A new internally heated diamond anvil cell system for time-resolved optical and x-ray measurements

We have developed a new internally heated diamond anvil cell (DAC) system for in situ high-pressure and high-temperature x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution has been seldom used in the past but proved to be very efficient to reduce the size of the heating spot near the sample region, improving heating and cooling rates as compared to other resistive heating strategies. The system has been widely tested under high-temperature conditions by performing several thermal emission measurements. A robust relationship between electric power and average sample temperature inside the DAC has been established up to about 1500 K by a measurement campaign on different simple substances. A micro-Raman spectrometer was used for various in situ optical measurements and allowed us to map the temperature distribution of the sample. The distribution resulted to be uniform within the typical uncertainty of these measurements (5% at 1000 K). The high-temperature performances of the DAC were also verified in a series of XAS (x-ray absorption spectroscopy) experiments using both nano-polycrystalline and single-crystal diamond anvils. XAS measurements of germanium at 3.5 GPa were obtained in the 300 K-1300 K range, studying the melting transition and nucleation to the crystal phase. The achievable heating and cooling rates of the DAC were studied exploiting a XAS dispersive setup, collecting series of near-edge XAS spectra with sub-second time resolution. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures during the application of constant power cycles, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device

Porous silicon nanowires phase transformations at high temperatures and pressures

Porous silicon nanowires (NWs) with homogenous lateral dimensions of 90 nm are investigated by Raman scattering experiments along isothermal pressure cycles in a diamond anvil cell. Experiments were performed at variable temperatures up to 400 °C for maximal pressures of about 30 GPa comparing directly with transformations in bulk Si and porous NWs. Scanning electron microscopy demonstrates the persistence of one-dimensional morphology after high pressure investigation. The diamond phase in porous nanowires persists upon compression up to around 20 GPa at room temperature (25 °C) and to about 14 GPa at 200 °C and 400 °C. However, the β−Sn high pressure phase is seen to coexist with the diamond phase above 12 GPa at 25 °C and above 6 GPa at 200 °C and 400 °C. The coexistence region of the two phases is found to be considerably enlarged as compared with crystal silicon at each temperature. Upon decompression from 30 GPa, nucleation to the β−Sn, followed by formation of amorphous structures, is observed for porous NWs. Returning to ambient pressure and temperature, amorphous silicon is the dominant form with a residual contribution of β−Sn. At higher temperatures, nucleation back to the diamond structure is triggered although coexistence of amorphous and crystalline phases is observed up to 400 °C. © 2021 Author(s)

Collapse of itinerant ferromagnetism in CoS2 under pressure: An x-ray absorption spectroscopy study

Cobalt L-edge x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of CoS2 have been measured at low temperature (4 K) in order to characterize the electronic and magnetic structure with a significant improvement in resolution as compared with previous measurements. The branching ratio of L3 and L2 x-ray absorption near-edge spectra was found to be consistent with a low-spin configuration (S=1/2). A total magnetic moment ⟨m⟩=0.78μB/Co atom with the orbital part ⟨ml⟩=0.049μB has been obtained from the XMCD signal, consistent with the nearly half-metallic character of CoS2 and with the itinerant nature of the ferromagnetism. The behavior under pressure of CoS2 was studied by Co K-edge XAS and XMCD experiments performed up to a pressure of about 30 GPa using diamond-anvil cell at a temperature of 10 K. No evidence of structural phase transition was observed, while the intensity of the XMCD signal was found to decrease continuously for increasing pressures, becoming negligible in the 8∼10 GPa pressure range, a fact consistent with the collapse of itinerant ferromagnetic ordering. The gradual change in the electronic and magnetic structure upon application of pressure was also monitored by the near-edge XAS evolution, found to be in agreement with full multiple-scattering calculations under uniform contraction of distances, and by the blue-shift of the K-edge energy. The Co K-edge energy was found to shift to higher energies (up to 0.64 eV at 30 GPa) and a change of slope was observed at pressures around 5 GPa that may correspond to the disappearance of half-metallicity, in agreement with previous studies

Crystal and electronic structure of Co3O4 spinel under pressure probed by XANES and Raman spectroscopy

Crystal and electronic structure of Co3O4 spinel have been investigated by x ray absorption near edge structure XANES at the Co K edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic P21 c phase above amp; 8764;52.7 GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above amp; 8764;21.9 GPa orthorhombic Fddd and amp; 8764;43.0 GPa monoclinic C2 m , in agreement with the previous x ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the Co2 sites, being completed upon transition to the monoclinic P21 c phase. The valence and spin states at two different Co sites remained unchanged at least up to the transition onset to the monoclinic P21 c phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previousl

