Origin of anomalous octahedral distortions and collapse of magnetic ordering in Nd1−xSrxFeO3 (0≤x≤0.5)

The effects of strontium doping on the structural properties and magnetic ordering of Nd1−xSrxFeO3 orthoferrite system have been studied by employing macroscopic and microscopic structural techniques like X-ray diffraction, scanning electron microscopy and 57Fe Mössbauer spectroscopy. X-ray diffraction confirmed that single phase materials have been synthesized. It has been observed that orthorhombic distortion, density and porosity have decreased; whereas, grain size, tolerance factor and symmetry have increased with increase in the strontium concentration. Mössbauer results showed an increase in the Fe4+/Fe3+ ratio, sagging and local octahedral distortions while decrease in the magnetic ordering with increase in the strontium content. The origin behind anomalous octahedral distortions in this system has been attributed to the decrease in the oxidation state and mismatch in the ionic radii of A-site cations and increase in the concentration of Fe4+, due to Sr2+ doping at Nd3+ sites. The collapse of magnetic ordering has been ascribed to the weakening of super-exchange interactions, dilution of strong long range magnetic sub-lattice of high spin Fe3+ (five unpaired electrons) by relatively lower spin of high spin Fe4+ (four unpaired electrons) and increase in the spin-spin relaxation frequency.