Metastable allotropes of silicon recovered from high-pressure conditions exhibit a wide range of crystal structures, physical properties and transformation pathways that remain only partially understood despite decades of study. This article combines original crystallographic observations with a critical review of phase transformations, nucleation mechanisms and crystal growth processes in elemental Si and Na-Si systems synthesized under high-pressure, high-temperature conditions. Using in situ diffraction data, structural characterization and computational approaches, we analyze how symmetry breaking, lattice instabilities and kinetic constraints govern the formation of dense polymorphs (Si-II, Si-III, Si-XI) and open-framework structures, including clathrate and channel phases. Particular attention is given to the role of large-volume synthesis and chemically assisted growth routes in controlling phase selection, defect formation and recoverability. The evolution of hexagonal polytypes, including nanostructured 6H silicon, is discussed in terms of stacking modifications driven by stress release and thermal treatment. By integrating crystallographic relations, thermodynamic considerations and growth kinetics, this work identifies phase-transformation mechanisms as the key factor linking structure, synthesis conditions and functional properties of silicon allotropes. The results provide a unified framework for understanding crystal growth at high pressure and offer guidance for the controlled synthesis of advanced silicon materials.

The pivotal role of silicon (Si) in semiconductor technology is well‐established; however, its hexagonal diamond (hd) crystal structure remains underexplored. This study addresses the paucity of microscopic evidence concerning the formation and phase transformation behavior of hd‐Si up to 1000 °C. Utilizing instrumented nanoindentation and subsequent annealing, the hd‐Si phase is obtained from a rhombohedral (R8) and body‐centered cubic (BC8) mixture within a diamond cubic silicon (dc‐Si) wafer. In situ characterization reveals that hd‐Si undergoes a reversible phase transition to a metallic β‐tin (Sn) phase under indentation loading, reverting to an R8/BC8 mixture upon unloading, thereby providing experimental confirmation of prior theoretical predictions. Thermal stability assessments indicate that hd‐Si remains stable beyond ≈700 °C and transitions to dominantly dc‐Si at 1000 °C, with traces of hd‐Si persisting. Notably, annealing at 500 and 700 °C yields large‐area textured hd‐Si nanocrystals with slight misorientations featuring 2H, 4H, and 6H polytypes. Interestingly, the dc‐Si formed from hd‐Si upon annealing at 1000 °C also transforms to a metallic β‐Sn phase during Berkovich indentation and reverts to an R8/BC8 mixture upon unloading. This work provides critical insights into the high‐pressure phases of Si, paving the way for future studies on phase engineering and stabilization for advanced semiconductor applications and material innovations.

Nanostructured polycrystalline cubic boron nitride, an advanced superhard and refractory covalent solid with hardness approaching that of diamond, exhibits extreme mechanical properties due to high chemical and phase purity, ideal crystallinity and high intergranular adhesion. It has been produced by direct phase transformation of pyrolytic turbostratic BN at high pressure – high temperature conditions (20 GPa, 2500 K) using a multianvil apparatus. The non-trivial Raman fingerprint of nano-cBN shows both bulk and surface/grain boundary modes in the phonon density of states. The anomalous heat capacity of nano-cBN is comparable to that of nanostructured metal bulks and shows universal behavior with a maximum at 20-25 percent of the Debye temperature. The intergrain boundaries and their intrinsic stacking faults structure is the only plausible cause of excessive heat capacity. We show that nanostructured cubic boron nitride hosts grain-boundary phonon modes that give rise to an anomalous excess heat capacity, a universal effect previously known only in metals. These findings establish grain boundaries as active thermodynamic degrees of freedom in covalent nanoceramics, opening perspectives for tuning vibrational and thermal properties of ultrahard materials.

Here we review the available experimental and theoretical data on phase transformations and thermodynamical properties of boron compounds that can be obtained by in situ methods at high-pressure and high-temperature (HPHT) conditions. The thermodynamic analysis allowed to establish the optimized sets of thermodynamic values and mechanical properties for boron allotropes and compounds such as boron suboxide B6O, boron subnitride B13N2, and boron carbide.

