Series of compositions Bi2(M′xM1−x)4O9 (M′, M=Al, Ga, Fe; 0≤x≤1) with mullite-type crystal structure: Synthesis, characterization and 18O/16O exchange experiment

Series of compositions Bi2(M′xM1−x)4O9 with x=0.0, 0.1,…, 1.0 and M′/M=Ga/Al, Fe/Al and Fe/Ga were synthesized by dissolving appropriate amounts of corresponding metal nitrate hydrates in glycerine, followed by gelation, calcination and final heating at 800 °C for 24 h. The new compositions with M′/M=Ga/Al form solid-solution series, which are isotypes to the two other series M′/M=Fe/Al and Fe/Ga. The XRD data analysis yielded in all cases a linear dependence of the lattice parameters related on x. Rietveld structure refinements of the XRD patterns of the new compounds, Bi2(GaxAl1−x)4O9 reveal a preferential occupation of Ga in tetrahedral site (4 h). The IR absorption spectra measured between 50 and 4000 cm−1 of all systems show systematic shifts in peak positions related to the degree of substitution. Samples treated in 18O2 atmosphere (16 h at 800 °C, 200 mbar, 95% 18O2) for 18O/16O isotope exchange experiments show a well-separated IR absorption peak related to the M–18Oc–M vibration, where Oc denotes the common oxygen of two tetrahedral type MO4 units. The intensity ratio of M–18Oc/M–16Oc IR absorption peaks and the average crystal sizes were used to estimate the tracer diffusion coefficients of polycrystalline Bi2Al4O9 (D=2×10−22 m2s−1), Bi2Fe4O9 (D=5×10−21 m2s−1), Bi2(Ga/Al)4O9 (D=2×10−21 m2s−1) and Bi2Ga4O9 (D=2×10−20 m2s−1).