Crystal structure, molecular dynamics, and polar properties of pyridinium fluorochromate

The crystal and molecular structure of pyridinium fluorochromate has been determined at 293, 240, and 150 K by x-ray diffraction. All three phases are orthorhombic with the following sequence of space groups and solid-solid phase transitions disclosed also by differential thermal analysis and differential scanning calorimetry, Cmcm-->258 K Cmc2(1)-->224 K Pbca. Only the low temperature phase is well ordered while the two high temperature phases reveal molecular disorder of both pyridinium and fluorochromate ions. H-1 and F-19 nuclear magnetic resonance study has been performed as a function of temperature to describe molecular dynamics of both ions. The dielectric and pyroelectric measurements together with these results allow us to characterize the low temperature phase III as antiferroelectric, the mesophase II as ferroelectric, with the Curie point at 258 K, and the high temperature phase I as paraelectric. (C) 2002 American Institute of Physics.