Thermal and magnetic anomalies ofα-iron: an exploration by extended x-ray absorption fine structure spectroscopy and synchrotron x-ray diffraction

Abstract

The local structure and dynamics of α-iron have been investigated by extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray diffraction (XRD) in order to shed light on some thermal and magnetic anomalies observed in the last decades. The quantitative EXAFS analysis of the first two coordination shells reveals a peculiar local vibrational dynamics of α-iron: the second neighbor distance exhibits anharmonicity and vibrational anisotropy larger than the first neighbor distance. We search for possible distortions of the bcc structure to justify the unexplained magnetostriction anomalies of α-iron and provide a value for the maximum dislocation of the central Fe atom. No thermal anomalies have been detected from the current XRD data. On the contrary, an intriguing thermal anomaly at about 150 K, ascribed to a stiffening of the Fe-Fe bonds, was found by EXAFS.

Probing disorder in high-pressure cubic tin (IV) oxide: a combined X-ray diffraction and absorption study

“…It should be noted that any uncertainties that may arise from a large ÁE 0 could not explain the large disorder apparent in the O positions, especially considering the highly ordered Sn positions. Thus, we analyze the level of disorder using the techniques set forth by Fornasini et al (2001) and Boccato et al (2016). Table 1 Atomic distances to 5 Å for Pa " 3 3 SnO 2 at 44.5 GPa from EXAFS and XRD along with parallel and perpendicular anharmonicity components, || and ?…”

“…However, for SnO 2 we expect disorder at these pressure conditions (Gira ˜o et al, 2018), and the Gaussian function is no longer sufficient, meaning that higher moments of the distribution must be considered. Thus, to study disordering, the cumulant expansion of the EXAFS equation was used (Boccato et al, 2016;Ravel et al, 2006),…”

“…It should be noted that any uncertainties that may arise from a large ÁE 0 could not explain the large disorder apparent in the O positions, especially considering the highly ordered Sn positions. Thus, we analyze the level of disorder using the techniques set forth by Fornasini et al (2001) and Boccato et al (2016).…”

“…, derived by comparison of the instantaneous relative atomic distances, hri, measured by EXAFS (given by hj " r a r a À " r b r b ji, where a is the absorbing and b the scattering atom) and the average atomic separations, R, provided by XRD (given by jh " r a r a i À h " r b r b ij), through h 2 ? i ' Rðhri À RÞ (Fornasini et al, 2001;Fornasini & Grisenti, 2015;Boccato et al, 2016).…”

Anomalous Conductivity in the Rutile Structure Driven by Local Disorder

“…EXAFS data is modeled here using the cumulant expansion of the EXAFS equation following Fornasini et al 26 for its precedent in describing structural anharmonicities and disorder, 27 provided an absence of anisotropic strain is ensured either by thermal annealing 13 or here by the use of a fluid He PTM. Upon fitting the data to the cumulant-expanded EXAFS equation (details of fitting are in the Supporting Information), the average Sn−O distance in the first coordination octahedron, as well as the mean squared relative displacements (MSRD) of the O, can be extracted.…”