Comparitive study of structure changes with temperature in univalent and divalent ionic nitrate crystals

• Published in: The Journal of physics and chemistry of solids 1980, Vol.41 (10), p.1127-1131
• Main Authors: Badr, Y.A., Kamel, R.
• Format: Article
• Language: English
• ISSN: 0022-3697; 1879-2553
• DOI: 10.1016/0022-3697(80)90069-4

The temperature dependence of the d.c. resistivity and dielectric constant of polycrystalline samples of Pb(NO 3) 2, Ba(NO 3) 2 and Sr(NO 3) 2 were found to show anomalies in the high temperature region. In addition, sharp peaks were obtained in both differential thermal analysis (DTA) and thermomechanical analysis (TMA) curves in the same temperature range. The observed anomalies were attributed to orientational disorder of the nitrate group leading to an order-disorder phase transition which is reported here for the first time. A comparison is made between the role of NO 3 −ions in both the divalent and univalent nitrates, taking NaNO 3 as representative. The TMA curve for Sr(NO 3) 2 showed a pronounced peak at 325°C. This peak was related to a sudden increase in the expansion coefficient associated with the rotation of the NO 3 − group leading to a solid state phase transformation. Energy diagrams describing the conduction mechanism and showing a fractionization of energy barriers in the case of divalent nitrates are introduced.