Raman scattering of vanadium ladder compounds from first principles

Abstract

We investigate phonon Raman scattering of the Ag modes in NaV2O5 and CaV2O5. We perform firstprinciples calculations within density functional theory (DFT) to obtain the ground state with relaxed atomic positions. Thereupon we determine eigenvectors and frequencies of the 8 Ag modes within the frozen phonon approach and compute the dielectric functions ε(ω) for structures distorted according to the phonon eigenvectors. Raman intensities are obtained using the derivatives of ε(ω) with respect to the normal coordinates Q taken at the incident photon energy ωI . For NaV2O5 the peak positions and the relative intensities of the theoretical results for the experimentally used frequency ωI = 2.5 eV excellently agree with measured data. Moreover, for the three different scattering geometries a strong dependency of the Raman intensity on ωI is revealed. IntroductionThe vanadium-based ladder compounds NaV 2 O 5 and CaV 2 O 5 are interesting examples of structures where charge, spin and lattice degrees of freedom strongly interact. NaV 2 O 5 and CaV 2 O 5 are iso-structural (space group P mmn), and the main building block of both compounds is a simple ladder formed by V-O rungs and V-O legs (Fig. 1), enabling electrons to hop within a rung and between the rungs of one ladder, whereas the hopping probability between neighboring ladders is considerably smaller. The half filled CaV 2 O 5 has one electron per V site while NaV 2 O 5 is a quarter filled structure with one electron equally distributed among the two V +4.5 sites of one rung [1]. At a temperature of T c = 35 K, NaV 2 O 5 undergoes a spin-Peierls like transition [2] accompanied by charge ordering and a large lattice distortion. It has been reported [3] that phonons induce strong charge fluctuations near T c that lead to fluctuations in the spin-spin exchange. Thus the investigation of lattice dynamics and electron (spin) -phonon coupling in vanadium ladder structures is a very promising instrument to understand their physical properties.Raman scattering is a particularly useful tool to get insight into both, phonons and electronic structure, and provides information about low-energy excitations and their coupling to light. For NaV 2 O 5 , the phonon Raman spectra demonstrate very pronounced changes when going below T c [4] such as variations in the electronic background [5] and the appearance of intense new Raman modes indicating a strong coupling of the lattice to the transition order parameter [3,4]. With the aim to understand similarities and differences in the optical properties of NaV 2 O 5 and CaV 2 O 5 we perform ab initio investigations within density functional theory (DFT). We calculate the fully symmetric A g phonon modes and their influence on the dielectric function. Such phonon-induced modulations of the polarizability are a measure for electronphonon coupling and the source of phonon Raman scattering of light [6].

Vibrational spectra of transition element compounds

“…184 Several other Raman spectroscopic studies have been made on VO x materials. 200 The Raman spectra of lithium-intercalated V 2 O 5 thin films show the formation of dand g-phases of Li x V 2 O 5 . 188 FTIR and Raman spectra were used to characterise vanadium oxide nanotubes.…”