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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.)

It has been theorized that at high pressure the increased energy of the zero-point oscillations in hydrogen would destabilize the lattice and form a ground fluid state at 0 K (ref. 1). Theory has also suggested that this fluid state, representing a new state of matter, might have unusual properties governed by quantum effects, such as superfluidity or superconductivity. Here, by combining Raman spectroscopy and in situ high-temperature, high-pressure techniques, we demonstrate that above 200 GPa a new phase transition occurs as temperature is increased, for example 480 K at 255 GPa. If the transformation is interpreted as melting, it would be the lowest melting temperature of any material at these high pressures. We also find a new triple point between phases I and IV and the new phase, and demonstrate that hydrogen retains its molecular character around this point. These data may require a significant revision of the phase diagram of hydrogen above 200 GPa.

Recent developments in high-pressure methods and advances in X-ray crystallography have led to a new level of understanding of phase diagrams and structures of materials under pressure. Recently discovered phenomena such as complex phases of alkali metals, incommensurate host-guest structures, and incommensurately modulated structures have rendered obsolete our conventional wisdom about the range of structures possible in the elements. Using new in situ diffraction techniques, we have resolved the long-standing problem of the phase-transition sequence of sulphur in its non-metallic state. We demonstrate that it is very different from that previously proposed, with only two phases stable between 1.5 GPa and 83 GPa (the pressure of metallization), and temperatures from 300 K to 1,100 K. The phases have a triangular chain and a squared chain structure. The same squared chain structure is found in the heavier group VI element selenium.

Alumina (alpha-Al(2)O(3)) has been widely used as a pressure calibrant in static high-pressure experiments and as a window material in dynamic shock-wave experiments; it is also a model material in ceramic science. So understanding its high-pressure stability and physical properties is crucial for interpreting such experimental data, and for testing theoretical calculations. Here we report an in situ X-ray diffraction study of alumina (doped with Cr(3+)) up to 136 GPa and 2,350 K. We observe a phase transformation that occurs above 96 GPa and at high temperatures. Rietveld full-profile refinements show that the high-pressure phase has the Rh(2)O(3) (II) (Pbcn) structure, consistent with theoretical predictions. This phase is structurally related to corundum, but the AlO(6) polyhedra are highly distorted, with the interatomic bond lengths ranging from 1.690 to 1.847 A at 113 GPa. Ruby luminescence spectra from Cr(3+) impurities within the quenched samples under ambient conditions show significant red shifts and broadening, consistent with the different local environments of chromium atoms in the high-pressure structure inferred from diffraction. Our results suggest that the ruby pressure scale needs to be re-examined in the high-pressure phase, and that shock-wave experiments using sapphire windows need to be re-evaluated.

There has been considerable interest in the synthesis of new nitrides because of their technological and fundamental importance. Although numerous metals react with nitrogen there are no known binary nitrides of the noble metals. We report the discovery and characterization of platinum nitride (PtN), the first binary nitride of the noble metals group. This compound can be formed above 45-50 GPa and temperatures exceeding 2,000 K, and is stable after quenching to room pressure and temperature. It is characterized by a very high Raman-scattering cross-section with easily observed second- and third-order Raman bands. Synchrotron X-ray diffraction shows that the new phase is cubic with a remarkably high bulk modulus of 372(+/-5) GPa.