S. P. Liu

Quantum-mechanical method for estimating ion distributions in spinel ferrites

A quantum-mechanical method for estimating the cation distribution in spinel ferrites is proposed, by which the ionization energy of the cations and the Pauli repulsion energy is considered, together with the magnetic ordered energy and the tendency toward charge density balance. Using this method, not only can the difference between the observed and the traditional theoretical magnetic moments of the spinel structure ferrites MFe2O4 (M=Mn,Fe,Co,Ni,Cu) be explained, but also the dependence of the magnetic moments of the ferrites M1−xZnxFe2O4 (M=Mn,Fe,Co,Ni,Cu) on the doping level x can be fitted.

Rietveld fitting of x-ray diffraction spectra for the double phase composites La0.7−xSr0.3Mn1−yO3−1.5(x+y)/(Mn3O4)y/3

The effects of lanthanum deficiency on the structural and magnetic properties of manganites with normal composition La0.7−xSr0.3MnO3 prepared by the sol-gel method with the highest heat treatment temperature at 800 °C have been investigated. X-ray diffraction (XRD) spectra indicate that the materials possess a single phase with the R3¯c perovskite structure for x≤0.05, and that they possess two phases with the R3¯c perovskite being the dominant phase and Mn3O4 being the second phase for x≥0.10. Using XRD analysis, these materials can be expressed as La0.7−xSr0.3Mn1−yO3−1.5(x+y)/(Mn3O4)y/3. On the basis of the thermal equilibrium theory of crystal defects, the ion ratios at the A, B, and O sites in the ABO3 perovskite phase were calculated. Those ion ratios were used in Rietveld fitting of the XRD spectra. It was found that the dependence of the Curie temperature TC on the content ratio RM4 of Mn4+ ions at B site is similar to that of the typical perovskite La1−xSrxMnO3.

Estimation of Mn4+ ion content ratio in self-doped compound La1−xMnO3−δ

A method based on the thermal equilibrium theory of crystal defects for estimating the Mn4+ ion content ratio (RM4) at B sites in ABO3 self-doped manganite La1−xMnO3−δ is presented. For this kind of manganite, the relationship between the Curie temperature (TC) and RM4 can be explained by the double exchange mechanism of Zener, which is similar to that in the perovskite manganite La1−xCaxMnO3.

Influence of cohesive energy on unit cell volume of perovskite manganites La1−xMxMnO3

Ionic size is considered to be an important factor influencing the unit cell volume of manganites with an ABO3 structure. For La1−xSrxMnO3, however, although the average effective ion radius of all cations taken together increases with increasing x, the unit cell volume decreases for x<0.5. In the case of La1−xNaxMnO3, the unit cell volume reaches a minimum when x=0.3. Up to now, no satisfactory explanation of these phenomena has been found. In this letter, an explanation based on the ionic cohesive energy with a small additional metallic cohesive energy is described.

Study of the free energy of the La1−xCaxMnO3 manganites based on the temperature dependence of the crystal cell volume

A study of the free energy of the perovskite manganites was performed by fitting experimental results for the temperature dependence of the crystal cell volume using a detailed semiclassical model of the free energy. The fitted results coincide with the experimental results for the samples La1−xCaxMnO3 (x=0.2, 0.25, 0.3, and 0.5). The free energy includes the energy corresponding to thermal vibration of the crystal lattice, the ionic cohesive energy, and additional energies produced by electrical carriers from the double exchange mechanism and the thermal activation of small polarons.

Structural and magnetic properties of self-doped perovskite manganites La0.8-xSr0.2MnO3−δ

Self-doped perovskite manganites with nominal composition La0.8−xSr0.2MnO3−δ (0 ≤ x ≤ 0.20) have been prepared by the sol-gel method. The highest heat treatment temperature used was 1073 K. The x-ray diffraction (XRD) patterns indicate that the samples had a single phase with the ABO3 perovskite structure when the doping content was x ≤ 0.10 and that when the doping content was x ≥ 0.15 the samples had two phases with the ABO3 perovskite structure being the dominant phase and Mn3O4 being the minor phase. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions were estimated for the perovskite phases in which there are Mn2+ ions and no vacancies at the A sites. The ion contents have been corrected by Rietveld fitting of the powder sample x-ray diffraction data. Magnetic measurements indicated that the evolution of the Curie temperature (TC) vs. the Mn4+ ion content ratio at the B sites of the ABO3 structure is in accord with experimental results for La1−xSrxMnO3 samples.

Behavior of Mn2+ in perovskite manganites with nominal composition La0.6−xNdxSr0.1MnO3

The fact that there are Mn2+ at the A sites in the ABO3 perovskite phase of manganites with the nominal composition La0.6−xNdxSr0.1-MnO3 showed by detailed experimental study and theoretical calculations. The magnetic moments of these Mn2+ are antiparallel to those of the Mn ions at the B sites. The content of the Mn2+ increases as the average ionic radius, 〈rA〉, of the ions at A sites decreases, resulting in the experimentally observed phenomenon that the content of the Mn3O4 phase in the manganites decreases with decreasing 〈rA〉.