結(jié)構及磁熵變的影響
(哈爾濱理工大學 材料科學與工程學院，哈爾濱 150080)
摘 要: 采用溶膠-凝膠法制備了鈣鈦礦La0.7Sr0.3-xNa xMnO3(0.05≤x≤0.3)系列樣品。結(jié)果表明：由于Na+離子半徑(0.102 nm)小于Sr2+離子半徑(0.127 nm)，導致La0.7Sr0.3-xNaxMnO3(0.05≤x≤0.3)樣品的結(jié)構隨著Na+離子摻雜量的增加由正交向單斜轉(zhuǎn)變。樣品的晶胞參數(shù)a隨x的增大而增大，而c隨x的增大而減小，c/a隨x的增加而減小；樣品的形貌呈現(xiàn)不規(guī)則的顆粒狀，中間還夾雜著棒狀物；隨著Mn4+與Mn3+摩爾比的增加，A位的平均離子半徑減小及A位離子失配效應減小的共同影響下，當x≤0.2時，居里溫度隨著Na+離子摻雜量的增加而增加；當x＞0.2時，居里溫度隨著Na+離子摻雜量的增加而下降；由于Na+離子摻雜引起的容差因子的減小，晶格收縮、鐵磁耦合變小，導致居里溫度附近的最大磁熵變隨x增加而減小。
關鍵字: 鈣鈦礦；雙交換；居里溫度；最大磁熵變
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
