Über die Diphosphate M4(P2O7)3 mit M = V,Cr und die Elektronenspektren von Vanadium (III)- und Chrom(III)-Phosphaten

On the Diphosphates M₄(P₂O₇)₃ with M = V, Cr and the Electronic Spectra of Vanadium(III) and Chromium(III) Phosphates Single crystals of ochre colored or dark brown V₄(P₂O₇)₃ ( I ) can be obtained by thermal transformation of an amorphous intermediate synthesized from V₂O₅ and aqueous H₃PO₃ and H₃PO₄; brown crystals of Cr₄(P₂O₇)₃ ( II ) are formed during thermal decomposition of Cr(PO₃)₃, C. I and II are isostructural, crystallizing in orthorhombic space group Pbn2₁ or Pbnm with Z = 4 and lattice constants a = 9.601(2), b = 21.425(5), c = 7.470(4) Å and a = 9.38(1), b = 21.00(4), c = 7.26(2) Å, respectively. Probably due to slight substitution of vanadium(V) for phosphorus atoms (P:V^v ～ 40:1) nonstoichiometic phase composition is found for I prepared at T ～ 1400°C. I and II are characterized by IR and electronic spectroscopy; their electronic spectra are discussed in comparison with those of fourteen other V^III and Cr^III phosphates. This includes a discussion of optical properties of CsCrP₂O₇ changing color from brown to green on change from daylight to artificial light. Some conclusions on the structural arrangement of I and II are drawn.