Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi₂Ga₄O₉and Bi₂Fe₄O₉between room temperature and about 1 200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1 280 K using high-resolution dilatometry and X-ray powder diffraction. The elastic constants at 295 K are in GPa c₁₁= 143.4(1), c₂₂= 161.9(1), c₃₃= 224.5(1), c₄₄= 68.4(1), c₅₅= 49.3(1), c₆₆= 76.6(1), c₁₂= 74.2(1), c₁₃= 62.2(1), c₂₃= 70.5(1) for Bi₂Ga₄O₉, and c₁₁= 106.7(1), c₂₂= 141.2(1), c₃₃= 183.7(2), c₄₄= 53.7(1), c₅₅= 41.9(1), c₆₆= 63.8(1), c₁₂= 63.5(1), c₁₃= 59.8(1), c₂₃= 63.4(2) for Bi₂Fe₄O₉. In both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the c_ijshows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge- and corner-linked coordination polyhedra.

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