Deformation and slip systems of CaCl2-type MnO2 under high pressure

Many nonmetals and metal dioxides MO2, including the dense form of SiO2 stishovite, crystalize in a rutile structure at low pressure and transform to a denser CaCl2 structure under high pressure. Structures and transformations in MO2 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 MO2 compounds in the CaCl2 structure is very poorly constrained. Here we use radial x-ray diffraction in a diamond-anvil cell and study MnO2 as a representative system of the MO2 family. We identify the dominant slip systems and constrain texture evolution in CaCl2-structured phases. After phase transition to a CaCl2 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}⟨0−11⟩ slip system, with an enhanced 001 texture at ∼50GPa. 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 CaCl2-structured dioxides. Finally, MnO2 transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the CaCl2 to rutile structural transformation. These findings provide key guidance regarding the plasticity of CaCl2-structured dioxides, with implications in materials and Earth and planetary science.