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Raman study of NaClO3 as a function of temperature into the melt and the novel high temperature phase

Raman spectra have been measured for oriented single crystals of NaClO3 II from 77 K to the melting temperature, for the molten salt and the high temperature phase, NaClO3 I. The results for NaClO3 II were in excellent agreement with unit-cell group analysis. Apparent anomalies associated with the presence of weak bands due to naturally abundant 37Cl and 18O substituted ions have been accounted for on the basis of intermolecular coupling models. The unusual high temperature, ambient pressure phase transition for NaClO3 has been characterised by spectroscopic and thermal methods. The novel feature about the high temperature phase is the fact that it may only be reached by cooling the melt and it transforms back to the room temperature phase at about 510 K as the sample is cooled. The longitudinal optical mode at about 1030 cm-1 which was characteristic of the acentric room temperature phase (P213, T4) was conspicuously absent in the Raman spectrum of the high temperature phase. The Raman spectrum of the high temperature phase indicated that the crystal has a centric structure but the similarity of many bands with the spectrum of the room temperature phase suggested that the phase transition involved only small changes in atomic positions. The structure remained ordered but there was evidence of considerable band broadening due to increased thermal motion. The Raman results are consistent with the diffraction studies of Meyer and Gasperin (1973) who reported a monoclinic structure similar to room temperature KClO3, space group P21/a. Measurements of the heats of fusion and transition by differential scanning calorimetry gave values of Delta Hf=21.7 kJ mol-1, Delta Sf=40.6 J mol-1, Delta Ht=4.1 kJ mol-1, Delta St=8.2 J mol-1 K-1. The relatively small values of the heat and entropy of transition compared to those of fusion also indicated that the high temperature phase was not disordered.