We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here we gain experimental insights, beyond the Born–Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy)3]2+ compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.

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Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption. Spin crossover (SCO) molecules are prototype systems having the ability to switch between low spin (LS) and high spin (HS) molecular states both at thermal equilibrium and after light irradiation. In the case of Fe(II) (3d(6)) complexes in a nearly octahedral ligand field, the two possible electronic distributions among the 3d split orbitals are S = 0 for the LS diamagnetic state and S = 2 for the HS paramagnetic state. In crystals, such photoexcited states can be long-lived at low temperature, as is the case for the photoinduced HS state of the [Fe(phen)2(NCS)2] SCO compound investigated here. We first show how such bistability between the diamagnetic and paramagnetic states can be characterized at thermal equilibrium or after light irradiation at low temperature. Complementary techniques provide invaluable insights into relationships between changes of electronic states and structural reorganization. But the development of such light-active materials requires the understanding of the basic mechanism following light excitation of molecules, responsible for trapping them into new electronic and structural states. We therefore discuss how we can observe a photomagnetic molecule during switching and catch on the fly electronic and structural molecular changes with ultrafast X-ray and optical absorption spectroscopies. In addition, there is a long debate regarding the mechanism behind the efficiency of such a light-induced process. Recent theoretical works suggest that such speed and efficiency are possible thanks to the instantaneous coupling with the phonons of the final state. We discuss here the first experimental proof of that statement as we observe the instantaneous activation of one key phonon mode precluding any recurrence towards the initial state. Our studies show that the structural molecular reorganization trapping the photoinduced electronic state occurs in two sequential steps: the molecule elongates first (within 170 femtosecond) and bends afterwards. This dynamics is caught via the coherent vibrational energy transfer of the two main structural modes. We discuss the transformation pathway connecting the initial photoexcited state to the final state, which involves several key reaction coordinates. These results show the need to replace the classical single coordinate picture employed so far with a more complex multidimensional energy surface.

Magnetic properties and bonding analyses of perovskite structure Co4N nitride have been investigated within density functional theory using both pseudo potential and all electron methods. In the same time, the structural and magnetic stability of pure cobalt in hexagonal close packed (HCP), face centered cubic (FCC) and body centered cubic (BCC) structures are reviewed. At equilibrium, non-spin polarized (NSP) and spin polarized (SP) calculations of the energy versus volume show that the ground state is ferromagnetic in both materials. HCP-Co is found to be more stable than the cubic ones. Magnetic moments of Co atoms in Co4N nitride respectively belonging to two different crystallographic sites are studied over a wide range of the cubic lattice constant, and a comparison with the FCC-cobalt one is given. The volume expansion in the nitride indicates that the corner Co I atoms show localized magnetism while face center Co II atoms exhibit an itinerant behavior. Like in FCC-Fe/Fe4N, a "low volume-low moment" and "large volume-high moment" behavior is observed for FCC-Co/Co4N. The density of states of the Co4N ferromagnetic ground state is interpreted within the rigid band model. The different bonding characters of Co I -N versus Co II -N are shown with help of electron localization fucntion ELF plots and spin resolved chemical bonding criteria.

Stoichiometric CrSi2 was prepared by arc melting and compacted by uniaxial hot pressing for property measurements. The crystal structure of CrSi2 was investigated using the powder x-ray diffraction method. From the Rietveld refinement, the lattice parameters were found to be a=4.42757 (7) and c=6.36804 (11)Å, respectively. The thermal expansion measurement revealed an anisotropic expansion in the temperature range from room temperature 800K with αa=14.58×10−6∕K, αc=7.51×10−6∕K, and αV=12.05×10−6∕K. The volumetric thermal expansion coefficient shows an anomalous decrease in the temperature range of 450–600K. The measured electrical resistivity ρ and thermoelectric power S have similar trends with a maxima around 550K. Thermal conductivity measurements show a monotonic decrease with increasing temperature from a room temperature value of 10Wm−1K−1. The ZT values increase with temperature and have a maximum value of 0.18 in the temperature range studied. An analysis of the electronic band structure is provided.

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2014 Utilisant la méthode de l'« onde sphérique augmentée » (augmented spherical wave) les auteurs ont calculé les structures électroniques et magnétiques de Fe4N et de Mn4N. Les deux nitrures s'ordonnent magnétiquement avec un alignement ferroet ferri-magnétique respectivement. Les valeurs obtenues des moments magnétiques sont en bon accord avec celles de la littérature. Une comparaison avec le modèle phénoménologique de Goodenough ainsi qu'avec les alliages de Heusler est donnée. Abstract. 2014 Using the augmented spherical wave method (A.S.W.) we calculate the electronic and magnetic structures of Fe4N and Mn4N. Both nitrides are found to order magnetically exhibiting ferroand ferrimagnetic spin alignment respectively. The magnetic moments found are in good agreement with neutron data. Both systems can be described via covalent magnetism. A comparison with Goodenough's phenomenological model as well as with Heusler alloys is given.