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Аннотация:Phase transitions in the elpasolite-type K3AlF6 complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures T-1 = 132 degrees C, T-2 = 153 degrees C and T-3 = 306 degrees C. The alpha-K3AlF6 phase is stable below T-1 and crystallizes in a monoclinic unit cell with a = 18.8588(2)angstrom, b = 34.0278(2)angstrom, c = 18.9231(1)angstrom, beta = 90.453(1)degrees (a = 2a(c)-c(c), b = 4b(c), c = a(c) + 2c(c); a(c), b(c), c(c)-the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate beta phase exists only in very narrow temperature interval between T-1 and T-2. The gamma polymorph is stable in the T-2 < T < T-3 temperature range and has an orthorhombic unit cell with a = 36.1229(6)angstrom, b = 17.1114(3)angstrom, c = 12.0502(3)angstrom (a = 3a(c)-3c(c), b = 2b(c), c = a(c) +c(c)) at 250 degrees C and space group Fddd. Above T-3 the cubic delta polymorph forms with a(c) = 8.5786(4)angstrom at 400 degrees C and space group Fm(3) over barm. The similarity between the K3AlF6 and K3MoO3F3 compounds is discussed. (C) 2005 Elsevier Inc. All rights reserved.