It was noted that bromides of this form crystallized in the so-called stuffed LiSbF6 structure (when M was Sm to Gd) or in the cryolite structure (when M was Gd to Lu). Single-crystal X-ray data indicated a trigonal space group of R3¯ with a = 740.8, c = 1998.9pm, when M was Sm, and a monoclinic space group of P21/n, with a = 721.3, b = 769.9, c = 1074.8pm, and  = 90.60 when M was Y. Reversible phase transitions from one structure to the other occurred. It was found that all of the compounds exhibited a Na-ion conductivity which decreased upon transformation to the cryolite structure. The conductivity of the bromide was always higher than that of the respective chloride. M.S.Wickleder, G.Meyer: Zeitschrift für Anorganische und Allgemeine Chemie, 1995, 621[3], 457-63