AN INFRARED SPECTROSCOPIC STUDY OF THE INTERNAL-MODES OF SODIUM-NITRATE - IMPLICATIONS FOR THE STRUCTURAL PHASE-TRANSITION

The temperature dependence of the infrared-active internal modes of NaNO3 has been investigated between 77 K and 700 K. The structural phase transition at 549 K is observed through the appearance of additional peaks subsidiary to the expected nitrate group internal modes. In particular, an extra peak is observed at slightly lower energies to the v2 mode at 830 cm-1, increasing dramatically in intensity at temperatures above Tc at the expense of the v2 mode. An interpretation of these results is presented in terms of the interaction of an additional structural element (which becomes relevant close to Tc) described by the order parameter P, causing a deviation from tricritical behaviour and leading to order parameter coupling in the Landau potential with Q, where Q is the driving order parameter which includes the orientational order-disorder part of the transition. It is further suggested that aplanarity of the nitrate groups plays a major part in the phase transition.