Looking for satellites: a powerful capability of neutrons at the ILL

Future progress will be defined by the development of new and innovative next-generation materials. Despite the magnitude of the endeavour, breakthroughs will depend on understanding at the smallest scale: fundamentally, the properties of a material depend on its structure. A recent study highlights the unique insights that can be provided by world-leading neutron expertise, instruments and technology at the Institut Laue-Langevin (ILL).

Metal-organic coordination polymers (MOCPs) are a type of material formed from metal ions connected by organic molecules into extended 2D or 3D networks. The versatility of their structure – that can be designed to tune specific properties – has generated significant interest regarding their potential as next-generation multifunctional materials.

Research into methylammonium metal formates, a family of MOCPs, has been developed at the ILL by Laura Cañadillas-Delgado and Oscar Fabelo, together with former ILL PhD student Madeleine Geers, and in collaboration with Matthew Cliffe, assistant professor at the University of Nottingham. “The cobalt- and nickel-based versions of these compounds are particularly unusual and interesting,” explains Cañadillas-Delgado. “Their structure changes from unmodulated to modulated as temperature decreases, likely causing a related change in the properties of the compound.”

An unmodulated structure refers to the classic crystal structure, composed of a unit cell that is repeated identically in three dimensions. In aperiodic modulated crystals, on the other hand, the unit cell is slightly shifted in at least one direction from the 3D lattice according to a periodic function, such as a sinusoidal wave. “Neutron diffraction is a particularly powerful technique for studying aperiodic modulated crystals,” explains Cañadillas-Delgado. “A sample placed in a beam of neutrons produces a diffraction pattern that provides information about the structure of the material. With crystals, a periodic diffraction pattern is produced, with the spacing between spots related to the dimensions of the unit cell. For a modulated structure, additional satellite reflections also appear in the diffraction pattern.”

In order to solve a modulated structure, however, a significant number of satellite reflections is required. “The ILL’s D19 diffractometer provided unique, world-leading capability for this study,” explains Cañadillas-Delgado. “The neutron wavelength enabled satellite reflections to be distinguished from the main central reflection, while also allowing information to be acquired at high angles.” The neutron flux available at the ILL – the most intense continuous flux in the world – is another requisite for the acquisition of high-angle satellite reflections, in addition to exploitable diffraction patterns for millimetre-sized samples.

The remarkably comprehensive nature of the study, recently published in International Union of Crystallography Journal (IUCrJ), was highlighted in the same issue by a commentary from Václav Petříček, professor at the Institute of Physics of the Czech Academy of Sciences and responsible for the development of the software used to analyse aperiodic crystal structures. “Generally, only the main reflections of the diffraction pattern are considered and the satellite reflections are disregarded due to the considerable effort required for their analysis,” explains Cañadillas-Delgado. “Modulated structures are thus largely ignored and I strongly suspect that a significant proportion of the compounds with interesting properties currently studied and published have unreported modulated phases.” Moreover, the findings suggest a low energy barrier between different structural phases. The transition between structures – and thus properties – could thus be controlled by an external stimulus such as pressure: a key finding for the development of smart materials.