Earl L. Muetterties Memorial Lecture: Beyond the unit cell: control of function by local structure and dynamics from fuel cell electrodes to metal-organic frameworks
Seminar | April 14 | 4-5 p.m. | Pitzer Auditorium, 120 Latimer Hall
The first lecture (April 7th) highlighted the opportunities that computation offers to accelerate the discovery of new extended solid materials. These approaches are centred on the crystal structures of the materials. Traditionally, we think about the structure of a solid in terms of the unit cell of the material derived from Bragg diffraction, but this involves averaging out local deviations such as short-range order, defects and dynamical behaviour. These deviations from the average structure are often decisive in determining the properties of the material, for example as an electrode or a catalyst. I will discuss how design that takes these non-average features into account can access important properties and interesting behaviour.
While the average structure computationally-enabled approach of the first lecture attained the difficult goal of combining polar and ferromagnetic ground states at room temperature in a single phase, the resulting material did not allow switching of the electrical polarisation, and thus did not afford the multiferroic needed for next-generation information storage. I will describe a non-computational strategy based on local structure information that produces a magnetoelectric multiferroic material able to switch both of these long-range orders at room temperature (1). The role of local chemistry in driving this real structure design emphasises the enduring importance of developing the chemical understanding that underpins classical approaches to materials design.
Synthesis of materials containing multiple distinct phases overcomes further limitations of the single phase average structure approach, and requires working outside the average structure. Using simple design ideas, we have controlled the structure of an oxide beyond the unit cell length scale by the chemical bonding-driven self-organisation during synthesis of two structurally-related phases over nanometre distances. The two phases have distinct functions that give the resulting chemically-controlled composite excellent performance as an intermediate temperature solid oxide fuel cell cathode (2). In particular, dynamic compositional exchange across the interfaces between the two phases allows the material to repair itself and remain stable under the operating conditions.
Dynamical processes are key to the function of proteins, and one of our longstanding research goals is to produce metal-organic frameworks with responses that are governed by the dynamics of the molecules that constitute them. By engineering weak interactions we can chemically control the structures adopted in response to guests, and exploit the porosity for recognition of complex molecules (3).
(1) P. Mandal et al., Nature 525, 363, 2015
(2) J.F. Shin et al., Nature Energy 2, 16214, 2017
(3) C. Marti-Gastaldo et al., Nature Chemistry 6, 343, 2014; J. Am. Chem. Soc. 2017 accepted
Light refreshments will be served at 3:50pm in The Coffee Lab