Abstract
AbstractA perspective on computational studies of ferroelectrics and multiferroics is given that emphasises what has yet to be done, along with some subtleties in previously studied systems. Beginning with the extensive data-mining studies of Abrahams and more recently, Rabe, a survey is given of magnetostrictive effects in antiferromagnetic antiferroelectrics (after Toledano and Toledano), which has an nonmagnetic analogy in the antiferroelectric phase of tris-sarcosine calcium chloride and a reminder of the unusual spin–phonon coupling of Holden et al. in systems such as KCoF3 and EuTiO3. Attention is also paid to field-temperature phase diagrams, finite non-periodic boundary conditions, and processing-dependent structures.
Highlights
The search for new ferroelectrics has been proceeding in at least three different ways: (1) data mining of existing crystallographic data;[1] (2) ab initio calculations, by density functional theory (DFT) of new families;[2] (3) new techniques of fabrication, including high-pressure synthesis or infiltration of the gaps between surfaces.[3]
Abrahams had a specific criterion for these choices: The ionic positions must be off-centred but within small specified distances from being centred
Where r is the distance from a centred position for the ion in question; the upper limit in Equation (1) is rather arbitrary but intended to be oo the bond length for the ion to nearest neighbour(s) and compatible with switching with a modest coercive field. Using this criterion Abrahams predicted a large number of new ferroelectrics, and a significant number of these were later shown to be ferroelectric.[7,8]
Summary
The search for new ferroelectrics has been proceeding in at least three different ways: (1) data mining of existing crystallographic data;[1] (2) ab initio calculations, by density functional theory (DFT) of new families;[2] (3) new techniques of fabrication, including high-pressure synthesis or infiltration of the gaps between surfaces.[3]. In a lengthy series of papers Sidney Abrahams, formerly head of the crystallography department at Bell Labs and president of the American Crystallography Society, presented a thorough review of materials that had a good but unproven probability of being ferroelectrics.[4,5,6] Abrahams had a specific criterion for these choices: The ionic positions must be off-centred (acentric symmetry) but within small specified distances from being centred. CRYSTALLOGRAPHERS’ PROTOCOL One of the problems arising with data mining is that researchers do not always recognise the most important protocol in crystallographic structural reports: The symmetry published must be the highest compatible with the highest point group symmetry for each ion that is possible within experimental uncertainty This means that structures are often reported with centred phases whereas, their ions, the lighter ions such as O, F, and H, are probably off-centred. L3 1⁄4 μ1 - μ2 - μ3 þ μ4 for magnetic moments μ on four spin sites 1, 2, 3 and 4
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
