Pressure-induced changes of the vibrational modes of spincrossover 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)](ClO4)2
ETP = [Fe ……………..butane)](ClO4)2
• first coordination sphere of iron is a distorted [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)
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
High resolution
monochromator
Sample
Detector
(APD)
NFS
Si(12 2 2)
Si(1 1 1)
Undulator
Detector
(APD)
Si(4 2 2)
Premonochromator
NIS
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 molecular 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
Orbital scheme for spin transition
LS becomes energetically favored over HS due to smaller volume
s antibonding
N
Fe
N
N
LS
(S=0)
HS
(S=2)
eg
N
Fe(3d)
N
p antibonding
Fe
N
N
t2g
Fe-N distance
N
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
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
NIS spectrum of single crystalline STP (T=30 K, k _I_ a )
-1
PDOS (meV )
k _|_ a
[Fe(tptMetame)](ClO4)2
tptMetame = 1,1,1-tris{[N(2-pyridylmethyl)-N-methylamino]-methyl}ethane)
0.2
E
Experiment
0.0
Theory
-0.2
-1
PDOS (meV )
0 k || a
Paulsen H et al. (2001) Phys Rev Lett 86: 1351
E
0.2
A2
20
40
Energy (meV)
Experiment
60
A1
0.0
→ two doubly degenerate E modes are connected
with a msd of the iron
Theory
-0.2
within the equatorial plane of the molecule ( _|_ C3 axis)
0
20
40
Energy (meV)
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
60
NFS spectrum of single crystalline STP (T=30 K, k || a)
-1
PDOS (meV )
k _|_ a
0.2 is connected with a Emsd of the
A
2
E
iron nucleus
along
C
Experiment
3
0.0
A1 reflects the slight deviation from
C3 symmetry
Theory
-0.2
-1
PDOS (meV )
k || a
Paulsen H et al. (2001) Phys Rev Lett 86: 1351
A2
0.2
Experiment
A1
0.0
Theory
-0.2
0
20
40
Energy (meV)
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
60
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: ΔEQ = 0.2 mm/s
HS: ΔEQ = 0.8 mm/s
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
Pressure-induced spin transition of STP monitored optically
ambient pressure
1.1 GPa
HS (S = 2)
LS (S = 0)
Photos: H Giefers and G Wortmann, Universität Paderborn, Germany
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
Pressure- and temperature-induced spin transition of STP
ambient pressure
and temperature
→ HS
p = 1.1 GPa
ambient temperature
→ LS
Absorption probability
monitored by NIS
ambient pressure,
T = 30 K
→ LS
0
20
40
E (meV)
AX Trautwein: Pressure-induced changes studied by synchrotron radiation
60
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)
AX Trautwein: Pressure-induced changes studied by synchrotron radiation