This work reviews evolution of search for novel high-pressure high-temperature (HP-HT) nitrides with nitrogen-to-metal ratio well above one which has commenced more than 25 years ago and brought today some remarkable blooms. The number of reports on synthesis of such nitrides grows permanently but their majority deals, in the last years, with synthesis at very high pressures where even characterisation of the products (structure, composition etc.) becomes vague. The number of binary nitrides and related ternary compounds or solid solutions, accessible as macroscopic samples using large volume HP-HT apparatuses, appears to have stabilised at a high but still monitorable level. Today, the Gretchen question is whether the outcome of the 25-years activity was rewarding, whether any of the nitrides recoverable at ambient conditions exhibits properties deserving transfer to the industry. An answer is not straight because the efforts needed to obtain reliable property data go beyond those invested during the synthesis and characterisation. Gathering of such data requires either laborious investment in samples quality or development/adaptation of non-traditional, for HP-HT research, in- and ex-situ techniques. Here, the exposé of the most notable novel nitrides and their HP-HT synthesis routes is followed by examples of several recent developments/adaptations of experimental approaches permitting access to mechanical and optoelectronic properties which were earlier difficult to measure due to imperfect polycrystalline, porous and eventually contaminated HP-HT samples. The revealed promising properties allow us to hope that the efforts were not futile and, at least, a couple of the discovered HP-HT nitrides could find industrial applications.

In this work, we investigate the evolution of the Raman-active modes of bulk WSe2 under hydrostatic compression up to 8 GPa. The in-plane E2g1 and out-of-plane A1g modes both exhibit linear shifts with pressure, from -253 cm-1 to -265 cm- 1 and from -258 cm-1 to -271 cm- 1, respectively, highlighting the responses of intra- and interlayer bonds to compression. Analysis of these spectroscopic shifts yields mode-specific Gr & uuml;neisen parameters of gamma(E2g1) = 0.51 f 0.02 and gamma(A1g) = 0.53 f 0.02, providing a mode-weighted average = 0.524 f 0.015, in close agreement with the estimated thermodynamic Gr & uuml;neisen parameter gamma th = 0.65 f 0.02. Using and previously reported bulk and shear moduli of WSe2 at ambient pressure, its thermal-shock resistance was estimated to be in the range of typical semiconductors and highly covalent thermal-shock-resistant ceramics. Our results provide quantitative insight into the vibrational behavior of WSe2 under high pressure.

Tungsten is an elemental metal known for its high melting point and hardness, but experimental information about its mechanical properties at high pressures is limited. Here, longitudinal sound velocity, V-L, of tungsten compressed in a diamond anvil cell up to P = 141 GPa was measured using picosecond laser ultrasonics. Thus, the range of static pressures for which its V-L is known was extended by more than one order of magnitude, corresponding to a 31% increase in density compared with that at atmospheric pressure. We have found that the V-L grows monotonically with pressure and reaches 7.75 +/- 0.18 km/s at P = 141 GPa. Applying an earlier published equation of state, rho(P), we derived the pressure-dependent shear modulus, G(P), and transversal sound velocity, V-T(P), of tungsten. Both parameters continuously increase with pressure, but their growth rates diminish upon compression. Based on these data, the evolution of tungsten ductility/brittleness with pressure was assessed using Pugh's ratio, B/G, and a transition from intermediate- to weakly-brittle state was recognized. Combining our G(P) with previously measured yield strength of tungsten at high pressures, sigma(y)(P), the ratio partial derivative sigma(y)/partial derivative G = 0.017 +/- 0.002 was established, and sigma(y) similar to 7.5 GPa at P = 141 GPa was estimated from linear extrapolation. Finally, we have found that the linear dependence of V-L on density, known as Birch's law, holds for tungsten up to the maximal pressure of our work.

