Local structural distortions in SnTe investigated by EXAFS.

Extended x-ray absorption fine structure (EXAFS) has been measured at theKedge of Sn in SnTe in the temperature range from 5 to 480 K. EXAFS results are consistent with the presence of a local rhombohedral distortion in the full temperature range from 5 to 300 K, even well above the ferroelectric transition temperature, suggesting a partial order-disorder character of the transition. At and above 300 K, the anomalous behaviour of the third and fourth EXAFS cumulants reveals a modification of the anharmonicity of the effective pair potential, possibly connected with the softening of high frequency modes or to the presence of multiple phases.

Bond compressibility and bond Grüneisen parameters of CdTe.

Extended x-ray absorption fine structure (EXAFS) at the Cd K edge and diffraction patterns have been measured on CdTe as a function of pressure from 100 kPa (1 bar) to 5 GPa using a cell with nano-polycrystalline diamond anvils and an x-ray focussing scanning spectrometer. Three phases-zincblende (ZB), mixed cinnabar-ZB and rocksalt (RS)-are well distinguished in different pressure intervals. The bond compressibility measured by EXAFS in the ZB phase is slightly smaller than the one measured by diffraction and decreases significantly faster when the pressure increases; the difference is attributed to the effect of relative vibrations perpendicular to the Cd-Te bond. The parallel mean square relative displacement (MSRD) decreases, the perpendicular MSRD increases when the pressure increases, leading to an increasing anisotropy of relative atomic vibrations. A constant-temperature bond Grüneisen parameter (GP) has been evaluated for the ZB phase and compared with the constant-pressure bond GP measured in a previous experiment; an attempt is made to connect the bond GPs measured by EXAFS and the more familiar thermodynamic GP and mode GPs; the comparisons suggest the inadequacy of the quasi-harmonic approximation to deal with the local vibrational properties sampled by EXAFS.

Nearest-neighbour distribution of distances in crystals from extended X-ray absorption fine structure.

Extended X-ray absorption fine structure (EXAFS) is a powerful probe of the distribution of nearest-neighbour distances around selected atomic species. We consider here the effect of vibrational disorder in crystals. The potential of EXAFS for the accurate evaluation of the coefficient of bond thermal expansion and its temperature dependence is discussed, with the aim of stimulating and facilitating the comparison with the results from total scattering experiments. The meaning of the distribution asymmetry in crystals and its connection with the effective potential anharmonicity and the bond expansion is quantitatively explored by comparing the results for a number of different systems. The extent of the relative atomic vibrations perpendicular to the bond direction and the perpendicular to parallel anisotropy are correlated with the extent of lattice negative thermal expansion as well as with the ionic mobility in superionic crystals.

On EXAFS Debye-Waller factor and recent advances.

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter-atomic equilibrium distance. By comparing the first-shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi-harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.

The coefficient of bond thermal expansion measured by extended x-ray absorption fine structure.

The bond thermal expansion is in principle different from the lattice expansion and can be measured by correlation sensitive probes such as extended x-ray absorption fine structure (EXAFS) and diffuse scattering. The temperature dependence of the coefficient α(bond)(T) of bond thermal expansion has been obtained from EXAFS for CdTe and for Cu. A coefficient α(tens)(T) of negative expansion due to tension effects has been calculated from the comparison of bond and lattice expansions. Negative lattice expansion is present in temperature intervals where α(bond) prevails over α(tens); this real-space approach is complementary but not equivalent to the Grüneisen theory. The relevance of taking into account the asymmetry of the nearest-neighbours distribution of distances in order to get reliable bond expansion values and the physical meaning of the third cumulant are thoroughly discussed.

Local vibrational properties of GaAs studied by extended X-ray absorption fine structure.

Extended X-ray absorption fine structure (EXAFS) has been measured at both the K edges of gallium and arsenic in GaAs, from 14 to 300 K, to investigate the local vibrational and thermodynamic behaviour in terms of bond expansion, parallel, and perpendicular mean square relative displacements and third cumulant. The separate analysis of the two edges allows a self-consistent check of the results and suggests that a residual influence of Ga EXAFS at the As edge cannot be excluded. The relation between bond expansion, lattice expansion, and expansion due to anharmonicity of the effective potential is quantitatively clarified. The comparison with previous EXAFS results on other crystals with the diamond or zincblende structure shows that the values of a number of parameters determined from EXAFS are clearly correlated with the fractional ionicity and with the strength and temperature interval of the lattice negative expansion.

Accuracy evaluation in temperature-dependent EXAFS measurements of CdTe.

The evaluation of uncertainty in temperature-dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K-edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest-neighbours parameters: bond thermal expansion, parallel and perpendicular mean-square relative displacements and the third cumulant. Different causes of uncertainty, a comparison of experimental results with theoretical models, the difference between EXAFS and crystallographic thermal expansions and the meaning of the third cumulant are discussed.

Negative thermal expansion in crystals with the zincblende structure: an EXAFS study of CdTe.

The extended x-ray absorption fine structure (EXAFS) has been measured at both the K edges of cadmium and tellurium in CdTe, from liquid helium to room temperature, in order to investigate the local thermodynamic behaviour. The temperature dependences of the structural parameters obtained from the separate analysis of the two edges are perfectly consistent. The positive contribution to the thermal expansion due to the bond stretching and the negative contribution due to the tension effects are disentangled and quantified in terms of the bond thermal expansion and the perpendicular mean square relative displacement. The comparison with previous EXAFS results for Ge and CuCl shows that relevant correlations can be established between a number of local parameters measured by means of EXAFS and the properties of the lattice negative thermal expansion of tetrahedrally bonded semiconductors. The effective force constants derived from the EXAFS are compared with the force constants of a valence force field model.

