Origin of the non-linear pressure effects in perovskite manganites: Buckling of Mn–O–Mn bonds and Jahn–Teller distortion of the MnO6 octahedra induced by pressure

High-pressure resistivity and X-ray diffraction measurements were conducted on La0.85MnO3 d at 6 and 7 GPa, respectively. At low pressures the metal–insulator transition temperature (TMI) increases linearly up to a critical pressure, P 3.4 GPa, followed by reduction in TMI at higher pressure. Analysis of the bond distances and bond angles reveals that a bandwidth increase drives the increase in TMI below P. The reduction in TMI at higher pressures is found to result from Jahn–Teller distortions of the MnO6 octahedra. The role of anharmonic interatomic potentials is discussed. & 2010 Elsevier B.V. All rights reserved. The RE1 xAxMnO3 (RE1⁄4rare earth and A1⁄4Ca, Sr, Ba) mixed valence (Mn(d,t3 2ge 1 g)/Mn 4+ (d,t3 2ge 0 g)) perovskite system exhibits complex and intriguing properties and an understanding of the basic physics of these materials has still not been realized [1,2]. This system is under detailed study both from a fundamental science and as well as application perspectives. It has been found that a strong coupling exists among the lattice, spin, and electronic degree of freedom that is manifested by complex phase diagrams. The properties of manganites depend strongly on subtle changes in the structure and chemistry of the system induced by changing the RE or A site ion size [3]. Separating the changes in structure from the changes in valence can be accomplished by using a series of RE cations of varying size at fixed RE/A ratio. However, precise control of the stable crystalline form produced by substitution is not typically possible. Modification of the structure by cation substitution may alter the system in unpredictable ways. A controllable way to explore the effect of strain or pressure on these systems is to apply hydrostatic pressure and then to measure the transport and structural properties [4]. Not many high-pressure measurements have been conducted on manganites. Early temperature dependent studies of high magnetoresistance phase for pressures were conducted below 2 GPa and predicted a linear increase in the metal–insulator (MI) transition temperature with pressure [5–7]. More recent studies on the changes in the metal–insulator transition temperature (TMI) at pressures up to 6 GPa reveal that an optimal pressure is reached ll rights reserved. beyond which the transition temperature decreases with pressure [8–10]. Meneghini et al. [11] conducted studies (up to 15 GPa) on the effects of high pressure on the transport properties of La1 xCaxMnO3 (x1⁄41/4), but the electrical transport data were presented only between 190 and 380 K—limiting the access to the MI transition at high pressures. The self-doped system La0.85MnO3 d shares similar transport, magnetic, and structural characteristics with the chemically doped system La1 xCaxMnO3 in the ferromagnetic phase [12], such as the classic metal–insulator transition from a high-temperature paramagnetic insulating phase to a low-temperature ferromagnetic metallic phase, yet it is chemically simple. It achieves a high magnetization and has a low resistivity at low temperature. In order to understand the origin of the changes in the electronic structure of manganites at both high and low pressures and to compare this system with the classical ion doped systems, we conducted detailed high-pressure transport and structural studies. A polycrystalline sample of La0.85MnO3 d was synthesized in air by the conventional solid-state reaction as in Ref. [12] with three calcinations cycles. The cation ratio La/Mn 0.85 was determined by the Ionic Coupled Plasma method. High-pressure synchrotron X-ray powder diffraction measurements were conducted and Rietveld refinements were carried out to extract the detailed atomic structure following the approaches in Ref. [8]. The magnetization at ambient-pressure at 4 K (0.5 T) yielded a saturation value of 3.5 mB/Mn site (close to the theoretical limit of 3.56 mB/Mn) with a Curie temperature (244.071.0 K) near the metal–insulator transition temperature (248.071.0 K) as shown in Fig. 1(c). Fig. 1(a) shows the electrical resistivity versus temperature for pressures ranging from ambient to 5.8 GPa. At 3.4 GPa (P, the