| Abstract
| - A newlayered structure type, KYP2Se6, has beenprepared that is a polytype of our earlier reportedKLaP2Se6. A series ofK(RE)P2Se6 (RE = Y, La, Ce, Pr, Gd) hasbeen found. A valence electron concentration treatment was appliedto these two structure types in order to find a structural link betweenthem. These compounds were also compared to three otherisoelectronic chalcophosphate-based structures. Although they allhave similar building blocks, the two lanthanide-basedselenodiphosphates assemble in unique, layered structuretypes.
- Two polytypes of potassium rare-earth-metalhexaselenodiphosphates(IV),K(RE)P2Se6 (RE = Y, La, Ce,Pr,Gd), have been synthesized from the stoichiometric reaction of RE, P,Se, and K2Se4 at 750 °C. Bothsingle-crystal and powder X-ray diffraction analyses showed that the structuresof these polytypes vary with the size ofthe rare earth metals. For the smaller rare-earth metals, Y andGd, K(RE)P2Se6 crystallized in theorthorhombicspace group P212121.The yttrium compound was studied by single-crystal X-raydiffraction with the cell parametersa = 6.7366(5) Å, b = 7.4286(6) Å,c = 21.603(2) Å, and Z = 4. Thisstructure type comprises a layered, squarenetwork of yttrium atoms that are bound to four distinct[P2Se6]4-units through selenium bonding. Each[P2Se6]4-unit possesses a Se atom that is not bound to any Y atom but ispointing out into the interlayer spacing, into anenvironment of potassium cations. For larger rare-earth metals,La, Ce, and Pr, K(RE)P2Se6crystallized in asecond, monoclinic polytype, the structure of which has been published.Both of these two different polytypescan be related to each other and several other isoelectronicchalcophosphate structures based on a Parthé valenceelectron concentration analysis. These structures includeAg4P2S6,K2FeP2S6, and the hexagonalMIIPS3 structuretypes. The magnetic susceptibilities of the title compounds havebeen studied, and the behavior can been explainedbased on a simple set of unpaired f-electrons. The diffusereflectance spectroscopy also showed that these yellowplates are moderately wide band gap (∼2.75 eV)semiconductors.
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