Pyroelectric 3La(IO3)3⋅HIO3⋅7H2O. Crystal structure of the transition metal iodates. V.

Abstract

3La(IO3)3⋅HIO3⋅7H2O is pyroelectric and piezoelectric at room temperature and crystallizes in the orthorhombic space group Aba2 with four formulas in the unit cell. The lattice constants at 298 °K are a=13.18008(10), b=21.72076(10), c=12.15514(18) Å (for λ CuKα1=1.540562 Å). The integrated intensities of most reflections within a hemisphere of reciprocal space with radius (sinϑ)/λ=1.02 Å−1 were measured on two different crystals, each with a different diffractometer, resulting in 2742 symmetry-independent Fmeas for one set and 4038 Fmeas for the second set of structure factors. The crystal structure was solved by use of Patterson and Fourier methods, and refined by the method of least squares. The final agreement factor R=0.063 for the first set and 0.040 for the second, for models in which all atoms but hydrogen undergo anisotropic thermal vibrations. Normal probability plot analysis is used to compare the two sets of data and parameters. The absolute configuration of the crystal has been determined. Evidence for lattice damage caused by x irradiation is presented. The iodate ions form an oxygen-sharing network, with one independent La3+ occupying distorted bicapped, the other distorted equatorially-girdled, trigonal prismatic interstices. Each of the five independent IO−3 ions form triangular pyramids with three short I–O bonds of average length 1.803 Å: Three iodine atoms are associated with two next-nearest oxygen neighbors at 2.80 Å, one has three next-nearest neighbors also at 2.80 Å, and the fifth has four next-nearest neighbors at 2.91 Å, on average. An extensive O⋅⋅⋅O hydrogen bond network, with shortest distance of 2.619 Å, connects water molecules and iodate ions. Evidence for H3O+ formation by the iodic acid proton, and for the positions of the hydrogen atoms associated with the four independent water–oxygen atoms, is discussed.