Effect of Ni substitution at Mn-site on structural and electronic properties of monovalent-dopedPr0.75Na0.25MnO3 manganite: Experimental and first principles LDA+U studies

Abstract

The structural, magnetic, electrical transport and electronic properties of Pr0.75Na0.25Mn1-xNixO3 (x = 0, 0.25 and 0.50) manganite were investigated using combined experimental and first principles approaches. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. Rietvield refinement analysis revealed unit cell volume slight increase with increase Ni concentration. AC susceptibility (χ′) against temperature (T) curve for x = 0 showed paramagnetic (PM) – antiferromagnetic (AFM) behaviorand weak ferromagnetic (FM) phase at lower temperature region. Meanwhile for x = 0.25 and 0.50 samples, the χ′(T) curve exhibited PM–FM transition with Curie temperature (TC), decreasing from 93 K (x = 0.25) to 92 K (x = 0.50) and the susceptibility values decrease at low temperature region below TC. Electrical resistivity measurement on all the samples displayed insulating behavior. UV–Vis absorption spectra curve showed optical energy gap (Eg) decreased from 2.48 eV (x = 0) to 1.75 eV (x = 0.50). For first principles calculation, the Hubbard U parameter for the treatment of strong Coulomb repulsion among electrons in 3d/4f orbitals was included in LDA + U functional. The U values for Mn3d, Pr4f and Ni 3d were set at 2, 6 and 6 eV, respectively. The calculated crystal volume structures for Ni free substitution (x = 0) and Ni-substituted samples (x = 0.25 and 0.50) were consistent with the experimental data for UMn values of 2 and 6 eV, respectively. The calculated total energy indicated that Pr0.75Na0.25Mn1-xNixO3 (x = 0, 0.25, 0.50) manganite with AFM phase was more stable (lower total energy) than that of the FM phase, which is in good agreement with experimental at lower temperatures. Furthermore, the results of calculation showed that the structural and electronic properties of Pr0.75Na0.25Mn1-xNixO3 (x = 0, 0.25, 0.50) were considerably influenced by Ni substitution. Consistent with the XRD data, the calculated crystal volume increased with Ni substitution. Interestingly, density of states (DOS) calculations revealed that Pr0.75Na0.25Mn1-xNixO3 (x = 0, 0.25, 0.50) was half-metallic. It found from calculated DOS that the spin down channel exhibited metallic behavior and the spin up channel showed insulating behavior with decreases in calculated energy band gap with Ni substitution. Partial DOS results showed that Mn, Ni and O atoms greatly contributed to the electronic states at Fermi energy for the spin down channel with high degree of hybridization between the Mn3d/Ni3d and O2p electrons. The average effective charge, bond population and length in the Mulliken population analysis indicated that Mn/Ni–O bonds possessed a higher degree of covalency than the Pr/Na–O bonds.