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

TL;DR

This study investigates Na-ion mobility in Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ using muon spin rotation and neutron diffraction, revealing that Ca doping enlarges diffusion channels but hampers overall Na-ion diffusivity due to defects.

Contribution

It provides the first detailed analysis of Na-ion dynamics in this solid solution, highlighting the impact of Ca doping and defects on diffusion mechanisms.

Findings

Na-ion diffusion occurs via interstitial mechanism.

Ca doping enlarges diffusion channels but reduces Na mobility.

Diffusivity is limited by local defects despite channel size increase.

Abstract

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.