Comprehensive study of the structural, microstructural, and electrical properties of RbZnPO4: insights into conduction mechanisms and the OLPT models

The structural, microstructural, compositional, vibrational, and electrical properties of the rubidium zinc orthophosphate RbZnPO₄ compound have been comprehensively investigated. X-Ray powder diffraction (XRPD) confirmed the crystallization of RbZnPO₄ in a monoclinic system (space group P2₁), adopting a stuffed tridymite-type structure. Elemental analysis via energy-dispersive X-ray spectroscopy (EDS) confirmed the expected stoichiometry and homogeneous elemental distribution, while scanning electron microscopy (SEM) revealed a dense microstructure with submicron grain sizes (∼0.4205 μm). Raman spectroscopy identified internal modes and external vibrational modes of the phosphate [PO₄]³⁻ units, confirming the structural integrity of the phosphate framework. Impedance spectroscopy highlighted the semiconducting behavior of the RbZnPO₄ compound, with grain and grain boundary contributions effectively modeled using an equivalent circuit (R₁//CPE₁) + (R₂//CPE₂), where R and CPE represent the resistance and the Constant Phase Element, respectively. Temperature-dependent measurements revealed thermally activated conduction, characterized by negative temperature coefficient of resistance (NTCR) behavior. Activation energies for grains, grain boundaries, and total conduction were determined as 0.775, 1.173, and 0.581 eV, respectively. AC conductivity analyses further indicated frequency-dependent transport, consistent with the Overlapping Large Polaron Tunneling (OLPT) mechanism. The conduction mechanism has been thoroughly studied and well understood. These results demonstrate that RbZnPO₄ is a chemically stable, structurally well-defined, and electrically active phosphate, suitable for potential applications in thermally activated ionic or electronic conduction systems, such as sensors.