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
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.
Scheme of spin crossover
LS
(S=0)
HS
(S=2)
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.
Taken from: Gütlich P, Hauser A,
Spiering H (1994) Angew Chem
Int Ed Engl 33: 2024
Simple scheme of NIS
Detector
(APD)
High resolution
monochromator
Sample
Si(12 2 2)
Si(1 1 1)
Undulator
Detector
(APD)
Si(4 2 2)
Premonochromator
E =0
Resonance
E<0
Deexcitation of vibrations
("Anti-Stokes-line")
-100
-50
NFS spectrum (see textbook)
E>0
Excitation of vibrations
("Stokes-line")
0
50
100
E (meV)
Typical NIS spectrum
ESRF, Grenoble
NIS spectrum of molecular crystals
Approximations: low temperature, isolated molecule
S ( E , kˆ )  e
 k u 2
L

2 
2
 ( E )  k   ( E  El ) k  u l  
l 1


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
NIS spectra of a single crystal (T=30 K)
of Fe(tptMetame)
-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 )
k || a
(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
E
A2
0.2
Experiment
A1
0.0
Theory
-0.2
0
20
40
Energy (meV)
60
NIS spectra of a single crystal (T=30 K)
of Fe(tptMetame)
-1
PDOS (meV )
k _|_ a
0.2
E
Experiment
0.0
Theory
-0.2
-1
PDOS (meV )
k || a
(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
E
A2
0.2
Experiment
A1
0.0
Theory
-0.2
0
20
40
Energy (meV)
60
k_|_a
0.1
-1
PDOS (meV )
NIS spectra of a single crystal of Fe(tptMetame)
(room temperature)
Experiment
0.0
Theory
0
20
40
60
Energy (meV)
Paulsen H, Benda R, Herta C, Schünemann V, Chumakov AI, Duelund L, Winkler H,
Toftlund H, Trautwein AX (2001) Phys Rev Lett 86: 1351
Absorption probability
Complete assignment of vibrational modes for
[Fe(phen)2(NCS)2]
200
E (cm-1)
400
Measured (symbols) and simulated (solid
lines) NIS spectra at 30 K (blue curves) and at
room temperature (red curves).
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 Phys 8: 4685-4693
Spectral distribution of entropy difference
(breathing mode)
4
40
b
s
-1
b
-1
30
bb
-1
s
2
20
b
1
b
s
s
10
b
Also low frequency
modes below 10
meV contribute
significantly.
i
b
-1
Svib(i) (J K mol )
3
1 Svib(i) (J K mol )
b
Only the first 30
vibrations out of
147 contribute to
the vibrational
entropy difference.
ss
0
0
0
10
20
30
40
Mode number i
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 Phys 8: 4685-4693
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.