Raman evidence for thermally disordered energy states in various phases of ionic nitrates

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Abstract
Detailed Raman studies of the symmetric stretching mode of a large number of nitrate salts have confirmed the presence of an anomalous second component not predicted by factor group analyses based on known crystal structures. Depolarization studies with oriented single crystals, decoupling studies with 18O‐enriched samples, and temperature variation studies support previous suggestions that the new components are due to the presence of alternate sites which result from a dynamic disorder within the lattice. The alternate sites are present even for structures normally considered to be completely ordered and do not destroy the long‐range interactions (correlation field coupling) of the ordered part of the lattice. Phase transitions may be precipitated by the increase in these impurity type disordered states until the long‐range coupling of the ordered lattice can no longer be maintained.