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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.

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The mixed fluorides with formula MIIZrF6 (MII=Co,Zn) exhibit an ordered ReO3-type structure: this structure is made of a three-dimensional arrangement of corner-shared octahedra with an ordered distribution of MII and ZrIV cations in the octahedra sites. These materials undergo around room temperature a first-order ferroelastic structural phase transition from a cubic phase (Fm3m) to a rhombohedral modification (R3), as established by neutron and X-ray diffraction experiments. From a structural determination of CoZrF6 at different temperatures through the phase transition, three order parameters have been measured, which enable one to describe completely the extent of lattice distortion in the rhombohedral phase: the spontaneous strain es, the coordinate of octahedra rotation around the threefold axis R and the octahedra internal deformation coordinate Q. Important thermal variations are observed for es and R, but the coordinate Q always remains very small, showing that the Co(Zr)F6 octahedra are essentially rigid bodies. X-ray diffuse scattering experiments on CoZrF6 at room temperature (cubic phase) are consistent with the existence of structural disorder due to rotations of the Co(Zr)f6 octahedra around the cubic fourfold axes.

Theoretical studies of the transition metal-carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V The atomic arrangement in Sr 3 (FeF 6 h determined by von der Miihll [CO R. Acad. Sci. Paris 278, 713 (1974)] differs from a structure of opposite polarity by atomic displacements of 1.3 A or less for F and 0.4 A or less for Sr and Fe. The structure is hence potentially ferroelectric, with an estimated transition temperature Tc to the paraelectric state of about 1160 K: experimentally, Tc = 922 K. The Pb 3 (MF 6h family is isomorphous with Sr 3(FeF 6b and the corresponding T elM) has been measured as 695 (Ti), 635 (V), 550 (Cr), 740 (Fe), and 645 (Ga) K. The respective nonlinear dielectric susceptibilities are 0.010,0.010,0.005,0.045, and O.OIOX 10-12 V-I m. A dielectric anomaly is observed close to Tc in each material at 1 kHz with a maximum permittivity €; of2740 (Ti), 300 (V), 2015 (Cr), 4000 (Fe), and 7800 (Ga). Dielectric losses in each material reach a minimum at the phase transition temperature.