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 Bi-2(M-x'M1-x)(4)O-9 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 degrees 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, Bi-2(GaxAl1-x)(4)O-9 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 O-18(2) atmosphere (16 h at 800 degrees C, 200 mbar, 95% O-18(2)) for O-18/O-16 isotope exchange experiments show a well-separated IR absorption peak related to the M-O-18(c)-M vibration, where O-c denotes the common oxygen of two tetrahedral type MO4 units. The intensity ratio of M-O-18(c)/M-O-16(c) IR absorption peaks and the average crystal sizes were used to estimate the tracer diffusion coefficients of polycrystalline Bi2Al4O9 (D=2 x 10(-22) m(2)s(-1)), Bi2Fe4O9 (D=5 x 10(-21) m(2)s(-1)), Bi-2(Ga/Al)(4)O-9 (D=2 x 10(-21) m(2)s(-1)) and (BiGa4O9)-Ga-2 (D=2 x 10(-20) m(2)s(-1)). (C) 2010 Elsevier Inc. All rights reserved.