Thermal Decomposition and Possible Polymerization in KH₂AsO₄ Crystal at High Temperatures

Abstract

The high-temperature transformation of KH₂AsO₄ was studied by means of impedance spectroscopy, optical microscopy, differential thermal analysis, thermogravimetric methods. The dielectric constant showed a high-temperature anomaly around T_P=185°C. The complex impedance spectra near T_P were fitted by superposition of two Cole-Cole types of relaxations. The fast component is interpreted by proton migration in the bulk, while the slow component is accounted for as a cluster formation due to breaking and reforming of the hydrogen bond at the surface. Upon heating above 185°C (T_p), the dark spots appeared at the surface and the their pattern grew up with increasing temperature and time under isothermal condition above 185°C. The thermal transformation which appears around T_P=187°C is endothermic in addition to showing weight loss. Our results show evidences that the high-temperature transformation of KH₂AsO₄ near T_P is not a structural phase transition, but an onset of thermal decomposition of KH₂AsO₄ into K_nH₂As_nO_3n+1 [probably, n» 1, (KAsO₃)_n] at reaction sites at the surface of crystal.