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.
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