Solid Solution Formation between Vanadium(V) and ­Tungs­ten(V) Oxide Phosphate.

The solid solutions (V1- xW x)OPO4 with β-VOPO4 structure type (0.0 ≤ x ≤ 0.01) and αII-VOPO4 structure type (0.04 ≤ x ≤ 0.26) were obtained from mixtures of VVOPO4 and WVOPO4 by conventional solid state reactions and by solution combustion synthesis. Single crystals of up to 3 mm edge length were obtained by chemical vapor transport (CVT) (800 → 700 °C, Cl2 as a transporting agent). Single crystal structure refinements of crystals at x = 0.10 [ a = 6.0503(2) Å, c = 4.3618(4) Å, R1 = 0.021, wR2 = 0.058, 21 parameters, 344 independent reflections] and x = 0.26 [ a = 6.0979(2) Å, c = 4.2995(1) Å, R1 = 0.030, wR2 = 0.081, 21 parameters, 346 independent reflections] confirm the αII-VOPO4 structure type ( P4/ n, Z = 2) with mixed occupancy V/W for the metal site. Due to the specific redox behavior of W5+ and V5+, solid solutions (V1- xW x)OPO4 should be formulated as (VIV xVV1-2 xWVI x)OPO4. The valence states of vanadium and tungsten are confirmed by XPS measurements. V4+ with d1 configuration was identified by EPR spectroscopy and magnetic measurements. Electronic spectra of the solid solutions show the IVCT(V4+ → V5+) and the LMCT(O2- → V5+). (V0.74W0.26)OPO4 powders exhibit semi-conducting behavior ( Eg = 0.7 eV). [ABSTRACT FROM AUTHOR]