Raman and dielectric studies of structurally modulated crystals: Thiourea and tetramethylammonium tetrachlorozincate.
Item
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Title
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Raman and dielectric studies of structurally modulated crystals: Thiourea and tetramethylammonium tetrachlorozincate.
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Identifier
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AAI9207058
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identifier
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9207058
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Creator
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Chen, Xiaoke.
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Contributor
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Adviser: Herman Z. Cummins
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Date
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1991
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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 complex dielectric constant of thiourea has been measured in the temperature range from 80 K to 300 K with different measuring-field levels and frequencies, with and without d.c. biasing fields. The temperature interval between the two peaks in the {dollar}\varepsilon{dollar}*{dollar}(T){dollar} curve near 177 K was found to extrapolate to 0.7 K at zero measuring field, which indicates that the {dollar}1\over 8{dollar} ferroelectric lock-in phase is intrinsic in a non-zero temperature range. Slowing-down of dielectric relaxation processes was observed at both the upper and lower transition points of this phase. The E-T phase diagram near the {dollar}1\over 8{dollar} lock-in region was surveyed by investigating the effects of biasing fields. D-E hysteresis loops were investigated in the vicinity of the {dollar}1\over 8{dollar} ferroelectric phase with measuring frequencies from 0.5Hz to 5mHz. The different D-E loops observed clearly demonstrate the difference between the intrinsic and "field-induced" lock-in mechanisms. The spontaneous polarization and the coercive field in the {dollar}1\over 8{dollar} phase were measured. The experimental results can be well explained by the phenomenological Landau theory of phase transitions.;Low-frequency Raman-scattering experiments on tetramethylammonium tetra-chlorozincate crystals have been performed in the temperature range from 310 K to 80 K, which includes the five successive phase transitions. As the temperature decreased, an 11 {dollar}cm\sp{lcub}-1{rcub}{dollar} mode appeared in the (aa) scattering geometry in the incommensurate phase II and lock-in phases III and IV, which have modulation wavevectors {dollar}{lcub}\bf q{rcub}\sb0=({lcub}2\over 5{rcub}+\delta){lcub}\bf a{rcub}*{dollar}, {dollar}{lcub}2\over 5{rcub}{dollar}a*, and {dollar}{lcub}1\over 3{rcub}{dollar}a*, respectively, disappeared in phase V (q{dollar}\sb0{dollar} = 0), and reappeared in phase VI (q{dollar}\sb0={lcub}1\over 3{rcub}{dollar}a*). The temperature dependence of the observed frequency shift demonstrates that this mode is the folded {dollar}\Sigma\sb3{dollar} acoustic mode scattering via a second-order process. A weak 15 {dollar}cm\sp{lcub}-1{rcub}{dollar} mode observed in phase VI can be interpreted as the folded {dollar}\Sigma\sb4{dollar} acoustic mode scattering via a third-order process.
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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.
