Session Y24: Static High Pressure

Computationally Directed High-Pressure Discovery of Transition Metal-Bismuth Materials

Under applied pressures, fundamental elemental properties are renormalized, revealing compounds that challenge our understanding. To efficiently direct high-pressure synthetic efforts, it is useful to employ computational approaches that predict material stability. In this work, we used high throughput ab initio random structure searches to explore high-pressure transition metal (TM)–Bi phase space. We observed that the CuAl₂ type structure is common, including for combinations of elements that are not miscible under ambient conditions. Turning to high-pressure experiments, we discovered the first Mo–Bi intermetallic material. Experimental structural elucidation of this material revealed how the CuAl₂ structure type imposes strict electronic limits on its Bi members. This structure-property relationship determines the dynamic and magnetic properties of these materials, but in situ techniques are required to elucidate the high-pressure properties. Using the optical response of nitrogen vacancies embedded in our diamond anvils, we observed the predicted magnetic ordering. This result confirms the promise of this high-pressure approach for designing and understanding new functional materials.   

Non hydrostatic Compression behavior of HfB2 to 220 GPa and Determination of Hydrostatic EoS and Shear Strength to 70 GPa.

Hafnium diboride belongs to the class of high strength transition metal diborides that exhibit high bulk moduli and known for its excellent resistance to thermal degradation in oxidizing environment at high temperatures. In this study, the compression behavior of hafnium diboride was investigated up to 220 GPa under nonhydrostatic conditions at ambient temperature using angle dispersive axial x-ray diffraction (A-XRD) with a diamond anvil cell (DAC). In addition, shear strength of hafnium diboride was measured to 70 GPa using radial x-ray diffraction (R-XRD) technique in DAC. The platinum was employed as an x-ray pressure standard in all measurements. R-XRD combined with lattice strain theory enables the determination of shear strength to high pressure and the determination of hydrostatic strain during compression. The static compression data and mechanical properties of Hafnium Diboride will be compared with the first-principle theoretical calculations.   

High-Pressure Studies of Topological Magnet EuSn2P2 via Synchrotron X-ray Techniques

Topological materials have attracted significant attention in recent years for their promises in applications of dissipationless electronics. Among these materials, EuSn₂P₂ is a unique topological semimetal [1]. It crystalizes in layered rhombohedral structure, similar to A₂B₃ topological insulators, and orders antiferromagnetically below 30 K. In this work we present systematic high-pressure studies of magnetism in EuSn₂P₂ using synchrotron X-ray diffraction, Moessbauer spectroscopy, X-ray absorption spectroscopy, and molecular orbital calculations. We observed a pressure-induced crystalline to amorphous transition in EuSn₂P₂ at 36 GPa, accompanied by a four-fold enhancement of magnetic ordering temperature attributed by increased RKKY interactions despite gradual Eu²⁺ to Eu³⁺ transition above 20 GPa. This study demonstrates the large tunability in EuSn₂P₂ under pressure, making it an excellent material to study interplay of magnetism, valence, crystal lattice, and topological state.   

HP-HT Phase Behavior of Melt-Castable Energetic of BNFF

High performance melt-castable energetic (MCE) materials offer an attractive alternative to commonly used pressed or cure-cast materials. An efficient application of MCEs requires, however, better understanding of this class of energetics, where a delicate balance between solid and molten states can control sensitivity and performance. Here, a novel, high density melt-castable compound of 3, 4-bis (3-nitrofurazan-4-yl) furoxan (BNFF or DNTF) was subjected to isothermal compression and isobaric heating, using a heated diamond anvil cell in conjunction with vibrational spectroscopy and optical imaging. Crystalline, molten and glassy forms of BNFF were investigated to gain insight into processes governing structural and chemical stability/reactivity over a broad range of pressures and temperatures. P-T domains for the existence of different states of BNFF were thoroughly examined to determine the melting and decomposition curves. The results revealed a robust response of crystalline BNFF to high pressure – structural stability to at least 20 GPa – and a rich response to temperature changes including formation of supercooled fluid. Implications of the observed interplay between different states of BNFF on material reactivity are discussed.   

Stabilization of hexazine and pentazolate rings in polynitrides of alkali metals at high pressure

Synthesis and characterization of polynitrogen species is of great importance for the design of novel high energy density materials as these compounds possess extremely energetic low-order nitrogen–nitrogen bonds. However, synthesis and stabilization of such compounds at ambient conditions remain a big challenge because they often require very high pressure to synthesize them. Here we report the synthesis of polynitrogen planar N₆^2- hexazine dianions, stabilized in K₂N₆, and several oxygen-free cyclo-N₅^- pentazolate salts stabilized in K and Na compounds, some of which are also containing slightly elongated dinitrogens N₂ [1,2]. The experiments have been performed in a laser heated diamond anvil cell above 45 GPa on NaN₃ and KN₃ azides using also N₂ as a medium and reagent in some runs. Synchrotron single-crystal and powder X-ray diffraction and Raman spectroscopy were used to identify this material, which have also distinct appearance and optical properties. The hexazine and pentazolate compounds documented here remain metastable down to approximately 20 GPa, suggesting that some of them may be metastable at low temperature.  The nitrogen ring compounds are likely to be present in many other high energy density materials stabilized by pressure.   

