Na^+掺杂对钙钛矿La_(0.7)Sr_(0.3-x)Na_xMnO_3的结构及磁熵变的影响

Effect of Na doping on structure and magnetic entropy of perovskite La_(0.7)Sr_(0.3-x)Na_xMnO_3 doped with Na^+

采用溶胶-凝胶法制备了钙钛矿La0.7Sr0.3-xNaxMnO3(0.05≤x≤0.3)系列样品。结果表明:由于Na+离子半径(0.102nm)小于Sr2+离子半径(0.127nm),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。

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 Mn^4＋ to Mn^3＋, 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.