Phase transitions of Fe$_2$O$_3$ under laser shock compression

Abstract

We present in-situ x-ray diffraction and velocity measurements of Fe$_2$O$_3$ under laser shock compression at pressures between 38-116 GPa. None of the phases reported by static compression studies were observed. Instead, we observed an isostructural phase transition from $α$-Fe$_2$O$_3$ to a new $α^\prime$-Fe$_2$O$_3$ phase at a pressure of 50-62 GPa. The $α^\prime$-Fe$_2$O$_3$ phase differs from $α$-Fe$_2$O$_3$ by an 11% volume drop and a different unit cell compressibility. We further observed a two-wave structure in the velocity profile, which can be related to an intermediate regime where both $α$ and $α^\prime$ phases coexist. Density functional theory calculations with a Hubbard parameter indicate that the observed unit cell volume drop can be associated with a spin transition following a magnetic collapse.

Figures (5)

• Figure 1: Radially integrated x-ray diffraction profiles of $\text{Fe}_2\text{O}_3$ under shock for targets with a sapphire window between 0-122 GPa. The vertical dotted lines corresponds to the ambient $\alpha$-$\text{Fe}_2\text{O}_3$ peak positions blake_refinement_1966. Miller (hkl) indices are displayed for the low- and high-pressure $\alpha$ structures in black and red, respectively. The inset highlights differences between patterns (ii) and (iv).
• Figure 2: Le Bail refinements of two radially integrated x-ray diffraction patterns just after shock breakout, at pressures of 47(1) GPa and 94(2) GPa. Each pattern was fitted using two $\alpha$-$\text{Fe}_2\text{O}_3$ phases with $R\bar{3}c$ symmetry: one for the $\alpha$-$\text{Fe}_2\text{O}_3$ ambient at 0 GPa (bottom green tick marks) and one for the compressed $\alpha$-$\text{Fe}_2\text{O}_3$ phase (top green tick marks). $R_\mathrm{p}$ factors were 10.5% and 12.4% for the 47 GPa and 94 GPa patterns, respectively. Fitted lattice parameters are $a_0=5.03$ Å; $c_0=13.03$ Å for ambient $\alpha$-$\text{Fe}_2\text{O}_3$; $a=4.87(2)$ Å; $c=12.94(2)$ Å for 47 GPa; and $a=4.56(2)$ Å; $c=12.54(2)$ Å for 94 GPa.
• Figure 3: Particle velocities measured at the rear of the $\text{Fe}_2\text{O}_3$ sample for the different pressure regimes. The time origin is set to the shock breakout time in the sapphire. In black we show a low-pressure shock where only a single $\alpha$ phase is observed in x-ray diffraction. The blue curves, taken at intensities between 1.9 and 2.4 TW/cm$^2$, correspond to thermodynamic conditions where both $\alpha$ and $\alpha'$ are observed in diffraction. The red curve corresponds to conditions where only a single compressed $\alpha^\prime$ phase is observed. The double-wave structure is indicative of a phase transition.
• Figure 4: V/V$_0$ for $\alpha$-$\text{Fe}_2\text{O}_3$ and $\alpha^\prime$-$\text{Fe}_2\text{O}_3$ determined from x-ray diffraction as a function of pressure, compared with other published work. Points corresponding to the observation of only the $\alpha$-Fe$_2$O$_3$ phase are plotted in black, of only the $\alpha'$-Fe$_2$O$_3$ phase in red, and of the coexistence of both phases in blue. The two empty symbols correspond to data points determined from the Le Bail fits shown in Fig. \ref{['lebail']}. A transition from $\alpha$-$\text{Fe}_2\text{O}_3$ to $\alpha^\prime$-$\text{Fe}_2\text{O}_3$ phase is observed at 54-62 GPa. It is associated with a relative volume jump $\Delta V/V_0$ equal to 11.0 % and pressure jump of $\Delta P\sim 9$ GPa.
• Figure 5: Lattice parameters for the $\alpha$ and $\alpha'$ phases as a function of pressure. All c/c$_0$ data are shifted vertically by -0.05 for clarity. Shock data are represented by solid blue dots (a/a$_0$) and empty blue dots (c/c$_0$). Dashed blue lines above 60 GPa are drawn to guide the eye. Lattice parameters for the $\alpha$ phase from static compression are given as solid black lines for ref. bykova_novel_2013liu_static_2003schouwink_high-pressure_2011finger_crystal_1980 and black dotted line for ref. rozenberg_high-pressure_2002. The results from DFT$+U$ using a rhombohedral phase are given in red. The inset figure shows the relative magnetic moment of Fe from DFT$+U$ as a function of pressure. The phase is high spin below 50 GPa, and low spin above 60 GPa.