Thesis

An EPR Study of Lithium Sulfate Monohydrate

May 1984

Thesis for: MS
Advisor: CE Bailey

Authors:

University of Puerto Rico System

Many of the sulfates have structural phase transitions at various temperatures as well as non-linear ferroelectric and/or piezoelectric characteristics. To understand the differences between the various sulfates and the reasons for the different properties.

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Article

An EPR study of the disorder and structural phase transitions of lithium potassium sulfate

December 1986 · Journal of Physics and Chemistry of Solids

The EPR of NH+3 free radicals has been used to probe the positional disorder and low-temperature structural phase transitions of LiKSO4 crystals. Anomalies at 215 K and at 190 K (on cooling) in the EPR spectrum are observed. The EPR results suggest that there is disorder in the atomic positions of the sulfate groups, and that the disorder has aggregated into small-grained crystal domains.

Article

EPR study of copper ions in Li2Ge7O15 crystals

August 2008 · Physics of the Solid State

The EPR spectra of Cu2+ ions (2D 5/2) located at two structurally nonequivalent positions Cu1 and Cu2 in crystals of lithium heptagermanate Li2Ge7O15 are recorded. The angular dependences of the EPR spectrum are measured in the paraelectric phase of the Li2Ge7O15 compound (T = 300 K). The components of the g factor and the hyperfine interaction tensor A are determined, and the orientation of the ... [Show full abstract] magnetic axes with respect to the crystallographic basis is established. The EPR spectra are recorded in the temperature range in the vicinity of the temperature T C = 283 K of the transition from the paraelectric phase to the ferroelectric phase. The position symmetry of the Cu1 and Cu2 centers is determined at temperatures above and below the phase transition temperature T C . The localization of paramagnetic centers in the structure is discussed, An analysis of the results obtained demonstrates that the Cu1 and Cu2 centers in the Li2Ge7O15 crystal lattice replace lithium ions located at two structurally nonequivalent positions with the symmetries described at temperatures above T C by the triclinic C i and monoclinic C 2 point groups, respectively.

Article

EPR studies of the low-temperature phase transition in LiKSO4 by SO4- and CRO43- centres

January 1987 · physica status solidi (a)

Lithium potassium sulfate (LPS) has a hexagonal symmetry. The aim of this note is the simultaneous study of two kinds of paramagnetic defects, i.e. SO4- and CrO43- in LPS in order to obtain information about the influence of defects on the low-temperature phase transitions. LPS crystals were grown isothermally at 315 K by the dynamical method from aqueous solutions containing the initial salts at ... [Show full abstract] stoichiometric ratio with 2 wt% of K2CrO4. LPS samples were heated during 24 h at 700 K and then were cooled down to room temperature and irradiated with 40 kV X-rays for 3 to 5 h. Two types of radicals were found from EPR studies and invesgigated in the temperature range 150 to 300 K in cooling as well as heating processes.

Article

EPR study of kinetic properties of a weak polar ferroelectric Li 2 Ge 7 O 15

January 2000 · Crystallography Reports

The EPR spectra of Mn2+ ions have been studied in the vicinity of the phase transition to the ferroelectric phase in lithium heptagermanate crystals. The anomalous broadening of the resonance lines observed in the vicinity of the transition temperature T C was attributed to an increase of the contribution of low-frequency (ω 7 Hz) fluctuations of the local order parameter. The critical index of ... [Show full abstract] the correlation length is determined to be ν ≈ 0.63, which indicates the Ising-type critical behavior of the crystal. The experimental data obtained in the vicinity of T C were interpreted with due regard for long-range dipole interactions. According to our estimates, the forces of dipole interactions are prevalent only in the close vicinity of the transition point, T-T C ≪ 0.1 K.

Article

Critical Properties of Weak Ferroelectric LGO Crystals Doped with Cr Ions

August 2005 · Ferroelectrics

Mikhail Trubitsyn

Angular diagrams of Cr EPR spectra in Li2Ge7O15 (LGO) crystals confirm that magnetic ions substitute for Ge and together with interstitial lithium ions form Cr −Li paired centers. On approaching ferroelectric phase transition point EPR lines of Cr broaden and split below TC = 283.4 K. Temperature dependence of the local order parameter η(T) determined from EPR lines splitting, shows power like ... [Show full abstract] behavior with critical exponent β = 0.31 of three-dimensional Izing model. Results of previous investigations evidence that on going away from TC the crossover from fluctuation regime to classical mean field one is characteristic for macroscopic and local LGO properties. For dependence η(T) in LGO:Cr such crossover is not detected. Absence of the crossover to classical mean field behavior is attributed to the defect nature of Cr−Li centers which weaken the order parameter correlations. Influence of Cr−Li centers on LGO properties is discussed on the basis of microscopic model of structural phase transitions.

Article

EPR of the ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals

June 1998 · Physics of the Solid State

A Cr3+ EPR study of lithium heptagermanate crystals, Li2Ge7O15 (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to ... [Show full abstract] occur below T C as a result of emergence of two structurally nonequivalent positions for Cr3+ ions.

Article

Ferroelectric Semiconductor SnTe: Mn EPR as a Probe of Phase Transitions

November 1979 · Journal of the Physical Society of Japan

We have investigated the EPR linewidth \varDelta H of Mn in a narrow-gap semiconductor, MnxSn1-xTe, at 9.2 GHz from 4.2 K to 400 K. An anomalous behavior of \varDelta H as a function of temperature was found around 75 K. We attribute this anomaly to the ferroelectric phase transition. The transition temperature obtained from our EPR study is consistent with the existing theory and resistivity ... [Show full abstract] data.

Article

An EPR study of local fluctuations in the vicinity of the ferroelectric phase transition in Li2Ge7O1...

January 1998 · Physics of the Solid State

EPR spectra of Mn2+ ions have been studied in the temperature interval containing the ferroelectric transition in crystalline lithium heptagermanate Li2Ge7O15. The EPR linewidth and shape were measured as functions of temperature and orientation. It is shown that the critical resonance-line broadening observed to occur in the vicinity of the phase transition is due to fluctuations in the local ... [Show full abstract] order parameter with frequencies below 107 Hz.

An EPR Study of Lithium Sulfate Monohydrate

Abstract

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