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NIS experiments on SCO compounds

Nuclear inelastic scattering on spin crossover complexes H. Paulsen and A.X. Trautwein Institut für Physik Universität zu Lübeck Ratzeburger Allee 160, D-23538 Lübeck, Germany E-mail: paulsen@physik.uni-luebeck.de. NIS experiments on SCO compounds.

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NIS experiments on SCO compounds

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  1. Nuclear inelastic scattering on spin crossover complexesH. Paulsen and A.X. TrautweinInstitut für PhysikUniversität zu LübeckRatzeburger Allee 160, D-23538 Lübeck, GermanyE-mail: paulsen@physik.uni-luebeck.de

  2. NIS experiments on SCO compounds Iron containing spin crossover (SCO) compounds (see for instance the contribution of P. Gütlich in this volume) have been investigated since long by conventional Mössbauer spectroscopy and by vibrational spectroscopies like IR or Raman. Those molecular vibrations that are connected with the iron center exhibit a considerable change of frequency upon spin crossover. For this reason such complexes are well suited for investigations with nuclear inelastic scattering (NIS) of synchrotron radiation, a vibrational spectroscopy that is based on the Mössbauer effect. In the following slides three examples are shown for [Fe(tpa)(NCS)2], [Fe(tptMetame)](ClO4)2 and [Fe(phen)2(NCS)2]. Together with the experimental NIS spectra simulated curves obtained by density functional theory (DFT) calculations are presented. In the case of [Fe(phen)2(NCS)2] the complete spectrum of frequencies and normal modes of molecular vibrations could be assigned by a combination of NIS, IR, Raman experiments and DFT calculations.

  3. Scheme of spin crossover The driving force for a temperature dependent spin crossover is the increase of entropy in the high spin (HS) state, that is mostly due to a softening of the iron-ligand bond stretching modes. This softening can be observed very well by NIS. LS (S=0) HS (S=2) Taken from: Gütlich P, Hauser A, Spiering H (1994) Angew Chem Int Ed Engl 33: 2024

  4. Simple scheme of NIS NFS spectrum (see textbook) Typical NIS spectrum ESRF, Grenoble

  5. NIS spectrum of molecular crystals Approximations: low temperature, isolated molecule k: wavevector of the X-rays E:energy of the X-rays (after subtraction of the resonance energy) El: energy of the lth normal mode ul: displacement vector of the iron nucleus Absorption probability  mean square displacement Paulsen H, Winkler H, Trautwein AX, Grünsteudel H, Rusanov V, Toftlund H (1999) Phys Rev B 59: 975

  6. NIS spectra of a single crystal (T=30 K)of Fe(tptMetame) [Fe(tptMetame)](ClO4)2 tptMetame = 1,1,1-tris{[N- (2-pyridylmethyl)-N-methyl-amino]-methyl}ethane) (click on the picture to start the animation) Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

  7. NIS spectra of a single crystal (T=30 K)of Fe(tptMetame) (click on the picture to start the animation) Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

  8. NIS spectra of a single crystal of Fe(tptMetame) (room temperature) Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H, Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351

  9. Complete assignment of vibrational modes for [Fe(phen)2(NCS)2] Ronayne KL, Paulsen H, Höfer A, Dennis AC, Wolny JA, Chumakov AI, Schünemann V,Winkler H, Spiering H, Bousseksou A,Gütlich P, Trautwein AX, McGarvey JJ (2006) Phys Chem Chem Phys8: 4685-4693 Measured (symbols) and simulated (solid lines) NIS spectra at 30 K (blue curves) and at room temperature (red curves).

  10. Spectral distribution of entropy difference Only the first 30 vibrations out of 147 contribute to the vibrational entropy difference. Also low frequency modes below 10 meV contribute significantly. Ronayne KL, Paulsen H, Höfer A, Dennis AC, Wolny JA, Chumakov AI, Schünemann V,Winkler H, Spiering H, Bousseksou A, Gütlich P, Trautwein AX, McGarvey JJ (2006) Phys Chem Chem Phys8: 4685-4693

  11. NIS on SCO compounds - summary In the NIS spectra of iron-containing spin crossover compounds one can observe predominant peaks that are shifted to lower energies upon crossover from the LS to the HS state. Comparison of the measured spectra with simulated spectra obtained by density functional calculations reveals that these peaks can be attributed to iron-ligand stretching modes. In this way NIS allows a direct view on the driving force of the temperature dependent spin crossover.

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