Phase coexistence and pressure-temperature phase evolution of ${\mathrm{VO}}_2\left(\mathrm{A}\right)$ nanorods near the semiconductor-semiconductor transition

A comprehensive understanding of the physical origins of the phase transition behaviors of transition metal oxides is still complex due to the interplay among competing interactions of comparable strengths tuning their nature. Widespread interest in such phase transitions has motivated explorations of nanocrystalline vanadium dioxide $\left({\mathrm{VO}}_2\right)$ in various forms and a long-running debate persists over the roles played by electron-electron correlation with lattice distortion. External stimuli like pressure and temperature have strong effects on the appearance, stability, and spacial distribution of the high-resistive (HR) and low-resistive (LR) phases accompanying their structural modification. Our comprehensive experiments establish the pressure-induced and thermally driven evolution of phase coexistence in ${\mathrm{VO}}_2\left(\mathrm{A}\right)$ nanorods. Our experimental evidence supports coexisting HR and LR phases, where compression suppressed coexistence at $\sim 7$ GPa, followed by a semiconductor-semiconductor transition at around $\sim 10$ GPa with the absence of pressure-induced metallization. X-ray diffraction revealed lattice distortion with local microscopic strain inhomogeneity in the nanorods, without any discontinuity in the pressure-volume data. We further investigated the vibrational modes and relaxations of the samples related to their thermal expansions. We also found that the pressure-dependent hierarchy of microstructural densification contributed significantly to the resulting transport properties.

Figure 1

(a) $P$ dependence of $R$ measured in the four-probe configuration (inset). (b) $I\text{−}V$ characteristics at different $P$. (c) $ln\left(V\right)\text{−}ln\left(I\right)$ data to focus on the inhomogeneous conduction mechanism with $P$ (see text). (d) Variations of total $R$ and $f_r$ with $P$ measured by ac impedance spectroscopy (solid and open symbols are for compression and decompression, respectively). (e) Pressure dependence of the band gap $\mathrm{\Phi }$ extracted from $R\text{−}P$ data. (f) Temperature-dependent $I\text{−}V$ characteristics show thermal hysteresis for $T\le T_C$ and vanish for $T>T_C$; inset shows the thermal reversibility of the nanorods.

Figure 2

(a) Temperature dependencies of $R$ where solid lines represent the model fit with Eq. (1). (b) Variation of the LR phase fraction $m_f$ at ambient pressure. Near the transition, $m_f$ changes drastically and is accompanied by significant changes in resistance.

Figure 3

(a) $P$ and $T$ dependencies of selective Raman mode ${\omega }_{v1}$. (b) Variation of ${\gamma }_{iT}$ with $T$ for four selective modes. (c) The implicit fraction $\eta ={\gamma }_{iT}/{\gamma }_{iP}$ for ${\omega }_{v1}$ and ${\omega }_{o1}$ with $T$ at 10 GPa. Variation of ${\gamma }_{iP}$ with $P$ (inset). (d) Crystal structures showing a reversible IMT with $T$. The crystallographic $\mathit{c}$ axis is along the $Z$ direction.

Figure 4

(a) XRD patterns for ${\mathrm{VO}}_2\left(\mathrm{A}\right)$ at a few selective pressure values, where the ambient phase is a tetragonal $\mathit{P}\mathit{4}/\mathit{ncc}$ structure. (b) SEM pictures were taken for decompressed samples; compression pressures are marked with arrows.

Figure 5

(a) $R$ vs $V/V_0$ shows the volume reduction related to the densification processes. The inset shows $P$ vs $V/V_0$ derived from XRD refinements. (b) In the $P\text{−}T$ phase diagram of ${\mathrm{VO}}_2\left(\mathrm{A}\right)$, the data from electrical transport, Raman spectroscopy, and XRD measurements are summarized. The high-pressure, low-resistive ${\mathrm{VO}}_2{\left(\mathrm{A}\right)}_{\mathrm{HP}}$ belongs to a HDPM form up to 20 GPa before PIA.