Measuring strain at the atomic-scale with Differential X-ray Absorption Spectroscopy

Strain-inducing phenomena, such as magnetostriction, lie at the heart of transducer technologies. Knowledge of their origin and mechanics, and how they manifest themselves in different materials, underpins the development and optimisation of sensor and actuator devices. DiffXAS has been developed to permit strain measurements at an atomic-scale, and thus verify theoretical models for transducer behaviour.Submitted versio

Atomic-scale mechanisms for magnetostriction in CoFe2O4 and La0.5Sr0.5CoO3 oxides determined by differential x-ray absorption spectroscopy

The atomic environments involved in the magnetostriction effect in CoFe2O4 and La0.5Sr0.5CoO3 poly-crystalline samples have been identified by differential extended x-ray absorption fine structure (DiffEXAFS) spectroscopy. We demonstrate that cobalt atoms at octahedral sites are responsible for their magnetostriction. The analysis of DiffEXAFS data indicates that the local-site magnetostrictive strains of Co atoms are reversed in these two oxides, in agreement with the macroscopic magnetostriction. For the CoFe2O4 spinel, a large negative strain along the (100) direction has been determined for the CoO6 octahedron causing a tetragonal contraction in contrast with the La0.5Sr0.5CoO3 perovskite, where a positive moderate strain along the (100) direction was found resulting in a tetragonal expansion. The different local-site magnetostriction is understood in terms of the different valence and spin state of the Co atoms for the two oxides. The macroscopic magnetostriction would be explained then by the relative change in volume, either contraction in CoFe2O4 or expansion in La0.5Sr0.5CoO3, when the tetragonal axis of the Co site is reoriented under an externally applied magnetic field

Electronic states of R F e2 O4(R=Lu, Yb, Tm, Y) mixed-valence compounds determined by soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism

We here report an investigation of the electronic states in the RFe2O4(R=Lu,Yb,Tm,Y) mixed-valence ferrites by means of soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements together with ab initio theoretical calculations. The presence of Fe+2 and Fe+3 pure ionic species is discarded in the XAS spectra at the OK edge in both experimental data and simulations based on the multiple scattering theory. Similarly, no trace of Fe+2/Fe+3 contributions is detected in the XMCD spectra at the FeK edge. On the other hand, the XAS and XMCD spectra at the FeL2,3 edges can be well described in terms of Fe+2/Fe+3 contributions, and are also supported by multiplet calculations. This finding can be interpreted as the existence of a mixture of 3d5/3d6 configurations at the Fe atoms. Alternative ferrimagnetic spin orderings based on a trimodal Fe valence distribution are also proposed and discussed. Finally, a possible explanation for the strong dependence of the FeL2,3 edges XMCD signal magnitude on both the sample surface preparation and detection method is presented

High pressure polymorphism of ?-TaON

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch–Murnaghan equation of state fit for baddeleyite structured ?-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K?o = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta–(O,N) and Ta–Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko 370 GPa, close to the theoretically predicted value.<br/

Dynamics of the magnetic and structural a -> e phase transition in Iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of Iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with theoretical prediction. The pressure domain of the transition observed (2.4 $\pm$ 0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full potential total energy calculations

Interplay of the electronic and lattice degrees of freedom in A_{1-x}Fe_{2-y}Se_{2} superconductors under pressure

The local structure and electronic properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$ are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium K-edges as a function of pressure. A combination of dispersive geometry and novel nanodiamond anvil pressure-cell has permitted to reveal a step-like decrease in the Fe-Se bond distance at $p\simeq11$ GPa. The position of the Fe K-edge pre-peak, which is directly related to the position of the chemical potential, remains nearly constant until $\sim6$ GPa, followed by an increase until $p\simeq 11$ GPa. Here, as in the local structure, a step-like decrease of the chemical potential is seen. Thus, the present results provide compelling evidence that the origin of the reemerging superconductivity in $A_{1-x}$Fe$_{2-y}$Se$_2$ in vicinity of a quantum critical transition is caused mainly by the changes in the electronic structure

One-year follow-up of mud-bath therapy in patients with bilateral knee osteoarthritis: a randomized, single-blind controlled trial

The objective of this prospective parallel randomized single-blind study was to assess that a cycle of mud-bath therapy (MBT) provides any benefits over usual treatment in patients with bilateral knee osteoarthritis (OA). Patients with symptomatic primary bilateral knee OA, according to ACR criteria, were included in the study and randomized to one of two groups: one group received a cycle of MBT at spa center of Chianciano Terme (Italy) in addition to the usual treatment, and one group continued their regular care routine alone. Clinical assessments were performed 7\ufffddays before enrollment (screening visit), at the time of enrollment (basal time), after 2\ufffdweeks, and after 3, 6, 9, and 12\ufffdmonths after the beginning of the study. All assessments were conducted by two researchers blinded to treatment allocation. The primary efficacy outcomes were the global pain score evaluated by Visual Analog Scale (VAS) and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) subscore for physical function (W-TPFS). Of the 235 patients screened, 103 met the inclusion criteria: 53 patients were included in the MBT group and 50 in the control group. In the group of patients treated with MBT, we observed a statistically significant (p\ufffd<\ufffd0.001) reduction of VAS and W-TPFS score at the end of the treatment; this improvement was significant (p\ufffd<\ufffd0.05) also at 3\ufffdmonths of follow-up. The control group did not show significant differences between baseline time and all other times. The differences between one group were significant for both primary parameters already from the 15th day and persisted up to the 9th month. This beneficial effect was confirmed by the significant reduction of symptomatic drug consumption. Tolerability of MBT seemed to be good, with light and transitory side effects. Our results confirm that a cycle of MBT added to usual treatment provides a beneficial effect on the painful symptoms and functional capacities in patients with knee OA that lasts over time. Mud-bath therapy can represent a useful backup to pharmacologic treatment of knee OA or a valid alternative for patients who do not tolerate pharmacological treatments