La0.25Ca0.75MnO3的晶体结构研究

用X射线衍射仪测量了La0．25Ca0．75MnO3多晶粉末在300K和70K时的晶体结构衍射谱，并用Rietveld全谱拟合法对这些数据进行了拟合．结果表明，在室温时，La0．25Ca0．75MnO3由于MnO6八面体的Jahn-Teller晶格畸变，其晶体结构已发生了扭曲，晶格常数分别为a=0．53559nm，b=0．75537nm，c=0．53587nm．温度降至70K时，X射线衍射谱上出现了比室温更多的衍射峰，有些峰发生了“双峰”劈裂，但仍然具有与室温相同的晶体对称性(空间群为Prima)．70K时的晶格常数分别为a=0．53673nm，b=O．74785nm，c=O．53718nm，与300K时的晶格常数比较，a和c分别增大了O．21％和O．24％，而b减小了1．O％，这一晶体结构的巨大变化源于在电荷有序态下形成了静态的合作Jahn-Teller效应．

STUDY OF CRYSTAL STRUCTURE OF La0.25Ca0.75MnO3

Crystallographic data of La-{0.25}Ca-{0.75}MnO-3 at room temperature and low temperature (70K) are obtained by Rietveld analyses of the high resolution powder X-ray diffraction data collected by using a powder X-ray diffraction instrument. It was found that both the low temperature and room temperature crystal structures have the Pnma symmetry. Even at room temperature, the MnO-6 octahedra are Jahn-Teller distorted, which lead to the orthorhombic lattice (a0.53559nm, b0.75537nm, c0.53587nm) of the sample. With decreasing temperature, there is a splitting of the Bragg peaks, indicating a large change of the lattice constants. Compared with those observed at 300K, the a and c axis increase about 0.21% and 0.24%, respectively, while the b axis decreased about 1.0%. These large changes of the lattice constants were attributed to the formation of static cooperative Jahn-Teller distortion of the MnO-6 octahedra in the charge ordered state.