Synthesis, structure and magnetic properties of ultra-high purity CrO2 prepared under high O2-gas pressure

Abstract Ultra-high purity CrO 2 was prepared by decomposing CrO 3 in O 2 with gas pressures up to 40 MPa, which were maintained throughout the decomposition process of CrO 3 to prevent the formation of any other phases of chromium oxides. Our method is different from the traditional methods that start from or under ambient pressures. The high oxygen pressure makes the meta-stable CrO 2 stable from the initial stage of preparation. As a result, the purity of the as-prepared CrO 2 is improved, and this has been further proved by the highest magnetization of the samples. The as-prepared CrO 2 particles show very large grains with flat surfaces, octagonal cross-section, and straight edges, owing to the high mobility of Cr ions in CrO 2 at temperatures above its melting point. The lattice parameters of CrO 2 are a  = 4.4176 A and c  = 2.9144 A. The maximum value of the magnetic entropy change of the high purity CrO 2 particles is ∼2.83 J/kg·K for an applied field of 1.5 T. The preparation of pure CrO 2 is important for studying its intrinsic properties and for applications in spintronic devices.