Antisite defects and valence state of vanadium in Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$

Electron paramagnetic resonance (EPR) studies have been performed with the aim of determining the valence state and local crystal structure of the nearest environment of vanadium ions in the initial, charged, and discharged samples of the cathode material Na$_{x}$V$_{2}$(PO$_{4}$)$_{3}$ (1 $\le x\le$ 3). It has been found that the charged sample ($x$ = 1) is characterized by an intense signal corresponding to V$^{4+}$ ions located in a highly distorted octahedral crystal field. An EPR signal with the $g$-factor close to the $g$-factor of the V$^{4+}$ ion has also been observed in the initial sample ($x$ = 3), where the intensity of the resonance signal is one order of magnitude lower than that in the charged sample. It has been revealed that the resonance signal under consideration is associated with the formation of antisite defects when a part of vanadium ions are located in sites of sodium ions. It has also been found that the intensity of this signal increases after a complete charge–discharge cycle ($x$ = 3).