Studies On The Synthesis And Raman Spectra Of The Lanthanide-Transition Metal Oxides And Metastable Phase, Of α-SiO₂

Since 1950s the doping induced antiferromagnetic-ferromagnetic, insulator-metal transitions at low-temperature have been discovered during the study of electric, magnetic properties in perovskite-structural manganese oxides. The strong coupling between the magnetic ordering and the electrical conductivity has exhibited a great variety of physical properties in such systems. The recent interest emanates from the discovery of colossal magnetoresistance (CMR) in hole-doped R1-xAxMnO3 manganite perovskite (R = La, rare earth; A = Ca, Sr, Ba), and its potential technological applications in magnetic recording and sensors, and the interesting effects of the strong correlation concerning metal-insulator transition. More recent research work has revealed a number of fascinating phenomena such as charge-ordering of the Mn³⁺ and Mn⁴⁺ ions observed in real lattice space, phase segregation occurred in the critical regimes of structural and magnetic transitions. How explain the interesting phenomena? It is necessary investigating the structure. No aforementioned phenomena present itself in normal perovskite, it appear in the distorted perovskites. Raman scattering is known to be a powerful technique to study the structure and crystal lattice. Raman-active phonons in orthorhombic perovskitelike RMnO3 were studied by measuring Raman spectra in various scattering configurations. The experimental Raman line wave numbers and the expected shapes for the phonon modes were compared to those reported for other perovskitelike compounds with the Pnma structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of RMnO3 were assigned to definite atomic motions. The remaining spectral weight can be explained by the presence of dynamic Jahn-Teller distortions that lower the symmetry of the cubic perovskite. We studied the Raman shift of perovskite RMnO3 (R=La, Pr, Nd, Sm, Eu, Gd, and Er). The properties of RMnO3 have been intensively studied since it was found that partial substitution of R by Ca, Sr, or Ba results in structural changes and the occurrence of colossal magnetoresistance near the temperatures of spin ordering of Mn ions. It is plausible to expect that structural changes and magnetic ordering will also influence the phonon spectra. Raman scattering is known to be a powerful technique to study the dynamical processes caused by phonons, charge carriers, and spins that may affect the magnetoresistance process. Although Raman spectra of other orthorhombically distorted perovskites with the Pnma structure, particularly in rare earth and yttrium aluminates and orthochromates, have been reported, there are practically no data on the Raman scattering from orthomanganites. In this thesis, by adopting a variety of experimental methods we systemically investigate lattice effects in perovskite-structure RMnO3(R=La,Pr,Nd,Sm,Eu,Gd,Dy,Er)and rare earth cobalt oxides RCoO3(R=La,Ce,Pr,Nd,Sm,Eu,Gd,Dy,Er). We have carried out X-ray diffraction measurements and Raman spectrum, and concentrated on thestudies of the behavior of Jahn-Teller distortions in RMnO3 and the unique characteristics of rare earth ion in such ABO3 compounds. RMnO3 compounds crystallize in the orthorhombic structure [space group Pnma (D2h16) and Z=4] for R (R=La to Dy) with larger ionic radius, and the hexangular structure [space group P63cm(C6v3),Z=6] for ErMnO3. We have studied the Raman phonons and the Raman shift of stoichiometric RMnO3 (R=La, Pr, Nd,Sm,Eu,Gd,Dy) compounds, with the orthorhombic Pnma structure, as a function of the rare earth ion, and the effect of the rare earth ion in the Raman spectrum, with specifically wave number moving. The Raman active modes of Pnma structure (mirror plane m is perpendicular to the long c-axis) are 7Ag + 7B1g + 5B2g + 5B3g and the non-zero components of the Raman tensors are (xx, yy, zz), (xy), (xz), and (yz) for Ag, B1g, B2g and B3g representations, respectively. In Raman spectrum, the three main peaks correspond to the following modes and symmetries: the symmetric stretching of the basal oxygen of the octahedra, around 610 cm-1 (B1g symmetry); the asymmetric stretching at 480 cm-1 (Ag symmetry) associated with the Jahn–Teller distortion; and the tilt of the octahedral at 280 cm-1 (Ag symmetry). The modes at the lowest energies are those related to the rare earth movements. In order to estimate the importance of both the terms in the observed phonon frequencies we have evaluated the expected changes in the phonon frequencies caused by the variation of the interatomic distances in the crystal. Taking into account a simple model for the interatomic force constants: K=Fij/rij and?