High-pressure elastic and plastic behavior of the amorphous solid, formed after solidification of the mixture of methanol and ethanol in the volume ratio 4:1 (4:1-ME), was investigated using the technique of time-domain Brillouin scattering (TDBS). Isotropic nature of this solid, whose bulk modulus exceeds that of diamond at pressures above 60-70 GPa (provided the earlier-measured dependence of its density on pressure, rho(P), holds at P>60 GPa), was confirmed thanks to the high 3D resolution of the TDBS technique. This permitted establishing of the relationship between elastic and plastic properties at high pressures because elastic response of the amorphous solid was not affected by anisotropy and texture, as is the case for crystalline solids. Using the TDBS technique, we measured pressure dependence of the Brillouin-oscillation frequencies, f(B)(P), of 4:1-ME and, consequently, of its longitudinal sound velocity, V-L(P), and shear modulus, G(P), to P = 78 GPa. To access pressure dependence of its yield strength, sigma(y)(P), we used the here-established secondary pressure scale, based on our experimental f(B)(P), and measured the maximal pressure gradients in the compressed samples which are proportional to sigma(y). We found that the rapid increase of bulk modulus of 4:1-ME with pressure, deduced from the rho(P) measured earlier to 60 GPa and extrapolated here to 78 GPa, is not accompanied by a similarly strong increase of its shear modulus and yield strength. The two parameters remain significantly below those of diamond at atmospheric and high pressures. Comparison of the here-measured sigma(y)(P) with the dependences B(P) and G(P) indicated that sigma(y) can be similarly well approximated by linear functions where B or G serves as the variable.

Single-crystal elastic moduli, Cij, and the B1-B2 phase transition of NaCl were investigated experimentally, using time-domain Brillouin scattering (TDBS), and theoretically, via density-functional-theory (DFT), to 41 GPa. Thus, we largely extended pressure range where Cij and elastic anisotropy of the solid are measured, including the first experimental data for the high-pressure B2 phase, NaCl-B2. NaCl-B1 exhibits a strong and growing with pressure anisotropy, in contrast to NaCl-B2. Theoretical values obtained using different advanced DFT functionals were compared with our measurements but no one could satisfactorily reproduce our experimental data for NaCl-B1 and NaCl-B2 simultaneously. For all available DFT results on the principal shear moduli and anisotropy, the deviation became pronounced when the degree of compression increased significantly. Similar deviations could be also recognized for other cubic solids having the same B1-type structure and similar bonding, such as CaO, MgO, or (Mg1-x,Fex)O. Furthermore, the available experimental data suggest that the B1-B2 phase transition of NaCl and the above mentioned compounds are governed by the Born stability criterion C44(P) - P > 0.

Seismic anisotropy in the Earth's lower mantle likely results from a combination of elastic anisotropy and lattice preferred orientations of its main constituent minerals. As the second most abundant component of the lower mantle, ferropericlase has been widely studied, and the experimental results demonstrated, in general, a growing with pressure elastic anisotropy up to 1 Mbar. However, the unique measurements on the endmember (MgO) at comparable pressure conditions contradict the above observations and theoretical results. Here, time‐domain Brillouin scattering was applied to measure longitudinal sound velocities in single crystals of MgO compressed in diamond anvil cell. Velocities along two specific crystallographic directions, [100] and [111], were independently collected to 43 GPa. Applying the known bulk modulus, a complete set of single‐crystal elastic moduli, elastic anisotropy and aggregate shear modulus were derived. Our results revealed a steadily increasing with pressure elastic anisotropy at P > 20 GPa, consistent with the previous theoretical predictions and measurements on ferropericlase with moderate amounts of iron.

Time-domain Brillouin scattering (TDBS) is a developing technique for imaging/evaluation of materials, currently used in material science and biology. Three-dimensional imaging and characterization of polycrystalline materials has been recently reported, demonstrating evaluation of inclined material boundaries. Here, the TDBS technique is applied to monitor the destruction of a lithium niobate single crystal upon non-hydrostatic compression in a diamond anvil cell. The 3D TDBS experiments reveal, among others, modifications of the single crystal plate with initially plane-parallel surfaces, caused by non-hydrostatic compression, the laterally inhomogeneous variations of the plate thickness and relative inclination of opposite surfaces. Our experimental observations, supported by theoretical interpretation, indicate that TDBS enables the evaluation of materials interface orientation/inclination locally, from single point measurements, avoiding interface profilometry. A variety of observations reported in this paper paves the way to further expansion of the TDBS imaging use to analyze fascinating processes/phenomena occurring when materials are subjected to destruction.

