title: NaF-KF-AlF3 System: Phase Transition in K2NaAl3F12 Ternary Fluoride.
abstract: Phase formation in the NaF-KF-AlF3 system, in the vicinity of the K2NaAl3F12 composition, has been studied. The samples have been prepared by melting the starting components at 650 °C. A new phase has been revealed, which appeared to be a low-temperature form of the well-known K2NaAl3F12 ternary fluoride obtained by the hydrothermal synthesis method. The high-temperature form melts at 598 °C and is stable in a narrow temperature region of about 15 deg below the melting point. Thermal analysis, high temperature X-ray diffraction, IR-spectroscopy, X-ray fluorescence, and X-ray powder diffraction crystal structure analysis have been applied to study the composition, crystal structure, and thermal properties of the low-temperature phase. The crystal structure consists of trigonal-hexagonal two-dimensional (2D) grids built from the [AlF6] octahedra connected via vertices. The 2D grids have a specific wave-like conformation with a wavelength of 11.88 Å and an amplitude of 0.46 Å. There is a shift of the adjacent grids relative to each other. Because of this shift, the space between the grids changes. The shift leads to the formation of pores adapted to potassium and sodium ions. The reasons for the wave-like structure of layers are discussed. It is shown that the two polymorphic forms differ in the order of cation occupations.
authors.affiliation: †Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: Sergei D
authors.initials: SD
authors.lastname: Kirik
authors.affiliation: ‡Institute of Chemistry and Chemical Technology SB RAS, 660036 Krasnoyarsk, Russia.
authors.forename: Yulia N
authors.initials: YN
authors.lastname: Zaitseva
authors.affiliation: †Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: Darya Yu
authors.initials: DY
authors.lastname: Leshok
authors.affiliation: †Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: Alexandr S
authors.initials: AS
authors.lastname: Samoilo
authors.affiliation: †Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: Petr S
authors.initials: PS
authors.lastname: Dubinin
authors.affiliation: †Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: Igor S
authors.initials: IS
authors.lastname: Yakimov
authors.affiliation: §UC RUSAL ETC 660111 Krasnoyarsk, Russia.
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Simakov
authors.affiliation: §UC RUSAL ETC 660111 Krasnoyarsk, Russia.
authors.forename: Alexandr O
authors.initials: AO
authors.lastname: Gusev