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5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Epitaxial registry and transport properties in V4O7 thin films

We report the growth of V4O7 thin films deposited simultaneously on amorphous SiO2 and crystalline c-cut Al2O3 substrates. x-ray diffraction shows that films grown on SiO2 are polycrystalline with no preferred orientation, while films grown on Al2O3 exhibit a well-defined out-of-plane orientation and an in-plane registry consistent with epitaxial growth. Transmission electron microscopy confirms the crystallographic relationship between the V4O7 film and the Al2O3 substrate. Atomic force microscopy indicates substantially lower surface roughness for films grown on Al2O3 (~6 nm) compared to those grown on SiO2 (~22 nm), and Raman spectroscopy confirms stabilization of the same V4O7 phase on both substrates. Electrical transport measurements reveal a metal–insulator transition near 244 K for both substrates, with thermal hysteresis not exceeding ~1 K. Although the transition temperature remains essentially unchanged, films grown on Al2O3 exhibit higher conductivity over the entire temperature range, exceeding that of films grown on SiO2 by approximately two orders of magnitude at 100 K and by a factor of five at 300 K. These results indicate that the conductivity differences are consistent with variations in microstructural connectivity associated with crystallographic order.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Quadratic magnetoresistance across the insulator–metal transition in V4O7 thin films

We present a comparative magnetotransport study of V4O7 thin films grown simultaneously on single-crystal Al2O3 (r-cut) and amorphous SiO2 substrates, to examine substrate-induced effects on electronic transport across the insulator–metal transition (IMT). Temperature-dependent resistivity reveals a broad electronic ordering regime below the IMT, with both films exhibiting similar transition temperatures but differing by nearly an order of magnitude in absolute resistivity. Despite this strong substrate dependence of the resistivity scale, the magnetoresistance remains predominantly quadratic in magnetic field over wide temperature intervals for both substrates. Analysis of the quadratic magnetoresistance defines an effective mobility scale that evolves systematically across the IMT and exhibits a similar temperature dependence for the two films. Structural characterization, including atomic-resolution scanning transmission electron microscopy of the V4O7/Al2O3 interface and azimuthal X-ray diffraction measurements, reveals a sharp interface and well-defined in-plane crystallographic registry for the r-cut film, in contrast to the rotationally averaged in-plane structure expected for films grown on amorphous SiO2. These results indicate that while substrate-controlled structural registry modifies the resistivity scale, the quadratic magnetotransport response across the IMT remains largely robust, providing a consistent diagnostic of electronic transport in V4O7 thin films.

25 ENERGY STORAGE↗
DOE OSTI2020

Materials Data on V4O7 by Materials Project

V4O7 crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are four inequivalent V+3.50+ sites. In the first V+3.50+ site, V+3.50+ is bonded to six O2- atoms to form distorted VO6 octahedra that share corners with two equivalent VO6 octahedra, corners with two equivalent VO6 pentagonal pyramids, corners with two equivalent VO5 trigonal bipyramids, edges with two VO6 octahedra, an edgeedge with one VO5 trigonal bipyramid, and a faceface with one VO6 pentagonal pyramid. The corner-sharing octahedra tilt angles range from 19–20°. There are a spread of V–O bond distances ranging from 1.81–2.22 Å. In the second V+3.50+ site, V+3.50+ is bonded to six O2- atoms to form distorted VO6 octahedra that share corners with two equivalent VO6 octahedra, corners with two equivalent VO6 pentagonal pyramids, corners with three equivalent VO5 trigonal bipyramids, edges with two VO6 octahedra, and an edgeedge with one VO6 pentagonal pyramid. The corner-sharing octahedra tilt angles range from 19–20°. There are a spread of V–O bond distances ranging from 1.91–2.24 Å. In the third V+3.50+ site, V+3.50+ is bonded to five O2- atoms to form a mixture of distorted edge and corner-sharing VO5 trigonal bipyramids. The corner-sharing octahedra tilt angles range from 35–75°. There are a spread of V–O bond distances ranging from 1.68–2.04 Å. In the fourth V+3.50+ site, V+3.50+ is bonded to six O2- atoms to form distorted VO6 pentagonal pyramids that share corners with four VO6 octahedra, a cornercorner with one VO5 trigonal bipyramid, an edgeedge with one VO6 octahedra, an edgeedge with one VO6 pentagonal pyramid, edges with two equivalent VO5 trigonal bipyramids, and a faceface with one VO6 octahedra. The corner-sharing octahedra tilt angles range from 7–64°. There are a spread of V–O bond distances ranging from 1.98–2.13 Å. There are seven inequivalent O2- sites. In the first O2- site, O2- is bonded in a see-saw-like geometry to four V+3.50+ atoms. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to four V+3.50+ atoms. In the third O2- site, O2- is bonded in a T-shaped geometry to three V+3.50+ atoms. In the fourth O2- site, O2- is bonded in a 2-coordinate geometry to two V+3.50+ atoms. In the fifth O2- site, O2- is bonded in a 4-coordinate geometry to four V+3.50+ atoms. In the sixth O2- site, O2- is bonded in a distorted T-shaped geometry to three V+3.50+ atoms. In the seventh O2- site, O2- is bonded in a T-shaped geometry to three V+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on V4O7 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2026

Harnessing the Second-Order Metal−Insulator Transition for Neuromorphic Computing

Vanadium oxides are widely studied phase change materials for brain-inspired computing architectures. Systems like VO 2 and V 2 O 3 exhibit first-order metal−insulator transitions (MITs) with hysteresis and percolative switching, increasing stochasticity and device variability. Here, we focus on the less explored Magnéli phase V 4 O 7 , which undergoes a continuous, non-hysteretic, second-order MIT. This surprisingly enables highly reproducible volatile resistive switching in spiking-neuron-type devices. We synthesize V 4 O 7 films, characterize their structural and transport properties, and demonstrate voltage and current-driven threshold switching with electrothermal feedback. In a Pearson–Anson oscillator, V 4 O 7 devices produce stable, tunable spiking across 20–200 kHz, with consistent operation among multiple devices. We introduce a numerical analog leaky-integrate-and-fire (aLIF) model that captures waveform shapes and their dependence on load resistance, temperature, and voltage. Furthermore, these findings suggest that second-order MIT materials like V 4 O 7 are promising for deterministic, scalable spiking neuron arrays for neuromorphic computing.

V4O7↗