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AX Trautwein , H Paulsen, H Winkler Universität Lübeck, Germany H Giefers, G Wortmann

Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation. AX Trautwein , H Paulsen, H Winkler Universität Lübeck, Germany H Giefers, G Wortmann Universität Paderborn, Germany H Toftlund

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AX Trautwein , H Paulsen, H Winkler Universität Lübeck, Germany H Giefers, G Wortmann

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  1. Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation AX Trautwein, H Paulsen, H Winkler Universität Lübeck, Germany H Giefers, G Wortmann Universität Paderborn, Germany H Toftlund Syddansk Universitaet, Odense, Denmark JA Wolny Technische Universität Kaiserslautern, Germany AI Chumakov, O Leupold ESRF, Grenoble, France

  2. Material • iron(II) complexes STP and ETP: STP = [Fe(1,1,1-tris{[N-(2-pyridylmethyl)- N-methylamino]methyl}ethane)](ClO4)2 ETP = [Fe ……………..butane)](ClO4)2 • first coordination sphere of iron is a distor- ted [FeN6] octahedron with C3 symmetry • STP and ETP exhibit transition from the low-spin (LS, S = 0) to the high-spin (HS, S = 2) state by increasing temperature • spin-transition complexes are promising materials for optical information storage gand display devices (Gütlich et al., Angew. Chemie Int. Ed. Engl. 33, 2024 (1994)

  3. Method nuclear resonant scattering (NFS and NIS) using synchrotron radiation storage ring at ESRF, Grenoble, France NFS NIS

  4. NIS absorption probability S(E, k) consists of a zero-phonon (elastic) and of a multi-phonon (inelastic) part S(E, k) = fLM(k) δ(0) + fLM(k) Σn Sn(E, k) • at 30 K (LS,open circles): fLM ~ 0.9; room temp. (HS,closed circles): fLM ~ 0.15 • negative energy accounts for annihilation, positive energy for creation of mole- cular vibrations • NIS spectrum reflects in its inelastic part (inset) drastic hardening of vibrational modes when decreasing temperature

  5. Orbital scheme for spin transitionLS becomes energetically favored over HSdue to smaller volume s antibonding N LS (S=0) HS (S=2) N Fe N eg N Fe(3d) N p antibonding Fe N N t2g Fe-N distance N

  6. Angular-resolved molecular vibrations•NIS study of a single crystal ofSTP at 30 K, with mol. orientations k_|_a and k || a (a is along the C3 symmetry axis of the molecular complex)•NIS intensity yields partial density of states: PDOSmol = Σjδ(E – hωj) (k · uj)2ωj = vibrational frequencies of the molecule uj = projection of the jth eigenvector in the subspace of iron coordinatesk = normalized wave vector of the photons (synchrotron beam)→only those vibrational modes are NIS visible that are connected with a mean square displacement of the iron nucleus along the wave vector k

  7. NIS spectrum of single crystalline STP (T=30 K, k _I_ a ) [Fe(tptMetame)](ClO4)2 tptMetame = 1,1,1-tris{[N- (2-pyridylmethyl)-N-methyl-amino]-methyl}ethane) 0 20 40 60 Energy (meV) Paulsen H et al. (2001) Phys Rev Lett 86: 1351 → two doubly degenerate E modes are connected with a msd of the iron within the equatorial plane of the molecule ( _|_ C3 axis)

  8. NFS spectrum of single crystalline STP (T=30 K, k || a) A2 is connected with a msd of the iron nucleus along C3 A1 reflects the slight deviation from C3 symmetry Paulsen H et al. (2001) Phys Rev Lett 86: 1351

  9. Pressure-induced spin transition•spin transition is induced by pressure, i.e. theLS state is energetically favoured when the volume becomes smaller • pressure was applied up to 2.6 GPa with a diamand anvil cell for studies with synchrotron radiation (SR) detection of NIS with avalanche photo diodes (APD) in 90º geometry

  10. Pressure-induced spin transition of ETP at room temperaturemonitored by NFSLS: ΔEQ = 0.2 mm/sHS: ΔEQ = 0.8 mm/s

  11. Pressure-induced spin transition of STP monitored optically 1.1 GPa LS (S = 0) ambient pressure HS (S = 2) Photos: H Giefers and G Wortmann, Universität Paderborn, Germany

  12. Pressure- and temperature-induced spin transition of STPmonitored by NIS ambient pressure and temperature→HS p = 1.1 GPaambient temperature→LS ambient pressure, T = 30 K→LS

  13. ConclusionSTP has transformed at room temperature under pressure into a new statewith molecular vibrations comparable to the high-energy streching modesof the LSstate at 30 KAcknowledgements• technical support from the ESRF services in Grenoble, France• fnancial support by the German Research Foundation (DFG) and by the German Ministery for Education, Science, Research and Technology (BMBF)

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