Synchrotron IR/THz Spectroscopy of Quantum Disordered Phase Dense H2O Ice to Megabar Pressures

Hydrogen bonds and proton mobility play a fundamental role for understanding the evolution from distinct molecular phases of ice to non-molecular forms under pressure, but the nature of the quantum disorder in the material across the transition has not been directly probed. We present the first synchron based broadband far-IR spectroscopy to megabar (100 GPa) pressures that reveal a significant increase in IR response and a multiplicity of excitations as a function of pressure between through the quantum disordered phase associated with hydrogen-bond symmetrization at 60-90 GPa. Features are found that were not observed or predicted in previous experimental and theoretical work, including recent simulations. The associated large static dielectric response accompanying the transition, as suggested by early simulations, is reproduced by first-principles calculations. The measurements provide crucial constraints on the many-body properties of ice in this quantum-disordered regime associated with the transition.    

Compression of Bi2Sr2Can-1CunO2n+4+δ to Megabar Pressures

Despite years of intense study, the superconducting mechanism seen in the cuprates is not yet fully understood. In recent years pressure has been used as a tuning parameter to probe the unusual electronic properties of cuprate superconductors, such as anomalous changes in T_c observed in the bismuth-based cuprates. We report the effects of quasi-hydrostatic megabar pressures (>100 GPa) on the structure of bismuth-based high-Tc cuprate superconductors - Bi₂Sr₂Ca_n-1Cu_nO_2n+4+δ (n=1,2,3) - using synchrotron x-ray diffraction and diamond anvil cells. To ensure nearly hydrostatic conditions during compression, neon was used as a pressure transmitting medium. A stiffening of the c axis occurs for n=1 and 2 at 10 GPa and 20 GPa, respectively, but no such incompressibility effect was observed in the n=3 composition. Below the stiffening structural properties agreed with previously reported data, but differ from earlier work as a result of non-hydrostatic stress in those experiments, as confirmed by our own measurements without a pressure transmitting medium. In all three compositions the changes in lattice parameters do not correlate with the changes in T_c including its monotonic rise seen in all compositions above a critical pressure.    

High-pressure studies on topological geometric ferroelectric LaTaO4

Ferroelectricity in layered and perovskite material has been of great scientific importance. Interestingly, topological ferroelectricity has been studied in layered perovskite compound LaTaO₄. At ambient conditions, LaTaO₄ crystallizes in monoclinic P21/c space group and further at 433 K transform to Orthorhombic Cmc21 and to Cmcm structure at 498 K. The high-temperature structure and phase transition is controversial. Very recently, it is reported that transition at around 500 K is incommensurate to commensurate in nature, in which macroscopic geometric modulation is of continuous temperature dependence until the transition to the basic unit cell. Anisotropic negative thermal expansion is also found in LaTaO₄, associated with the soft phonon modes. However, the high-pressure properties of LaTaO₄ are largely unknown. Here our aim is to understand its high-pressure behavior, structural phase transitions, and vibrational properties related to its anisotropic thermal behavior. We present in situ high-pressure Raman spectroscopic and synchrotron x-ray diffraction studies on LaTaO₄ in a diamond-anvil cell up to 32 GPa under hydrostatic and non-hydrostatic conditions to get more insight into its microscopic Gruneisen parameters, thermal and ferroelectric transition behavior.   

Magnetic structure of antiferromagnetic high-pressure phases of dysprosium and holmium

Dysprosium (Dy) and holmium (Ho) have been studied using neutron diffraction under high pressures and low temperatures at a spallation neutron source by employing a large-volume diamond anvil cell. Companion measurements at different central wavelengths allow the collection over extended reciprocal space with momentum transfer Q covering the range from 0.5 Å -1 to 5.5 Å -1 . Upon cooling to 15 K, magnetic ordering was observed in the hexagonal close-packed ( hcp ), alpha-samarium ( α-Sm ), and double hexagonal close packed ( dhcp ) phases of Dy to 22 GPa as well as Ho to 31 GPa. We report on previously undetected magnetic superlattice reflections signaling antiferromagnetic transition for both the α-Sm and dhcp phases of Dy and Ho. Magnetic structure refinements for the α-Sm phase of Dy shows a complex phase comprising two magnetic propagation vectors k = (1/2, 1/2, 1/2) and k = (1/2, 0, 0). Magnetic structure refinements for the dhcp phase of Dy yield a single magnetic propagation vector k = (1/2, 0, 1/3) and a possible magnetic space group P_bnma. Similar magnetic structures were observed in Ho in the α-Sm phase. Very weak magnetic reflections were observed in the dhcp phase of Ho, however the quality is not sufficient for magnetic structure refinement. The observed magnetic structures are presented to the highest pressure of 22 GPa for Dy and 31 GPa for Ho.   

