Deformation and slip systems of ${\mathrm{CaCl}}_2$-type ${\mathrm{MnO}}_2$ under high pressure

Many nonmetals and metal dioxides $M{\mathrm{O}}_2$, including the dense form of ${\mathrm{SiO}}_2$ stishovite, crystalize in a rutile structure at low pressure and transform to a denser ${\mathrm{CaCl}}_2$ structure under high pressure. Structures and transformations in $M{\mathrm{O}}_2$ dioxides hence serve as an archetype for applications in materials science and inside the Earth and terrestrial planets. Despite its significance, however, the deformation behavior of $M{\mathrm{O}}_2$ compounds in the ${\mathrm{CaCl}}_2$ structure is very poorly constrained. Here we use radial x-ray diffraction in a diamond-anvil cell and study ${\mathrm{MnO}}_2$ as a representative system of the $M{\mathrm{O}}_2$ family. We identify the dominant slip systems and constrain texture evolution in ${\mathrm{CaCl}}_2$-structured phases. After phase transition to a ${\mathrm{CaCl}}_2$ structure above 3.5 GPa, the dominant (010)[100] and secondary {110}[001] and {011}[0-11] slip systems induce a 121 texture in compression. Further compression increases the activity of the $\left\{011\right\}\langle 0\text{−}11\rangle$ slip system, with an enhanced 001 texture at $\sim 50\mathrm{GPa}$. During pressure release, the 001 texture becomes dominant over the original 121 texture. This clearly demonstrates the effect of pressure on the deformation behavior and slip systems of ${\mathrm{CaCl}}_2$-structured dioxides. Finally, ${\mathrm{MnO}}_2$ transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the ${\mathrm{CaCl}}_2$ to rutile structural transformation. These findings provide key guidance regarding the plasticity of ${\mathrm{CaCl}}_2$-structured dioxides, with implications in materials and Earth and planetary science.

Figure 1

Radial x-ray diffraction patterns of ${\mathrm{MnO}}_2$. (a)–(f) Unrolled diffraction images showing stress and texture information at different pressure points during both compression and decompression. Analyzed diffraction patterns during compression (g) and decompression (h). The small peak at $2\theta =7.6^{\circ }$ in (g) and (h) which does not move during the whole compression and decompression range is a signal from the gasket. Red stars in (h) indicate peaks from Pt. The x-ray wavelength is 0.4246 Å.

Figure 2

X-ray diffraction results in axial geometry for hydrostatically compressed ${\mathrm{MnO}}_2$. Integrated patterns of ${\mathrm{MnO}}_2$ under (a) compression and (b) decompression. Lattice parameters (c) and volume (d) as a function of pressure. Solid symbols refer to points obtained upon compression and open symbols to those obtained on decompression. Dashed line in (d) is the third-order Birch-Murnaghan EOS fitting result. X-ray wavelength is 0.6199 Å.

Figure 3

Lattice parameters (a), volume (b), and $t/G$ (c) of ${\mathrm{MnO}}_2$ as a function of pressure. Solid symbols refer to points obtained upon compression and open symbols to those obtained on decompression. In (b) and (c), the square and circle correspond to tetragonal (rutile-type) and orthorhombic (${\mathrm{CaCl}}_2$-type) phases, respectively. Dashed line in (b) is the EOS fitting result.

Figure 4

Inverse pole figures of the compression direction showing the texture evolution in ${\mathrm{MnO}}_2$ during compression and decompression. Pressure unit: GPa, $de$ indicates decompression. Pole densities are measured in multiples of a random distribution (m.r.d.), where 1 m.r.d. represents random texture. Equal area projections.

Figure 5

Pole figures showing texture development in ${\mathrm{CaCl}}_2$-type phase during both compression and decompression, and the texture inheritance during phase transition back to rutile phase. The viewed crystallographic lattice plane normal is indicated above each pole figure. The compression direction lies in the center of the pole figure, while the axis perpendicular to the compression direction lies in the peripheral. Equal area projection.

Figure 6

Theoretical modeling texture results. (a) Slip systems used for VPSC modeling for ${\mathrm{CaCl}}_2$-type phase. (b)–(d) VPSC modeling texture results corresponding to Table 1, model 1 to model 3. (e) Modeled transformation texture of quenched rutile phase.

Figure 7

Burgers vector of three slip systems used in VPSC. (a) Length of Burgers vector as a function of pressure. (b) Ratio of Burgers vector's length to that at 8.6 GPa. The length of the $\langle 011\rangle$ Burgers vector decreases much faster with pressure than the other two. Solid symbols represent data obtained during compression and open symbols represent decompression data.