Investigation of the structural connectivity with the physical properties of ilmenite and pyroxene based oxides (PhD)

Abstract

Transition metal oxides exhibit emergent phenomena like colossal magnetoresistance, high TC superconductivity, multiferroicity etc. Such phenomena have given us ample opportunities to explore both from application and fundamental physics point of view. In this thesis, we have studied the structural link with the physical properties of doped MnTiO3, MnTiO3 prepared under different conditions and LiFeSi2O6 compounds. These compounds are potential ferrotoroidic materials which fall under the fourth primary ferroic order. The samples were studied using temperature dependent x-ray diffraction, dc susceptibility, heat capacity measurements, room temperature electrical resistivity, photocurrent measurements, optical absorbance spectroscopy and x-ray photoemission spectroscopy. MnTiO3, a quasi two dimensional system, stabilises in hexagonal structure with the space group R . This compound exhibits paramagnetic to antiferromagnetic transition around 64 K. This compound shows magnetoelectric property where ferroelectricity occurs with the application of magnetic field and also optical absorption in the wavelength range covering ultra-violet (UV) and visible regions. With the motivation to understand if spin lattice coupling occurs even at zero field, we have carried out temperature dependent x-ray diffraction studies on MnTiO3. Our results show the signature of the onset of intra and inter layer magnetism at temperature ~ 200 K and 100 K, respectively which is much higher than reported ones. In this compound, the magnetic contribution occurs due to the Mn ion. To understand the role of Ti ions, we have prepared MnTi1-xRuxO3 (x= 0 and 0.2) compounds. Our temperature dependent x-ray diffraction and dc susceptibility studies results show that, with Ru doping, there occurs increase in the three dimensional character. To understand the origin of the optical absorbance in the visible region, we prepared two compounds of MnTiO3. Both were prepared by the usual solid state route only with the difference that in one of the sample, the cooling was natural (MTOuq) while in the other it was quenched (MTOq) in liq. nitrogen. Our combined results of room temperature structural studies, optical absorbance, core level spectra, valence band spectra and the electrical resistivity suggest even MTOuq sample show signature of Mn3+ ions in addition to Mn2+ ions. The fraction of Mn3+ ions has been found to increase on quenching. This further leads to an increase in the optical absorbance in the visible region, a persistent photo-resistance when the incident light is terminated after illuminating it and a decrease in the electrical resistivity. The features of the optical absorbance and valence band spectra were identified based on band structure calculations. LiFeSi2O6, pyroxene family, stabilizes in monoclinic structure with C2/c space group. It undergoes structural transition from C2/c to P21/c space group of monoclinic structure. This compound also shows magetoelectric property. This compound undergoes paramagnetic to antiferromagnetic transition around 18 K. There are reports that it shows short range magnetic ordering but the onset temperature is not clear. In this compound, it shows ferrotoroidicity in the ab-plane. The crystal structure of this compound suggests that it appears that its magnetism is of quasi one dimensional. But the observation of ferrotoroidicity suggests that magnetism should be three dimensional in character. Our temperature results show the clear connection between the structural parameters and the pre-ordering of magnetic interactions. The magnetism exisiting in this compound appears to be of three dimensional in character.