Study on phase transitions in the systems K(3)MO(3)F(3)-A(3)MO(3)F(3)(A=Na,Rb; M=Mo,W)

X-ray diffraction, differential thermal analysis, polarized light microscopy, and dielectric measurements have been used to study phase transitions and solid solutions in the K(3)MO(3)F(3)-A(3)MO(3)F(3) (A = Rb, Na; M = Mo, W) systems, K(3)MO(3)F3-Na(3)MO(3)F(3) (M = Mo, W) systems show two solid solutions; the first (SSI) near x = 0 (K(3-x)Na(x)MO(3)F(3)) involves only one transition at T-C which is practically independent of composition, The sharp transition at T-1 (<T-C) for x = 0 disappears or vanishes for x = 0.03, The second solid solution (SS2) near K(2)NaMO(3)F(3) retains cubic symmetry down to 80 K, Rb(3)MO(3)F(3)-K(3)MO(3)F(3) systems show three solid solutions, The SS1 and SS3 exist close to the rubidium and potassium extremes of the binary lines. The pure phases display transitions at T-1 and T-C. For compositions far from y = 0 or y = 3 (Rb(3-y)K(y)MO(3)F(3)), T-C decreases and T-1 decreases abruptly and for SS1 becomes indetectable. The solid solution SS2 (near y = 1) has cubic symmetry down to 80 K for M = W, and a cubic-noncubic transition at low temperature for M = Mo. In any case, the phase transitions are of first order, reversible, and improper character. The transition at T-C is ferroelectric, ferroelastic-paraelectric, prototype. The extent of each solid solution depends on the ratio of alcaline cations present in the 6- and 12-fold coordination sites of the perovskite structure. (C) 1996 Academic Press, Inc.