Electron density mapping analysis using XRD and tailoring of the optical and magnetic properties in Ni doped SnO₂

Single-phase Ni-doped SnO₂ with concentrations of 2.5%, 5%, and 7.5% was prepared by the simple co-precipitation method and studied to tailor the optical energy gap and magnetism useful for diluted magnetic semiconductor (DMS) applications. In all compositions, XRD analysis shows Ni doping with no peak shift. Ni doping in SnO₂ increases the energy band gap from 3.51 eV to 4.09 eV, reducing optical absorbance. Room-temperature PL emission with excitation at 320 nm produces strong red emissions. Room-temperature magnetic investigations show that 2.5% Ni doping in pure SnO₂ changes diamagnetism to soft ferromagnetism with 0.5 emu/g magnetisation and 149 Oe coercivity. More Ni doping reduces magnetisation and leads to paramagnetism, which is attributed to interstitial charge accumulation. The Rietveld analysis of the XRD data reveals a cation deficiency, while the maximum entropy method (MEM) analysis of the electronic structure and bonding behaviour reveals the maximum dilution of Ni²⁺ ions in the Sn⁴⁺ host site at 2.5% doping. Further doping increases interstitial charge accumulation, defects, and magnetisation reduction. MEM also shows a new empirical conclusion: for magnetism to be fully developed, the Ni²⁺ needs to be diluted correctly in the Sn⁴⁺ host lattice so that there is no charge buildup between the atoms (interstitial) and the Sn/Ni-O covalency needs to be maximum in both apical and equatorial directions. In conclusion, a dilute Ni-doped SnO₂ system allowed to achieve a soft ferromagnetic transition and optical band tuning.