“…However, space group P2cm requires there to be two symmetry-independent pairs of anions and cations in 2c x,y, sites (labeled Tl1, Tl2, F1, and F2), this structure being identical to the Pbc2 model of Barlow and Meredith (11) if the ionic coordinates satisfy the conditions y 2 "!y 2 and y $ "!y $ . On increasing temperature a structural transition has been observed at &355 K by di!erential thermal analysis (DTA) (9), drop calorimetry (15), and X-ray di!raction (11) methods. The structure of the high-temperature modi"cation [labeled TlF-I (9)] has been studied by X-ray di!raction and the results are summarized in Table 1.…”