Abstract

The microstructure of La 2− x Sr x CuO 4± δ crystals has been studied by neutron diffraction. Profile analysis of the intensity of Bragg reflections shows that the large crystals consist of domains of four different orientations, which are related to the symmetry reduction of the phase transition I4/mmm-Abma. The domain structure has a strong influence on the extinction, therefore it may be studied macroscopically. The microstructure is formed at the phase transition and does not change in the orthorhombic phase. After a first cooling cycle the formation of the domains appears to be reversible without a temperature hysteresis. The same domain structure appears in consecutive cooling cycles. This indicates a pinning of the domain structure by lattice defects. A data set of Bragg reflection intensities obtained on a twinned La 1.93Sr 0.07CuO 4 crystal at room temperature could be refined with special consideration of the microtwinning. The achieved precision of structural parameters is almost comparable to structural analyses on monodomain single crystals.

Highlights

  • The system (La/RE)2_xMxCuO4±, (RE=rare earth, M = Sr, Ba) contains a number of structurally different phases whose stabilities depend on the incorporated RE metal and on the earth alkali substitution as well as on the oxygen stoichiometry

  • At high temperatures pure La2CuO4 crystallizes in a tetragonal structure (HTT) of K2NiF4 type. The stability of this phase can be discussed with reference to the tolerance factor between the bond lengths of the CuO and L a / R E / M - O polyhedra [ l ]

  • The first response of the system to this mismatch is a strong Jahn-Teller distortion of the CuO6 octahedra, which are elongated in the c-direction, thereby shifting the apical oxygen closer to the La/RE/M sites

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Summary

Introduction

At high temperatures pure La2CuO4 crystallizes in a tetragonal structure (HTT) of K2NiF4 type. The stability of this phase can be discussed with reference to the tolerance factor between the bond lengths of the CuO and L a / R E / M - O polyhedra [ l ]. Two further structural modifications exist in the mixed system (La/ RE)2_xM~CuO4, commonly called T' and T* [ 12,13 ] They are both tetragonal at room temperature and become superconducting by doping with electrons and holes, respectively. Crystals seems to be valuable as such crystals are increasingly used in recent research

Experimental
Method
Characterization o f the microstructure
Influence of the microstructure on the extinction
Temperature dependence of the domain structure
Structure analysis
Comparison of different structural models
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