“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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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)](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)
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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) = 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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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 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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
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)
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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
A2 is connected with a msd of the iron nucleus along C3
A1 reflects the slight deviation from C3 symmetry
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
Pressure-induced spin transition of ETP at room temperaturemonitored by NFS
LS: ΔEQ = 0.2 mm/s
HS: ΔEQ = 0.8 mm/s
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
0 20 40 60
E (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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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)