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Thomas Meier, Dominique Laniel & Florian Trybel (2023). , 8(1), 018401. 

Severe limitations in determining the chemical formula of metal hydrides synthesised at extreme conditions, especially with regards to their hydrogen content, impedes a deep understanding of the realised phases and can lead to significantly erroneous conclusions. We found a way to access the hydrogen content of metal hydride solids synthesized at high pressures in (laser-heated) diamond anvil cells directly in-situ at high pressure using nuclear magnetic resonance spectroscopy. Photo credit Thomas Meier

Thomas Meier, Florian Trybel, et al. (2022). 

The experimental study of hydrogen-bonds and their symmetrization under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrization has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution in-situ ¹H-NMR experiments in diamond anvil cells investigating a range of systems containing linear O-H 鈰� O units at pressure ranges of up to 90 GPa covering their respective H-bond symmetrization. We found pronounced minima in the pressure dependence of the NMR resonance line-widths -- independent of the chemical environment of the O-H 鈰� O unit -- at an average critical oxygen-oxygen distance of 2.44 to 2.45 脜.

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Dominique Laniel, Florian Trybel, et al. (2022). 

The lanthanum-hydrogen system has attracted significant attention following the report of superconductivity in LaH10 at near-ambient temperatures and high pressures. Phases other than LaH10 are suspected to be synthesized based on both powder X-ray diffraction and resistivity data, although they have not yet been identified. We present the results of our single-crystal X-ray diffraction studies and DFT calculations on this system, which reveal an unexpected chemical and structural diversity of synthesised lanthanum hydrides.

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Florian Trybel, et al. (2020).

Using a density-functional-theory-based approach, we explore the symmetrisation and proton dynamics in the high pressure H₂O ice-VII phase, for which recent high-pressure NMR experiments indicate significant proton dynamics in the pressure-range of 20– 95 GPa. We directly sample the potential seen by the proton and find a continuous transition from double- to single-well character over the pressure range of 2 to 130 GPa accompanied by proton dynamics, in agreement with NMR experiments.

Thomas Meier, Florian Trybel, et al. (2019). , 9, 031008 (2019)

Knowledge of the behaviour of hydrogen in metal hydrides is the key for understanding their electronic properties. Performing high-pressure ¹H-NMR and density-functional theory-based calculations on cubic FeH up to 202 GPa, we observe a distinct deviation from ideal metallic behaviour between 64 and 110 GPa that suggests the emergence of pressure-induced H-H interactions. Ab-initio calculations reveal the formation of an intercalating sub-lattice of electron density, which enhances the hydrogen contribution to the electronic density of states at the Fermi level. This study shows that pressure-induced H-H interactions can occur in metal hydrides at much lower compression and larger H-H distances than previously thought.

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Dominique Laniel, Florian Trybel, et al.:  

We present the first synthesis of a compound featuring aromatic nitrogen 6-rings. The hexazine anion [N₆]⁴⁻ was realised in the high-pressure potassium nitrogen compound K₉N₅₆ formed at P > 60 GPa and temperatures > 2000 K. The complex structure of K₉N₅₆—composed of 520 atoms per unit cell—was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N₆]⁴⁻ is planar and proposed to be aromatic.

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Dominique Laniel, Florian Trybel, et al. (2022). , 28(62), e202201998.

We studied the phosphorus鈥搉itrogen system at up to 137 GPa in laser-heated diamond anvil cells. Three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. 未-P3N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two compounds are ultra-incompressible, having a bulk modulus of K0=322 GPa for 未-P3N5 and 339 GPa for PN2. Upon decompression below 7 GPa, 未-P3N5 undergoes a transformation into a novel 伪鈥�-P3N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of 伪鈥�-P3N5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.

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Andrey Aslandukov, Florian Trybel et al.:  (2022)

Photo credit Andrey Aslandukov Two novel yttrium nitrides, YN6 and Y2N11, were synthesized by direct reaction between yttrium and nitrogen at 100 GPa and 3000 K in a laser鈥恏eated diamond anvil cell, featureing a unique organization of nitrogen atoms鈥攁 previously unknown anionic N18 macrocycle and a polynitrogen double helix, respectively. Density functional theory calculations, confirming the dynamical stability of the YN6 and Y2N11 compounds, show an anion鈥恉riven metallicity, explaining the unusual bond orders in the polynitrogen units.

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Laniel, D., Trybel, F., et al. (2023). , 2308030.

Carbon nitrides featuring three-dimensional frameworks of CN4 tetrahedra are one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, the high-pressure high-temperature synthesis of three carbon鈥搉itrogen compounds, tI14-C3N4, hP126-C3N4, and tI24-CN2, in laser-heated diamond anvil cells, is reported. Their structures are solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations show that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions.

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News item LiU: Theoretical physicists at Link枚ping University play a crucial role in discovery of ultra hard material that can rival diamond

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Laniel, D., Trybel, F., et al. (2023). , 14(1), 6207.

The allotropy of solid molecular nitrogen is the consequence of a complex interplay between fundamental intermolecular as well as intramolecular interactions. Understanding the underlying physical mechanisms hinges on knowledge of the crystal structures of these molecular phases. That is especially true for ζ-N2, key to shed light on nitrogen’s polymerization. Here, we perform single-crystal X-ray diffraction on laser-heated N2 samples at 54, 63, 70 and 86 GPa and solve and refine the hitherto unknown structure of ζ-N2. In its monoclinic unit cell (space group C2/c), 16 N2 molecules are arranged in a configuration similar to that of ε-N2. The structure model provides an explanation for the previously identified Raman and infrared lattice and vibrational modes of ζ-N2. Density functional theory calculations give an insight into the gradual delocalization of electronic density from intramolecular bonds to intermolecular space and suggest a possible pathway towards nitrogen’s polymerization.


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