Chemistry in Molten Alkali Metal Polyselenophosphate Fluxes. Influence of Flux Composition on Dimensionality. Layers and Chains in APbPSe4, A4Pb(PSe4)2 (A = Rb, Cs), and K4Eu(PSe4)2

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Abstract
The reaction of Pb and Eu with a molten mixture of A2Se/P2Se5/Se produced the quaternary compounds APbPSe4, A4Pb(PSe4)2 (A = Rb,Cs), and K4Eu(PSe4)2. The red crystals of APbPSe4 are stable in air and water. The orange crystals of A4Pb(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) Å, b = 7.096(4) Å, c = 6.612(4) Å, and Z = 4. Rb4Pb(PSe4)2 crystallizes in the orthorhombic space group Ibam (No. 72) with a = 19.134(9) Å, b = 9.369(3) Å, c = 10.488(3) Å, and Z = 4. The isomorphous K4Eu(PSe4)2 has a = 19.020(4) Å, b = 9.131(1) Å, c = 10.198(2) Å, and Z = 4. The APbPSe4 have a layered structure with [PbPSe4]nn- 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. A4M(PSe4)2 (M = Pb, Eu) has an one-dimensional structure in which [M(PSe4)2]nn- 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 °C region.

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