Deformation and slip systems of CaCl2 -type MnO2 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 ${011}\ensuremath{\langle}0\text{\ensuremath{-}}11\ensuremath{\rangle}$ slip system, with an enhanced 001 texture at $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. 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.