结构及磁熵变的影响
(哈尔滨理工大学 材料科学与工程学院，哈尔滨 150080)
摘 要: 采用溶胶-凝胶法制备了钙钛矿La0.7Sr0.3-xNa xMnO3(0.05≤x≤0.3)系列样品。结果表明：由于Na+离子半径(0.102 nm)小于Sr2+离子半径(0.127 nm)，导致La0.7Sr0.3-xNaxMnO3(0.05≤x≤0.3)样品的结构随着Na+离子掺杂量的增加由正交向单斜转变。样品的晶胞参数a随x的增大而增大，而c随x的增大而减小，c/a随x的增加而减小；样品的形貌呈现不规则的颗粒状，中间还夹杂着棒状物；随着Mn4+与Mn3+摩尔比的增加，A位的平均离子半径减小及A位离子失配效应减小的共同影响下，当x≤0.2时，居里温度随着Na+离子掺杂量的增加而增加；当x＞0.2时，居里温度随着Na+离子掺杂量的增加而下降；由于Na+离子掺杂引起的容差因子的减小，晶格收缩、铁磁耦合变小，导致居里温度附近的最大磁熵变随x增加而减小。
关键字: 钙钛矿；双交换；居里温度；最大磁熵变
perovskite La0.7Sr0.3−xNaxMnO3 doped with Na+
（School of Materials Science and Engineering, Harbin Polytechnic University, Harbin 150080, China）
Abstract: La0.7Sr0.3-xNaxMnO3(0.05≤x≤0.3) powders were prepared by sol-gel technique. A structure transition from orthorhombic to monoclinic was observed with increasing x values in La0.7Sr0.3-xNaxMnO3. The crystal parameter a increases and c decreases with increasing x values. The c/a ratio becomes smaller with increasing addition of Na+. The morphologies of La0.7Sr0.3−xNaxMnO3 sample are irregular particles and part of sticks. This can be induced to three aspects with increasing molar ratio of Mn4+ to Mn3+, decreasing average ionic radius and lessening ionic mismatch effect. The Curie temperature TC and magnetization of La0.7Sr0.3−xNaxMnO3 powders increase with increasing x in the rang of x≤0.2. However, the maximal magnetic entropy around TC decreases with increasing x values, which can be induced to the tolerance factor decrease, crystal lattice shrink and ferromagnetic coupling decrease.
Key words: perovskite; double exchange; Curie temperature; maximal magnetic entropy
