Study Of Infrared Spectrum In Perovskite Manganites La_1-x-yR_xCa_yMnO₃

Since the discovery of colossal magnetoresistance effect (CMR) in perovskites manganites, it has sparked considerable renewed interests in these long-known materials with an eye towards both an understanding of the CMR and related properties and potential applications in magnetic information store and low-field magnetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal and structure transition induced by applied magnetic-field or photo radiation, charge ordering, orbital ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics. Infrared spectroscopy is certainly one of the most important analytical techniques available to today's scientists. One of the great advantages of infrared spectroscopy is that virtually any sample in virtually any state may be studied. Liquids, solutions, pastes, powders, films, fibres, gases and surfaces can all be examined with a judicious choice of sampling technique. The application of Infrared spectroscopy in perovskites manganites is still in a initial stage, a lot of work need to be done.In this thesis, the author devoted his effort to the study of IR property in CMR oxides La1-x-yRxCayMnO3(R=Pr,Y,Ho), and the stretching mode abortion peaks for these perovskites manganites have been investigated systematically. The main results are generalized as follows:For the polycrystalline samples La_1-xCa_xMnO₃ (x=0.00¹.00), the infrared absorption spectrum of these samples were collect by a Fourier-transform IR spectrophotometer at different temperature. The stretch-mode absorption peak near ⁶20cm-1 was investigated thoroughly, the result indicates that the peak split into three peaks due to different mode of vibrations in MnO6 octahedron, And peak B and C show significant frequency shifts near the Curie temperature TC. The variation of effective carrier number obtained from the infrared absorption spectra dependence on temperature is according with the phase transition of FM to PM.In the polycrystalline samples La_0.67-xPr_xCa_0.33MnO₃(x=0⁰.67), La³⁺ is replaced by Pr³⁺ gradually,which can induce a complexity phase separation phenomenon. The infrared absorption spectrum at different temperature shows a notability difference from La0.67-xCaxMnO3 compounds. The analysis of stretch-mode absorption peak indicates that the frequency of peak A, B and C shift in differently in different magnetic phase region. In FM and AFM region, all the frequencies are stabilized. In the PM region, the frequencies of peak B and C decrease with the increase of temperature, and the frequency of peak A has a tendency to ascend with the rise of temperature followed by the increase of x. In the region of FM-PM, the shift of frequencies is too complicated to explain readily.With the same radii, Ho³⁺ and Y³⁺ were chosen to substitute La³⁺ in La0.67Ca0.33MnO3. The magnetism of ion only effects on the frequency of peak C, in we opinion. With lower value of x, the frequency of peak C decreases with the increase of temperature in the PM region. With higher value of x, the frequency of peak C is a little higher than the La_0.67-xY_xCa_0.33MnO₃ compound with same x. In addition, the scope of the frequency increase with x is much higher than the samples contained Y³⁺.