Features of Structures, Ionic Conductivity and Phase Transitions of Polycrystals Na3Fe2(1-х)Cr2х(PO4)3 (0≤х≤0.12)

Crystal structures, ionic conductivities and phase transitions a→b, b→c of polycrystals in the system Na3Fe2(1-х)Cr2х(PO4)3 (0≤x≤0.12) were studied. It has been established that samples of solid solutions Na3Fe2(1-х)Cr2х(PO4)3 (0≤x≤0.12) are monoclinically distorted (space group P 21/n), and as the concentration of dopants increases, the structural parameters of the samples are linear are decreasing. In this case, a slight increase in the conductivity of solid solution samples in the a-phase and a decrease in the b- and c-phases are observed, and a decrease in the temperatures Тa→b and Тb→c occurs as the concentration of dopants increases. It is concluded that the indicated changes in the parameters of crystal structures, conductivity and temperatures Тa→b and Тb→c solid solutions are associated with local «compression » deformations and a violation of the regularity of the anion part of the crystal structure {[Fe2(1-х)Cr2х(PO4)]3-}3∞, which in turn determine the nature of the mobility of the cationic part. The results of doping iron cations with chromium atoms into the Na3Fe2(PO4)3 structure make it possible to establish the mechanism for increasing ionic conductivity and indicate the «elasticity» of the crystal structure of {[Fe2(1-х)Cr2х(PO4)]3-}3∞ solid solutions. Therefore, it becomes possible to search for new solid solutions with higher conductivity by substituting Fe→M in the solid solution system Na3Fe2(1-х)М2х(PO4)3, where M is the substitution cation.