ABSTRACT
Detailed powder neutron diffraction studies as a function of temperature is performed on NdFe0.5Mn0.5O3 in the temperature range 400-1.5 K. Diffused magnetic scattering is observed due to three dimensional short range ordering (SRO), between Fe3+/Mn3+ spins, over the whole temperature range 400-1.5 K. The presence of SRO is independent of long range ordering (LRO) in this compound which has never been observed in any Fe3+/Mn3+ based compounds. Further, in this compound two-fold spin reorientation is discussed in the temperature range 300-1.5 K. Development of long range ordering at 300 K is due to the mixture of Γ4 and Γ1 magnetic structure, not like other orthoferrites which have Γ4 structure at 300 K. This occurs due to the presence of large single ion anisotropy of Mn3+ ions. Volume fraction of Γ4 decreases with temperature leading to pure Γ1 magnetic structure in the temperature range 150-90 K. Another spin reorientation of Fe3+/Mn3+ spins occurs from Γ1 to Γ2 in the temperature range 70-25 K.
ABSTRACT
The electronic structure of double perovskite Pr2MnNiO6 was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+ and 2+ states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U - J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p - d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.
ABSTRACT
We have carried out dielectric and transport measurements in NdFe1-xMnxO3 (0 ≤ x ≤ 1) series of compounds and studied the variation of activation energy due to a change in Mn concentration. Despite similar ionic radii in Mn(3+) and Fe(3+), large variation is observed in the lattice parameters and a crossover from dynamic to static Jahn-Teller distortion is discernible. The Fe/Mn-O-Fe/Mn bond angle on the ab plane shows an anomalous change with doping. With an increase in the Mn content, the bond angle decreases until x = 0.6; beyond this, it starts rising until x = 0.8 and again falls after that. A similar trend is observed in activation energies estimated from both transport and dielectric relaxation by assuming a small polaron hopping (SPH) model. Impedance spectroscopy measurements delineate grain and grain boundary contributions separately both of which follow the SPH model. Frequency variation of the dielectric constant is in agreement with the modified Debye law from which relaxation dispersion is estimated.
ABSTRACT
Temperature-dependent Raman and dielectric measurements have been carried out on (C2H5NH3)2CdCl4 single crystals. Raman studies reveal the presence of two structural phase transitions below room temperature at 216 K and 114 K. The phase transitions are marked by anomalies in temperature dependence of wave-number and full width half maximum (FWHM) of several vibrational modes. The transitions are also accompanied by anomalies in dielectric measurements. Raman and dielectric data indicate that the transition at 216 K is order-disorder in nature and is driven by re-orientation of organic ions, while the transition at 114 K is due to coupling between the CdCl6 octahedron and the organic chain. Further high temperature dielectric measurements reveal the presence of one more structural phase transition around 473 K across which dispersion in dielectric parameters is observed. The activation energies and relaxation time obtained for high temperature dielectric phases are characteristic of combined reorientation motions of alkyl ammonium cations.