Boron-rich compounds within the B-C-N-O and B-C-Si systems exhibit exceptional functional properties, making them highly attractive for industrial applications such as those requiring superhardness, nuclear technologies, or thermoelectricity. High-pressure, high-temperature (HPHT) conditions allow obtaining the ingots of ceramics with best physical properties, as well as to explore advanced materials by means of in situ crystallography, high-pressure chemistry, and nanoscience. This review summarizes recent experimental and theoretical advances on high-pressure, high-temperature (HPHT) phase equilibria up to 20 GPa and 3000 K, focusing on boron carbide (B4C), boron suboxide (B6O), boron subnitride (B13N2), boron silicides, and some of their solid solutions. Emphasis is placed on in situ X-ray diffraction (XRD), density functional theory (DFT) calculations, and CALPHAD thermodynamic modeling. Despite recent progress, significant methodological challenges remain, requiring enhanced experimental accuracy and refined theoretical approaches. Future work should address these gaps to fully leverage the potential of these superhard materials.

The phase diagram of the Mg-C system has been constructed up to 20 GPa and similar to 4000 K based on complementary Thermo-Calc simulations and experimental data obtained in both ex situ and in situ experiments using X-ray diffraction with synchrotron radiation. Three high-pressure magnesium carbides, namely, beta-Mg2C3, its high-temperature form gamma-Mg2C3, and antifluorite Mg2C, have p-T domains of thermodynamic stability. At the same time, the carbides accessible by ambient-pressure synthesis, alpha-Mg2C3 and MgC2, are either metastable or unstable, depending on the temperature, at least up to 20 GPa. Experimental observations show that at ambient conditions, all carbides are metastable and remain unchanged at least for years.

For complete and reproducible recovery of a high-pressure polymorph of zinc oxide, rock-salt ZnO (rs-ZnO), nanostructured wurtzite ZnO (w-ZnO) is typically used as a precursor for high-pressure synthesis. In the case of polycrystalline w-ZnO, only small amounts (less than 30 vol.%) of disordered/nanosized rs ZnO were occasionally observed in the recovered products. Here we report the conditions for the synthesis of single-phase rs ZnO from microcrystalline (40-50 µm) w ZnO powder at 7.7 GPa and 2000 K. The complete recovery of metastable rs-ZnO is possible only by using a multianvil apparatus for quasi-hydrostatic (triaxial) compression/decompression and pyrolytic boron nitride as a pressure medium. Single-phase rs ZnO was produced as colorless nanocrystalline well-sintered bulks with Vickers hardness up to 7 GPa - the record value for ZnO due to a fortunate combination of Hall-Petch nanostructuring effect and high intergranular purity. This unexpected phenomenon can be related to the mechanism of the direct and reverse phase transitions in ZnO, which requires "uniaxial" tensile deformation. Texture analysis of the recovered samples, as well as previous kinetic studies and ab initio simulation of strain-structure relationships, strongly support this model. Thus, 3-axial decompression is a more efficient tool – neglected until now – for reproducible recovery of high-pressure ZnO-based materials than the nature of the w-ZnO precursor and the presence of isostructural rock-salt phases.

Ice‐templating, also known as directional freezing or freeze‐casting, features the tunability of microstructure, the wide applicability of functional nanomaterials, and the fabrication of multiscale well‐controlled biomimetic materials. Recently, integrating ice‐templating with other materials’ processing technologies (such as, spraying, spinning, filtration, and hydrothermal), it has been investigated to tailor pore morphology of scaffolds for emerging applications. Such integration endows materials with various structures (cellular, dendritic, and lamellar) and dimensions (0D, 1D, 2D, and 3D), which opens up a new avenue for improving material properties and developing new materials. Herein, this review probes into the relationship of integrative ice frozen assembly with structure and describes the fundamental principles and synthesis strategies for preparing multi‐scale materials with complex biomimetic structures via ice‐templating. Focusing on ice crystal nucleation and growth, it summarizes the performance of ice‐templating in constructing pore geometries. Additionally, the review analyzes in depth the correlation between microstructure and macromorphology of final scaffolds, highlighting the application of integrative ice frozen assembly in electrochemical energy storage and conversion, and prospects for future research directions for this field.

