Preparation Of La_1-xCa_xMnO₃and Their Laser Raman And Infrared Optical Spectra Analysis

The manganese oxide materials with perovskite structure are widely used in modern society. With varieties of special electromagnetic, optical and catalytic characteristics, they are researched by scientists as a hot point since being discovered. In such materials, the competitions and couplings among the charges, orbits, spinning and crystal lattices is easy to cause the heterogeneity in micro-nano scales (phase separation), and affect the macroscopic physical characteristics of the materials significantly. Phase transitions will be generated in different temperature ranges, and the coexistence, competitions and evolution will occur among these phases. The different physical characteristics such as conductivity and conductivity among different small zones can lead to different responses in their Raman and infrared spectra. Conversely, these different responses in their Raman and infrared spectra can be used to discover the microscopic heterogeneity in materials.In this paper we have prepared manganese oxide samples La1-xCaxMnO3(x=0,0.33and0.75) with perovskite structure by a standard solid-state method, and measured, characterized and analyzed their physical and optical properties such as crystal structures, magnetism, resistance infrared reflectance spectra and Raman scattering spectra.We characterized the crystal structures of the samples with the X-ray diffractometer, acquired the positions and FWHM (full width at half maximum) of diffraction peaks by Jade software, and calculated the average crystal grain size of each sample with the Scherrer formula. Each diffraction peak in the XRD diagram of the samples is relatively sharp, which is indicating the single phase crystals with perovskite structure of the samples. With the doping of Ca, the crystal cell parameters are becoming smaller. The intensity of the characterized peaks in the (002) direction of each sample is largest, which is indicating the preferential growth and best crystallinity in the c-axis direction. In the same preparation process, the crystallinity of doped samples is better than the undoped LaMnO3. For the doped samples, the crystallinity of La0.67Ca0.33MnO3is better the La0.25Ca0.75MnO3’s. We used a laser Raman spectrometer to measure and analyze the Raman scattering spectra of samples with changing temperature (300K-80K). Three obvious characteristic peaks appear in Raman spectrum for each sample, and they correspond to the symmetric stretching vibration, anti-symmetric stretching vibration, and rotating vibration modes of oxygen atoms in octahedral MnO6With the increasing of Ca’s doping amount, the positions of low wave number peaks appeared no significantly offset at the room temperature, compared with the significantly offset for the medium wave number peaks and the lesser extent offset for the high wave number peaks. It indicated that at room temperature the doping of A ion didn’t have a significant impact on the Mn-O-Mn bond angles. The doping of Ca made Mn4+ions appear. It led to the differences of Mn-O bond length in different directions within plane, which had impacted the vibration mode of Mn-O in plane. With the increasing of Ca’s doping, the quantity of Mn4+ions were increasing, which resulting in the increase of Mn-O bond length in plane. The doped Ca had no effect on the O ions at the octahedron vertex. When the temperature decreased, the high wave number peaks showed significant changing behavior and characteristic temperatures, which corresponded to the changing behavior and characteristic temperatures of magnetic properties and electron transport with the temperature of the material. However, the other two peaks, the medium and low wave number peaks, didn’t show changing regularity with temperature.We used a synchrotron radiation infrared spectrometer to measure the infrared reflectance spectra of samples with changing temperature (300K-80K). Three characteristic reflectance peaks of each sample corresponded to the external vibration mode of La(Ca), bending vibration mode of Mn-O-Mn and stretching vibration mode of Mn-O in octahedral Mn06. For the undoped sample, its infrared spectra did not show changing behavior significantly. For the doped samples (x=0.33and0.75), the intensity of medium and high wave number peaks decreased with as the temperature decreased. In the meantime, the positions of peaks also moved towards the direction of higher wave numbers, and the movements had outstanding performances at some certain temperatures, which corresponded to the transition temperatures of the magnetic and conductive behavior of the samples.In addition, we used a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM) to measure the changing behavior of samples’magnetization at different external magnetic fields with changing temperature, and acquired the M-T curves. We also used the standard four-probe method to measure the changing resistivity of samples with changing temperature, and acquired the p-T curves.Through the analysis we can know that LaMnO3is typically anti-ferromagnetic at its ground state. At the high temperature, the thermal fluctuations make magnetic moments disordered, and the system appears paramagnetic. As the temperature decreases, the thermal fluctuations are weakened, and the system translates from paramagnetic to antiferromagnetic. Under150K, it has the A-type antiferromagnetic structure, in which the magnetic moments of Mn3+ions in plane are arranged parallelly and appear as ferromagnetic coupling while the magnetic moments of Mn3+ions at adjacent layers appear as ferromagnetic coupling. For the La0.67Ca0.33MnO3sample, it will appear paramagnetic above260K, and turn to be ferromagnetic metals due to the double exchange interaction under260K. From the quantum mechanics point of view, the emergence of ferromagnetic and antiferromagnetic is the result of overlap exchange integral of electron cloud. For La0.25Ca0.75MnO3, it is a typical kind of charge-ordered material. As the temperature decreases, the charge-ordered transition firstly occur in the sample, and later the transition from paramagnetic phase at high-temperature to antiferromagnetic phase at low temperature will emerge.