Na-ion Dynamics in the Solid Solution Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$ Studied by Muon Spin Rotation and Neutron Diffraction

In this work we present systematic set of measurements carried out by muon spin rotation/relaxation ($\mu^+$SR) and neutron powder diffraction (NPD) on the solid solution Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$. This study investigates Na-ion dynamics in the quasi-1D (Q1D) diffusion channels created by the honeycomb-like arrangement of CrO$_6$ octahedra, in the presence of defects introduced by Ca doping. With increasing Ca content, the size of the diffusion channels is enlarged, however, this effect does not enhance the Na ion mobility. Instead the overall diffusivity is hampered by the local defects and the Na hopping probability is lowered. The diffusion mechanism in Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$ was found to be interstitial and the activation energy as well as diffusion coefficient were determined for all the members of the solid solution.