We report on the synthesis of tin(IV) nitride with spinel structure,gamma -Sn3N4, from the elements at high pressures and temperatures using a laser-heated diamond anvil cell, and on the Rietveld refinement of the product structure. The procedure described here is, in our opinion, the most reliable method of obtaining high-purity nitrides which are thermodynamically stable only at high pressures. Raman spectroscopy and powder X-ray diffraction were used to characterize the synthesis products. Pressure dependences of the Raman-band frequencies of gamma-Sn3N4 were measured and used to determine its average mode Gruneisen parameter, gamma = 0.95. Using this value, we estimated the thermal-shock resistance of gamma-Sn3N4 to be about half that of gamma -Si3N4, which, in turn, is moderately surpassed by ss-Si3N4, known to be highly thermal-shock resistant. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Germanium nitride, having cubic spinel structure, γ-Ge3N4, is a wide band-gap semiconductor with a large exciton binding energy that exhibits high hardness, elastic moduli and elevated thermal stability up to approximately 700°C. Experimental data on its bulk and shear moduli (B0 and G0, respectively) are strongly limited, inconsistent and, thus, require verification. Moreover, earlier first-principles density functional calculations provided significantly scattering B0 values but consistently predicted G0 much higher than the so far available experimental value. Here, we examined the elasticity of polycrystalline γ-Ge3N4, densified applying high pressures and temperatures, using the techniques of laser ultrasonics (LU) and Brillouin light scattering (BLS) and compared with our extended first-principles calculations. From the LU measurements, we obtained its longitudinal- and Rayleigh wave sound velocities and, taking into account the sample porosity, derived B0 = 322(44) GPa and G0 = 188(7) GPa for the dense polycrystalline γ-Ge3N4. While our calculations underestimated B0 by approximately 17%, most of the predicted G0 matched well with our experimental value. Combining the LU- and BLS data and taking into account the elastic anisotropy, we determined the refractive index of γ-Ge3N4 in the visible range of light to be n = 2.4, similarly high as that of diamond or GaN, and matching our calculated value. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Time-domain Brillouin scattering uses ultrashort laser pulses to generate coherent acoustic pulses of picoseconds duration in a solid sample and to follow their propagation in order to image material inhomogeneities with sub-optical depth resolution. The width of the acoustic pulse limits the spatial resolution of the technique along the direction of the pulse propagation to less than several tens of nanometres. Thus, the time-domain Brillouin scattering outperforms axial resolution of the classical frequency-domain Brillouin scattering microscopy, which uses continuous lasers and thermal phonons and which spatial resolution is controlled by light focusing. The technique benefits from the application of the coherent acoustic phonons, and its application has exciting perspectives for the nanoscale imaging in biomedical and material sciences. In this study, we report on the application of the time-domain Brillouin scattering to the 3D imaging of a polycrystal of water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided the opportunity to identify the phase for individual grains and evaluate their crystallographic orientation. Monitoring the propagation of the acoustic waves in two neighbouring grains simultaneously provided an additional mean for the localisation of the grain boundaries.

Cubic solids such as NaCl, crystalline argon, or H2O-ice VII exhibit significant elastic anisotropy strongly increasing upon compression. As earlier recognized for solid argon and H2O-ice (both exhibiting Zener ratio A > 1), longitudinal sound velocities of their polycrystals, VLav, measured using Brillouin light scattering (BLS) or pulse-echo ultrasonics are much closer to VL⟨111⟩ than to VL⟨100⟩, the VL-extremes in any cubic single crystal. Here, we experimentally confirm, using the technique of time-domain Brillouin scattering, the same tendency for NaCl exhibiting the opposite anisotropy type, A < 1. To understand this tendency, we modelled orientational distribution and the frequency of occurrence of VL values in texture-free polycrystalline samples of NaCl and solid argon. We found a remarkable and predictable asymmetry of VL distributions with maxima at VL⟨110⟩ that is always much closer to VL⟨111⟩. This asymmetry persists in BLS peaks but can be obscured in experiments. In the case of solid argon at 49 GPa, the asymmetry can lead to a moderate deviation of experimental VLav from VLH (obtained from elastic-stiffness constants Cij applying the Hill approximation) by ∼7%. The latter can cause, however, a significant overestimation of the aggregate shear modulus by δG/G ∼ 50% or of the bulk modulus by δB/B ∼ 20% if just one BLS peak of longitudinal modes is detectable. A similar analysis, performed for transverse sound velocities, VT and VTav, has shown that by the use of a BLS spectrum showing peaks of both longitudinal and transverse modes, overestimation of B is similarly high but that of G is much less dramatic.

