AX Trautwein: Pressure-induced changes studied by synchrotron radiation 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

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Material iron(II) complexes STP and ETP: STP = [Fe(1,1,1-tris{[N-(2-pyridylmethyl)- N-methylamino]methyl}ethane)](ClO 4 ) 2 ETP = [Fe ……………..butane)](ClO 4 ) 2 first coordination sphere of iron is a distor- ted [FeN 6 ] octahedron with C 3 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)

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Method nuclear resonant scattering (NFS and NIS) using synchrotron radiation storage ring at ESRF, Grenoble, France NIS NFS

AX Trautwein: Pressure-induced changes studied by synchrotron radiation NIS absorption probability S(E, k) consists of a zero-phonon (elastic) and of a multi-phonon (inelastic) part S(E, k) = f LM (k) δ(0) + f LM (k) Σ n S n (E, k) at 30 K (LS, open circles): f LM ~ 0.9; room temp. (HS, closed circles): f LM ~ 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

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Fe N N N N N N N N Fe(3d) egeg t 2g Fe-N distance  antibonding LS (S=0)  antibonding HS (S=2) Orbital scheme for spin transition LS becomes energetically favored over HS due to smaller volume

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Angular-resolved molecular vibrations NIS study of a single crystal of STP at 30 K, with mol. orientations k_|_ a and k || a (a is along the C 3 symmetry axis of the molecular complex) NIS intensity yields partial density of states: PDOS mol = Σ j δ(E – hω j ) (k · u j ) 2 ω j = vibrational frequencies of the molecule u j = projection of the jth eigenvector in the subspace of iron coordinates k = 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

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Paulsen H et al. (2001) Phys Rev Lett 86: 1351 [Fe(tptMetame)](ClO 4 ) 2 tptMetame = 1,1,1-tris{[N- (2-pyridylmethyl)-N-methyl- amino]-methyl}ethane) NIS spectrum of single crystalline STP (T=30 K, k _I_ a ) Energy (meV) → two doubly degenerate E modes are connected with a msd of the iron within the equatorial plane of the molecule ( _|_ C 3 axis)

AX Trautwein: Pressure-induced changes studied by synchrotron radiation NFS spectrum of single crystalline STP (T=30 K, k || a) Paulsen H et al. (2001) Phys Rev Lett 86: 1351 A 2 is connected with a msd of the iron nucleus along C 3 A 1 reflects the slight deviation from C 3 symmetry

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Pressure-induced spin transition spin transition is induced by pressure, i.e. the LS 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

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Pressure-induced spin transition of ETP at room temperature monitored by NFS LS: ΔE Q = 0.2 mm/s HS: ΔE Q = 0.8 mm/s

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Photos: H Giefers and G Wortmann, Universität Paderborn, Germany ambient pressure HS (S = 2) 1.1 GPa LS (S = 0) Pressure-induced spin transition of STP monitored optically

AX Trautwein: Pressure-induced changes studied by synchrotron radiation ambient pressure and temperature → HS p = 1.1 GPa ambient temperature → LS ambient pressure, T = 30 K → LS Pressure- and temperature-induced spin transition of STP monitored by NIS

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Conclusion STP has transformed at room temperature under pressure into a new state with molecular vibrations comparable to the high-energy streching modes of the LS state at 30 K Acknowledgements 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)