Raman spectra of polycrystalline lithium, sodium and potassium sulfates in the pretransition temperature region below the structural phase transition

Molecular relaxation processes in lithium sulfate (Li$_{2}$SO$_{4}$), sodium sulfate (Na$_{2}$SO$_{4}$), and potassium sulfate (K$_{2}$SO$_{4}$) are studied using Raman spectroscopy (RS). The order parameter decreases in the low-temperature phase as it approaches the phase transition. For example, this is typical of a first-order phase transition close to a second-order phase transition. The existence of the pretransition region in the investigated sulfates Li$_{2}$SO$_{4}$, Na$_{2}$SO$_{4}$, and K$_{2}$SO$_{4}$ is proven.