結構和輸運性質
(1. 華中科技大學 材料科學與工程學院, 武漢 430074;
2. 南昌大學 材料科學與工程學院， 南昌 330047；
3. 南京大學 固體微結構國家重點實驗室， 南京 210093)
摘 要: 用固相反應法制備了La(1-x)2/3Ca1/3MnO3（V-LCMO）和
(La(0.7-x)Y0.3)2/3Ca1/3MnO3)(V-LYCMO)(x=0, 0.02, 0.04, 0.08, 0.10)系列La空位錳氧化物樣品。 X射線衍射（XRD）分析表明： 所有的樣品均為單相， 具有正交對稱性(Pnma)。 La(1-x)2/3Ca1/3MnO3系統(tǒng)和(La(0.7-x)Y0.3)2/3Ca1/3MnO3系統(tǒng)的晶格參數(shù)以及Mn—O鍵長和Mn—O—Mn鍵角隨La空位濃度不同而改變，說明在室溫系統(tǒng)中存在Jahn-Teller效應。 對于V-LYCMO系統(tǒng)，其晶格參數(shù)、 Mn—O鍵長和Mn—O—Mn鍵角都比V-LCMO相應的值更小, 這可能與離子半徑小的Y3+部分替代La3+導致更大的晶格畸變有關。 V-LCMO系統(tǒng)隨著空位濃度的增大，絕緣體-金屬的轉變溫度TMI幾乎不變。 在x=0.04時MR值在溫度TMI處達到最大值， 約為220%（8T）。 而對V-LYCMO系統(tǒng)， 其轉變溫度TMI約為50 K, 在溫度40~50 K左右最大的磁電阻值超過106%。認為很大的磁電阻效應與用離子半徑小的Y部分替代離子半徑大的La有關，它會破壞雙交換作用， 從而導致Jahn-Teller效應。
關鍵字: La(1-x)2/3Ca1/3MnO3； (La(0.7-x)Y0.3)2/3Ca1/3MnO3； La空位； 正交對稱； Jahn-Teller效應
manganese oxides (La(1-x-y)Yy)2/3Ca1/3MnO3
with rare earth vacanties
（ 1. School of Materials Science and Engineering,
Huazhong University of Science and Technology, Wuhan 430074, China;
2. School of Materials Science and Engineering,
Nanchang University, Nanchang 330047, China;
3. State Key Laboratory of Solid Microstructures,
Nanjing University, Nanjing 210093, China）
Abstract: Some La-vacancy samples of manganese oxides La(1-x)2/3Ca1/3MnO3 and (La(0.7-x)Y0.3)2/3Ca1/3MnO3(x=0, 0.02, 0.04, 0.08, 0.10) were prepared by solid-state reaction method. All the samples are single phase, with orthorhombic symmetry(Pnma), analyzed by XRD. The lattice parameters, Mn—O bond length and Mn—O—Mn bond angle of both systems vary with different La-vacancy concentration, which indicates there exists Jahn-Teller effect in the system. For V-LYCMO system, its lattice parameters, Mn—O bond length and Mn—O—Mn bond angle are smaller than those of V-LCMO system， which may be related to the bigger lattice distortion. For V-LCMO system, a metal-insulator transition is observed at about -267 K. The maximum of the magnetoresistance value is about 220% at TMI when x is 0.04. For V-LYMCO system, TMI is observed at about 50 K, a very larger magnetoresistance up to about 106% is observed at 40-50 K. It is suggested that it may be related to the partial substitution of La3+ cation with smaller Y3+, which may destroy the double exchange interaction of Mn—O—Mn, giving rise to Jahn-Teller effect.
Key words: La(1-x)2/3Ca1/3MnO3； (La(0.7-x)Y0.3)2/3Ca1/3MnO3； La-vacancy； orthorhombic symmetry； Jahn-Teller effect
