Infrared investigation of some ferroelectric and related compounds : NH₄NCS, (NH₄)₂SO₄, RbHSO₄, and NH₄HSO₄ Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gm80j0743

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  • At liquid nitrogen temperature the spectra of sublimed films of NH₄NCS, both as phase I (the usual form stable at room temperature) and phase II (the low temperature form), exhibit detail usually reserved for gases at low pressures. In both phases the symmetry of ammonium ion has degenerated from tetrahedral to a condition of no symmetry, The observed frequencies of ammonium ion and its completely deuterated analog were sufficient to calculate a complete quadratic potential function with which vibrational frequencies of the partially deuterated species NH₃D⁺, NH₂D₂⁺, and NHD₃⁺ were calculated. The complex spectrum to which all species contribute was thus assigned, The number of components observed for some bending vibrations of the partially deuterated species was consistent with the hypothesis that species of C₃v and C₂v symmetry could occupy their sites on the crystal lattice in four and six distinguishable orientations, respectively. The previous observations preclude the possibility that ammonium ion rotates freely in ammonium thiocyanate. Vibrations of ammonium ion in phase II exhibited 2n components where n was the number of components observed for corresponding vibrations of phase I. From this evidence a likely crystal structure for phase II was deduced. (NH₄)2SO₄, RbHSO₄, NH₄HSO₄, and their deuteroderivatives were studied with respect to their ferroelectric transitions and low temperature spectra. Samples were produced by evaporation of aqueous solutions upon insoluble BaF₂ substrate windows and by mulling with nujol and fluorolube. The observed multiplicity of the low temperature spectra was assigned on the following bases (1) equipoint splitting which originates in the exact theory of crystal spectra whenever like ions occupy inequivalent positions on the crystal lattice, (2) site group splitting which may arise for degenerate modes of free ions whenever the site symmetry is less than the point symmetry of the free ion, and (3) factor group splitting which is produced by coupling of like vibrational modes among equivalent ions within a unit cell. Often the number of components and their relative positions were sufficient to differentiate among the three sources of multiplicity. It was found that site group splitting was the dominant effect followed by equipoint and factor group splitting in that order. First order phase changes in all three compounds were associated with marked and reproducible changes in the spectra which were reversible with temperature inversion, The infrared spectrum was found completely indifferent to second order transitions, On the bases of spectral changes at their respective Curie points and their low temperature spectra, dielectric polarization in ammonium sulfate, rubidium bisulfate, and ammonium bisulfate was attributed to the existence of oriented systems of hydrogen bonds.
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