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Structure and
lattice dynamics of condensed matter studied using neutrons, X-ray and
synchrotron radiation
Main activity concerns static and dynamic properties of condensed matter.
Crystal and magnetic ordering in materials such as: magnetic materials,
disordered systems, ferroelectrics-antiferromagnets, protonic conductors and
bio-crystals. Interactions in condensed matter.
Main achievements
(2000-2008)
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The
modulated ordering of atomic positions was observed in CaCuxMn7-xO12
(x=0 and x=0.1) by using synchrotron radiation diffraction [4]. The atomic
positions are modulated and the propagation vector of this modulation is
(0,0,0.92).
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The
magnetization of BiFeO3 has been studied in high magnetic fields
up to 58 T [1]. Our studies have shown an anomaly due to a change of the
modulated cycloidal magnetic ordering of Fe3+ magnetic moments
near 18 T.
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The
crystal structure of CaCO3 biocrystals extracted from coral
skeletons was studied by using synchrotron radiation diffraction [2,5,6,9].
Our studies have shown that the biogenic origin changes considerably the
crystal structure of CaCO3 [5,6,9]. The thermal expansion of
biogenic CaCO3 diffres considerably from that of geological CaCO3
[2].
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The
process of grain growth of electrodeposited nanocrystalline chromium nano-Cr
[8] was studied by synchrotron radiation diffraction and small angle
scattering. During annealing one observes gradual changes of the
fractal-like density autocorrelation function, a decrease of the microstrain
fluctuation and increase of the crystallite size [8].
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The
crystal and magnetic structure of the multiferroic material BiFeO3
has been studied by neutron diffraction [7,11]. Our studies show that the
character of the modulated magnetic ordering do not change between 4 K and
300K
[11]. It has been also shown
[12] that several modulated magnetic ordering models can describe the
high resolution neutron powder diffraction patterns of BiFeO3 with the same
accuracy as the circular cycloid one proposed in [J. Phys. C 15 (1982)
4835]. These orderings are: the elliptical cycloid and the spin density wave
(SDW).
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High
resolution synchrotron radiation diffraction
[13] does not show any sign of charge ordering nor any crystal symmetry
breaking in BiFeO3 at temperatures from 5 K up to 1000 K. There
is a local minimum of the rhombohedral angle arh around the Néel
temperature suggesting a strong spin–lattice coupling. Mössbauer
spectroscopy studies support the magnetic modulation of the hyperfine fields
observed by NMR which are related to the modulated Fe3+ magnetic
moments ordering observed by neutron diffraction.
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The
energy splitting of Nd nuclear levels and Nd nuclear polarization in NdFeO3
have been studied by using high resolution inelastic neutron backscattering
with simultaneous neutron diffraction at temperatures between 100mK and 15 K
[14]. Inelastic peaks are observed below 4.5K with a corresponding
energy splitting DE = 1.24meV below 0.9 K. The Nd nuclear magnetic moments
are polarized below 1K with a maximal polarization of 17% observed at 100
mK. Both these phenomena directly observed in NdFeO3 are
described by assuming a magnetic hyperfine coupling model. It is found that
the present experimental data on NdFeO3 and the literature data
concerning Nd2CuO4 can be consistently described by
using the same value of the magnetic hyperfine coupling constant A =1.10(5)
meV/ mB.
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The
crystal structures and charge ordering in the manganites of CaCuxMn7-xO12
have been studied with synchrotron radiation and neutron diffraction
[15]. The x = 0.10 and 0.20 compounds both show an apical-type
Jahn–Teller distortion of the MnO6 octahedra around Mn3+
ions. Both compounds undergo a structural phase transition to a
high-temperature cubic structure (space group Im-3) with coexistence of both
phases between 375 and 415K for x = 0.10 and between 10 and 380K for x =
0.20. The domain sizes of the coexisting phases are at least 200 nm for both
x = 0.10 and 0.20 compounds.
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Neutron
[17] and synchrotron radiation [10] powder diffraction studies of NdFeO3
have shown a spin reorientation transition with gradual changes of the
directions of the Fe3+ ordered magnetic moments Between 100K and
200K
[17]. The spin reorientation temperature range is associated with
changes of the crystal structure. The b lattice parameter has a broad
local minimum in the spin reorientation region
[10,17]. There is also a coherent rotation of the FeO6 octahedra with an
increase of the Fe–O–Fe angles with increasing temperature. These structural
changes tend to increase the strength of the in-plane (a, b) Fe–Fe
interactions and to decrease the strength of Fe–Fe interactions along the
c-axis as the temperature increases.
