Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; hence the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. Therefore, it is necessary for the material science community to develop new strategies to create these materials.
Recently, the Greenblatt lab has developed a new approach for the synthesis of unique polar oxide materials. Polar and magnetic materials with LiNbO3-type (ABO3), corundum-related structure, for the first time, were extended to the well known A2BB'O6 family with two kinds of cations over the B-site, by high pressure (5-10 GPa) and high temperature (up to 1500 C) synthesis in collaboration with Prof. Dave Waker in Columbia University. In addition to the polar and magnetic properties, second harmonic generation and pyroelectric behavior were also observed in these materials. The discovery of these A2BB'O6 phases opens up a huge range of possibilities in A2BB'O6 oxides, due to the variety of possible cations and structural versatility of cation arrangements at both the A and B sites. Both experimental and theoretical studies show that the coexistence of B'-site cations with d0 electronic configuration with unusually small A-site cations lead to structural distortions under pressure that result in polar properties. This finding is of major significance to the current renaissance in polar oxide research. Moreover, the appropriate combination of nonmagnetic A, magnetic B and d0 B’ ions in new A2BB'O6 oxides could lead to new magnetoelectric oxides critical for the fundamental understanding and applications of the emerging field of spintronics.
Man-Rong Li, David Walker, Maria Retuerto, Tapati Sarkar, Joke Hadermann, Peter W. Stephens, Mark Croft, Alexander Ignatov, Christoph P. Grams, Joachim Hemberger, Israel Nowik, P. Shiv Halasyamani, T. Thao Tran, Swarnakamal Mukherjee, Tanusri Saha Dasgupta, Martha Greenblat, Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis, Angew. Chem. Int. Ed. 2013, 52, 8406-8410. DOI: 10.1002/anie.201302775