High-pressure studies of neodymium-doped lanthanum superhydrides

Hydrogen-rich hydrides are well known to show superconductivity in the vicinity of room temperature under pressure. The discovery of very high-T_c superconductivity in binary hydride systems accelerated the field of hydride superconductivity greatly. There is current interest in ternary hydrides to further enhance the critical temperature and reduce the synthesis pressure. Here we investigated the effect of doping and substitution of magnetic ions in these hydrides focusing on the La-H system including its superhydrides. We explore the effect of magnetic neodymium ions on the structure, stability, and T_c of these lanthanum-based hydrides. The work involves in-situ powder x-ray diffraction for the synthesis of the ternary Nd-La-H system at high pressure-temperature conditions.   

Pressure control of magnetic state in EuCd2As2

EuCd₂As₂ has been predicted to be an ideal platform for studies of intrinsic topological magnetic systems for the possibility of hosting a single pair of Weyl points when tuned into the ferromagnetic phase from the antiferromagnetic state by external pressure, external magnetic field, or chemical doping [1-2]. In search of the ideal topological state, we have systematically investigated pressure controlling of the magnetic state up to 43 GPa using synchrotron Mössbauer spectroscopy and valence state verification using X-ray absorption spectroscopy along with structural refinement using high-resolution X-ray diffraction. A detailed discussion of experimental results will be presented.

References:

[1] E.  Gati, S.  L.  Bud’ko, L.-L.  Wang, A.  Valadkhani, R.  Gupta, B.  Kuthanazhi, L.  Xiang, J.  M.  Wilde, A. Sapkota, Z. Guguchia, R. Khasanov, R. Valenti, and P. C. Canfield, Pressure-induced ferromagnetism in the topological semimetal EuCd2As2, arXiv:2108.01871 (2021).

[2] J.-R. Soh, F. de Juan, M.G. Vergniory, N.B.M. Schröter, M.C. Rahn, D.Y. Yan, J. Jiang, M. Bristow, P.A. Reiss, J.N. Blandy, Y.F. Guo, Y.G. Shi, T.K. Kim, A. McCollam, S.H. Simon, Y. Chen, A.I. Coldea, and A.T. Boothroyd, Phys. Rev. B 100, 201102 (2019).   

Strucural Phase Transitions in Praeseodymium to 205 GPa

On compression, the trivalent lanthanides undergo a common sequence of structural transitions to phases comprising different stackings of hcp layers, with transition pressures that increase as Z increases across the series. Further compression results in transitions to "volume-collapsed" phases, typically with hexagonal (hP3 in Pearson notation), or orthorhombic (oF8, oF16. oC4) structures. oF8 and oC4 are also found in Pu and U, respectively, at ambient pressure, and also in Am, Cm and Cf at high pressure. Pr is the lowest-Z lanthanide in which the oC4 phase is observed, from 20 GPa to 147GPa, above which it is reported to transform to a distorted-oC4 structure, space group P2₁2₁2₁ (oP4), different to the tetragonal post-oC4 structure (tI2) reported in neighbouring Ce, and the distorted-oC4 structures, space group Pnma, reported in Am, Cm and Cf. As the trivalent lanthanide metals are expected to continue exhibiting a common transition sequence at pressures higher than they have been studied to date (typically 150-300 GPa), confirming the existence and structure of the oP4 phase in Pr is important, as it is currently seen only in that element. In this presentation we will describe the results of recent x-ray diffraction studies of Pr to 205 GPa, which focussed on determining its structural behaviour above 150 GPa.   

Highly Coordinated Tungsten Nitride with U7Te12-type Structure Synthesized Using the Laser-Heated Diamond Anvil Cell

Tungsten nitride (WN) has gained growing interest because of its potential as a functional compound. In our experiment, a novel WN was found when compressing and heating MoC-type WN using Laser-Heated Diamond Anvil Cell at a pressure of 70 GPa. Unknown peaks, different than those of the starting sample, were found in the XRD pattern. The unknown peaks were indexed using DICVOL calculation. By comparing the model structures base on the lattice parameters, space group, and EDS results, these unknown peaks were assigned to U₇Te₁₂-type W₇N₁₂. Rietveld refinement confirmed that this new structure has lattice parameters of a = 8.2398(3) Å, c = 2.94948(14) Å, and unit cell volume of V = 173.423 (13) ų. DFT calculations predicted that U₇Te₁₂-type W₇N₁₂ has a metallic behavior. This compound is the third example of a U₇Te₁₂-type structure ever found. The high nitrogen-coordinated tungsten polyhedra with coordination numbers of 8 and 9 has never been reported for other nitrides synthesized. Finally, this finding of a novel tungsten nitride compound confirms the feasibility of high-pressure synthesis techniques for the nitridation of tungsten. Moreover, it opens new possibilities and potentials in the W-N system that still need to be explored.