Relationship between the structures of ferroelectric Pb₅Cr₃F₁₉ and antiferroelectric Pb₅Al₃F₁₉ at 295 K and the phase III–phase IV transition in Pb₅Al₃F₁₉ on cooling to about 110 K

Abstract

Antiferroelectric phase III of lead aluminium fluoride, PbsAI3Fw, Mr = 1477.9, tetragonal space group P4/n. At T = 295 K, a = 20.1738 (4) and c = 7.2205 (1) A,, V = 2939 (1) A 3, Z = 8, O m = 6.66 (5), Dx = 6.681 Mg m -3. For A(Mo Ka) = 0.71069 A,, /x = 58.0 mm-~, F(000) = 4960. The structure was determined from 18502 (1276 independent) Fm 2> -3~r(F~) with (sin 0)/,~ -< 0.703 A,-~ Least-squares refinement on wF~ resulted in R(Fm)= 0.0579 with Rint(Fm) = 0.048. PbsA13F19 undergoes a first-order phase transition from antiferroelectric to ferroelectric at about 110 K, with a wide thermal hysteresis.Transformation of the atomic coordinates of ferroelectric PbsCr3Fi9 previously measured at 295 K and comparison with those of antiferroelectric Pb5AI3FI9 at 295 K reveal differences between related atomic positions that range from 0.10 to 1.56 A. The origin of the first-order transition from the antiferroelectric phase III to ferroelectric phase IV in PbsAI3FI9 is shown to be associated with the orientational change from an eclipsed arrangement of A1F6 octahedra along the inversion and rotation-tetrad axes in phase III to a staggered arrangement along the rotationtetrad axes in phase IV.

Structure relationship to dielectric, elastic and chiral properties

“…The transition from antiferroelectric Pb5ml3F19 phase HI in space group P4/n to ferroelectric phase IV in space group 14cm is associated with a structural change from an eclipsed arrangement of A1F6 octahedra along the inversion and rotation-tetrad axes in the former to a staggered arrangement of octahedra along the rotation-tetrad axes in the latter (Andriamampianina, Gravereau, Ravez & Abrahams, 1994). It may be noted that the barrier to the resulting octahedral rotations of ,,,45 ° at the Tc phase transition leads to an unusually large thermal hysteresis of about 170 K in the transition temperature as observed between heating and cooling, in addition to a wide and stable thermal regime over which the two phases coexist.…”

Structural Phase Transition Nomenclature. Report of an IUCr Working Group on Phase Transition Nomenclature

“…Lead aluminium¯uoride (Pb 5 Al 3 F 19 ) (Andriamampianina et al, 1994;Sarraute et al, 1995Sarraute et al, , 1996Omari et al, 1998) I |> 670 K |I4/mcm (140) |Z = 4 |non-ferroic |structure not yet unambiguously determined.…”

Structure–property correlation over five phases and four transitions in Pb5Al3F19

“…Lattice constants for phase I are estimated. The primitive unit cell of phase IV at 295 K used for structure re®nement in the space group P4/n has a = 20.1738 (4) A Ê , c = 7.2205 (1) A Ê and Z = 8 (Andriamampianina et al, 1994). The smaller transformed cell, see x7.1, is also bodycentered with symmetry reduced effectively to I "…”

“…The structure of phase IV|315±260 K| (Andriamampianina et al, 1994), unlike phase V |< 260 K|, contains in®nite chains of eclipsed corner-sharing AlF 6 octahedra as well as individual octahedra. The arrangement and number of octahedra, PbF n polyhedra and non-octahedral F À ions, as well as the averaged bond distances dhPbÐFi and dhAlÐFi, are summarized in Table 2.…”