INCOMMENSURATE PHASE TRANSITION IN SODIUM-NITRITE.
Item
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Title
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INCOMMENSURATE PHASE TRANSITION IN SODIUM-NITRITE.
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Identifier
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AAI8601686
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identifier
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8601686
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Creator
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QIU, SHEN LI.
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Contributor
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Herman Z. Cummins
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Date
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1985
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Condensed Matter
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Abstract
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The incommensurate phase transition in NaNO(,2) has been studied by the following experiments: (1) Dielectric constant measurements under E-field along the b-axis; (2) Elastic neutron scattering under E-field along the b-axis; (3) Elastic neutron scattering under E-field along the a-axis; (4) Elastic neutron scattering under E-field along the c-axis; (5) Brillouin scattering under E-field along the c-axis; (6) Inelastic neutron scattering; (7) Diffuse neutron scattering; (8) Dielectric constant measurements under hydrostatic pressure; (9) Brillouin scattering under hydrostatic pressure.;By numerically minimizing the Landau free energy the predictions of different Landau free energy expansions for the phase diagram and the behaviour of the modulation wave vector under E-field along the b-axis were explored. All the coefficients in the free energy have been determined from the dielectric constant measurements, the elastic neutron scattering results and the spontaneous polarization data.;Some discussions of the free energy model are given. Comparison of the diffuse neutron scattering results with the predictions of the free energy model indicates that the (eta)-term in the free energy cannot be neglected, and that temperature dependence of the coefficient (alpha) should be included. There is another reason why the (eta)-term cannot be neglected: without an (eta)-term the para (--->) INC phase transition will be of first order. However, the observed para (--->) INC transition is of second order.;Extension of the Lifshitz point theory to the case of a first order transition from the disordered phase to the ordered phase has for the first time been discussed in detail. By fitting our data of the elastic neutron scattering under transverse electric field to the numerically calculated results we can determine where the virtual Lifshitz point should occur and construct the whole phase transition diagram.;Using the ANNNI model, numerical calculations of dielectric constants under longitudinal electric field were carried and some discussions are presented.;The microscopic theory of Michel and coworkers is reviewed. Some of the results of the microscopic theory are cited for comparison with the results of the phenomenological theory and the Ising model.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
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Program
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Physics
