Electric And Magnetic Characteristics Of Several Ba Based ABCO₄-Type Oxide Ceramics

ABCO₄-type oxides form a novel group of significant electronic materials, with abundant structure information and physical properties. In this thesis, three kinds of Ba based compounds with ABCO₄ structure were investigated: i) BaLaA1O₄ microwave dielectric ceramics;ii) BaLaNiO₄ antiferromagnetic and semiconductor ceramics;and iii) Ba_1-xLa^1+xCoO₄ ferromagnetic and semiconductor ceramics.There are two kinds of primary structures with formula of ABCO4, namely, tetragonal K₂NiF₄ and orthogonal K₂SO₄. The former with better properties was the primary object of the present work. For the bigger size of Ba²⁺, when it occupied A site, the strong mismatch energy would lead to the instability of the system. To improve the stability, it was inclined to reduce the system energy by forming compounds in nonstoichiometric proportion with lower content of big size ions, or by distortion of the lattice. So Ba_0.89La_1.11NiO₄å’Œ Ba_1.52La_0.48CoO₄ were prepared in this project. If it could not eliminate the excess energy in this way, it was likely to become K₂SO₄-type, such as BaLaAlO₄ in this project.The ceramics with dense and homogeneous microstructure could be obtained by sintering temperature at 1475°C, which would have high stability of frequency and lower influence by temperature. BaLaA104 dense ceramics showed good microwave dielectric characteristics: dielectric constant of 15.4 and Qf of 5259GHz. Its ε_r was lower than that of Sr, Ca based ABCO4 ceramics, which derived from the smaller tetrahedron of AlO₄ in different orientation compared with that of Sr, Ca based ABCO4.Both BaLaNiO₄ and Ba_1-xLa_1+xCoO₄ are magnetic compounds. The former transformed from paramagnetism to supperparamagnetism at 35K, while the latter from paramagnetism to ferromagnetism at 185K and from ferromagnetism to at 60K, accompanied. Ba_1-xLa_1+xCoO₄ ceramics have magnetic resistance effect below 215K. The compounds with x=0 behave strong minus magnetic resistanceeffect with MR≈-63.5%, while that with x=0.15 have weak positive magnetic resistance effect with MR≈12% . The resistivity decreases with increasing field for the enhancement ofmagnetic order. By contrast, no magnetic resistant was observed.It was found that Griffiths abnormal phenomenon existed in many ferromagnetic oxide with K2NiF4-type ABCO4 structure including Ba].xLai+xCoO4. That was the deviation of 1/%—T curve to Curie-Weiss rule, which increased with reducing field. It was believed that big-spin short-order ferromagnetic cluster accounted for it very well. Meanwhile, spin glass was observed in ceramics, the compound with x=0.2 have spin glass transformation at Tsg=8K. The M-T curve under ZFC and FC model separate from each other at Tb =180K, closed to the temperature where paramagnetism transformed to ferromagnetism.They share many electronic magnetic properties in common. The temperature dependence of resistivity from 2K to 500°C was effected by the interaction of spin magnetic moment and heat energy. The transition point at lower temperature was observed near magnetism transition point for the change of the effect of magnetic moment. The inflection point at higher temperature where it transit from semiconductive to metallic properties derived from the rapid elevation of heat energy. The variation of dielectric constant with temperature from 150°C to 280°C for BaLaNiO4 ceramics showed two broad peaks at 150°C and 280°C. while Bai.xLai+xCoO4 at -100°C and 400°C. At the same time, both dielectric losses displayed peaks at -15 °C and 200 °C, 150°C and 400 °C, respectively. For the effect of magnetic moment declined and was lower than that of heat energy at higher temperature, the appearance of peaks at high temperature could be explicated by traditional mechanism of electronic polarization and ionic polarization. The dielectric constant peak of Bai_xLai+xCoO4 ceramics at -100°C as well as two peaks of loss at lower temperature resulted from the interaction between confinement of electron spin and heat attractive polarization.