A key aspect of cobalt oxides that eclipses the conventional charge, spin and orbital degrees of freedom, is the possible variety of spin states of the cobalt ions. For this reason, cobalt layered perovskites represent a unique system that allows the study of the effect of spin-state degrees of freedom on the metal-insulator charge- and spin-ordering transitions.

The complex oxide TbBaCo2O5.5 contains Co3+ ions in octahedral or pyramidal coordination of oxygen atoms. Each Co3+ ion can be found in a low, intermediate, or high spin state (LS, IS, or HS) having different ionic radii. TbBaCo2O5.5 shows an insulator-to-metal transition on heating, and at the same temperature, Tc~335 K, there is a structural change from Pmma (2acx2acx2ac) to Pmmm (acx2acx2ac). This transition agrees with a partial ordering of cobalt ions in different spin and orbital states. The deactivation of a “spin blockade” mechanism (associated with the LS states) has been suggested for this insulator-to-metal transition, assuming that heating would promote HS states and suppress LS states [1].

There are several reasons to also expect a change in structural and transport properties of insulating oxides of 3d-metals as a function of pressure. Pressure could favour the orbitally-disordered phase thus giving an increase of conductivity. A change of spin-state for cobalt ions from HS or IS to LS is accompanied by a decrease of ionic radius, thus pressure could favour the spin conversion toward LS states. In this case one would not expect an increase of conductivity under pressure since LS states effectively block electron mobility via the “spin blockade” mechanism. One more possible pressure effect may be foreseen from symmetry analysis that links the structure and conductivity via an electron transfer integral through the Co-O-Co path. For this mechanism, one would expect a symmetry change under pressure.

As no change in physical and structural properties has been found so far between ambient pressure and 1.2 GPa [2], the “spin blockade” has been considered the primary mechanism for the temperature induced metal-to-insulator transition in rare earth cobaltites.

Fig. 6: Diffraction patterns for TbBaCo2O5.48 with experimental points, fitted profile, difference curve (bottom), and positions of Bragg reflections (vertical bars).
a) P=1.48 GPa; the inset shows the unit cell dimensions as a function of pressure.
b) P=16.55 GPa; the inset shows the high temperature/high pressure orthorhombic Pmmm (acx2acx2ac) structure. Colour code: Tb (black), Ba (grey), cobalt ions are coordinated by oxygen forming octahedrons and pyramids.

By combining diffraction and transport measurements, we have recently shown that there is an insulator-to-metal transition, but at pressures higher than probed before, at ~10 GPa (Figures 6 and 7). In the same pressure range we observe changes in resistivity and in the ratio of the lattice constants of the average orthorhombic Pmmm (acx2acx2ac) cell; the same behaviour holds for the spontaneous strains. The change in the ratio of the lattice constants is very similar to that observed as a function of temperature (Figure 7), thus suggesting a common mechanism for the two transitions. We can exclude the spin blockade mechanism as candidate, as it should be suppressed under pressure due to the higher ionic radius of HS ions. Temperature favours disorder via the entropy term, but pressure does not; ordering of spin states can therefore be excluded as well as common mechanisms. We propose instead that the change of the Co-O-Co angle affects the electron transfer integral, and consequently the conductivity. This mechanism assumes a strong link between structural deformation and transport properties while the spin-state degree of freedom only plays a secondary role.

Fig. 7: Ratio of unit cell dimensions as a function of a) temperature, and b) pressure. c) Temperature dependence of the resistivity measured at different pressures. The inset shows the pressure dependence of the resistivity at 298 K.



[1] A. Maignan, V. Caignaert, B. Raveau, D. Khomskii and G. Sawatzky, Phys. Rev. Lett. 93 026401 (2004).
[2] A. Podlesnyak, S. Streule, K. Conder, E. Pomjakushina, J. Mesot, A. Mirmelstein, P. Schützendorf, R. Lengsdorf, and M.M. Abd-Elmeguid, Physica B-Condensed Matter 378-80, 537 (2006).


Principal publication and authors

D. Chernyshov (a), G. Rozenberg (b), E. Greenberg (b), E. Pomgakushina (c) and V. Dmitriev (a), Phys. Rev. Lett. 103, 125501 (2009).
(a) Swiss-Norwegian Beam Lines at ESRF, Grenoble (France)
(b) School of Physics & Astronomy, Tel-Aviv University (Israel)
(c) Laboratory for Developments and Methods, PSI Villigen (Switzerland)