Transcript Ditellurides of 3d transition metals studied by 57Fe and 125Te

Ditellurides of 3d transition metals studied
by 57Fe and 125Te Mössbauer spectroscopy
Piotr Fornal
Cracow University of Technology
Institute of Physics
30-083 Krakow, Poland
Jan Stanek
Jagiellonian University
Marian Smoluchowski Institute of Physics
30-059 Krakow, Poland
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
The series of the 3d transition metal
ditellurides is an ideal object for the
study of the interplay between
crystal structure and bonds, here
described by the local electronic
states of anions observed by 125Te
and cation states by 57Fe Mössbauer
spectroscopy, if 57Fe probes are
introduced into the structures
without its altering.
25
26
27
Mn Fe Co
3d54s2
3d64s2
3d74s2
28
Ni
3d84s2
52
Te
5s25p4
For MeTe2, Me=Mn, Fe, Ni, the crystal structures evolve from cubic pyrite type (Pa3)
for MnTe2 through orthorhombic marcasite type for FeTe2 (Pnnm) to hexagonal CdI2
(C3m) type for NiTe2. Metal ions are six coordinated by Te atoms which form
compressed, quasi-planar trygonal ant-prism in MnTe2 and NiTe2 or compressed
octahedra in FeTe2 with quasi-linear coordination. Te atoms form Te2 molecules in
MnTe2 and FeTe2, while in NiTe2 each Te is coordinated by 3 Te atoms, forming
double anion layers.
Coordination of Te and cations in MnTe2 , FeTe2 and NiTe2
MnTe2
NiTe2
FeTe2
Methodological aspects of 125Te Mössbauer spectroscopy
The most popular source is thermal
neutron activated metastable 125mTe in
Mg3TeO6, as the matrix
[H. Binczycka, S.S. Hafner, J. Stanek, M. Tromel,
Its Debye
temperature, ΘD = 352(3)K, yields the
recoil free fraction fs=0.392(5) at 295 K
which enables measurements to be
made with this source at room
temperature. The activation of 25 mg
of Mg3TeO6, enriched in 124Te to 90%,
in a thermal neutron beam of 1014
neutrons/cm2·s for 20 days results in
the final 125mTe source activity of 150
mCi (5.6 GBq),which allows
measurements for many months.
Phys. Lett. A, 131, 135 (1988) ].
The decay scheme for 125mTe
The observed ranges of isomer shift and quadrupole splitting are of the order of
the experimental line width
Simplified plot of the ranges of the 125Te isomer shift and quadrupole
splitting compared to the natural line width.
The relatively high energy of Mössbauer transition in 125mTe leads to the strong
temperature dependence of the recoil-free fraction between 80 K and 300 K which
facilitates the dynamical studies.
125Te
125Te
Mössbauer spectra of FeTe2 and
NiTe2 at different temperatures.
Mössbauer spectra of MnTe2 at
different temperatures. At 78 K (below the
Néel transition) the spectrum exhibits a
magnetic splitting.
Results
from 125Te Mössbauer spectroscopy
The experimentally determined hyperfine interaction parameters may be
transparently interpreted in terms of 5s and 5p shell population. One hole in
the 5pz -orbital produces a quadrupole splitting between 12 mm/s and 15
mm/s (assuming that the 5px and 5py orbitals remain fully populated ) and one
5s electron increases the isomer shift by 2.4 mm/s which is modified by
contributions of 5p electrons - one 5p electron reduces the isomer shift by 0.4
mm/s, due to enhanced shielding. Consequently, it was possible to determine
the electron configuration of Te and its effective charge in the investigated
series
[J. Stanek, A.M. Khasanov, S.S. Hafner, Phys. Rev. B, 45, 56 (1992)].
-2.0
Ni
MnTe 2
8
CrTe
Te effective charge
QS [ mm/s ]
Results from 125Te Mössbauer spectroscopy: electronic states of tellurium
6
FeTe
4
CoTe
Co
-1.5
Fe
-1.0
2
Mn
NiTe
0
1.0
1.2
1.4
1.6
IS [mm/s]
QS-IS relation of 125Te
in 3d transition-metal ditellurides
2.5
3.0
o
Te - Te distance [ A ]
Effective charges of Te
in 3d transition metal ditellurides
3.5
Results from 125Te Mössbauer spectroscopy: electronic states of tellurium
Mn+2
(Te-Te)
-2
The effective charges of Te
imply the corresponding
cationic charges:
Fe+3
(Te-Te)
-3
Mn
+2
Co, Fe +3
Ni
+4
Te-2
-
Te-2
Ni,
+4
Results from 57Fe Mössbauer spectroscopy
Samples
Samples of Mn1-x57FexTe2, FeTe2 and Ni1-x57FexTe2 (x=0, 0.03, 0.1) were synthesized
from high purity elements employing evacuated silica tube technique. All specimens
were heated, quenched, reground and annealed at 400ºC for several weeks until
single phase, tested by X-ray diffraction, was attained.
