Selenophosphate Phase Diagrams Developed in Conjunction with the Synthesis of the New Compounds K₂La(P₂Se₆)_1/2(PSe₄), K₃La(PSe₄)₂, K₄La_0.67(PSe₄)₂, K_9-xLa_1+x/3(PSe₄)₄ (x = 0.5), and KEuPSe₄

Five new rare-earth metal polyselenophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction:  K₂La(P₂Se₆)_1/2(PSe₄) (I), K₃La(PSe₄)₂ (II), K₄La_0.67(PSe₄)₂ (III), K_9-xLa_1+x/3(PSe₄)₄ (x = 0.5) (IV), and KEuPSe₄ (V). Compound I crystallizes in the monoclinic space group P2₁/n with a = 9.4269(1) Å, b = 7.2054(1) Å, c = 21.0276(5) Å, β = 97.484(1)°, and Z = 4. Compound II crystallizes in the monoclinic space group P2₁/c with a = 9.5782(2) Å, b = 17.6623(4) Å, c = 9.9869(3) Å, β = 90.120(1)°, and Z = 4. Compound III crystallizes in the orthorhombic space group Ibam with a = 19.0962(2) Å, b = 9.1408(1) Å, c = 10.2588(2) Å, and Z = 4. Compound IV crystallizes in the orthorhombic space group Ccca with a = 18.2133(1) Å, b = 38.0914(4) Å, c = 10.2665(1) Å, and Z = 8. Compound V crystallizes in the orthorhombic space group Pnma with a = 17.5156(11) Å, b = 7.0126(5) Å, c = 6.9015(4) Å, and Z = 4. Optical band gap measurements show that compound V has an optical band gap of 1.88 eV. Solid-state Raman spectroscopy of compounds II−V shows the four normal vibrations expected for the (PSe₄)^3- unit. The observation of compounds I−V in several reactions has allowed the creation of a quasi-quaternary phase diagram for potassium rare-earth-metal polyselenophosphates. This phase diagram can qualitatively be separated into three regions on the basis of the oxidation state of phosphorus in the crystalline products observed and takes the next step in designing solid-state compounds.