Vibrational and Electronic Properties of Np2O5 from Experimental Spectroscopy and First Principles Calculations

TL;DR

This paper tackles the limited understanding of high-valence actinide oxides by focusing on neptunium pentoxide, $Np_2O_5$. It introduces a combined experimental approach—high-resolution Raman spectroscopy and scanning tunneling spectroscopy (STS)—with first-principles DFT+$U$+SOC calculations to link lattice dynamics to electronic structure. Key findings include dominant Raman features at 567 and 782 cm$^{-1}$ and an STS band gap of $1.5$ eV, with DFT predicting an indirect gap of $1.68$ eV and a direct gap of $1.81$ eV, and $5f$-electron states dominating near the band edges. Overall, the work provides a benchmark dataset and demonstrates a robust framework for connecting lattice dynamics and electronic structure in actinide oxides, enabling future exploration of external controls on $5f$-electron behavior.

Abstract

High-valence actinide oxides are critical to understanding the behavior of 5f-electrons, yet their structural and electronic properties remain poorly understood due to challenges in synthesis and handling. We report the first Raman spectroscopic study of single-crystalline Np2O5 and the first scanning tunneling spectroscopy (STS) measurement on any neptunium-containing material. Hydrothermally synthesized crystals were structurally verified by X-ray diffraction. Raman spectra revealed sharply resolved vibrational features, including previously unreported low-frequency modes. STS measurements revealed a band gap of 1.5 eV. Density functional theory (DFT) enables vibrational mode assignments, reveals neptunium-dominated low-frequency phonons, oxygen-dominated high-frequency modes, and predicts an indirect band gap of 1.68 eV. This predicted value is in excellent agreement with the experimentally measured STS gap. This combined Raman, DFT, and STS approach provides a robust framework for correlating lattice dynamics and electronic structure in actinide materials, providing benchmark data for Np2O5, and opening new avenues for probing structure-property relationships in complex f-electron materials.

Figures (5)

• Figure 1: XRD pattern of Np$_{2}$O$_{5}$ collected using a Mo-source at room temperature together with the Rietveld refinement fit (red line) and Bragg positions (vertical blue lines).
• Figure 2: Experimental and computed (using LDA+$U$+SOC ($U=3$ eV)) Raman spectra of Np$_2$O$_5$.
• Figure 3: Computed phonon vibrational modes correspond to Raman peaks (a) 257 cm$^{-1}$, (b) 567 cm$^{-1}$, and (c) 782 cm$^{-1}$. Grey (large) and red (small) spheres, respectively, represent neptunium and oxygen atoms, and the neptunium plane is represented by the green plane. The phonon eigenvectors are indicated by the black arrows. Two side views, both within and outside the neptunium plane, are also provided. (b) and (c) represent the two major Raman peaks, where both symmetric bending and stretching vibrations exist in the out-of-plane O-Np-O bonds, with the bending vibrations having a larger contribution.
• Figure 4: Computed (a) electronic band structure and DOS and (b) phonon dispersion and DOS of Np$_2$O$_5$ along the high symmetry path, using LDA+$U$+SOC ($U=3$ eV).
• Figure 5: Representative room temperature STS measurements of Np$_2$O$_5$. a) Density of states (dI/dV) directly measured using the lock-in technique. b) I(V) curves recorded concurrently with the measurements taken in a).