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We have investigated the local and electronic structure of solid rubidium by means of x-ray absorption spectroscopy up to 101.0 GPa, thus doubling the maximum investigated experimental pressure. This study confirms the predicted stability of phase VI and was completed by the combination of two pivotal instrumental solutions. On one side, we made use of nanocrystalline diamond anvils, which, contrary to the more commonly used single crystal diamond anvils, do not generate sharp Bragg peaks (glitches) at specific energies that spoil the weak fine structure oscillations in the x-ray absorption cross section. Second, we exploited the performance of a state-of-the-art x-ray focussing device yielding a beam spot size of 5 × 5 μm², spatially stable over the entire energy scan. An advanced data analysis protocol was implemented to extract the pressure dependence of the structural parameters in phase VI of solid Rb from 51.2 GPa up to the highest pressure. A continuous reduction of the nearest neighbour distances was observed, reaching about 6% over the probed pressure range. We also discuss a phenomenological model based on the Einstein approximation to describe the pressure behaviour of the mean-square relative displacement. Within this simplified scheme, we estimate the Grüneisen parameter for this high pressure Rb phase to be in the 1.3-1.5 interval. [ABSTRACT FROM AUTHOR]

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ~8 GPa. We find this substructure to be cubic with space group I43m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) À at 8.3(2) GPa, which is a factor of larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter 14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of α-manganese at ambient conditions. [ABSTRACT FROM AUTHOR]

We have determined the full crystal structure of the high-pressure phase methane A. X-ray single-crystal diffraction data were used to determine the carbon-atom arrangement, and neutron powder diffraction data from a deuterated sample allowed the deuterium atoms to be located. It was then possible to refine all the hydrogen positions from the single-crystal x-ray data. The structure has 21 molecules in a rhombohedral unit cell, and is quite strongly distorted from the cubic close-packed structure of methane I, although some structural similarities remain. Full knowledge of this structure is important for modeling of methane at higher pressures, including in relation to the mineralogy of the outer solar system. We discuss interesting structural parallels with the carbon tetrahalides. [ABSTRACT FROM AUTHOR]

Polycrystalline Fe3C (cementite) was compressed in a neon pressure medium to 30.5 GPa at 300 K using diamond-anvil cell techniques. Angular dispersive X-ray diffraction of Fe3C was measured using monochromatic synchrotron radiation and imaging plates. No phase transition was observed up to the highest pressure studied. The pressure–volume data were fitted to a third-order Birch–Murnaghan equation of state. With V0 constrained to a measured value of 155.28 Å3, the best fit yielded a 300-K isothermal bulk modulus K0 = 174 ± 6 GPa, and its pressure derivative at constant temperature K 0 ′ =(K 0 /P) T = 4.8±0.8. [ABSTRACT FROM AUTHOR]

Solid nitrogen exhibits a panoply of phenomena ranging from complex molecular crystalline configurations to polymerization and closing band gap at higher densities. Among the elemental molecular solids, nitrogen stands apart for having phases, which can only be stabilized following particular pressure-temperature pathways, indicative of metastability and kinetic barriers. Here, through the combination of Raman spectroscopy and dynamic compression techniques, we find that the appearance of the whole nitrogen phase diagram is determined by the P-T paths taken below 2 GPa. We reveal the existence of the path- and phase-dependent triple point between the β - N 2 , δ loc - N 2 and γ - or ϵ - N 2 . We further show that the β - N 2 towards γ - N 2 path below the triple point, that evades δ ( δ loc )- N 2 , results in the formation of γ - N 2 , which in turn becomes a dominant phase. We then demonstrate, that the β - N 2 through δ ( δ loc )- N 2 above the triple point path leads to the formation of ϵ - N 2 and the "well-established" phase diagram. An additional pathway, which by-passes the rotationally inhibited modifications δ ( δ loc )- N 2 , via rapid compression is found to produce γ - N 2 at higher temperatures. We argue that the pathway and phase sensitive triple point and the compression rate dependent phase formation challenge our understanding of this archetypal dense molecular solid., (© 2024. The Author(s).)

Using optical spectroscopy, X-ray diffraction, and electrical transport measurements, we have studied the pressure-induced metallization in BaH 2 and Ba 8 H 46 . Our combined measurements suggest a structural phase transition from BaH 2 -II to BaH 2 -III accompanied by band gap closure and transformation to a metallic state at 57 GPa. The metallization is confirmed by resistance measurements as a function of the pressure and temperature. We also confirm that, with further hydrogenation, BaH 2 forms the previously observed Weaire-Phelan Ba 8 H 46 , synthesized at 45 GPa and 1200 K. In this compound, metallization pressure is shifted to 85 GPa. Through a comparison of the properties of these two compounds, a question is raised about the importance of the hydrogen content in the electronic properties of hydride systems.

