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Giant Magnetoelastic Effect in a 5d Transition-metal Oxide
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Figure 1 (left). Crystal structure of Ba3BiIr2O9, showing Ir2O9 face-sharing octahedral dimers (grey), BiO6 octahedra (purple), and Ba atoms (green). Figure 2 (right). Electron localization function from ab initio calculations for the Ba3BiIr2O9 structure above (HT) and below (LT) the transition temperature T*. The red localization domain encloses a single electron localization function maximum of the Ir−Ir bond at high temperature, which vanishes at low temperature. |
The phenomenon of negative thermal expansion, where the volume of a material expands anomalously on cooling, can arise through a range of mechanisms. Some are essentially mechanical, based on the thermal motion of coupled rigid units (e.g., ZrW2O8), while others involve a redistribution of electron density, often associated with changes in magnetic properties (e.g., the Ni-Fe alloy known as Invar).
The 6H-type perovskite compound Ba3BiIr2O9V is an entirely novel example of the latter, magnetoelastic, case [1]. Its structure contains face-sharing Ir2O9 bi-octahedra with direct bonds between Ir4+ cations (Fig. 1). On cooling through T* = 74 K, the length of this Ir–Ir bond suddenly increases by 4%, producing a giant 1.0% thermal volume contraction (Fig. 3, left), accompanied by a sharp drop in magnetic susceptibility (Fig. 3, right).
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| Figure 3. Temperature dependence of (a) the unit cell volumeV, (b) the lattice parameters a, b/√ 3, and c , (c) the Ir − Ir intradimer distance and mean Ir − O− Ir bond angles, and (d) bond valence sums (BVS) for Ir and Bi in Ba3BiIr2O9. |
Data were obtained from Rietveld-refinement against neutron powder diffraction data from the ECHIDNA high-resolution powder diffractometer. Where not apparent, error bars are smaller than symbols. Solid lines are guides to the eye.
Based on the experimental data analysis we have ruled out the onset of any long-range magnetic, charge, or orbital order, as well as intermetallic charge transfer or disproportionation, which might otherwise explain the drastic changes in structure and physical properties below T *. Instead, the transition appears to be driven by a dramatic change in the interactions among Ir 5d orbitals, at the crossover between two competing ground states: one that optimises direct Ir–Ir bonding (at high temperature); and one that optimises Ir–O–Ir magnetic superexchange (at low temperature).
Above T*, the distribution of 5d electrons on Ir4+ is dominated by the drive to optimize the Ir−Ir bond, shortening that distance to maximize orbital overlap, resulting in electron delocalization, while below T*, the distribution is dominated by the drive to optimize superexchange, increasing the Ir−O−Ir bond angles toward 90° by lengthening the Ir−Ir distances resulting in electron localization and local moment formation.
The transition lowers the total energy of the system, but must occur in each isolated dimer coherently throughout the crystal, hence its first-order nature. Analysis of the electron localization function based on Density Functional Theory calculations using the ANSTO Computing Cluster supports this interpretation. Above T*, a single bonding domain consisting of an electron localization function maximum (a so-called “bonding attractor”) is present between Ir sites in the dimer (Fig. 2). Below T*, this bonding attractor vanishes, the Ir−Ir intra-dimer distance increases, and the unpaired Ir 5d electrons are localized closer to the ionic core.
There is nomagnetoelastic transition in the analogous Bi3RIr2O9, when R is a 3+ rare-earth element, and this suggests that bismuth in the nominal +4 oxidation state plays a critical role in forming the electronic structure of Ba3BiIr2O9. This will be further investigated using chemically doped compositions. Also, the spin-gap opening expected to accompany the transition in Ba3BiIr2O9 will be probed with inelastic neutron scattering or muon spectroscopy.
Authors
Wojciech Miiller, Qingdi Zhou, Brendan Kennedy, Siegber Schmid, Peter Blanchard and Chris Ling (Sydney University), Maxim Avdeev, Neeraj Sharma, Ramzi Kutteh and Gordon Kearley (Bragg Institute, ANSTO) and Kevin S. Knight (ISIS).
References:
- W. Miiller, M. Avdeev, Q. D. Zhou,B. J. Kennedy, N. Sharma, R. Kutteh, G. J. Kearley, S. Schmid, K. S. Knight, P. E. R. Blanchard and C. D. Ling, J. Am. Chem. Soc. 134, 3265-3270 (2012).