Carbon framework of graphite structure can host alkali metals between layers forming graphite intercalation compounds (GIC). In the case of GICs with multiple layers separated by metallic layer, one can imagine graphite-to diamond transformation in carbon framework, leading to “diamond intercalation compounds” ( DIC). The design of such material(s) was the purpose of our work. GIC with metals such as Li, Na and K form different compositions (and crystal structures) are produced by stacking along c-axis of metal (Me) and n carbon (A, B or C) layers. The n number indicate the stage of intercalation. Typically ordered compounds are obtained for n = 1 to 6 with various stacking sequences depending on metals: n = 1 for MeAMeB, n = 2 for MeABMeBAMeCA, n = 3 for MeABCMeBCAMeCAB, etc. Experiments show that both high pressure and high temperature leads to increasing n . We will discuss the structural features of GIC, the XRD, Raman and other structurally related data, as well as corresponding DIC structurally related to GIC. The pressure and temperature range of formation of DICs from GICs coincide with industrially accessible conditions, that allows considering them as new promising materials. We have also shown that powder XRD is a method o fchoice for study of such transformations.

The effect of doping of cubic boron nitride with beryllium, silicon, sulfur, and magnesium on the lattice parameters, electrical conductivity, and ESR spectra has been studied. It is established that the degree of doping increases significantly in the case of crystallization of cubic boron nitride from BN solutions in supercritical ammonia at 3.9-4.2 GPa and 1100°C in comparison with the conventional synthesis from melts of the Mg–B–N system at 4.2 GPa and 1400°C. Doping with silicon and beryllium results in semiconductor properties of cubic boron nitride.

High-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the “large volume” presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under “traditional” industrial pressure range (3–8 GPa). We illustrated the power of such methodology by (i) two classical examples of “reference” superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years.

HP research on Si started more than 50 years ago and since then several allotropes, displaying a wide variety of physical properties, have been reported. The narrow-bandgap semiconductorSi-III with BC8 structure (originally believed to be semimetal) can be obtained from the high-pressure tetragonal metallic phase, Si-II, formed during compression of common silicon according to Si- I→Si -II. Such a transformation during decompression can be either direct, Si- II→Si -III, or with an intermediate step Si- II→Si - XII→ SiIII. Our in situ studies of pure Si in oxygen-free environment indicated that in the absence of pressure medium, Si-I remains metastable at least up to ~14 GPa, while the pressure medium allows reducing the onset pressure of transformation to ~10 GPa. Upon heating Si-III at ambient pressure a hexagonal structure, named Si-IV, was observed. This allotrope was believed to be a structural analogue of the hexagonal diamond found in meteorites (called also lonsdaleite) with the 2H polytypestructure. Calculations have predicted several hexagonal polytypes of Si and of other Group-IV elements to be metastable, such as 2H (AB), 4H (ABCB) and 6H (ABCACB). Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking), in contrast with Si-2H (AB stacking) proposed previously. The sequence of transformations Si- III→Si - IV(4H)→Si -IV(6H) has been observed in situ by powder X-ray diffraction. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab-initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region, for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics. The study of silicon allotropic transformation in Na-Si and K-Si systems at high pressure led to new open-framework allotrope of Si, Si24 with zeolite structure and promising optoelectronic properties.

