We perform first principles simulations for the structural, elastic, vibrational, electronic and optical properties of orthorhombic samarium orthoferrite SmFeO3 within the framework of density functional theory. A number of different density functionals, such as local density approximation, generalized gradient approximation, Hubbard interaction modified functional, modified Becke-Johnson approximation and Heyd–Scuseria–Ernzerhof hybrid functional have been used to model the exact electron exchange-correlation. We estimate the energy of the ground state for different magnetic configurations of SmFeO3. Its crystal structure is characterized in terms of calculated lattice parameters, atomic positions, relevant ionic radii, bond lengths, bond angles and compared with experimental values. The stability of its orthorhombic structure is simulated in terms of elastic properties. The vibrational phonon modes are calculated using density functional perturbation theory and are shown to be consistent with recent experimental observations. In case of electronic properties, we provide estimates based on density functionals with varying degrees of computational complexities in the Jacob's ladder. We show Heyd–Scuseria–Ernzerhof density functional theory provides better modelling for localized d and f orbitals in SFO which is in line with theoretical work on other rare-earth materials. The linear optical properties in terms of complex dielectric function and other standard optical functions are derived for Hubbard corrected generalized gradient approximation in combination with Fermi's golden rule.These provide a good theoretical analysis of structural, elastic, vibrational, electronic and optical properties of SFO.

Structural, elastic, vibrational, electronic and optical properties of SmFeO3 using density functional theory

Spin-phonon coupling in Mn-Bi co-doped SmFeO3: An experimental study

Nanocrystalline samples of Nd0.7Bi0.3Fe1−xNixO3 (x = 0, 0.1, 0.2, and 0.3) are synthesized using sol-gel auto-combustion process to investigate their structural, electrical, magnetic, and thermal properties. Phase purity and crystallinity of the samples are determined through x-ray diffraction (XRD) data. XRD patterns along with Rietveld refinement reveal orthorhombic structure with Pbnm space group. Unit cell parameters, bond angles, and bond lengths for all the samples have been determined. Crystallite size calculated by Scherrer equation is found to be in the range of 15–21 nm. The characteristic bands in the FTIR spectra further confirm the formation of our samples. FE-SEM images indicate the homogeneous distribution of particles on large scale and Ni-doped samples show some wall-like nanostructure in the morphology. EDX spectra confirm the elemental composition without any impurity element. The polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) loops exhibit the multiferroic nature of the material. The sample with concentration x = 0.2 shows the maximum value of polarization (Pm) while maximum magnetization is achieved for x = 0.1 concentration. The specific heat capacity (Cp), endothermic, and exothermic peaks of the materials have been determined in the broad temperature range with the help of DTA measurements in the controlled nitrogen atmosphere. The Neel temperature of the parent system is found to be 742 K and that decreases up to 573 K on Ni doping.

Room temperature dual ferroic behavior induced by (Bi, Ni) co-doping in nanocrystalline Nd 0.7 Bi 0.3 Fe 1−x Ni x O 3 (0 ≤ x ≤ 0.3)

We report the electronic and local structural investigation of double pervoskites Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) using x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$-edges. The $ab$ $initio$ simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of 1$s$ electrons to the mixed $p-d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive Co$^{2+}$ ions with $x$ . On the other hand, the $z$-in distortion in NbO$_6$ octahedra remains unaltered up to $x=$ 0.4 and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x \geqslant$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white line feature in the XANES spectra and possible reason for this are discussed. More interestingly, we observe the signature of KN$_1$ double electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z$+1 approximation. Further, the Co L$_{2,3}$ edge shows the reduction in the crystal field strength and hence an increase in the charge transfer energy ($\Delta_{ct}$) with the La substitution. 

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Probing the electronic and local structure of 
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We report the electronic and local structural investigation of double pervoskites Sr 2 − x La x CoNbO 6 ( x = 0–1) using x-ray absorption near edge structure (XANES) and extended x-ray absorption ﬁne structures (EXAFS) at the Nb, Co, and Sr K -edges. The ab initio simulations and detailed analysis of the Nb and Co K -edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of 1 s electrons to the mixed p − d hybridized states. We reveal a z -out Jahn-Teller (JT) distortion in the CoO 6 octahedra, which decreases monotonically due to an enhancement in the JT inactive Co 2+ ions with x . On the other hand, the z -in distortion in NbO 6 octahedra remains unaltered up to x = 0.4 and then decreases with further increase in x . This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in x (cid:62) 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white line feature in the XANES spectra and possible reason for this are discussed. More interestingly, we observe the signature of KN 1 double electron excitation in the Sr K -edge EXAFS spectra for all the samples, which is found to be in good agreement with the Z +1 approximation. Further, the Co L 2 , 3 edge shows the reduction in the crystal ﬁeld strength and hence an increase in the charge transfer energy (∆ ct ) with the La substitution.

Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures

Structural, dielectric and magnetic properties of Bi–Mn doped SmFeO3

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr2−xLaxCoNbO6 (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P21/n) is observed between x = 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic thermal expansion in case of the x = 0.4 sample, whereas the x = 0.6 sample shows the anisotropy where the thermal expansion along the c-axis significantly deviates from the Grüneisen function. We observe the z-out JahnTeller distortion in the CoO6 and consequently z-in distortion in the adjacent NbO6 octahedra. With increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter ∆ around the B-site atoms.

Observation of anisotropic thermal expansion and Jahn-Teller effect in double perovskites Sr$_{2-x}$La$_x$CoNbO$_6$ using neutron diffraction

We report the evolution of structural, magnetic, transport, and electronic properties of bulk polycrystalline La$_{0.5}$Sr$_{0.5}$Co$_{1-x}$Nb$_x$O$_3$ ($x =$ 0.025--0.25) samples. The Rietveld refinement of the x-ray diffraction patterns with R$\bar3$c space group reveals that the lattice parameters and rhombohedral distortion monotonously increase with the Nb$^{5+}$(4$d^0$) substitution ($x$). The magnetic susceptibility exhibits a decrease in the magnetic ordering temperature and net magnetization with $x$, which manifests that the Nb substitution dilutes the ferromagnetic (FM) double exchange interaction and enhances the antiferromagnetic (AFM) super-exchange interaction. Interestingly, for the $x>$ 0.1 samples the FM order is completely suppressed and the emergence of a glassy state is clearly evident. Moreover, the decrease in the coercivity (H$\rm_{C}$) and remanence (M$\rm_{r}$) with $x$ in the magnetic isotherms measured at 5~K further confirms the dominance of AFM interactions and reduction of FM volume fraction for the $x>$ 0.1 samples. More interestingly, we observe resistivity minima for the $x=$ 0.025 and 0.05 samples, which are analyzed using the quantum corrections in the conductivity, and found that the weak localization effect dominates over the renormalized electron-electron interactions in the 3D limit. Further, a semiconducting resistivity behavior is obtained for $x>$ 0.05, which follows the Arrhenius law at high temperatures ($\sim$160--320~K), and the 3D-variable range hopping prevails in the low-temperature region ($<$160~K). The core-level photoemission spectra confirm the valence state of constituent elements and the absence of Co$^{2+}$ is discernible.

Raveena Dhaka

Papers

Structural, dielectric and magnetic properties of Bi–Mn doped SmFeO3

Observation of anisotropic thermal expansion and Jahn-Teller effect in double perovskites Sr$_{2-x}$La$_x$CoNbO$_6$ using neutron diffraction

Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures

Evolution of complex magnetic phases and metal-insulator transition through Nb substitution in La$_{0.5}$Sr$_{0.5}$Co$_{1-x}$Nb$_x$O$_3$