The binary Xe-Ar system has been studied in a series of high pressure diamond anvil cell experiments up to 60 GPa at 300 K. In-situ x-ray powder diffraction and Raman spectroscopy indicate the formation of a van der Waals compound, XeAr2, at above 3.5 GPa. Powder x-ray diffraction analysis demonstrates that XeAr2 adopts a Laves MgZn2-type structure with space group P63/mmc and cell parameters a = 6.595 Å and c = 10.716 Å at 4 GPa. Density functional theory calculations support the structure determination, with agreement between experimental and calculated Raman spectra. Our DFT calculations suggest that XeAr2 would remain stable without a structural transformation or decomposition into elemental Xe and Ar up to at least 80 GPa., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Pressure-induced magnetic phase transitions are attracting interest as a means to detect superconducting behaviour at high pressures in diamond anvil cells, but determining the local magnetic properties of samples is a challenge due to the small volumes of sample chambers. Optically detected magnetic resonance of nitrogen vacancy centres in diamond has recently been used for the in situ detection of pressure-induced phase transitions. However, owing to their four orientation axes and temperature-dependent zero-field splitting, interpreting these optically detected magnetic resonance spectra remains challenging. Here we study the optical and spin properties of implanted silicon vacancy defects in 4H-silicon carbide that exhibit single-axis and temperature-independent zero-field splitting. Using this technique, we observe the magnetic phase transition of Nd 2 Fe 14 B at about 7 GPa and map the critical temperature-pressure phase diagram of the superconductor YBa 2 Cu 3 O 6.6 . These results highlight the potential of silicon vacancy-based quantum sensors for in situ magnetic detection at high pressures., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Spin defects in silicon carbide appear to be a promising tool for various quantum technologies, especially for quantum sensing. However, this technique has been used only at ambient pressure until now. Here, by combining this technique with diamond anvil cell, we systematically study the optical and spin properties of divacancy defects created at the surface of SiC at pressures up to 40 GPa. The zero-field-splitting of the divacancy spins increases linearly with pressure with a slope of 25.1 MHz/GPa, which is almost two-times larger than that of nitrogen-vacancy centers in diamond. The corresponding pressure sensing sensitivity is about 0.28 MPa/Hz -1/2 . The coherent control of divacancy demonstrates that coherence time decreases as pressure increases. Based on these, the pressure-induced magnetic phase transition of Nd 2 Fe 14 B sample at high pressures was detected. These experiments pave the way to use divacancy in quantum technologies such as pressure sensing and magnetic detection at high pressures.

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the H 2 δ - units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the H 2 δ - bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to H 2 δ - and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH 4 , where the precompression of H 2 δ - units at 50 GPa results in bond lengths comparable to that of pure H 2 above 275 GPa.

There is an ever increasing interest in studying dynamic-pressure dependent phenomena utilizing dynamic Diamond Anvil Cells (dDACs), devices capable of a highly controlled rate of compression. Here, we characterize and compare the compression rate of dDACs in which the compression is actuated via three different methods: (1) stepper motor (S-dDAC), (2) gas membrane (M-dDAC), and (3) piezoactuator (P-dDAC). The compression rates of these different types of dDAC were determined solely on millisecond time-resolved R 1 -line fluorescence of a ruby sphere located within the sample chamber. Furthermore, these different dynamic compression-techniques have been described and characterized over a broad temperature and pressure range from 10 to 300 K and 0-50 GPa. At room temperature, piezoactuation (P-dDAC) has a clear advantage in controlled extremely fast compression, having recorded a compression rate of ∼7 TPa/s, which is also found to be primarily influenced by the charging time of the piezostack. At 40-250 K, gas membranes (M-dDAC) have also been found to generate rapid compression of ∼0.5-3 TPa/s and are readily interfaced with moderate cryogenic and ultrahigh vacuum conditions. Approaching more extreme cryogenic conditions (<10 K), a stepper motor driven lever arm (S-dDAC) offers a solution for high-precision moderate compression rates in a regime where P-dDACs and M-dDACs can become difficult to incorporate. The results of this paper demonstrate the applicability of different dynamic compression techniques, and when applied, they can offer us new insights into matter's response to strain, which is highly relevant to physics, geoscience, and chemistry.

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH&#95;{4}-H&#95;{2} system. We find that pressures as low as 4.8 GPa can stabilize CH&#95;{4}(H&#95;{2})&#95;{2} and (CH&#95;{4})&#95;{2}H&#95;{2}, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H&#95;{2} units within the structure. On further compression, a unique structural composition, (CH&#95;{4})&#95;{3}(H&#95;{2})&#95;{25}, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H&#95;{2}. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.