Structural Properties of Porous Silicon Nanowires: A Combined Characterization by Advanced Spectroscopic Techniques

Advanced characterization techniques including synchrotron radiation have been used to investigate the structural and electronic properties of doped silicon nanowires (NWs). Si L-edge, O K-edge and F K-edge XAS (x-ray absorption spectroscopy) spectra of silicon NWs at different doping levels have been collected at the BEAR beamline of the ELETTRA synchrotron radiation facility. XAS results show that the NWs structures are modified changing the type and level of doping and by the etching process. Optical Raman spectroscopy of NWs shows shifted and broadened first order optical mode, corresponding to a decrease in size of the crystallite domains inside the nanowires. The observed Raman shifts are compatible with the occurrence of a larger crystallite size in p-type NWs and smaller one in n-type NWs, in line with XAS results. Fabricated low-doped p-type NWs were also pressurized up to 24 GPa in a diamond anvil cell at room temperature and Raman scattering was recorded at selected pressures. The Si diamond crystal structure (dc-Si) is observed to persist up to ∼ 22 GPa, much higher than the phase transition onset (∼ 11 GPa) occurring in bulk silicon in the same experiment. © 2021, Springer Nature Switzerland AG

Crystal and electronic structure of Co₃O₄ spinel under pressure probed by XANES and Raman spectroscopy

Crystal and electronic structure of Co3O4 spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co K edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic P2(1)/c phase above similar to 52.7 GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above similar to 21.9 GPa (orthorhombic Fddd) and similar to 43.0 GPa (monoclinic C2/m), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the Co2+ sites, being completed upon transition to the monoclinic P2(1)/c phase. The valence and spin states at two different Co sites remained unchanged at least up to the transition onset to the monoclinic P2(1)/c phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously

Local Structure of Ga85:8In14:2 Eutectic Alloy and Its Pressure–Temperature Melting Line

The structure of the (Formula presented.) eutectic liquid alloy is investigated both under ambient conditions and at high pressure/high temperature using X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The local structure of the liquid alloy at ambient conditions is analyzed using double-edge refinements of the XAS data. Solid–liquid phase transitions under high-pressure and high-temperature conditions are monitored by combined XAS and XRD measurements along several quasi-isobaric heating runs, allowing to draw a melting line up to 10 GPa. The established melting line is found to be slightly below the one of pure gallium (Ga) and to follow its trend as expected from the eutectic nature of the compound. A series of Ga K-edge X-ray absorption fine structure (XAFS) spectra measured at different pressures indicates the absence of large structural modifications at local Ga sites in the liquid within the investigated pressure and temperature range. © 2021 The Authors. physica status solidi (RRL) Rapid Research Letters published by Wiley-VCH GmbH

Crystal and electronic structure of Co3O4 spinel under pressure probed by XANES and Raman spectroscopy

Crystal and electronic structure of Co3O4 spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co K edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic P21/c phase above ∼52.7 GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above ∼21.9 GPa (orthorhombic Fddd) and ∼43.0 GPa (monoclinic C2/m), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the Co2+ sites, being completed upon transition to the monoclinic P21/c phase. The valence and spin states at two different Co sites remained unchanged at least up to the transition onset to the monoclinic P21/c phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously

A new internally heated diamond anvil cell system for time-resolved optical and x-ray measurements

We have developed a new internally heated diamond anvil cell (DAC) system for in situ high-pressure and high-temperature x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution has been seldom used in the past but proved to be very efficient to reduce the size of the heating spot near the sample region, improving heating and cooling rates as compared to other resistive heating strategies. The system has been widely tested under high-temperature conditions by performing several thermal emission measurements. A robust relationship between electric power and average sample temperature inside the DAC has been established up to about 1500 K by a measurement campaign on different simple substances. A micro-Raman spectrometer was used for various in situ optical measurements and allowed us to map the temperature distribution of the sample. The distribution resulted to be uniform within the typical uncertainty of these measurements (5% at 1000 K). The high-temperature performances of the DAC were also verified in a series of XAS (x-ray absorption spectroscopy) experiments using both nano-polycrystalline and single-crystal diamond anvils. XAS measurements of germanium at 3.5 GPa were obtained in the 300 K-1300 K range, studying the melting transition and nucleation to the crystal phase. The achievable heating and cooling rates of the DAC were studied exploiting a XAS dispersive setup, collecting series of near-edge XAS spectra with sub-second time resolution. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures during the application of constant power cycles, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device