Scandium Selenophosphates: Structure and Properties of K₄Sc₂(PSe₄)₂(P₂Se₆)

The new compound K₄Sc₂P₄Se₁₄ was synthesized via the polychalcogenide flux method. It crystallizes in the space group <i>C</i>2/<i>c</i>, and the structure is composed of ¹/_∞[Sc₂P₄Se₁₄^4–] chains that are separated by K⁺ cations. The structural motif features two [PSe₄]^3– units and one [P₂Se₆]^4– unit bridging the Sc centers and has not been reported for any other compound. The ¹/_∞[Sc₂P₄Se₁₄^4–] chains pack in a crosshatched pattern perpendicular to the <i>c</i> axis of the crystal, forming channels for half of the K⁺ atoms while the other half occupy empty space between the chains. The orange-yellow crystals of K₄Sc₂P₄Se₁₄ are air-sensitive and gradually turn red over the course of a couple hours. The band gap of the phase is 2.25(2) eV, and Raman spectroscopy shows the symmetric stretches of the selenophosphate groups to be at 231 and 216 cm^–1 for the [PSe₄]^3– and [P₂Se₆]^4– units, respectively. Solid-state ³¹P MAS NMR of K₄Sc₂P₄Se₁₄ shows two prominent peaks at 11.31 and −23.07 ppm and one minor peak at −106.36 ppm, most likely due to degradation of the product or an unknown second phase