T-by Exafs Study The Lattice Dynamics And Its Isotope Effect Of Transition Metal Borides

Synchrotron radiation (SR) sources can provide a powerful x-ray beam in a wide energy range, extremely intense and with a high polarization rate. Nowadays, there are 55 SR facilities operational in the world that provide a great deal of x-ray experimental opportunities. Among the different SR techniques the x-ray absorption spectroscopy (XAS) is one of the most powerful to investigate the local electronic and geometrical structure around a photoabsorber with sub-Angstrom resolution. Up to date, XAS has been extensively applied to material science, nano-science, enviromental science, life science, etc. According to the latest statistics, the XAS station represent about 1/4, probably -1/3 of all experimental stations in the SR facilities existing all over the world.In this thesis, we first introduced XAS principles, than the experimental methods, the BSRF facility and finally a detailed discussion of the data analysis will be presented. For the first time, we discuss the applications of the multiple-scattering theory to XANES data and the possible extention to the treatment of EXAFS. In addition, we will introduce a special EXAFS method: the temperature-dependent EXAFS (T-EXAFS), which may provide unique information about the lattice dynamics. The superconductive mechanism of MgB₂, a new exotic superconductor was investigated with such technique.Magnesium diboride is superconducting at 39 K and its discovery dates back at the beginning of this century, characterized by the highest known transition temperature for a non-copper-oxide bulk material. Its discovery is a relevant breakthrough because is the first conventional superconductor system that overcome the theoretical value of the BCS theory set to T_C = 30 K. The MgB₂ superconductivity triggered the research towards other diboride compounds, in particular transition-metal diborides, systems that all crystallize in the well known AlB₂ structure, although none of them showed superconductivity above 10 K. Moreover, great interest both from the experimental as well as the theoretical point of view is the understanding of the superconductive mechanism in this binary compound characterized by:1) a simple lattice structure and the lack of both magnetic contributions and a strong electron-electron correlation;2) a critical temperature (i.e., for MgB₂) very close to those of the high-Tc superconducting cuprates (HTSC).Theoretical researches regarding HTSC superconductive mechanism did not produced relevant discoveries in the last two decades and the discovery the MgB₂ superconductor represent a new opportunity. Many teams are focusing their interest to MgB₂ and its related binaries, and since 2001 hundreds of investigations are now available. Still, the superconducting mechanism of this system is an open problem.Due to their identical geometrical structure associated to different (and lower) critical temperature, transition-metal diborides (TMB₂) represent ideal reference models of the superconductive mechanism in MgB₂. Compared with this latter system, transition-metal diborides differ by:1) the occupancy of the d state;2) the large mass of the metal atoms.Clearly, the number of d electrons affect the electronic structure. As an example, among all transition-metal diborides, only ScB₂ and YB₂ share similar band structures with MgB₂, and in fact, in these systems the p_xy bands lie at the A point above the Fermi level. Energy band calculations of MgB₂ shows that Bσband may significantly affect the superconductive mechanism. The mass difference of metal atom should modulate the lattice vibrational property because:1) the light mass of metal enhances the phonon frequency;2) the vibrations of an heavy transition metal do not couple with the vibration of boron due to their large mass difference.Nevertheless, theoretical studies predict a coupling dynamic that plays a key role in order to increase the MgB₂ critical temperature not yet confirmed by experiments. Inelastic neutron scattering technique represents a common tool to investigate the lattice dynamics of these systems, but this method cannot return local information of lattice vibration or correlation information of the atomic pair. However, it is well known that a significant coupling between the local structure and other properties of the system such as superconductivity occurs. Due to its element selectivity EXAFS can give local structure information such as the type of coordination atom, the coordination number, the coordination distance and the disorder degree around the absorbing atom. Moreover, the Debye-Waller factor has two components: the static disorder and the thermal disorder. This latter is correlated to the lattice dynamics and these information may be extracted by EXAFS, e.g., by the pair distribution function because the EXAFS signal is associated to the photoelectron scattering between the absorbing atom and its first neighbors. The same correlation can not be obtained by the XRD technique. Moreover, a detailed temperature-dependent study (T-EXAFS) of the width of the Pair Distribution Function (PDF),σ, may allow to separate the different contributions and to provide a connection between macroscopic observations and the underlying atomic interactions.In this thesis, we present first XANES data at the metal K-edge of 3d transition-metal diborides and ab initio full multiple-scattering calculations. Moreover, we measured the temperature dependence of the Debye-Waller factors in 4d and IVB serial transition-metal diborides have been characterized with this technique. An anomalous behavior of the Debye-Waller factor of the TM-B pair has been detected for the first time. The anomalous behavior is not described by the Einstein model. Therefore, a modified Einstein model taking in consideration two frequencies was introduced to describe the observed behavior. Using this model, the observed behavior is interpreted as the superposition of an optical mode corresponding to phonon vibrations induced by the boron sublattice and an acoustic mode corresponding to the metal sublattice vibration.By comparison of the XANES spectra of 4d transition-diborides which have similar metal mass but different number of 4d electron, because electronic structure affects the strength between metal and boron atoms we found that the vibrational frequency is affected by the number of 4d electrons.By comparison of the XANES spectra of IVB serial transition-diborides which have similar electronic structures but different metal masses we observed an effective decoupling between metal and boron vibrations. The phenomenon may be characterized by the parameter a in the modulated mode and the decoupling is a function of the metal mass.For the first time we combined T-EXAFS and the modulated Einstein model, we investigated also the boron isotope effect and the lattice dynamics of YB₂, a system that has the same crystal structure and a similar electronic structure of MgB₂. An evident energy shift in the lattice vibration (- 4.3 meV) for the optical mode and about 1.2 meV for the phonon mode, was observed for both boron isotopic compounds. Data also confirm the hypothesis that B-B vibrational mode plays an role in the superconductive mechanism in the transition-metal diborides and in particular in MgB₂.To summarize, two are the relevant results obtained by the lattice dynamics study of transition-metal diborides presented in this thesis:a) in agreement with theoretical calculations, the B-B vibrational mode plays a key role in the electron-phonon coupling, an important parameter of the BCS theory.b) experimental data support the model in which the vibrational decoupling between metal and boron atoms is correlated to the mechanism that significantly reduce the superconducting critical temperatures of TMB₂ respect to the MgB₂ compound;These results are important to improve the knowledge of the MgB₂ system and its superconducting properties. In metal diborides, the B-B vibration is a relevant item for the electron-phonon interaction, but a pure the B-B vibration makes the superconductive property of transition-metal diboride go to commonness. Moreover in MgB₂ system, a strong coupling between boron and metal vibrations occurs due to the large difference of mass, then introducing a high critical temperature.