Raffaella Torchio

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

Methodology for in situ synchrotron X-ray studies in the laser-heated diamond anvil cell

ABSTRACT A review of some important technical challenges related to in situ diamond anvil cell laser heating experimentation at synchrotron X-ray sources is presented. The problem of potential chemical reactions between the sample and the pressure medium or the carbon from the diamond anvils is illustrated in the case of elemental tantalum. Preliminary results of a comparison between reflective and refractive optics for high temperature measurements in the laser-heated diamond anvil cell are briefly discussed. Finally, the importance of the size and relative alignment of X-ray and laser beams for quantitative X-ray measurements is presented.

Probing the local, electronic and magnetic structure of matter under extreme conditions of temperature and pressure

ABSTRACT In this paper we present recent achievements in the field of investigation of the local, electronic and magnetic structure of the matter under extreme conditions of pressure and temperature. These results were obtained thanks to the coupling of a compact laser heating system to the energy-dispersive XAS technique available on the ID24 beamline at the ESRF synchrotron. The examples chosen concern the melting and the liquid structure of 3d metals and alloys under high pressures (HPs) and the observation of temperature-induced spin crossover in FeCO3 at HP.

The Melting Curve of Nickel Up to 100 GPa Explored by XAS

Precise knowledge of the melting temperatures of iron, nickel, and their alloys at pressures of the deep Earth would allow us to better constrain the parameters used for the Earth’s heat budget and dynamics. However, melting curves of transition metals at pressures approaching 100 GPa and above are still controversial. To address this issue, we report new data on the melting temperature of nickel in a laser-heated diamond anvil cell up to 100 GPa obtained by X-ray absorption spectroscopy (XAS), a technique rarely used at such conditions. We couple this for the first time to ex situ analysis of the sample, providing a further validation of the melting criterion adopted here. Finally, a Simon-Glatzel fit to the melting data obtained in this work, combined with those obtained in the most recent X-ray diffraction experiments, gives TM(K) = 1727 × [ PM 17±3 + 1 ] 1 2.5±0.1 , defining the most up-to-date X-ray-determined melting curve for Ni. This result confirms that Ni could be ignored in the discussion on melting properties and thermal profile of the Earth’s core, as it should affect the Fe melting point by only 10–20 K at 90 GPa.

Structure and magnetism in compressed iron–cobalt alloys

Combined Co K-edge XANES-XMCD and XRD measurements were used to shed light on the magnetic and structural phase diagram of the Fe1−x Co x alloy under HP in the Co-rich region (x≥0.5). At 0.5≤x≤0.75, the alloy shows a pressure-induced structural/magnetic phase transition from bcc-FM to hcp-non-FM phase just like pure iron but at higher pressures. The x=0.9 sample has an fcc structure in the pressure range investigated but presents an FM to non-FM transition at P=64 GPa, a significantly lower pressure compared with pure Co (predicted ≈120 GPa), showing that Fe impurities strongly affect the HP Co response.

Measurement of temperature in the laser heated diamond anvil cell: comparison between reflective and refractive optics

ABSTRACT In this article we present a direct comparison between reflective (Schwarzschild mirrors) and refractive (achromatic doublets) optics commonly used in spectroradiometric temperature measurements in laser heated diamond anvil cells. Emission spectra are fitted with the Plancks law and are further analysed with the two-colour technique; theoretical simulations are used to compare the temperature measurement accuracy of the two optical systems. The first result obtained is that achromatic doublets with large numerical aperture () produce extensive accuracy errors in the full T range (1500–3000 K). When reduced apertures () are used a good agreement is found with measurements from reflective optics up to 2600 K while systematic differences (around 200 K) appear at higher temperatures. However, these temperature differences cannot explain the discrepancies between the iron melting curves obtained in the past years using laser heated diamond anvil cells.

Exploring liquid–liquid transitions in ZnSe at extreme conditions

Here, we report on the fingerprints of 4-fold to 6-fold transition in liquid ZnSe at extreme pressure and temperature conditions up to 42 GPa and more than 3000 K using X-ray absorption spectroscopy (XAS) combined with laser-heated diamond anvil cell techniques and complementary ab initio molecular dynamics simulations. Among pressure induced structural modifications, liquid–liquid’ (L-L’) transitions have attracted much interest both experimentally and theoretically due to their peculiarity. Unlike most L-L’ transitions that only show slight modifications of the bond distances between atoms (i.e., P, Na, supercooled Si) [1, 2, 3], strong L-L’ transitions have been theoretically predicted for a few II-VI semiconductor compounds such as ZnSe, CdSe and CdTe, given the following two conditions are satisfied: Firstly, the melting of the sp3 bonded phase (4-fold coordination) at ambient pressure results in a liquid structure that remains approximately 4-fold coordinated; and secondly, the presence of a semiconductor to metal phase transition under pressure [4, 5, 6, 7]. Our results show that solid and liquid ZnSe undergoes a series of structural modifications at various P, T values that satisfy the conditions above. The red shift in Zn K edge energy observed at around 7 GPa upon increasing temperature suggests a metallization event before melting to a possible 6-fold coordinated liquid structure. These findings are supported by our simulation results showing a pronounced difference in the first diffraction peak of the calculated structure factor at high pressures indicating a 4-fold to 6-fold coordination change in liquid ZnSe. Our results may provide additional insight for such transitions that may be observed for similar tetrahedrally coordinated II-VI systems.

High pressure dynamic XAS studies using an energy-dispersive spectrometer

ABSTRACT We present in this paper recent advances in the high pressure domain provided by the introduction of time-resolved energy-dispersive XAS (EDXAS) techniques at synchrotrons. We highlight technical aspects and describe two modes of acquisition: the ‘movie’ mode, where the time resolution is given by the detector acquisition speed and the ‘pump-and-probe’ mode, where the time resolution is given by the delay between the pump and the probe. These two modes define a frontier in the time resolution, respectively above and below the ∼10 μs regime. In the former, examples of applications are chemical stability and reactions at high pressure and high temperature or probing the warm dense matter regime using rapid current ramps. In the latter, an example is given on studies of dynamically compressed matter, by coupling single-bunch EDXAS at high-brilliance synchrotron to a nanosecond high-power laser.