Structures of the paraelectric and ferroelectric phases of NaD/sub 3/(SeO/sub 3/)/sub 2/ by neutron diffraction: A vertex model for the ordered ferroelectric state
Neutron diffraction studies have been performed on layered sodium trideuterium selenite Na(H/sub 0.06/D/sub 0.94/)/sub 3/ (SeO/sub 3/)/sub 2/ in both the paraelectric and the ferroelectric phases. The crystal data are a = 10.365(2), b = 4.850(1), c = 5.792(1) A, ..beta.. = 91.16(3)/sup 0/, Z = 2, space group P/sub 21//n for the paraelectric phase at 298 K; and a = 10.314(3), b = 9.663(2), c = 5.768(2) A, ..beta.. = 91.23(3), Z = 4, space group Pn for the ferroelectric phase at 173 K. An external field of 15 KV/cm was applied to the crystal at the transition temperature before the low temperature measurements. The atomic parameters of the two structures were determined by full-matrix least-squares methods. The final agreement indices are R(F/sup 2/) = 0.038, R/sub w/(F/sup 2/) = 0.049 on 748 reflections for the paraelectric structure and R(F/sup 2/) = 0.035, R/sub w/(F/sup 2/) = 0.044 on 1636 reflections for the ferroelectric structure. The results resolve a controversy concerning the ordered hydrogenic state in the ferroelectric phase. Local charge neutrality is found for each molecular unit NaD/sub 3/(SeO/sub 3/)/sub 2/ rather than previously proposed charged local configurations NaD/sub 4/(SeO/sub 3/)/sub 2//sup 1 +/ and NaD/sub 2/(SeO/sub 3/)/sub 2//sup 1 -/. However, the ground state is not as simple as might be expected. To help in understanding the ordering we develop a phenomenological vertex or site model which also includes interactions between pairs of hydrogen bonds. This model does indeed yield the observed ground state in each layer upon suitable choice of parameters, although the mechanism for three-dimensional ordering is still unclear. Various bond lengths, bond angles, and their perturbation upon hydrogenic ordering are discussed.
- Research Organization:
- Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973
- OSTI ID:
- 5227298
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 77:1
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
DEUTERIUM COMPOUNDS
CRYSTAL STRUCTURE
FERROELECTRIC MATERIALS
PHASE TRANSFORMATIONS
SODIUM COMPOUNDS
LOW TEMPERATURE
NEUTRON DIFFRACTION
SELENITES
ALKALI METAL COMPOUNDS
COHERENT SCATTERING
DIFFRACTION
HYDROGEN COMPOUNDS
SCATTERING
SELENIUM COMPOUNDS
360602* - Other Materials- Structure & Phase Studies