Probing the structure of framework materials by high pressure and the example of a magnetic, non-porous coordination polymer.

Nestled between the modest pressures employed to study protein folding (a few hundred megapascals) and the very high pressures obtained in shock-wave experiments (in the order of terapascals), a vast pressure scale can be explored experimentally and computationally in the physical and life sciences. Pressure is a powerful thermodynamic variable that enables the structure, bonding and reactivity of matter to be altered. In materials science it has become an indispensable research tool in the quest for novel functional materials. Examples include the target synthesis of new classes of materials with unique physical properties, see for instance the recently reported ultra-thin ‘diamond nanothreads’ obtained by decompressing benzene from 20 GPa to ambient pressure (Fitzgibbons et al., 2015). Materials scientists can exploit the effectiveness of pressure for probing and tuning structural, mechanical, electronic, magnetic and vibrational properties of materials in situ; crystallography plays a crucial role here, enabling on one hand the unravelling of structural phenomena through a better understanding of interactions, and on the other hand the derivation of structure–property relationships. Crystalline framework materials have long established themselves as the subject of intense interdisciplinary research activity; several classification systems and topological descriptors exist and the terminology found in the literature is rich and varied. In the simple and somewhat arbitrary view of this author, these materials can be divided into two main classes based on property, namely materials that exhibit porosity and those that do not. Porosity allows for potential applications in, inter alia, energy storage, catalysis, separation and capture technologies. Examples of industrially relevant framework materials displaying porosity include zeolites, at the forefront of the family of inorganic open-framework materials (Cheetham et al., 1999), and metal–organic frameworks (MOFs), which in the literature appear under the heading of hybrid inorganic organic framework materials (Cheetham et al., 2006) and, following the preferred definition by IUPAC (Batten et al., 2012, 2013), under the heading of coordination networks, a subclass of coordination polymers. In the past decade MOFs have sparked an international frenzy of research activity. Porous framework materials have been widely investigated by highpressure crystallography: zeolites have been recently reviewed (Gatta & Lee, 2014) and a review on the subject of ‘MOFs under high pressure’ by S. A. Moggach will appear later in 2015 as a feature article in Acta Crystallographic Section B. In a very recent perspective in Chemistry of Materials, Coudert (2015) provides an inspiring overview of porous and non-porous ‘stimuli-responsive’ framework materials, i.e. those framework materials that by virtue of their flexibility undergo marked structural changes (‘changes of large amplitude’) in response to external stimuli such as pressure, temperature or light. High-pressure structural investigations of dense, i.e. non-porous, framework materials based on coordination compounds have received somewhat less attention compared with their porous counterparts. However, with pressure promoting effects such as magnetic crossover, spin transitions, negative linear compressibility (NLC; for a very recent and elegant review on this subject see Cairns & Goodwin, 2015), changes in proton conductivity, or even phase transitions that generate porous structures, high-pressure crystallographic studies on dense framework materials are markedly on the rise. More generally, coordination compounds are a fascinating class of materials for high-pressure crystallographic studies (Tidey et al., 2014; Moggach & Parsons, 2009). Compared with purely organic compounds, they have an inherent extra degree of flexibility for ISSN 2052-5206