High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide

Chromium dioxide has been synthesized by direct combustion process between Cr and O-2 in a diamond anvil cell at 14 GPa using laser heating. Addition of nitrogen into the reactants' environment substantially facilitated the oxidation of Cr and resulted in the formation of an orthorhombic CaCl2-structured phase of CrO2 after laser heating at 37 GPa. High-pressure synchrotron x-ray diffraction and Raman spectroscopy measurements of quenched CrO2 samples at ambient temperature showed a tetragonal (rutile-type structure)-to-orthorhombic (CaCl2-type structure) phase transition at about 17 GPa. The first principle calculations reproduce well the experimentally observed pressure domains of the stability of the tetragonal and orthorhombic phases of CrO2. The calculations show that the orthorhombic CrO2 is also half-metallic and that the ferromagnetic arrangement of spins of d electrons persists in orthorhombic phase of CrO2 up to 56 GPa.