Phase transitions in the Pb₅(Cr_1−xAl_x)F₁₉system

Abstract

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.

Structure relationship to dielectric, elastic and chiral properties

“…This series was later extended to include A = Pb and M = A1, Ti, V, Cr, Fe, Ga (Ravez, Andriamampianina, Simon, Grannec & Abrahams, 1991). PbsA13F19, which forms four distinct phases including all those known among the other members of the family, may be regarded as the family prototype (Ravez, Andriamampianina, Simon, Rabardel, Ihringer & Abrahams, 1994). Ferroelectric PbsAI3F19 phase IV, formed on cooling phase III below l l0K, is isomorphous with PbsCr3F19 phase II at 295 K in space group I4cm with a = 14.384 and c = 7.408/~.…”

Crystal Structure of Ferroelastic Pb5Al2.96Cr0.04F19 at 300 K

“…A study of the Pb (Al \V Cr V ) F system has recently shown the III}II phase transition temperature to decrease with a rising rate x of Cr}Al substitution (7,8). The phase diagram in Fig.…”

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

“…The primitive unit cell of phase III is rotated 45 ° about c with respect to the orientation of phases I, II or IV (Ravez et al, 1994). The only systematically absent reflections are hkO with h + k = 2n + 1, hence the most likely space group is P4/n.…”

“…PbsCr3Ft9 forms a continuous series of ferro-electric solid solutions with Pb5A13FIg; the transition temperatures of the resulting materials, as measured on heating, decrease smoothly from 555 to 280 K with increasing PbsA13Fw component (Ravez, Simon, Andriamampianina, Grannec, Hagenmuller & Abrahams, 1990;. The phase diagram of the Pbs(Crl-xAlx)3F19 system has been determined, under both heating and cooling conditions, by Ravez, Andriamampianina, Simon, Rabardel, Ihringer & Abrahams (1994).…”

“…Roomtemperature phase III of PbsAI3FI9 is referred to as antiferroelectric, rather than as paraelectric, since phase I is the prototypic paraelectric phase stable above 345 K (Ravez et al, 1994) and point group 4/m for phase III is not a supergroup of 4mm for ferroelastic phase IV, the stable phase at lower temperatures. Roomtemperature phase III of PbsAI3FI9 is referred to as antiferroelectric, rather than as paraelectric, since phase I is the prototypic paraelectric phase stable above 345 K (Ravez et al, 1994) and point group 4/m for phase III is not a supergroup of 4mm for ferroelastic phase IV, the stable phase at lower temperatures.…”