AX Trautwein: Pressure-induced changes studied by synchrotron radiation Pressure-induced changes of the vibrational modes of spin- crossover complexes

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

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 WinklerUniversität Lübeck, Germany

H Giefers, G WortmannUniversität Paderborn, Germany

H ToftlundSyddansk Universitaet, Odense, Denmark

JA WolnyTechnische Universität Kaiserslautern, Germany

AI Chumakov, O LeupoldESRF, Grenoble, France



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)



Method

nuclear resonant scattering (NFS and NIS) using synchrotron radiation

Undulator

Premonochromator

Detector (APD)

Detector (APD)

Sample

High resolutionmonochromator

Si(4 2 2)

Si(12 2 2)Si(1 1 1)

storage ring at ESRF, Grenoble, France

NIS

NFS



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 molecular vibrations• NIS spectrum reflects in its inelastic part (inset) drastic hardening of vibrational modes

when decreasing temperature



Fe

FeN

N

N

N

N

N

N

N

Fe(3d)

eg

t2g

Fe-N distance

antibonding

LS (S=0)

antibondingHS

(S=2)

Orbital scheme for spin transitionLS becomes energetically favored over HS due to smaller volume



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 C3 symmetry axis of the molecular complex)

• NIS intensity yields partial density of states: PDOSmol = Σj δ(E – hωj) (k · uj)2

ωj = vibrational frequencies of the molecule uj = 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



Paulsen H et al. (2001) Phys Rev Lett 86: 1351

[Fe(tptMetame)](ClO4)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 )

0 20 40 60 Energy (meV)

→ two doubly degenerate E modes are connected with a msd of the iron within the equatorial plane of the molecule ( _|_ C3 axis)



NFS spectrum of single crystalline STP (T=30 K, k || a)

Paulsen H et al. (2001) Phys Rev Lett 86: 1351

A2 is connected with a msd of the iron nucleus along C3

A1 reflects the slight deviation from C3 symmetry



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



Pressure-induced spin transition of ETP at room temperaturemonitored by NFS

LS: ΔEQ = 0.2 mm/s

HS: ΔEQ = 0.8 mm/s



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



0 20 40 60E (meV)

Abs

orpt

ion

prob

abili

ty

ambient pressure and temperature

→ HS

p = 1.1 GPaambient temperature

→ LS

ambient pressure, T = 30 K→ LS

Pressure- and temperature-induced spin transition of STPmonitored by NIS



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)

Documents

AX Trautwein: Pressure-induced changes studied by synchrotron radiation Pressure-induced changes of the vibrational modes of spin- crossover complexes