination geometry around M is a distorted dodecahedron. Two [PSe4](3-) ligands bridge two adjacent metal atoms, using three selenium atoms each, forming in this way a chain along the c-direction. The solid state optical absorption spectra of the compounds are reported. All compounds melt congruently in the 597-620 degrees C region.