Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

The ruby luminescence method is widely used for pressure measurement in the diamond anvil cell and other optically transparent pressure cells. With this application in mind, we briefly review the ground-state physical properties of corundum (α-Al2O3) with some emphasis on its behavior under high pressure, survey the effects of temperature and stress on the R-line luminescence of ruby (Cr-doped corundum), and address the recent efforts towards an improved calibration of the R-line shift under hydrostatic pressures beyond the 50 GPa mark.

Ruby under pressure

Silicon wafers subject to depth-sensing indentation tests have been studied using Raman microspectroscopy. We report a strong correlation between the shape of the load-displacement curve and the phase transformations occurring within a nanoindentation. The results of Raman microanalysis of nanoindentations in silicon suggest that sudden volume change in the unloading part of the load-displacement curve (“pop-out” or “kink-back” effect) corresponds to the formation of Si–XII and Si–III phases, whereas the gradual slope change of the unloading curve (“elbow”) is due to the amorphization of silicon on pressure release. The transformation pressures obtained in nanoindentation tests are in agreement with the results of high pressure cell experiments.

Effect of phase transformations on the shape of the unloading curve in the nanoindentation of silicon

mentally. In this report, we present in situ TEM observation of the elastic-plastic-fracture processes of a single Si NW recorded at atomic resolution. The study directly shows the strain-induced structural evolution process of Si NWs and its largestrain plasticity (LSP). Our results indicate that the LSP of Si NWs via a brittle-ductile transition originates from a dislocation-initiated amorphization. This behavior is in contrast to the mechanical behavior of bulk Si. Our observation reveals a phenomenon and mechanism of the Si NW mechanics and is fundamentally important for Si-NW-based nanotechnology. The in situ Si NW elastic, elastic-plastic, and plastic deformation investigations were carried out by ultrahigh-resolution TEM (UHRTEM) by axially extending the Si NWs by means of a mechanical force created by a TEM specimen-supporting grid under electron beam irradiation, as reported in detail elsewhere [17,18] (also see the Experimental section and Sup

http://www.nanoscience.gatech.edu/zlwang/paper/2007/07_AM_7.pdf

Low-Temperature In Situ Large-Strain Plasticity of Silicon Nanowires

This paper discusses the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential. By monitoring the positions of atoms within the model, the microstructural changes as silicon transforms from its diamond cubic structure to other phases were identified. The simulation showed that diamond cubic silicon transforms into a body-centred tetragonal form (β-silicon) upon loading of the indentor. The change of structure is accomplished by the flattening of the tetrahedron structure in diamond cubic silicon. Upon unloading, the body-centred tetragonal form transforms into an amorphous phase accompanied by the loss of long-range order of the silicon atoms. By performing a second indentation on the amorphous zone, it was found that the body-centred-tetragonal-to-amorphous phase transformation could be a reversible process.

http://iopscience.iop.org/article/10.1088/0957-4484/11/3/307/pdf

Molecular dynamics simulation of phase transformations in silicon monocrystals due to nano-indentation

X-ray-diffraction data have been obtained on Si in a diamond anvil cell to pressures of \ensuremath{\sim}50 GPa. Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, \ensuremath{\beta}-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III [body-centered-cubic (BC8)] and on the coexistence of the phases. Comparison is made between these data and the predictions of ab initio calculations for these structures and their equations of state.

Crystal data for high-pressure phases of silicon

Variations in the fluorescence line positions, linewidth, and lifetimes with hydrostatic pressure up to about 80 kbar are reported for samples of NdP5O14 and Nd0.1Y0.9P5O14. The results for the lightly doped sample are consistent with changes in the radiative decay rate while the very different results obtained on the heavily doped sample are interpreted in terms of increased radiationless quenching due to energy migration to trapping sites. Results are also reported on the variations in the fluorescence lifetimes of the R lines, and the positions of the N lines and vibronic peaks of ruby as a function of hydrostatic pressure.

Effects of Pressure on the Spectra and Lifetimes of Nd(x)Y(1-x)P55O14 and Ruby,

Etude de l'absorption de l'etat excite (ESA) dans NaF:Eu 2+ et KM g F 3 :Eu 2+ en utilisant une technique de sonde de pompage dans le domaine du visible. La dependance de la fluorescence et de la transmission avec le flux de pompage met en evidence l'ESA dans le domaine ultraviolet. On observe une large bande d'ESA aux longueurs d'onde inferieures a environ 600 nm dans NaF:Eu. La comparaison de ce resultat avec des donnees anterieures suggere que l'ESA est due a la photoionisation. Dans KMgF 3 :Eu, il n'y a aucune evidence quant a l'ESA dans le visible, mais on observe une forte ESA a la longueur d'onde de pompage de 193 nm

Larry D. Merkle

Papers

Crystal data for high-pressure phases of silicon

Effects of Pressure on the Spectra and Lifetimes of Nd(x)Y(1-x)P55O14 and Ruby,

Excited-state absorption by Eu2+ in NaF and KMgF3.