Mixed‐Metal Monophosphate Tungsten Bronzes Containing Rhodium and Iridium

Solution combustion synthesis followed by annealing in air led to the MPTB‐related phosphates (Rh1/6W5/6O3)8(PO2)4, (Ir1/6W5/6O3)8(PO2)4 (a=5.258(2) Å, b=6.538(3) Å, c=17.322(8) Å), (Rh1/9W8/9O3)12(PO2)4 and (Rh2/21W19/21O3)14(PO2)4. Single‐crystals of the mixed‐metal (Rh,W)‐MPTBs at m=4 and at m=7 were grown by chemical vapor transport (CVT). Their crystal structures have been refined from X‐ray single‐crystal data {(Rh,W)‐MPTB at m=4: P212121, Z=1, a=5.2232(3) Å, b=6.4966(3) Å, c=17.3819(9) Å, R1=0.032, wR2=0.075 for 1714 unique reflections, 1524 with Fo>4σ(Fo), 66 variables, 1 constraint, composition from refinement (Rh0.15W0.85O3)8(PO2)4; (Rh,W)‐MPTB at m=7: P21/n, Z=1, a=5.2510(4) Å, b=6.4949(5) Å, c=26.685(2) Å, β=90.30(1)°, R1=0.060, wR2=0.163 for 2074 unique reflections, 1894 with Fo>4σ(Fo), 100 variables, comp. from ref. (Rh0.07W0.93O3)14(PO2)4}. These structure refinements show unexpected distribution of Rh and W over the available metal sites. Further characterization (powder reflectance and magnetic measurements) of the (Rh,W)‐MPTB at m=4 and at m=7 suggest for both phases a homogeneity range with respect to the Rh/W ratio and the presence of small amounts of W5+ besides Rh3+ and W6+. Results of the ligand field analysis for the reference material Rh(PO3)3, which is containing the octahedral chromophore [RhIIIO6], are reported (Δo=23200 cm−1, B=490 cm−1). Incorporation of rhodium or iridium in phosphates of the MPTB structure family is possible. SXRD analysis of (Rh1/6W5/6O3)8(PO2)4 and (Rh2/21W19/21O3)14(PO2)4 reveals an unexpected distribution pattern for Rh3+ over the available metal sites. Results of further characterization by TEM, powder reflectance spectroscopy and magnetic measurements are reported.