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Designing Polar and Magnetic Oxides in the A2BB'O6-⁠Type Corundum Derivatives

Friday, 25.08.2017 10:00 to 11:00

Speakers: Martha Greenblatt (Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey)
Place: lecture hall of IoP, Slovanka
Presented in English
Organisers: Department of Dielectrics
Abstract: Polar and magnetic oxides are fundamentally and technically important, but difficult to prepare. Recently, we were able to synthesize, at high pressure and temperature (HPT) in a Walker-⁠type multi-⁠anvil cell, a number of new compounds, A2BB′O6 in the corundum-⁠derived and perovskite structure with unusually small A-⁠site cations [1–5]. At HPT the crystal structures of these A2BB′O6 phases allow the incorporation of strong magnetic transition metal ions on all cation sites for magnetic and potentially multiferroic, or magnetoelectric behavior and applications in spintronics. Our aim was to design room-⁠temperature polar ferri-⁠ or ferro-⁠magnets by composition modulation of A2BB′O6 phases. So far, we have successfully prepared a series of polar and magnetic oxides and systematically investigated the relationship between the crystal, magnetic, and electronic structure and physical properties. The discovery of polar antiferromagnetic LiNbO3-⁠type (R3c) Mn2FeMO6 (M = Nb, Ta) [1] predicted new polar structures with second-⁠order Jahn-⁠Teller effect ions (such as Nb5+ and Ta5+, d0) at the B′-⁠site and small ions at the A-⁠site of A2BB′O6, which has been confirmed by the preparation of Zn2FeTaO6 [2]. In the Ni3TeO6-⁠type (R3) ferrimagnetic semiconductor Mn2FeMoO6 (Tc ~ 340 K) [3], the polarization of the structure, is found to be stabilized by the spin structure at high pressure, while at ambient pressure, a new spin structure with lower energy state induces an unusually low-⁠temperature (~400 – 550 K) cationic rearrangement, which provides a new way to tune the physical properties at the atomic-⁠scale, under relatively mild conditions, of bulk oxides. In polar ferrimagnetic Mn2FeWO6 with Ni3TeO6-⁠type structure the charge and size difference between Fe2+ and W6+ leads to a fully ordered Fe/⁠W lattice and several exotic magnetic phases [4]. Other A2BB′O6 compounds with perovskite or distorted perovskite structures and interesting magnetic properties were also synthesized at HPT, such as Mn2FeReO6 which is half-⁠metallic with large magnetoresistance and orders ferri-⁠magnetically at 520 K [5]. While all of these materials are multiferroic, none studied thus far exhibits ferroelectric switching; the search continues.

[1] M.-⁠R. Li et al., Angew. Chem. Int. Ed. 52, 8406 (2013).
[2] M.-⁠R. Li et al., J. Am. Chem. Soc. 136, 8508 (2014).
[3] M.-⁠R. Li et al., Angew. Chem. Int. Ed. 53, 10774 (2014).
[4] M.-⁠R. Li et al., Adv. Mater. 27, 2177 (2015).
[5] M.-⁠R. Li et al., Angew. Chem. Int. Ed., 54, 1 (2015).
[6] G.-⁠H. Cai et al., Polar Magnets in Double Corundum Oxides, Chem. Mater. (in press).