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)

1. The modulated ordering of atomic positions was observed in CaCu_xMn_7-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).
2. 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.
3. 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].

4. 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].

5. 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).

6.  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 a_rh  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.

7. 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/ m_B.

8. The crystal structures and charge ordering in the manganites of  CaCu_xMn_7-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.

9. 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.

10. 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].

11.  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].

12. 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].

13. The phase separation phenomenon was also studied in Cu-doped mixed manganese oxides CaCu_xMn_7-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].

14. 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].

15. 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.

16. The magnetic ordering of the magnetic moments of Mn³⁺ and Mn⁴⁺ ions in the mixed valence system CaCu_xMn_7-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].

17. 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].

18. 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.

19. 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].
20. 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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