Electrophysical Properties of Methylammonium Alum Single Crystals: The Role of the Hydrogen Bond Network in the Formation of Temperature Anomalies

Abstract

Pyroelectric, dielectric properties, and thermodepolarization currents of methylammonium alum crystallohydrate single crystals were studied in temperature range 120–200 K. Temperature dependences of these quantities demonstrated peaks at ∼143, ∼160, and 176.4 K (Curie point). The behavior, which is typical to many crystals with a network of hydrogen bonds with an O–H...O distance of ∼2.7 ± 0.1 ˚A, is considered from the point of view of a quantum chemical model of a hydrogen bond network containing water clusters in addition to individual H2O molecules. The movement of bridging protons in molecular clusters in electric fields is coherent and correlated, and all bridging protons of the system are involved in it. These displacements are associated with oscillations of the oxygen frame of the compression-tension type. The discreteness of the temperature peaks of the electrophysical properties is associated with the discreteness of the energy levels of the double-well potential, caused by the splitting of the excited levels.