A third type of fluorine exchange is that between Ba-rich and Sn-rich environments, which we probe using variable-temperature 19 F MAS NMR spectroscopy. Fig. shows spectra recorded at 40 • C, 65 • C and 90 • C. As the temperature increases, the relative intensity of the peak assigned to fluoride ions in Sn-rich environments also increases, from 54 % to 60 %, at the expense of the peak assigned to fluoride ions in Ba-rich environments, confirming some degree of fluoride-ion exchange between Ba-rich and Sn-rich environments.