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It has
been shown that the widely accepted Yoshimori model can not be used for a
description of the long range ordering of the Mn4+ ions in the
manganese oxide ß-MnO2. Neutron diffraction studies
[8,12] have shown that the propagation vector of this screw-type
modulated structure differs from the value of 7/2c* given by Yoshimori. The
length of the propagation vector changes with temperature with a local
maximum at about 90 K i.e. near the Néel temperature of 92 K
[18]. The c-lattice parameter has a local maximum near 92 K what shows
that the importance of the spin–lattice coupling in ß-MnO2. Our
studies have shown that the critical exponent of b-MnO2 is equal
to 0.18
[18].
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It has
been shown that the mixed valence system CaMn7O12
undergoes a charge ordering between 410 K and 440 K. The low temperature,
charge ordered phase coexists with the high temperature charge disordered
phase from 410 K up to 440 K. The influence of internal strains on this
phase separation phenomenon was studied by performing high resolution
synchrotron radiation diffraction studies on annealed CaMn7O12
samples. The phase separation phenomenon in CaMn7O12
is not sensitive to internal strains
[23].
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The low
temperature crystal structure of CaMn7O12 has been
studied by using resolution neutron diffraction and synchrotron radiation
diffraction
[19]. Our studies have shown an anisotropic thermal lattice expansion
of CaMn7O12 with a local maximum and minimum of the c
lattice parameter at 50 K and 250 K, respectively
[19]. The maximum coincides with a magnetic phase transition in CaMn7O12
while the minimum coincides with the onset of weak diffraction maxima which
are interpreted as a sign of a charge ordered state
[19].
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The phase
separation phenomenon was also studied in Cu-doped mixed manganese oxides
CaCuxMn7-xO12 with x=0.4 and 0.7. High
resolution synchrotron radiation diffraction studies of the (x=0.4) compound
have shown a coexistence of a high temperature cubic phase with a low
temperature trigonal phase in a temperature range from 250 K down to 10 K
[19]. At higher Cu doping, i.e. x=0.7, the material has a single cubic
phase at temperatures down to 10 K
[20].
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The
magnetic ordering of the manganese oxide a-Mn2O3 has
been studied by using neutron powder diffraction. Our studies have shown
that the magnetic ordering model given by Grant et al. is not correct. A new
collinear model of the magnetic ordering of a-Mn2O3
at 10 K is presented [15]. The main antiferromagnetic Bragg peaks have
different temperature dependence of their intensities, suggesting that the
magnetic ordering in a-Mn2O3 cannot be described by a
single order parameter
[21].
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he
magnetic ordering of nanocrystalline Cr (nano-Cr) was studied by neutron
diffraction
[24]. These studies have shown that nano-Cr has a spin density wave
modulated magnetic ordering characteristic for single crystals of Cr.
However the magnetic phase transitions observed in nano-Cr occur at
different temperatures as compared with Cr single crystals.
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The
magnetic ordering of the magnetic moments of Mn3+ and
Mn4+ ions in the mixed valence system CaCuxMn7-xO12
was studied by neutron diffraction. The system without Cu doping (x=0) shows
a behaviour characteristic for 3-dimensional Ising systems
[26]. The system with small Cu doping (x=0.3) shows a modulated magnetic
ordering with a reduced coherence length
[28].
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Neutron
diffraction studies have shown that the modulation of the Fe3+
magnetic moments in BiFeO3 changes drastically when a part
of the iron ions are replaced by manganese ions
[25,29,35].
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The
crystal microstructure of electrodeposited nanocrystalline nano-Ni, nano-Co
[30] and nano-Cr
[31] was studied by small angle neutron scattering. All these
nanocrystalline materials show a fractal-like density autocorrelation
function. This specific microstructure is probably due to the
electrochemical preparation method.
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It has
been shown that the magnetic ordering of the magnetic moments of Mn3+
and Mn4+ ions in the mixed valence system CaMn7O12
undergoes a commensurate-to-incommensurate magnetic phase transition. The
modulation vector as well as the coherence lengths of this ordering changes
considerably at the transition temperature. The modulated magnetic ordering
exists also in an external field of 4 Tesla
[34].
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Neutron
and X-ray diffraction studies of the protonic conductor Ba3Ca1+yNb2-yO9-d
have shown what are the possible positions occupied by the protons in the
crystal lattice [32,36]
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