Probing local site environments and distribution of manganese in SrZrO3:Mn; PL and EPR spectroscopy complimented by DFT calculations

Abstract

In order to understand the local environment, valence state and cationic distribution of manganese ions in gel-combustion derived SrZrO₃ (SZO), a combined experimental and theoretical approach was formulated based on photoluminescence (PL), electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations. An attempt was also made to investigate the same as a function of manganese ion concentration. The phase identification of the samples was confirmed using powder X-ray diffraction technique (PXRD). In all the doped compounds, manganese was found to be stabilized as divalent Mn²⁺ and preferentially occupying the 8-coordinated Sr²⁺ ion site. However, the proportion of manganese ions residing at zirconium sites was enhanced at higher concentrations. The cohesive energies from DFT calculations explained the stability of Mn²⁺ ions at different sites. It was also observed from the density of states (DOS) that the substitution of manganese at strontium sites leads to the generation of shallow defect states, whereas that at zirconium site generates both shallow and deep defect states within the band gap of the material. A change in the host emission due to these defect states with varied concentrations of Mn²⁺ was also observed, which further supported the observed cationic distribution's trend. The decrease in the band gap energy explained the red shift of the emission spectra. PL decay study also suggested the existence of shallow and deep trap states. The intensity of the EPR signal at g ≈ 1.976, due to paramagnetic oxygen vacancies, was found to increase at higher Mn-concentration because of more substitution at Zr⁴⁺ sites. Two additional EPR sextets with g ≈ 1.993 and 2.013 in Mn doped SZO compounds were attributed to lattice and surface bound Mn²⁺ ions, respectively, which disappeared at higher Mn-concentrations, giving a broad signal.