Pyroelectric α-Cu(IO3)2. Crystal structure of the transition metal iodates. III

Abstract

Alpha-cupric iodate, α-Cu(IO3)2, is pyroelectric and piezoelectric at room temperature and crystallizes in the monoclinic space group P21 with two formulas in the unit cell. The lattice constants at 298 °K are a =5.56902 ± 0.00010, b=5.11098 ± 0.00009, c=9.26976 ± 0.00036 Å and β=95.82 ± 0.02 ° (λCuKα1 − 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 using two different diffractometers, resulting in 1814 Fmeas for one set and 4272 Fmeas for the second set of independent structure factors. The crystal structure was solved using Patterson and Fourier series and refined by the method of least squares, the final agreement factor R becoming 0.063 for the first set, and 0.035 for the second set of Fmeas. The iodate ions form a nearly-regular hexagonal closest-packed array, with Cu2+ in the resulting octahedral interstices. The two independent iodate groups, each with three short I–O distances averaging 1.82 Å, share oxygen atoms to form a three-dimensional framework of corner-sharing polyhedra. One iodate ion forms three additional I–O distances averaging 2.66 Å, the other has four that average 2.95 Å. The CuO6 octahedron is tetragonally distorted, with four bonds averaging 1.97 Å and two apical Cu–O bonds averaging 2.39 Å. The two sets of Fmeas, and the resulting derived structural parameters, are compared using normal probability plot analysis.