Absttract: 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).