Raman studies of the cadmium chloride-alkali metal chlorides, lead chloride-cesium chloride and lithium carbonate-cesium carbonate systems

Wang, Jianfang (1992) Raman studies of the cadmium chloride-alkali metal chlorides, lead chloride-cesium chloride and lithium carbonate-cesium carbonate systems. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Raman studies have been performed on following systems: CdCl₂/CsCl and CdCl₂/RbCl solids and melts; CdCl₂/KCl solids; PbCI₂/CsCl solids and melts; Li₂CO₃/Cs₂CO₃ solids: and Li₂CO₃/LiCl and Li₂CO₃/K₂CO₃ melts. The phase diagrams suggested by this work are in excellent agreement with the literature and the framework of the phase diagram for the Li₂CO₃/Cs₂CO₃ binary system has been reported for the first time. -- The Raman spectra of the compounds with known structures have been discussed in terms of factor group analysis. It has been possible to characterize the Raman spectra for the discrete octahedrally coordinated cadmium, CdCl⁴⁻₆ in Rb₄CdCl₆ and K₄CdCl₆ and the discrete tetrahedrally coordinated CdCl₄²⁻ in Cs₃CdCl₅ and the high temperature phase of Cs₂CdCl₄. The Raman studies of the solids have provided useful reference frequencies to assist the analysis of the Raman studies of the molten salts. -- In the CdCl₂/CsCl binary system, a total of five compounds have been identified, three congruent: Cs₃CdCl₅, Cs₂CdCl₄, CsCdCl₃, and two incongruent: Cs₃Cd₂Cl₇ and CsCd₅Cl₁₁. Cs₃CdCl₅ is isostructural with Cs₃CoCl₅ and thermodynamically stable only above 390°C. Below 390°C Cs₃CdCl₅ decomposes into a mixture of Cs₂CdCl₄ and CsCl, but the sample quenched to room temperature from the melt may give a metastable Cs₃CdCl₅. Cs₂CdCl₄ has the K₂NiF₄ structure in the LTP (low temperature phase), but the salt undergoes a SPT (structural phase transition) at about 435°C to a β-K₂SO₄ type HTP (high temperature phase). The transition takes place over a temperature range from 435°C up to just below the melting point. At any temperature within this temperature range Raman bands for each of the separate phases may be detected simultaneously with relative intensities that are dependent of the temperature. Relative intensity measurements of bands characteristic of each cadmium coordination as a function of temperature indicated an equilibrium distribution. The enthalpy and entropy associated with the change of coordination were of the order of those usually observed on fusion. These large values are consistent with the major structural rearrangement from a network octahedral structure to a discrete tetrahedral coordination. Cs₃Cd₂Cl₇ decomposes into CsCdC₃ and Cs₂CdCl₄ at temperatures above 450°C. Raman spectra suggest that the Cd atom has octahedral coordination within a network structure for solids Cs₃Cd₂Cl₇ and CsCd₅Cl₁₁. CsCdCl₃ is hexagonal and there is no SPT from 25°C to the melting point. -- For the CdCl₂/RbCl and CdCl₂/KCl solid systems, six compounds: Rb₄CdCl₆, Rb₂CdCl₄, Rb₃Cd₂Cl₇, Rb₄Cd₃Cl₁₀, RbCdCl₃ and RbCd₅Cl₁₁ in the RbCl system; and two compounds: K₄CdCl₆ and KCdCl₃ in the KCl system have been identified. Both K₄CdCl₆ and Rb₄CdCl₆ contain discrete octahedral CdCl⁴⁻₆ species. The other compounds have network structures with CdCl₆ local units and chloride bridges. Unlike KCdCl₃ and CsCdCl₃, the anhydrous RbCdCl₃ gave no intense peaks in the Cd-Cl stretching region of the Raman spectra and probably has an octahedral structure. When RbCdCl₃ was opened to the atmosphere it gave a different structure which is isostructural with NH₄CdCl₃ and KCdCl₃. The discrete tetrahedral CdCl₄²⁻ structure is favored by larger and more polarization alkali cations and higher temperature. -- In the CdCl₂/CsCL, CdCl₂/RbCl and PbCl₂/CsCl melts, the tetrahedral structure was indicated. As the Cl⁻/M²⁺ ≥ 4 (M²⁺ = Cd²⁺ and Pb²⁺ ) the discrete tetrahedral MCl₄²⁻ species were the principle species present, while as Cl⁻/M²⁺ < 4 short-lived tetrahedral units with bridge structures were suggested. Exchange processes were too fast to permit detection of separate peaks for the discrete tetrahedron and the bridge species. Raman spectra indicate that the bond strength between Pb²⁺ and Cl⁻ ions is much weaker than that between Cd²⁺ and Cl⁻ ions. -- For the Li₂CO₃/Cs₂CO₃ system, only a congruent compound LiCsCO₃ has been identified and the compound forms eutectic mixtures with Li₂CO₃ or Cs₂CO₃. This compound LiCsCO₃ undergoes a SPT at about 435°C. The quenched sample from the melt to room temperature may give a metastable HTP which is stable for a long time at room temperature or even 77K. Unlike the SPT in Cs₂CdCl₄, there is no dynamic equilibrium between the HTP and the LTR but the transition is kinetically sluggish. Raman analysis indicates that the SPT in LiCsCO₃ is associated with the rotation of CO²⁻ in the primitive unit cell. The carbonate ion in molten alkali metal carbonates was found to be relatively unperturbed by the cations.

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