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August 22, 2003

Pressure Induced Atomic Structure Transformations in Manganites

C. Cui¹, T.A. Tyson¹, J.P. Carlo¹, Y. Qin¹ and Z. Zhong^2
1Physics Department, New Jersey Institute of Technology, Newark, NJ; ²National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY

The evolution of the atomic structure of the manganite La_0.60Y_0.07Ca_0.33MnO₃ 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.

The atomic structure of the MnO₆ octahedra plays an important role in manganite systems of Ln_1-xA_xMnO₃ (Ln = La, Pr, Nd, etc; A = Ca, Sr, etc.) with colossal magnetoresistance. Pressure has been used to modify the lattice and various bulk properties. Below ~2 GPa, it is argued that pressure enhances the ferromagnetic metallic state in the La_1-xA_xMnO₃ system (A = Ca, Sr, and x~0.2~0.5) by compressing the lattice constants, increasing the Mn-O-Mn bond angle, making the unit cell more cubic and hence reducing the local distortion of the MnO₆ octahedra and electron-lattice coupling. In this range pressure effects are similar to and can be scaled to those of chemical doping. In our work, we have found that this is not true at higher pressures.

La_0.60Y_0.07Ca_0.33MnO₃, the system studied, is a typical manganite system with a very high magnetoresistance of ~10000% at 6 T and a Curie temperature T_C and metal-insulator transition temperature T_MI 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 T_MI and conductivity below P*~3.8 GPa, but T_MI 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 T_MI. 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 T_MI and resistivity behavior. According to the double exchange theory: the overlap of Mn³⁺ e_g and O^2- 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, , increases abruptly (Figure 2d); the Mn-O1-Mn bond angle increases by ~20° while the Mn-O2-Mn bond angle decreases slightly. The competing mechanisms of increasing JTD and bond angle may lead to the saturation of T_MI and resistivity at P*. Above ~4 GPa, the JTD is stabilized, the Mn-O1-Mn bond angle, characterizing the tilting of MnO₆ octahedra, drops with pressure. The overlap of the O^2- 2p orbital and the Mn³⁺ e_g orbital decreases and the charge carriers are more localized with further pressure increases. Therefore, reversed metal-insulator, resistivity, and magnetic behavior occur.

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
X17B1

FUNDING
National Science Foundation Career Grant DMR-9733862 and DMR-0209243

PUBLICATION
Congwu Cui, Trevor A. Tyson, Zhong Zhong, Jeremy P. Carlo, and Yuhai Qin, "Effects of pressure on electron transport and atomic structure of manganites: Low to high pressure regimes", Phys. Rev. B 67, 104107 (2003).

ACKNOWLEDGEMENT
We are indebted to Dr. Jingzhu Hu for much experimental assistance in the pressure calibration and high pressure techniques.

FOR MORE INFORMATION
Trevor A. Tyson
Department of Physics
New Jersey Institute of Technology
Tiernan Hall, University Heights
Newark, NJ
Email: tyson@adm.njit.edu