Chih-Hung Shen

Effect of Co doping in LiMn2O4

Abstract Lithium transition-metal oxides Li(Mn2−xCox)O4 (0≤x≤0.5) are synthesized by solid state reaction. X-ray and electrochemical data show that the replacement of Mn3+ (d4) ions by Co3+ (d6) in the octahedral framework of the spinel eliminates the local disorder present in the lattice around the [Mn3+O6] octahedra. The capacity loss observed in the undoped Li/LiMn2O4 cell is about 25% after 20 cycles, whereas that for the x=0.1 and 0.2 doped spinel materials is about 0.48 and 1%, respectively. The good capacity retention of Li(Mn2−xCox)O4 (0.1≤x≤0.5) electrode is attributed to the stabilization of the spinel structure by Co doping for Mn ion sites. The chemical substitution of Co3+ for Mn3+ in LiMn2O4 improves the efficiency in maintaining electrochemical capacity over a large number of cycles without sacrificing initial reversible capacity at room temperature.

Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8−xCax)Mn2O7 manganites

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in La1.2(Sr1.8−xCax)Mn2O7 (x=0–0.8) are investigated. The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H=1.5 T) at a temperature of 102 K was observed for the x=0.4 sample. The Curie temperatures decreased from 135 to 102 K for x=0–0.4, respectively. Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration. The antiferromagnetic behavior with Neel temperature around 30 K was found in the x=0.8 sample. The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Neel temperatures.

Cr doping in the La1:2Sr1:8Mn2O7 system

The effect of doping Cr in the Mn site of the La1.2Sr1.8Mn2O7 system has been studied. Addition of Cr modifies the transport and magnetic properties of the parent phase. With increasing Cr, the insulator-metal transition observed in the parent phase is suppressed and insulating behaviour is induced. In the range of doping studied (25%), the compositions show ferromagnetic behaviour with the Curie temperature decreasing with increasing Cr. The unit cell volume also shows a decrease. However, the magnetoresistance ratio is not significantly affected. We compare these results with an earlier study of doping Cr in the three-dimensional LaMnO3 structure, where it was seen that the ferromagnetic and magnetoresistance characteristics are sensitive to Cr doping. The results of the present study suggest that the layered manganates are more accommodative to doping compared to the three-dimensional perovskites.

Phase stability study of La1.2Ca1.8Mn2O7

Abstract We report that the composition of La 1.2 Ca 1.8 Mn 2 O 7 does not form as a single phase, and consists of a mixture of La 0.6 Ca 0.4 MnO 3 and CaO. The XRD patterns show the presence of a peak at 2θ ∼ 37°, which is a specific peak to indicate the existence of CaO with space group Fm 3 m and a unit cell of a = 4.8152(3) A. After a few days, the intensity of this peak decreases and CaO transforms to Ca(OH) 2 . The electrical and magnetic measurements show that the colossal magnetoresistance (CMR) behavior corresponds to that exhibited by La 0.6 Ca 0.4 MnO 3 .

The chemical control of colossal magnetoresistance (CMR) in the new two-dimensional La1.2(Sr1.8−xCax)Mn2O7 system

The magnetic and electrical properties of the colossal magnetoresistance (CMR) La1.2(Sr1.8−xCax)Mn2O7 materials have been investigated. Based on the chemical titration method, the valence of Mn was determined to be 3.39±0.04 for all measured La1.2(Sr1.8−xCax)Mn2O7. The magnetoresistance ratio [ρ(0)/ρ(H)] was efficiently increased from ∼192% (135 K, 1.5 T) for x=0 to 208% (102 K, 1.5 T) for x=0.4 in La1.2(Sr1.8−xCax)Mn2O7. Moreover, the Curie temperature (Tc) decreased with increasing x and was strongly dependent on the apical bond lengths of Mn–O(2).

Chemical pressure controlled colossal magnetoresistance effects in La0.6(Sr0.4−xCax)MnO3☆

The effects of structural and paramagnetic to ferromagnetic transition with the isovalent chemical substitution of smaller Ca2+ into the bigger Sr2+ sites in La0.6(Sr0.4−xCax)MnO3 (0≤x≤0.4) are investigated. With increasing x, a change from the rhombohedral cell (space group: R-3c) to an orthorhombic cell (space group: Pbnm) is observed. For x≥0.3, the structural transformation leads to an increase of the Mn–O(1) bond length and the bending of the Mn–O–Mn bond along the c-axis which corresponds to the Jahn–Teller distortion of the distorted MnO6 octahedra in the orthorhombic cell. Such effects give rise to a decrease in TC (the temperature of the paramagnetic to ferromagnetic transition) with increasing Ca content.

Dimensional and Chemical Control of Colossal Magnetoresistance in New Manganites

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in new series of two-dimensional La1.2(Sr1.8-xCax)Mn2O7 compounds (x=0~0.8) and three-dimensional La0.6(Sr0.4-xCax)MnO3 compounds (x=0~0.4) are investigated. The Curie temperatures (TC) decreased from 135 K to 102 K and 370 K to 270 K for x = 0 to 0.4 in La1.2(Sr1.8-xCax)Mn2O7 and La0.6(Sr0.4-xCax)MnO3, respectively. The antiferromagnetic behavior with Neel temperature around 30 K was found in the x=0.8 sample in La1.2(Sr1.8-xCax)Mn2O7. The composition induced structural variation has been found in La0.6(Sr0.4-xCax)MnO3. All the results demonstrate that the dimensionality and chemical size play an important role in controlling the colossal magnetoresistance in manganites.

New Advanced Materials of La 1. 2 (Sr 1. 8- x Ca x )Mn 2 O 7 with Colossal Magnetoresistance

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in new series of the La1.2(Sr1.8-x Ca x )Mn2O7 compounds (x = 0 ~ 1.8) are investigated. The highest magnetoresistance (MR) ratio [ρ(0)/ρ(H)] of 208 % (H = 1.5 T) at a temperature of 102 K was observed for the x = 0.4 sample. The static strain within the compound for x = 0.4 has also been found by high resolution transmission electron microscopic techniques. The Curie temperature decreases with Ca doping from 135 K to 71 K for x = 0 to 0.8 and from 355 K to 260 K for x = 1.2 to 1.8 respectively.