Intraatomic vs Interatomic InteractionsJohn B. Goodenough (University of Texas at Austin) DMR 055663

Intellectual MeritLocalized electrons have stronger intraatomic interactions, itinerant electrons have stronger interatomic interactions. Presently we are monitoring the evolution of structural and electronic properties at the crossover from localized to itinerant electronic behavior in AMO3 perovskites in which only the π-bonding orbitals of the transition-metal M-cation d orbitals are occupied. The orthorhombic RVO3 perovskites (R = rare earth) have two localized π-bonding 3d electrons per V3+ ion, and a threefold orbital degeneracy in a cubic site leaves an orbital angular momentum L to give an intraatomic spin-orbit coupling L S to compete with the interatomic spin-spin Si Sj interactions responsible for magnetic order below a TN. We have previously shown that a local VO6/2-site distortion accompanies orthorhombic crystal symmetry to bias an orbital ordering below a T OO > TN that suppresses L for R = Pr Lu. We show here that in LaVO3 where TN ≈ TOO, an interatomic spin-spin frustrated by competition from (L S) coupling is resolved by the orbital ordering.

Intraatomic vs Interatomic Interactions John B. Goodenough (University of Texas at Austin) DMR 055663

Broader ImpactOur use of high pressure as a variable for studying the evolution of physical properties at the crossover from localized to itinerant electronic behavior has resulted in collaboration with the Institute of Physics, Chinese Academy of Science in Beijing, China, and with the Institute for the Study of the Earth’s Interior, Okayama University in Japan. We illustrate with a figure from a collaborative paper, C.-Q. Jin, J.-S. Zhou, J.B. Goodenough, Q.Q. Liu, J.G. Zhao, L.X. Yang, Y. Yu, R.C. Yu, T. Katsura, A. Shatskiy, and E. Ito, Proc. Nat. Acad. Sci. 105, 7115 (2008). Our synthesis of BaRuO3 has interested the geoscientists as it represents a phase that can exist under the pressure of the Earth’s interior. Sr1-

xBaxRuO3 is an itinerant-electron ferromagnet. In Sr1-xCaxRuO3, breaking of spin-spin interactions between Ru neighboring Ca is responsible for the Griffiths phase.

Phase diagram of the magnetic transition temperatures versus the tabulated average A-site ionic radius 〈 rA 〉 for Sr1-yBayRuO3 and Sr1-

xCaxRuO3

Notes

The evolution of electronic and structural properties at the crossover from localized to itinerant electronic behavior continues to reveal surprises. In oxides, the occupied d orbitals of an octahedral-site transition-metal ion lie in the energy gap between empty and filled s and p bands of the cations and anions, respectively. In an AMO3 perovskite, the strength of the interatomic MOM interactions vis à vis the intraatomic interactions on a transition-metal M cation determine whether the 3d electrons remain localized or are transformed into itinerant electrons occupying one-electron band states. Cubic crystalline fields at an MO6/2 site split the fivefold-degenerate d orbitals into a threefold-degenerate set of t orbitals that π-bond with the bridging oxygen and a higher energy twofold-degenerate set of e orbitals that -bond with the bridging oxygen. This splitting does not completely quench the orbital angular momentum of a tne0 configuration that is orbitally threefold-degenerate (i.e. n = 1,2; 5,6).

The strength of the MOM π-bond interactions depends on the amount of O2p wavefunction covalently admixed into the ligand-field t orbitals; as the covalent mixing increases, there is a transition from localized to itinerant electronic character. Since the O2p orbitals that π-bond with the Mcation t orbitals -bond with the A cations, the strength of the π-bond interactions can be modulated in an isoelectronic MO3 array by the acidity and size of the A-site counter cation. Therefore, the perovskite structure allows modulating the strength of the MOM interactions of π-bonding electrons by substitution of isovalent A-site cations, as also does the application of hydrostatic pressure. It is this situation that underlies the experimental strategies on the RTiO and RVO3 perovskites (R = rare earth) and on the ACrO3 and ARuO3 perovskites (A = alkaline earth) that we are studying. Reported here are experiments on LaVO3 where the V3+: t2e0 configuration is

localized. Note particularly how the thermal conductivity (T) is low (Figure shows -1(T)) in the paramagnetic phase due to the orbital fluctuations associated with spin-orbit coupling whereas (T) rises (-1(T) falls) sharply below TN and climbs to the phonon values of LaGaO3 below 100 K. We have also been studying the isoelectronic system Sr1-xCaxCrO3 where the t2 configuration is transformed to itinerant electrons in a π* band of t-orbital parentage; it exhibits a transition from an enhanced Pauli paramagnetism in SrCrO3 to itinerant-electron antiferromagnetism in CaCrO3.

We have been collaborating with the Chinese and Japanese on the Sr1-xCaxCrO3 and the Sr1-

xBaxRuO3 systems that are prepared under high pressure.

See attached figure for the structure of the AMO3 perovskites.

John B. Goodenough (University of Texas at Austin) DMR 055663

Ideal cubic AMO3 Perovskite StructureCooperative rotations of the MO6/2 octahedra about the [001] axis give tetragonal symmetrys about the [i10] axis give orthorhombic symmetry.

AM

John B. Goodenough (University of Texas at Austin) DMR 055663