A metastable Cr4+-compound AlCrO3+δ, δ ≤ 0.5, grown at a nanocolloid, and its ultraviolet absorption, phonon bands, and dielectric properties

AlCrO3+δ, δ ≤ 0.5, is a Cr4+:CrO2 product of a metastable compound of two types Cr4+−3d2 electrons that determine its spin degree of freedom and allied magnetic/optical properties at a magnetic semiconductor. A nanocolloid of Cr6+ bonded via Al3+ and O2– in small micelles (a hydrogel) in water is explored to grow up AlCrO3+δ, δ ≤ 0.5, at hydrothermal reactions at microwave irradiation. It forms hexagonal plates (20–40 nm thickness) as annealed at 500 °C − 1000 °C (microwave). It has 4.69% larger X-ray crystal density of 4.848 g/cm3 than a usual phase, AlCrO3.5 → AlCrO3 + ¼O2↑, turned up on anneals at ≥ 1000 °C, at a larger bandgap, ≥ 4.2 eV, of a Cr3+−3d3 insulator. X-ray diffraction, XPS, electronic absorption, and phonon bands studied at duly annealed samples confer a Cr5+ → Cr4+ → Cr3+ charge ordering at anneals. A 700 °C annealed sample (others behave as an antiferromagnet, AFM) exhibits a ferromagnetic (FM) loop, with an unsaturated magnetization 9.2 emu/g (at 50 kOe fields), a coercivity 3.67 kOe, and a 0.1 MGOe energy-product at room temperature. The two types FM−AFM spins channels govern electrical conduction at major charge carriers Cr4+ → Cr3+ − e–, O2– vacancies, and ‘e–−h+’ electron-hole pairs as per applied frequencies. The results have renewed interest to design and develop spintronics-photonics at Cr4+−3d2 spin doped Al2O3 type oxides.