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short communications
The thermal expansion of cyclopentadienylthallium, TlC5H5, was determined in the range 78 to 295 K, using X-ray powder diffractometry. Expansion is very anisotropic: α1 = 125 ± 12, α2 = 97 ± 5, α3 = 0 ± 4 (all 10−6 K−1). Directions and ratio of the two greatest expansion coefficients: α1 and α2, are explained by considering the shortest distances Tl+–Tl|+ in this ionic structure.