On the origin of the differences in the Cu K-edge XANES of isostructural and isoelectronic compounds.

Cu K-edge x-ray absorption near-edge structure (XANES) spectra of trigonal (3R) CuScO(2) and CuLaO(2) and of hexagonal (2H) CuScO(2) were investigated experimentally and theoretically, in order to study differences between spectra of isostructural and isoelectronic compounds. Significant differences were found in the Cu K-edge XANES of 3R CuScO(2) and 3R CuLaO(2); these differences can be understood by considering the calculated polarization dependence of the XANES spectra and the differences between the phaseshifts of Sc and La. Spectra of the 3R and 2H polytypes of CuScO(2) differ only weakly and the difference originates from the long-range order. The pre-edge peak around 8980 eV is generated by the same mechanism as the pre-edge peak in Cu(2)O, i.e. involving scattering by the Cu atoms in the plane which is perpendicular to the O-Cu-O axis.

Correlation between I-Ag distance and ionic conductivity in AgI fast-ion-conducting glasses.

A large number of AgI-based fast-ion-conducting glasses have been investigated by K-iodine extended x-ray absorption fine structure spectroscopy (EXAFS) measurements at liquid nitrogen temperature. A general correlation between the I-Ag distance measured by EXAFS and the glass activation energy for dc ionic conductivity has been found out: glasses with longer I-Ag distances display higher ionic conductivity, independently from the chemical composition of their host glassy matrix. This behavior can be related to the progressive increase of the "pathway volume" for ionic conduction.

Isotopic effect in extended x-ray-absorption fine structure of germanium.

Extended x-ray absorption fine structure has been measured on two powdered samples of (70)Ge and (76)Ge as a function of temperature from 20 to 300 K. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller factors. The isotopic effect is also detected on the difference of nearest-neighbor average ineratomic distances, thanks to a resolution better than 10 fm.

Einstein and Debye models for EXAFS parallel and perpendicular mean-square relative displacements.

The correlated Einstein and Debye models for EXAFS parallel mean-square relative displacement (MSRD) are derived from the general expression in terms of eigenfrequencies and eigenvectors of the dynamical matrix, without ad hoc assumptions. The two models are generalized to parameterize also the EXAFS perpendicular MSRD. The physical meaning of Einstein frequencies, as well as the application of the Debye model to crystals with more than one atom per cell, are critically discussed.

Local thermal expansion in a cuprite structure: the case of Ag(2)O.

The local thermal behavior of the Ag(2)O framework structure has been studied by extended x-ray absorption fine structure. The average Ag-O nearest-neighbor distance expands upon heating, while the Ag-Ag next-nearest-neighbor distance contracts. An original implementation of the cumulant analysis shows that the Ag-O expansion is a joint effect of potential anharmonicity and geometrical deformation of the Ag(4)O basic tetrahedral units. Accordingly, the negative thermal expansion of the lattice parameter in Ag(2)O cannot be explained uniquely in terms of rigid unit modes.

On the cumulant analysis of EXAFS in crystalline solids.

The analysis of temperature-dependent EXAFS spectra based on the cumulant expansion is critically reviewed, seeking for accurate relations between EXAFS parameters and physical properties of crystals. The treatment is based on the distinction between the real and effective distribution of distances, and is divided into three logical steps. (a) The connection between lattice dynamics and cumulants C(n)* of the real distribution is studied and the extent of the usual approximations are numerically evaluated. Atomic vibrations perpendicular to the bond direction make the EXAFS thermal expansion larger than the crystallographic one; the difference is connected to a shift of the effective pair potential rather than to its asymmetry. Peculiar information on lattice dynamics of crystals can be obtained from accurate EXAFS measurements and their cumulant analysis. (b) The differences between cumulants of the real and effective distribution (C(n)* and C(n), respectively) are calculated for various physically realistic distributions. The largest discrepancy concerns the first cumulant: C(1)* measures the thermal expansion of the interatomic bond, while C(1) is a better estimate of the crystallographic thermal expansion. (c) The convergence properties of the cumulant series are discussed and some phenomenological procedures are suggested to monitor and possibly work out the connected failures of the cumulant method. Benefits and risks of the use of an effective pair potential are at last debated.

EXAFS Debye-Waller factor and thermal vibrations of crystals.

The EXAFS Debye-Waller factor depends on the correlation of atomic motion and can yield original information on the vibrational dynamics of crystalline solids. In this paper an introductory treatment of thermal disorder in EXAFS, based on thc cumulant approach, is given. Thc general relation between mean-square relative displacement (MSRD) measured by EXAFS and atomic thermal vibrations in harmonic approximation is explored. Strengths and limitations of the phenomenological Einstein- and Debye-correlated models are discussed. Some of the most significant results so far obtained are reviewed. A relatively simple method for estimating anharmonic corrections to the MSRD is proposed.

PIN Silicon Diodes as EXAFS Signal Detectors.

The properties of PIN silicon diodes as X-ray detectors for EXAFS measurements with synchrotron radiation have been investigated. Electronic stability, linearity and noise current have been analyzed. The effects of diffraction peaks resulting from the crystalline nature of the diodes have been minimized by mounting the diodes on a simple device that continuously changes its orientation by a few degrees with respect to the X-ray beam. An accurate comparison between EXAFS signals monitored by ionization chambers and PIN photodiodes is presented. It is shown that good-quality EXAFS measurements with PIN photodiodes are possible if diffraction effects are eliminated.