A new family of ferroelectric materials with composition A₂BMO₃F₃(A, B = K, Rb, Cs, for r_A+ ⩾ r_B+ and M = Mo, W)

Raman spectroscopic study of the lattice dynamics in the Rb2KMoO3F3 oxyfluoride

“…In [2,[11][12][13][14]18], it was found that the Rb 2 KMoO 3 F 3 elpasolite undergoes two successive phase transitions at T = 328 K (with appearance of a spontaneous polarization) and at T = 182 K. The stud ies of the compound were restarted several years ago. In [17], the properties of the Rb 2 KMoO 3 F 3 elpasolite were studied on many samples synthesized by various methods.…”

“…Depending on the ratio of the ionic radii R A and R A' and also on the type of M atoms, the structure can be distorted as a result of two [7][8][9][10][11][12][13][14][15] or one phase transition [1,7,[11][12][13][14]16], or it can remain stable down to the liquid helium temperature [7,[11][12][13][14]. For instance, despite very close values of the ionic radii of Mo (R Mo = 0.073 nm) and W (R W = 0.074 nm) atoms, the Rb 2 KMoO 3 F 3 elpasolite undergoes a phase transition on cooling [17], while the Rb 2 KWO 3 F 3 elpasolite remains cubic and does not undergo any phase transformations.…”

Paraelectric–paraelastic Rb2KMoO3F3 structure at 343 and 473 K

“…The average entropy change observed (Ravez, Peraudeau, Arend, Abrahams & Hagenmuller, 1980) at the cubic to rhombohedral transition of six A 3MO3F3 compounds (A = K, Rb or Cs, M = Mo or W) is 3.9 (3) J mol -~ K -1, which compares well with that expected for a change at the phase transition in the number of available orientations corresponding to Rln 3/2, i.e. 3.4 J mol -~ K-k Busey et al (1962) find an average entropy change of 5.1 J mol -~ K -1 at each of the three transitions in K2ReC16, corresponding to R In 2: the symmetry progression in K2ReC16 has been reported as cubic-to-tetragonal-to-monoclinic PARAELECTRIC-PARAELASTIC Rb2KMoODF3 STRUCTURE AT 343 AND 473 K for the three higher-temperature phases (O'Leary & Wheeler, 1970).…”

“…One such family with composition A2BMO3F 3 has ten members in which A, B = K, Rb, Cs and M = Mo, W, each of which exhibits at least one phase transition above 78 K (Ravez, Peraudeau, Arend, Abrahams & Hagenmuller, 1980). The transition at T c = 328 K for Rb2KMoO3F 3 is from rhombohedral to cubic symmetry: a dielectric anomaly is associated with the transition and the spontaneous polarization decreases to zero above 328 K. The crystal becomes ferroelastically twinned in the rhombohedral phase (Abrahams, Bernstein & Ravez, 1981).…”

Локальная структура и процессы гидратации галогензамещенных перовскитов на основе Ba-=SUB=-4-=/SUB=-In-=SUB=-2-=/SUB=-Zr-=SUB=-2-=/SUB=-O-=SUB=-11-=/SUB=-

“…Метод введения галогенид-ионов в кислородную подрешетку сложного оксида с целью изменения структуры и улучшения физико-химических свойств широко используется для различных классов перовскитоподобных систем, таких, например, как сегнетоэлектрические и пьезоэлектрические материалы [1][2][3][4], сверхпроводники [5][6][7][8], магнитные материалы [9,10], катодные материалы [11][12][13].…”