Mn1-x57FexTe2
Below 85 K, in the antiferromagnetic state, the spectra showed mixed magneticquadrupole interaction with H=9.3 T at 20 K, the electric field gradient being axial
with the main axis parallel to H and positive Vzz, as calculated in the point charge
approximation
Results from 57Fe Mössbauer
spectroscopy : Mn1-x57FexTe2
293 K
102 K
85 K
The 57Fe Mössbauer spectra of Mn1-x57FexTe2.
The magnetic spectra were fitted using a
Hamiltonian-solving program for mixed
magnetic-quadrupole interactions.
75 K
20 K
-10
0
velocity [mm/s]
10
Results from 57Fe Mössbauer spectroscopy : FeTe2
The well known Mössbauer spectrum
of FeTe2 is in form of weakly
temperature dependent quadrupole
doublet. The spectrum recorded in
external magnetic field, fitted using
the Gabriel-Ruby procedure, shows
that electric field gradient is fairly
axial (η<0.2) and Vzz is negative, as
calculated in the point charge
approximation [J. Stanek, P. Fornal.
Nukleonika, 49 (2004) 63-65]. The fitted
hyperfine field is reduced in
comparison to the applied one.
4.2 K
0T
4.2 K
4.5 T
-3
0
3
velocity [mm/s]
The 57Fe Mössbauer spectra of
FeTe2 at 4.2 K at zero field (top)
and in external magnetic field of
4.5 T (bottom)
Results from 57Fe Mössbauer spectroscopy : Ni1-x57FexTe2
The asymmetry of the quadrupole doublet depends on the orientation of the sample against the
gamma beam which suggests the occurrence of texture, confirmed by the electron microscopy
study. If one assumes that the main axis of the field gradient is perpendicular to the sample
plane, the more intense high energy line shows that Vzz is positive, as calculated in the point
charge approximation
[J. Stanek, S.S. Hafner, P.Fornal, Hyperfine Interaction C, 5, (2001) 355-358 ].
4.2 K
293 K
o
90
293oK
45
-2
0
2
velocity [mm/s]
The 57Fe Mössbauer spectra of Ni1-x57FexTe2 at
4.2 K (top) and at room temperature: middle:
sample perpendicular to the gamma beam,
bottom: sample at 45º to the gamma beam.
The scanning electron microscopy
picture of the powder sample of
Ni1-x57FexTe2.
57Fe
Summary
Mössbauer parameters
T [K]
 [mm/s]
 [mm/s]
293
0.78
(+)1.28
247
0.82
(+)1.29
197
0.85
(+)1.31
102
0.91
(+)1.34
85
0.91
+ 1.34
2.76
75
0.92
+ 1.37
4.60
20
0.94
+ 1.45
9.32
Mn0.97Fe0.03Te2
293
0.78
(+)1.28
FeTe2
293
0.47
(-)0.52
4.2*
0.46
- 0.52
293
0.42
(+)0.33
150*
0.44
(+)0.41
4.2*
0.40
(+)0.44
293
0.42
(+)0.31
Sample
Mn0.9Fe0.1Te2
Ni0.9Fe0.1Te2
Ni0.97Fe0.03Te2
H [T]
1. The increase of the
125Te
isomer shift is
correlated with the decrease of the
57Fe isomer shift.
2. The quadrupole splitting of 57Fe is
weakly temperature dependent.
(*) source and absorber at the same temperature
3. The sign of Vzz acting on 57Fe is the
same as that obtained from calculation
within the point charge approximation
lattice contribution.
4. The value of the hyperfine magnetic ion
57Fe in MnTe is close to the
2
transferred magnetic field on 125Te.
5. The measured magnetic field at FeTe2
was less by 2% than the applied one.
Discussion (1)
1.
The starting point for the discussion is FeTe2, where Fe is in the diamagnetic
low spin FeII state with nominal 5d6 configuration. This spin state is
confirmed by Mössbauer spectroscopy (cf. lattice origin of efg, reduced vs.
applied value of the magnetic field. However, the charge of about 3 electrons
is transferred from Fe to Te2 molecule and the charge of Fe should be +3.
The only possibility is to write the electronic configuration as 3(d6)x with x=
5/6.
2.
Applying the same arguments for Mn1-x57FexTe2 and Ni1-x57FexTe2 one may
write the Fe configuration as 3(d6)1 and 3(d6)4/6 which lead to the +2 effective
charge of Fe and +4 in Mn1-x57FexTe2 and Ni1-x57FexTe2, respectively.
3.
These simple minded arguments point out that the terms “electronic state”
and “charge state” or “valence state” should not be used equivalently. In the
studied samples iron is in FeII low spin electronic state, with “charge state”
varying from +2 in Mn1-x57FexTe2 through +3 in FeTe2 up somehow
controversial +4 state in Ni1-x57FexTe2.
Discussion (2)
Assuming that the local states of Fe reasonably reproduce the properties of the
substituted Mn and Ni cations the simple ionic model well reproduce the
bonding-structure interplay in the investigated series, according to the
following scenario:
Increase in the number of 3d electrons (Mn, Fe, Ni)
Increase of the charge transfer to Te
Increase of the Te-Te distances
Weakening of the Te-Te bonds in Te2 molecule
Structure evolution from pyrite-type through marcasite-type to CdI2-type