Formation and evolution of defect levels in the electronic structure of silicon nitride with cubic spinel structure, gamma-Si3N4, after the irradiation with He+ ions was investigated using spectroscopic techniques. Strong changes of cathodoluminescence (CL), photoluminescence (PL), photoluminescence excitation (PLE) and Raman spectra were detected. In particular, excitonic PL was significantly inhibited and a new near-IR band appeared with the band gap excitation h nu >= E-g = 5.05 eV. This was explained by an effective trapping of photoinduced electrons and holes by charged defects. The spectral shift of PL with the excitation photon energy indicated heterogeneous nature of the defect sites. The energetic position of near-IR and visible PL bands correlate, suggesting an interaction with the common cation defect to be an origin. The visible PL of exciton bound to a neutral defect Si-x was red shifted, which was attributed to the permutations between empty and occupied octahedral and tetrahedral sites, inherent to the spinel structure, after collisions with He+ ions. The positively charged cation sites in the spinel structure are compensated by V-N'''nion vacancies. The local deformation of the spinel lattice affects PL intensity of the self-trapped exciton at 4.35 eV.

Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters.

Research has shown that group IVB nitrides have a narrow direct electronic band gap, but there have been few investigations into the dependence of the electronic band gap on cation defects and whether tuning the electronic band gap is possible by controlling the level of substitutional oxygen in c-Zr3-x(N1-xOx)(4). We use a combination of soft X-ray spectroscopy and density functional theory to study the electronic structure and determine the electronic band gap as well as the exciton binding energy of an oxygen-bearing defect zirconium nitride, c-Zr-2.86(N0.88O0.12)(4), and oxygen-free hafnium nitride, c-Hf3N4. Moreover, we extend our structural model to consider the dependence of the electronic band gap on oxygen substitution in c-Zr3-x(N1-xOx)(4). The results suggest that the electronic band gap can be precisely controlled between 1.47 and 1.84 eV (674-844 nm) by adjusting the stoichiometry, without adversely affecting the electronic structure. The exciton binding energy of c-Zr-2.86(N0.88O0.12)(4) is estimated to be 37 meV, much larger than current materials being used (GaAs). This larger exciton binding energy combined with both the direct narrow electronic band gap and stable electronic structure demonstrates that this material is an ideal candidate to replace currently used materials, such as GaAs, in infrared light-emitting diode applications.

Time-domain Brillouin scattering uses ultrashort probe laser pulses and coherent acoustic pulses of picoseconds duration, generated and detected by ultrashort pump laser pulses, for imaging material inhomogeneities with sub-optical depth resolution. The width of the acoustic pulse limits the spatial resolution of the technique along the direction of the pulse propagation to less than several tens of nanometers. Thus, the time-domain Brillouin scattering outperforms axial resolution of the classical frequency-domain Brillouin scattering microscopy, which uses continuous lasers and thermal phonons and which spatial resolution is controlled by light focusing. The technique benefits from the application of the coherent acoustic phonons, and its application has exciting perspectives for the nanoscale imaging in bio-medical and material sciences. In this study, we report on the application of the time-domain Brillouin scattering to the 3D imaging of a polycrystal of water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided the opportunity to identify the phase of individual grains and evaluate their orientation. Monitoring the propagation of the acoustic waves in two neighbour grains simultaneously provided an additional mean for the localisation of the grain boundaries.

Aims.This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.Methods.SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.Results.SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission’s highly elliptical orbit.