|
|
August 22, 2003 Pressure Induced Atomic Structure Transformations in ManganitesC. Cui1, T.A. Tyson1, J.P. Carlo1, Y. Qin1 and Z. Zhong2 The evolution of the atomic structure of the manganite La0.60Y0.07Ca0.33MnO3 under pressure was explored through high-pressure x-ray powder diffraction measurements. It is shown that an atomic structure transformation occurs between ~2-4 GPa, in which the Jahn-Teller distortion (JTD) of the MnO6 octahedra is enhanced, in parallel with an increase in the Mn-O1-Mn bond angle. Above ~4 GPa, the Mn-O1-Mn bond angle is reduced with the JTD remaining unchanged. These structural measurements enable a better understanding of the electrical, magnetic, and the metal-insulator transition behavior in manganites under pressure.
La0.60Y0.07Ca0.33MnO3, the system studied, is a typical manganite system with a very high magnetoresistance of ~10000% at 6 T and a Curie temperature TC and metal-insulator transition temperature TMI coincide at ~150 K. In this material, there exist strong electron-lattice and spin-lattice coupling. Our high-pressure electron transport measurements indicate that pressure increases TMI and conductivity below P*~3.8 GPa, but TMI and conductivity decrease quickly with pressures above P*. The conductivity in the range of liquid nitrogen to room temperature and the localization length due to spin disorder in the paramagnetic phase follow the same trend as TMI. This possibly suggests a pressure induced crystal structure change (local-long range).
To understand the changes in atomic structure with pressure, high-pressure powder x-ray diffraction measurements with a diamond anvil cell were carried-out. It is found that pressure induces a structural transformation within the MnO6 octahedra in the range ~2-4 GPa, in which the Jahn-Teller distortion (JTD) is enhanced and the MnO6 octahedra align. Above this range, the JTD is stable, but the MnO6 octahedra buckle as in the low pressure regime (Figure 1). The diffraction patterns were refined with the Rietveld method on
the basis of the 1 atm Pbnm space group. While the unit cell is
compressed (Figure 2a), the Mn-O bond length and Mn-O-Mn bond angle
change with pressure in a complicated manner (Figure 2b and 2c).
Below ~2 GPa, all three Mn-O bonds are compressed and the bond
angles are unchanged. This may explain the TMI and resistivity
behavior. According to the double exchange theory: the overlap of
Mn3+ eg and O2- 2p band and hence the hoping integral is increased
due to the Mn-O bond compression. From ~2 to ~3 GPa, the splitting
of the two in-plane Mn-O2 bonds increases, hence the coherent Jahn-Teller
distortion,
Transport studies reveal pressure-induced distortions of the MnO6 octahedra in the whole class of the colossal magnetoresistive materials with varying bandwidth. The identification of the structural changes under pressure may lead to a better understanding of the physics of these materials. BEAMLINE FUNDING PUBLICATION ACKNOWLEDGEMENT FOR MORE INFORMATION |