Neutron powder diffraction studies of two spin transition Fe^IIcomplexes under pressure

Abstract

The pressure dependences of the lattice parameters of two spin transition compounds have been derived from neutron powder diffraction measurements at ambient temperature. The study of Fe(PM–AzA)2(NCS)2[PM isN-2′-pyridylmethylene and AzA is 4-(phenylazo)aniline] has been used to validate this new type of investigation of spin crossover compounds, and the study of Fe(PM–BiA)2(NCS)2(BiA is 4-aminobiphenyl) has allowed the atypical spin crossover behaviour of this compound under pressure to be explained. In addition, this complex exhibits a pressure-induced structural transition with an associated symmetry change, inducing the transformation of Fe(PM–BiA)2(NCS)2into a different polymorph that avoids a first-order spin transition in favour of continuous transition.

Piezo‐ and Photo‐Crystallography Applied to Spin‐Crossover Materials

“…For instance, high-pressure neutron diffraction on two [Fe(PM-L) 2 (NCS) 2 ] complexes has confirmed pressure-induced SCO at room temperature above 0.6 GPa in [Fe(PM-AzA) 2 (NCS) 2 ], and has revealed a pressure-induced transition between two well known polymorphs of [Fe(PM-BiA) 2 (NCS) 2 ] at 0.75 GPa. 33 The latter result appeared consistent with the change in magnetic behaviour observed above 0.7 GPa from high-pressure SQUID measurements, 30 but is contradicted by more recent diffuse reflectivity measurements under high pressure. 68 The latter confirmed the pressure-induced phase transition, but rules out its assignment as the previously described polymorphism.…”

“…6 Since then, in the last four decades, a large variety of techniques has been successively used to investigate pressure-induced SCO, including Mössbauer, 25 infrared and optical spectroscopies, 9,26 reflectivity, 27 X-ray absorption, 28, 29 magnetometry, 30 X-ray diffraction, 31 Raman, 32 neutron diffraction 33 and inelastic X-ray scattering. 34 The interesting point is that while many investigations support expectations (i)-(vii) (Fig.…”

“…This is the case for the possible disappearance or occurrence of steps in the course of the SCO under pressure, 46,47 as well as potential pressure-induced polymorphism in SCO materials. 33,43 All the above comments emphasize the need for crystallography under high pressure, that is piezo-crystallography, in the field of SCO.…”

Thermo- and piezochromic properties of [Fe(hyptrz)]A2·H2O spin crossover 1D coordination polymer: Towards spin crossover based temperature and pressure sensors

“…Regarding this issue, very few attempts have been noticed to record the (p-T) phase diagram by using various detection techniques such as optical reflectivity [19][20][21][29][30][31], magnetic techniques [15][16][17][18][32][33][34], X-ray and neutron diffraction [35][36][37][38][39]. This lack of data could be explained by the difficulty to obtain small variations in the applied pressure in both loading and unloading processes, at variable temperatures.…”

Pressure-induced two-step spin transition with structural symmetry breaking: X-ray diffraction, magnetic, and Raman studies

“…25 Indeed, the majority of high-pressure investigations involving SCO systems have used spectroscopic techniques or magnetic studies to characterize the effect of applied pressure on the spin transition. 26 Very few studies to date have used diffraction techniques to explore the relationship between structure and properties of SCO materials under pressure, [27][28][29][30] but such investigations are essential to establishing the driving forces behind the pressure-induced spin transition, as well as to thorough investigation of more complex phenomena such as stepped transitions.…”