Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO2

TL;DR

This study visualizes hydrogen diffusion in VO2 single crystals, revealing one-dimensional diffusion, phase segregation, and nonlinear behavior, providing new insights into hydrogen behavior in complex materials.

Contribution

It introduces a novel optical method to monitor hydrogen diffusion in VO2, uncovering highly nonlinear diffusion and spontaneous segregation phenomena.

Findings

Hydrogen diffuses along the rutile c-axis only

Diffusion coefficient is ~0.01 e^(-0.6 eV/k_B T) cm^2/sec

Hydrogen segregation into stripes occurs spontaneously

Abstract

Hydrogen intercalation in solids is common, complicated, and very difficult to monitor. In a new approach to the problem, we have studied the profile of hydrogen diffusion in single-crystal nanobeams and plates of VO2, exploiting the fact that hydrogen doping in this material leads to visible darkening near room temperature connected with the metal-insulator transition at 65 {\deg}C. We observe hydrogen diffusion along the rutile c-axis but not perpendicular to it, making this a highly one-dimensional diffusion system. We obtain an activated diffusion coefficient, ~0.01 e^(-0.6 eV/k_B T) cm2sec-1, applicable in metallic phase. In addition, we observe dramatic supercooling of the hydrogen-induced metallic phase and spontaneous segregation of the hydrogen into stripes implying that the diffusion process is highly nonlinear, even in the absence of defects. Similar complications may occur in hydrogen motion in other materials but are not revealed by conventional measurement techniques.