=(K/m)1/2, where Fij are constants that depend on i and j ions, and rij are, in the present case, Mn-O distances, we can estimate the changes in the stretching modes (at 610 cm-1 and 480 cm-1) due to the changes of these distances. Then, the dominant mechanism for the observed frequency shift is due to anharmonic scattering of phonons.The observed frequency shift at 280 cm-1 is more obvious than the others. The tilt of the octahedron at 280 cm-1 (Ag symmetry) is due to the influence of rare earth ions. The phonon modes nearby 280 cm-1 of different ions are different, which depend on the atomic mass and the ionic radius of the rare earth element. We can find the shift of wave number (nearby 280 cm-1) in Raman spectrum increasing with the augment of atomic mass of the rare earth element. We have studied the Raman active phonons of stoichiometric RMnO3 compounds with orthorhombic structure. We have correlated the frequencies of the three most intense modes of the orthorhombic samples, with some structural parameters such as Mn-O bond distances, atomic radius of the rare earth and Jahn-Teller distortion. As the ionic radii of the rare earth decreases, the frequency of the mode at 280 cm-1 increases dramatically, following a linear dependence with the Raman shift. The stretching mode at 610 cm-1 behaves inversely with the mean distances of Mn-O bonds in the basal plane. We correlated the Raman peak at 480 cm-1 was correlated with the Jahn-Teller distortion for compounds with large ionic radii. Rare-earth cobalt oxides, RCoO3(R=rare-earth element) have been extensively studied because of their interesting electrical and magnetic properties. The electrical conduction mechanism and the observed insulator–metal transition have been reported for many years. Although Raman spectra of other orthorhombically distorted perovskites with the Pnma structure, particulary in rare-earth and yttrium aluminates, orthochromates, orthoferrites, and manganites, have been reported, there are practically no data on the Raman scattering from rare earth cobalt oxides. In this paper, we analyze the Raman spectra and Raman active phonon modes in RCoO3 (R=La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy) rare earth cobalt oxides. We have studied the Raman phonons of stoichiometric RCoO3 (R=La, Ce, Pr,Nd), with cubic structure [space group Pm3m (Oh1) ], as a function of the rare earth ion, as well as the Raman phonons of orthorhombic [space group Pnma (D2h16) and Z=4]RCoO3 (R=Sm, Eu, Gd, Dy). RCoO3 compounds present the Pm3m cubic structure for R=La, Ce, Pr, Nd while compounds with smaller ionic radii (R=Sm, Eu, Gd and Dy) can be obtained in the orthorhombic structure [space group Pnma (D2h16), z=4,]. Their magnetic and transport properties are strongly dependent on the rare earth and, therefore, on their detailed structure. We have studied these compounds through the Raman phonons and XRD of both structures as a function of the rare earth ion. The phonon frequencies of the main peaks of Pnma structure have been correlated to structural characteristics and the presence of dynamic Jahn–Teller distortions. The observed Raman lines in the spectra of RCoO3 were assigned to definite atomic motions. According to the structural and Raman behavior, these compounds can be classified into three groups: one includes La, Ce, Pr, Nd, the other one includes Sm, Eu, Gd, Dy, and the other includes the rest. The research of the coesite appeared in the earth's surface is thought as the "window"of knowing the earth. For a long time the found of the coesite in the earth's surface is regarded as the good evidence for the theory of the earth's plate exhumation. In this thesis, based on the research of the crystallization mechanism of the mechanical milling, the amorphous SiO2 was obtained by high-energy milling.When the SiO2 was pulverized, the metastable phase of SiO2 was brought.We studied the metastable phase of SiO2 by measuring Raman spectra, and crystallize it is in the tetragonal structure [space group P41212(92)]. The structure is easy to come into being the coesite.