feature articles 252 http://dx.doi.org/10.1107/S2052520615005867 Acta Cryst. (2015). B71, 252–257 Received 6 February 2015 Accepted 23 March 2015 Edited by A. J. Blake, University of Nottingham, England Keywords: high pressure; magnetic molecular framework material; MOFs. CCDC references: 1055794; 1055795; 1055796; 1055797; 1055798; 1055799; 1055800; 1055801; 1055802 Supporting information: this article has supporting information at journals.iucr.org/b Pressure-induced structural phase transformation in cobalt(II) dicyanamide Andrey A. Yakovenko, Karena W. Chapman and Gregory J. Halder* X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S Cass Ave, Argonne, Illinois 60439, USA. *Correspondence e-mail: halder@aps.anl.gov In situ synchrotron powder diffraction has been used to probe the pressure- dependent structural properties of the magnetic molecular framework material Co(dca) 2 [dca = dicyanamide or N(CN) 2  ]. An orthorhombic (Pmnn) to monoclinic (P2 1 /n) transformation to a high-pressure phase, namely  -Co(dca) 2 , occurs at 1.1 GPa. Structural determination of  -Co(dca) 2 shows that the rutile- like topology of the pristine material is retained at high pressures, with the lower symmetry allowing a progression of volume-reducing structural distortions.  -Co(dca) 2 was stable at the maximum pressure measured of 4.2 GPa. Both phases were soft, with bulk moduli (B 0 ) for -Co(dca) 2 and  -Co(dca) 2 of 13.15 (18) and 9.0 (6) GPa, respectively. Modest uniaxial negative linear compressibility (K) of the order of 4 TPa 1 was observed over the entire measured pressure range. 1. Introduction Pressure offers a novel means to perturb the structure and function of molecular materials, including molecular magnets and metal–organic frameworks (MOFs; Tan & Cheetham, 2011). The rich structural chemistry and complex topologies of these materials can give rise to more unusual pressure-induced behaviour than traditional solid-state materials (e.g. metal oxides and intermetallics) more commonly explored under high-pressure conditions (Halder et al., 2011; Chapman et al., 2008; Lapidus et al. , 2013; Graham et al., 2011; Ogborn et al., 2012; Bennett et al., 2011). For instance, the structurally simple molecular material Zn(CN) 2 exhibits a diverse range of pressure-induced transformations, most significantly a pronounced dependence on the fluid used to transmit hydro- static pressure whereby small molecule fluids can trigger major structural rearrangements. The typically flexible and low density nature of molecular materials allows access to such pressure-induced phenomena under relatively mild condi- tions, such as could be routinely encountered in practical applications. This has been demonstrated for the nanoporous MOF system ZIF-8 [Zn(2-methylimidazolate) 2 ], where the porosity of the system is irreversibly modified following a crystalline-amorphous transition during pelletization (< 0.5 GPa) to optimize volumetric gas storage capacity (Chapman et al., 2009, 2011). To this end, thorough exam- ination of their pressure-dependent structures represents a critical step in the advancement of the field toward a diverse range of important applications (Furukawa et al., 2013). The topological versatility of molecular framework mate- rials, whereby magnetically active metal centres can be connected into regular networks, underlies a wide range of interesting magnetic and electronic properties (Kurmoo, 2009). For example, frameworks constructed from transition ISSN 2052-5206 # 2015 International Union of Crystallography