Investigation of lattice dynamics, magnetism and electronic transport in $\beta$-Na$_{0.33}$V$_2$O$_{5}$

TL;DR

This study explores the lattice dynamics, magnetism, and electronic transport in $eta$-Na$_{0.33}$V$_2$O$_5$ using various spectroscopic and magnetic measurements, revealing spin-phonon coupling, mixed-valence V ions, and semiconducting behavior.

Contribution

It provides a comprehensive analysis of the interplay between lattice vibrations, magnetic properties, and electronic transport in $eta$-Na$_{0.33}$V$_2$O$_5$, highlighting spin-phonon coupling and mixed-valence states.

Findings

Phonon frequencies show anharmonic behavior with temperature.

Spin-phonon coupling causes phonon hardening below 40 K.

V ions exhibit a mixed-valence state with V$^{4+}$ and V$^{5+}$ proportions.

Abstract

We investigate the electronic and magnetic properties as well as lattice dynamics and spin-phonon coupling of $\beta$-Na$_{0.33}$V$_2$O$_5$ using temperature-dependent Raman scattering, dc-magnetization and dc-resistivity, x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of XRD pattern with space group C2/m confirms the monoclinic structure. The analysis of temperature-dependent Raman spectra in a temperature range of 13--673~K reveals an anharmonic dependence of the phonon frequency and full width at half maximum, which is accredited to the symmetric phonon decay. However, below about 40 K, the hardening of the phonon frequency beyond anharmonicity is attributed to the spin-phonon coupling. Interestingly, the estimated effective magnetic moment $\mu_{\rm eff}=$ 0.63~$\mu_B$ from the magnetization data manifests a mixed-valence state of V ions in 4+ (18$\pm$1\%) and 5+ (82$\pm$1\%). A similar ratio of V$^{4+}$ to V$^{5+}$ is also observed in the x-ray photoemission and x-ray absorption near-edge spectra and that is found to be consistent with the sample stoichiometry. In addition, the V$^{4+}$ ions are distributed between different vanadium (V1 and V3) sites. The analysis of extended x-ray absorption fine structure at different V-sites gives the corresponding V--O bond lengths, which are utilized in the assignment of Raman modes. Moreover, the temperature-dependent resistivity resembles a semiconducting behavior where the charge carrier transport is facilitated by the band conduction at higher temperatures and via hopping $\le$260~K.