Phase transitions have been found in Pb5Al3F19 at Tc = 285 K, Pb5Ti3F19 at 695 K, Pb5V3F19 at 630 K, Pb5Cr3F19 at 545 K (reported previously), Pb5Fe3F19 at 740 K, and Pb5Ga3F19 at 945 K. In striking contrast to the variation of Tc with M atom in Sr5M3F19 and Ba5M3F19 (reported previously), for which materials the phase transition temperature increases with increasing atomic number from M=Ti to M=Cr but with a lower Tc for M=Fe before increasing again, Tc in Pb5M3F19 decreases from M=Ti to M=Cr with a sharp increase for M=Fe before it decreases again. The transition temperature for Pb5Al3F19 is below room temperature, that for isotypic Sr5Al3F19 is not detectable before reaching a dissociation temperature around 1000 K. All phase transitions in the Pb5M3F19 family are first order. Each new material has been fully characterized. Differences between materials with A=Pb, M=Al or Cr, and those with A=Sr, M=Al or Cr have been investigated through the formation of solid solutions. Six separate systems have been studied. Complete solid solutions form between end components with identical A and different M atoms. Partial solid solutions form with different A and different M atoms. Solid solutions do not form with different A and identical M atoms. M atom replacement, hence, appears to be energetically favorable, A atom replacement unfavorable, probably due to the role of the Pb atom lone pair of electrons. Two models outlining this role are presented.
The Pb5(Cr1 −xAlx)3F19 system forms four phases. Ferroelectric phase IV, with point group 4mm, undergoes a first‐order transition on heating to paraelectric phase I, point group 4/m, in the composition range 0 ≤x≲ 0.80. The Curie temperature varies linearly with x over this range, from 555 K at x = 0 to 365 K at x = 0.80. Phase IV transforms to monoclinic phase II in the range 0.80 ≲x≲ 0.88 in a first‐order transition before undergoing a second‐order transition at higher temperatures to phase I. In the range 0.88 ≲x≲ 1, phase IV initially transforms on heating to phase III, point group 4/m, in a first‐order transition; at higher temperatures, this phase transforms to phase II in another first‐order transition; at still higher temperatures, it transforms to phase I in a second‐order transition. Multiple phase formation at x≲ 0.80 is a result of structural instabilities that are related to the small size of the Al3+ cation and to the decreased interaction strength between the 6(sp)2 electron lone pair of Pb2+ and the residual 3d electron spin moment at the Cr‐atom site as it is increasingly substituted by aluminum.
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.
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