Raman spectroscopic studies of phase equilibria in binary monovalent metal nitrates

Xu, Kangcheng (1997) Raman spectroscopic studies of phase equilibria in binary monovalent metal nitrates. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Raman spectroscopy has been developed as a complementary tool to X-ray and thermal methods for phase equilibrium studies of new materials. Raman measurements for mixtures as a function of mole fraction can give the composition for congruently and incongruently melting compounds. Stable and metastable compounds may be identified by comparison to the characteristic spectra obtained from the rapidly quenched or slowly cooled samples from the melts. The presence of longitudinal optic ( LO ) modes in the Raman spectrum provides a method to identify non-linear crystals. Temperature induced phase transitions can be followed by the measurement of the Raman spectra as a function of temperature. Kinetic information may be obtained for phase transitions with slow conversion. -- For the first time Raman spectroscopy has been applied to systematically investigate the structural phase transitions in binary nitrate systems. The binary nitrate systems studied in the present work are KNO3-RbNO3 , NaNO3-KNO3 , LiNO3-MNO3 ( M = K, Rb, Cs ), and AgNO3-MNO3 ( M = K, Rb, Cs ). -- Raman studies of the KNO3-RbNO3 system indicated that rubidium ion may substitute for potassium ion up to 67 mol% in the KNO3 II crystal and up to 80 mol% in the KNO3 III crystal. The substitutional crystals retained the transitions of phase I to III to II of KNO3 on cooling and the metastable property of KNO3 III at room temperature. Chemical substitution of this type was used for a lattice dynamical analysis of potassium nitrate to assign the bands in the external vibrational region. The results supported the assignment of the 52 cm-1 band in KNO3 II to a mode primarily due to translatory motion of cations against anions. A transition of the KNO3 III structure to the RbNO3 IV structure was observed for crystals containing 80 and 90 mol% RbNO3. The R3m microscopic structure in the KNO3 III solid solutions seemed to become disordered when Rb ̄ was more than 50 mol% in the substitutional crystal. These structural details offer a possible explanation for the abnormal solid solubility in the KN03-RbN03 system. -- Raman spectra were measured for mixed crystals of NaNO3-KNO3 quenched from different temperatures. Temperature dependent Raman spectra were measured to actually follow the phase transitions. The Raman studies indicated that there existed limited solid solutions Nal1-xKxNO3 (NaII) and (KIll) as well as the R3m solid solution and there existed structural phase transitions of the R3m solid solution to these limited solid solutions in addition to exsolution of the components from the R3m solid solution. It was suggested that two vertical lines be added to the published phase diagram, one to indicate the solid solution Na0.95K0.05NO3 (NaII) and the other to indicate the solid solution N0.15K0.85NO3 (KIll). Three slightly different structures were found for the R3m solid solution below the solidus: in the NaN03 terminal phase the solid solution behaved like NaNO3 I, in the KNO3 terminal phase the solid solution behaved like KNO3 I, in the intermediate concentration phase the solid solution was in a new disordered state. The new disordered state was quenchable and the Raman studies suggested that it was caused by phase separation of the NaNO3 I and KNO3 I structures on a microscopic or submicroscopic scale. The existence of the three slightly different structures suggested that NaNO3-KNO3 was a limited solid solution system rather than a continuous solid solution system. -- Raman studies of the LiNO3-KNO3 system revealed a new compound KLi(NO3)2 which could exist in a narrow temperature range just below the solidus and in the vicinity of equimolar point. The existence of the new compound suggested that LiNO3-KNO3 is a simple syntectic system with a congruently melting compound instead of a eutectic system. -- The fact that the congruently melting compounds KLi(NO3)2, RbLi(NO3)2 and CsLi(NO3), had very similar Raman spectra in both external and internal vibrational regions suggested that they have the same structure. A detailed analysis of the Raman spectra revealed that in the crystals the lithium ion was tetrahedrally coordinated by the nitrate ions to form polymeric complexes in which part of the nitrate ions acted as unidentate ligands and the other part acted as bidentate ligands. The heavier alkali metal ions acted as counterions. -- Raman studies of the compounds KAg(NO3)2 and RbAg(NO3)2 were consistent with the published crystal structure. There existed polymeric complexes of silver and nitrate ions and the nitrate ions acted as both unidentate and bidentate ligands. Potassium or rubidium ions acted as counterions. Raman studies suggested that the Li-NO3 complexes in KLi(NO3)2, RbLi(NO3)2 and CsLi(NO3)2 were similar to the Ag-NO3 complexes in KAg(NO3)2 and RbAg(NO3)2. However, the nature of the bond in the lithium complexes was predominately ionic while the nature of the bond in the silver complexes was considerably covalent. -- Judged by the notable differences in Raman spectra, the coordination geometry of Ag-NO3 changed dramatically in CsAg(NO3)2. The weak Rayleigh wing and a well separated band at 224 cm-1 suggested that the polymeric complexes in KAg(NO3)2 may have broken down to form simple and discrete entity in CsAg(NO3)2.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/1631
Item ID: 1631
Additional Information: Bibliography: leaves 238-246
Department(s): Science, Faculty of > Chemistry
Date: 1997
Date Type: Submission
Library of Congress Subject Heading: Raman spectroscopy; Nitrates--Spectra;Binary systems (Metallurgy); Phase transformations (Statistical physics)

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