Forming a Cr4+(3d2) spin doped Zr1−xCrxO2 (x ≤ 0.2) of small crystallites at moderate pressure: A spin-semiconductor

A spin Cr4+(3d2) doped ZrO2 dielectric phase has tailored magnetism, ionic conductivity and optical properties. The synthesis is made with a polymer complex of Cr6+ and Zr4+ salts in glycerol that gives a controlled ionic conversion Cr6+ → Cr4+ in ambient air. At ∼320 K in open air, a CrO3 solution (orange color) forms a metal ion–polymer complex which darkens to a blackish color of Cr4+ species. A hydrogel dried at ∼330 K is heated in an autoclave at a small pressure ∼0.5 bar at 420–470 K so that it yields Zr1−xCrxO2 (x ≤ 0.2) buried in carbon which is burnt out when annealing at higher temperatures ≥673 K in air. Small crystallites, with D = 3–10 nm diameter, adapt a cubic ZrO2-type phase at ≤1073 K. As much CrO2 as 20 mol% (x = 0.2) is doped in this way. An optimal annealing at 773 K gives D = 5 nm sample plates with magnetization M = 9.08 emu g−1 (in the CrO2 part) and coercivity Hc = 54.2 Oe at 295 K, or M = 85.79 emu g−1 (Hc = 100 Oe) at 5 K in a magnetic hysteresis loop (not saturating until 60 kOe field). The Curie point ∼482 K is enhanced over CrO2 (387 K) by strong Cr4+–O2−–Cr4+ superexchange interaction. Spin-assisted electrical conductivity drops largely by 85–100%, depending on the applied frequency 10–100 kHz, in the paramagnetic → ferromagnetic order. Small core–shell crystallites facilitate both the magnetic and transport properties.