In this work we present a detailed structural characterization of Si-4H, a newly discovered bulk form of hexagonal silicon (Si) with potential optoelectronic applications. Using multi-scale imaging, we reveal a hierarchical structure in the morphology of Si-4H obtained from high-pressure synthesis. We demonstrate discrete structural units, platelets, at an intermediate length-scale between the bulk pellets synthesized at high pressures and the flake-like crystallites inferred in previous studies. Direct observation of the platelets reveals their 2D structure, with planar faces spanning hundreds of nanometers to a few micrometers and thicknesses of only tens of nanometers. We separated and dispersed small packets of quasi-single platelets, which enabled us to analyze the crystalline domains within each grain. With this view, we demonstrate that Si-4H platelets represent the smallest crystalline structural units, which can bend at the single-domain level. Our characterization of the quasi-2D, flexible platelets of hexagonal Si-4H and proof of concept that the platelets can be dispersed and manipulated quite simply demonstrate opportunities to design novel optoelectronic and solar devices.

Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2–315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K.

The p-V-T equation of state of magnesium metal has been measured up to 20 GPa and 1500 K using both multianvil and opposite anvil techniques combined with synchrotron X-ray diffraction. To fit the experimental data, the model of Anderson-Gr\"uneisen has been used with fixed parameter {\delta}T. The 300-K bulk modulus of B0 = 32.5(1) GPa and its first pressure derivative, B0' = 3.73(2), have been obtained by fitting available data up to 20 GPa to Murnaghan equation of state. Thermal expansion at ambient pressure has been described using second order polynomial with coefficients a = 25(2)x10-6 K-1 and b = 9.4(4)x10-9 K-2. The parameter describing simultaneous pressure and temperature impact on thermal expansion coefficient (and, therefore, volume) is {\delta}T = 1.5(5). The good agreement between fitted and experimental isobars has been achieved to relative volumes of 0.75. The Mg melting observed by X-ray diffraction and in situ electrical resistivity measurements confirms previous results and additionally confirms the p-T estimations in the vicinity of melting.

A brief overview of the high-pressure synthesis of superhard and ultrahard materials is presented in this tutorial paper. Modern high-pressure chemistry represents a vast exciting area of research which can lead to new industrially important materials with exceptional mechanical properties. This field is only just beginning to realize its huge potential, and the image of “terra incognita” is not misused. We focus on three facets of this expanding research field by detailing the following: (i) the most promising chemical systems to explore (i.e., “where to search”); (ii) the various methodological strategies for exploring these systems (i.e., “how to explore”); and (iii) the technological and conceptual tools to study the latter (i.e., “the research tools”). These three aspects that are crucial in this research are illustrated by examples of the recent results on high-pressure–high-temperature synthesis of novel super- and ultrahard phases (orthorhombic γ-B28, diamondlike BC5, rhombohedral B13N2, and cubic ternary B–C–N phases). Finally, some perspectives of this research area are briefly reviewed.

Orientation-dependent aloof-beam vibrational electron-energy-loss spectroscopy is carried out on uniaxial icosahedral B12P2 submicron crystals. We demonstrate that the high sensitivity of the signal to the crystal orientation allows for an unambiguous determination of the symmetry of normal modes occurring at the Brillouin zone center of this anisotropic compound. The experimental results are assessed using first-principles quantum mechanical calculations (density functional theory) of the dielectric response of the specimen. The high spatial resolution inherent to this technique when implemented in the transmission electron microscope thus opens the door to nanoscale orientation-dependent vibrational spectroscopy.

The inorganic chemistry of the Na-Si system at high pressure is fascinating, with a large number of interesting compounds accessible in the industrial pressure scale, below 10 GPa. In particular, Na4Si4 is stable in this whole pressure range and thus plays an important role in understanding the thermodynamics and kinetics underlying materials synthesis at high pressures and high temperatures. In the present work, the melting curve of the Zintl compound Na4Si4 made of Na+ and Si44- tetrahedral cluster ions is studied at high pressures up to 5 GPa, by using in situ electrical measurements. During melting, the insulating Na4Si4 solid transforms into an ionic conductive liquid that can be probed through the conductance of the whole high-pressure cell, i.e., the system constituted of the sample, the heater, and the high-pressure assembly. Na4Si4 melts congruently in the studied pressure range, and its melting point increases with pressure with a positive slope dTm/dp of 20(4) K/GPa.