Crystal structure and pyroelectric coefficient of Co(IO 3) 2 and structural relationships among the anhydrous noncentrosymmetric 3 d-transition metal lodates

Abstract

Cobalt iodate, Co(IO 3) 2, is pyroelectric at room temperature and crystallizes in the trigonal space group P3 with four formulas in the unit cell. The pyroelectric coefficient p 3 = 5.1(5) × 10 −5 Cm −2 K −1, and the lattice constants at 298 K are a = 10.9597(1) and c = 5.0774(1)Å. The crystal structure was solved from the Patterson function and a consideration of the structural similarities among the anhydrous 3 d-transition metal iodates. Refinement was by the method of least squares using 1825 observed reflections measured with a four-circle diffractometer. The final agreement factor R = 0.050. Two of the three independent octahedrally coordinated cobalt atoms are disordered. The absolute orientation of the polar axis was determined with respect to the crystal morphology. The iodine atoms in the trigonal pyramidal iodate ions point toward the morphologically undeveloped end of the crystals, as does the positive sense of the spontaneous polarization: It is this end that develops a positive polarization on heating. The spontaneous polarization is estimated at about 22 × 10 −2 Cm −2. The iodate ion arrangement is very similar to that in α-LiIO 3. The anhydrous noncentrosymmetric 3 d-iodates may be classified into three isomorphous sets, with space group P6 3 and corresponding maximal subgroups P3 and P2 1. All three sets are isostructural with α-LiIO 3 except for the cation distribution. The ability of the iodate ion framework to accept a range of cations of different valences is discussed in terms of the tricapped trigonal-prismatic environment about the iodine atoms.