The magnetic behavior of materials is an intellectually interesting subject and also one of enormous technological importance. In both these respects, one group of materials, the 3d transition metal oxides, has attracted a great deal of attention in recent years. Research at the institute has concentrated in particular on doped LaMnO3 compounds, which show a range of unusual properties including colossal magnetoresistance.
Pressure-induced break-down of Jahn-Teller distortion and insulator to metal transition in LaMnO
3I. Loa, P. Adler, A. Grzechnik and K. Syassen; U. Schwarz (MPI-CPFS Dresden);
M. Hanfland (ESRF Grenoble)
Perovskite-type manganites have recently gained renewed interest after the observation of a colossal negative magnetoresistance (CMR) effect in La1 x(Ca;Sr)xMnO3. Materials of the class R1 xAxMnO3 (where R is a trivalent rare earth and A a divalent alkaline earth element) are currently under intense investigation. The interest stems partly from a wealth of interesting physical effects: CMR, charge and orbital ordering, metal-insulator, mag-netic, and magnetic-field induced phase transitions, which are due to an intimate interplay of structural, spin, and electronic degrees of freedom. In addition, these compounds are examined as possible magnetoresistance sensor materials.
We have investigated the effect of hydrostatic pressure on the structural and electronic properties of LaMnO3– the parent compound of the CMR materials – by angle-dispersive synchrotron X-ray powder diffraction and optical reflectivity spectroscopy. This study was motivated in part by the observation that the CMR effect is strongly pressure sensitive and the close relation between structural, magnetic, and electronic properties. Only for a small number of distorted perovskites the structural changes under pressure have been studied up to date. Deviation from the ideal cubic perovskite structure is generally attributed to a mismatch of ionic sizes. Decreasing as well as increasing distortion under pressure have been found. In contrast to other perovskites studied under pressure so far and also differing from the typical CMR materials, an additional pronounced distortion of MnO6 octahedra occurs in LaMnO3due to the Jahn-Teller effect arising from a localized t32ge1gelectron con-figuration of the Mn3+ions. Application of pressure is expected to enhance the Mn–O–Mn interactions and consequently the itinerancy of the electronic system. This leads to the ex-pectation that the Jahn-Teller effect in LaMnO3 could be reduced at high pressure which might result in structural phase transitions to higher symmetry perovskite structures.
Figure 36: Crystal structure of LaMnO3at ambient conditions (space group Pnma).
The crystal structure of orthorhombic LaMnO3 is sketched in Fig. 36 for ambient con-ditions. The Mn ions are surrounded by corner-sharing oxygen octahedra which form a three-dimensional network. Cooperative Jahn-Teller distortion of the octahedra leads to al-ternating long and short Mn–O distances in the ac-plane. The tiltings of the octahedra can be described by rotations about two axes which would be parallel to the a- and b- axis in an undistorted structure. The La ions are located in the voids between the octahedra. Qualita-tively, the same structure is adopted by the lightly doped manganites La1 xAxMnO3with x<0.1.
The pressure dependence of the crystal structure of LaMnO3 up to 40 GPa under hydro-static conditions was studied by angle-dispersive X-ray powder diffraction at the European Synchrotron Radiation Facility (Grenoble) using diamond anvil cell (DAC) techniques.
The relative changes of the lattice parameters with pressure are illustrated in Fig. 37a).
At low pressures, the a-axis compressibility is 4 times larger than that of the b and c axes. This can be attributed to a pressure-induced reduction of the Jahn-Teller distortion of the MnO6octahedra which is evident from the evolution of the Mn–O distances shown in Fig. 37b). Extrapolation of the experimental data suggests the Mn–O bond lengths to become nearly equal around 17 GPa, where the a-axis compressibility exhibits a distinct change (cf Fig. 37a)). Besides the usual overall compression of the lattice, the important effect of pressure on the crystal structure of LaMnO3 is a reduction of the Jahn-Teller induced distortion of the MnO6octahedra and also a decrease of the octahedral tiltings.
Figure 37: a) Relative changes of the lattice parameters of LaMnO3as a function of pressure. Solid lines are guides to the eye. b) Mn–O bond lengths of the MnO6distorted octahedra versus pressure.
The extrapolated distances (solid lines) become nearly equal around 17 GPa – close to the pressure, where the compressibility of the a-axis changes markedly.
With respect to the doped manganites, these findings support the suggestion that the in-creasing temperature of maximum resistivity and maximum magnetoresistance with pres-sure is due to a straightening of the Mn–O–Mn bond angles towards 180Æ and decreasing Mn–O distances. This would induce an increase of the overlap between the manganese d orbitals and the oxygen p orbitals which is a crucial parameter for the electronic conduc-tivity. Altogether, it would lead to a stabilization of the low-temperature metallic state and hence to the observed pressure effects in the doped manganites.
The structural changes of LaMnO3under pressure strongly affect its electronic properties.
Figure 38 shows optical reflectivity spectra in the pressure range 11–38 GPa at ambient temperature. The broad spectral feature around 2 eV corresponds to an optical excita-tion gap in agreement with an insulating ground state. Up to 20 GPa, there is no ma-jor change in the optical response, although the Jahn-Teller distortion of the octahedra is mostly suppressed in this pressure range. Above 25 GPa, the near-infrared reflectivity in-creases strongly. The inset in Fig. 38 shows an increase of the reflectance at 0.6 eV by two orders of magnitude, indicating an insulator to metal transition. The important effect of pressure appears to be a strengthening of the Mn–O–Mn interactions, leading to enhanced itinerancy of the system.
Figure 38: Optical reflectivity spectra of LaMnO3 at room temperature as a function of energy.
Rd denotes the absolute reflectivity of the interface between sample and diamond of the pres-sure-generating DAC. The inset shows the pressure dependence of Rdat 0.6 eV.
Apparently, high pressure in LaMnO3leads to a similar electronic situation as in the cubic perovskite SrFeO3 where isoelectronic Fe4+ ions occur. Enhanced covalency of the iron-oxygen bonding results in a suppression of the Jahn-Teller effect and metallic conductivity in SrFeO3 already at ambient pressure. Also, doping of LaMnO3 with divalent alkaline earth elements, which leads to Mn3+=Mn4+mixed-valence compounds, gives rise to sim-ilar structural and electronic changes. Moderate pressures up to15 GPa continuously suppress the Jahn-Teller effect – as does doping up to x0.1. In an intermediate range (pressure 15–25 GPa; doping x0.1–0.2), there is no Jahn-Teller distortion, but the sys-tem remains insulating. At higher doping levels, the syssys-tem becomes metallic and adopts a rhombohedral structure. In analogy, pressure above 25 GPa moves LaMnO3 into a metal-lic regime and there are in fact indications for a transition to a higher symmetry crystal structure.