AlPO4 belongs to the berlinite quartz homeotype family, which has been the subject of intense high-pressure research triggered by the supposed existence of reversible pressure-induced amorphization. X-ray diffraction experiments, complemented with ab initio calculations, demonstrate the existence of two high-pressure crystalline polymorphs and show that AlPO4 shares the same two-stage densification mechanism as silica. In the first step, a compact hexagonal sublattice of oxygen atoms is formed. In the second step, the cations redistribute in the interstices giving rise to a monoclinic distorted CaCl2 phase. The most outstanding feature of the phase is that phosphorous becomes six-fold coordinated by oxygen, adopting a configuration unknown so far in solid-state science. This finding opens possibilities in the high-pressure chemistry of phosphorus. The close relationship of AlPO4 with silica suggests the existence of completely unexplored families of compounds analogous to those of six-fold-coordinated silicates but based on PO6.

Six-fold-coordinated phosphorus by oxygen in AlPO 4 quartz homeotype under high pressure

\ensuremath{\alpha}-quartz-type ${\mathrm{GeO}}_{2}$ was found to undergo a transition to a crystalline phase above 6 GPa at room temperature from in situ, angle-dispersive, x-ray-diffraction measurements. There was no evidence of amorphization, although this phase is poorly crystallized. Hydrostatic conditions play an important role in the crystallization process. The crystal structure of this high-pressure form of ${\mathrm{GeO}}_{2}$ was found to be monoclinic, space group ${P2}_{1}/c,$ $Z=6$ and is built up of $3\ifmmode\times\else\texttimes\fi{}2$ kinked chains of edge-sharing ${\mathrm{GeO}}_{6}$ octahedra. This phase is 45% denser than \ensuremath{\alpha}-quartz-type ${\mathrm{GeO}}_{2}$ and 1% less dense than the rutile-type phase and is metastable from ambient pressure up to at least 50 GPa at room temperature. Upon heating this monoclinic phase at pressures up to 22 GPa, rutile-type ${\mathrm{GeO}}_{2}$ is formed, whereas at 43 GPa a mixture of the ${\mathrm{CaCl}}_{2}$-type and ${\mathrm{Fe}}_{2}\mathrm{N}$-type phases is obtained.

Transition to a crystalline high-pressure phase in α-GeO 2 at room temperature

Studies at extreme pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as semiconductors, superconductors or minerals. This is connected with the opportunity of tuning the pressure by many orders of magnitude. Diamond-anvil and large-anvil pressure cells installed at dedicated synchrotron beamlines are efficient tools for examination of crystal structure, equation of state, compressibility and phase transitions. One of basic methods in such studies is powder diffraction. This review is devoted to methods of powder X-ray diffraction at high-pressures generated by devices installed at synchrotron radiation sources, in particular to the principles of operation of high-pressure–high-temperature cells. General information on high-pressure diffraction facilities installed at 11 synchrotron storage rings in the world is provided. Measurement aspects are considered, including (i) pressure generation and calibration, (ii) strain in the sample, the pressure marker and the pressure-transmitting medium and (iii) pressure and temperature distributions within the cells. Sources of interest in high-pressure diffraction studies (design of new materials, observation of new phenomena, confrontation of theory with experiment) are briefly discussed. Recent developments of high-pressure methods make that pressure becomes a variable playing a key role in investigation of condensed matter. The paper ends with some remarks on the possible future developments of the technique.

High-pressure powder X-ray diffraction at the turn of the century

Abstract α-Quartz and its homeotypes are of great importance for both materials and Earth sciences. The properties of these materials depend strongly on their crystal structures and particularly the intertetrahedral bridging angle and the tetrahedral tilt angle. These angles are highly dependent on composition and the external parameters pressure and temperature. The behavior of the eleven known α-quartz homeotypes, along with examples of α-quartz-type solid solutions, are compared. The distortion in α-quartz-type structures decreases as a function of temperature and increases as a function of pressure. Thermal stability depends on initial structural distortion and on the electronic configuration of the cation. Pressure stability also depends on the former and on cation size. Transitions to new crystalline and/or amorphous forms, often with increased cation coordination number, are commonly observed at high-pressure. The combined use of high-pressure and high-temperature can be used to synthesize novel α-quartz homeotypes in compounds with small cations.

The effects of pressure, temperature and composition on the crystal structures of α-quartz homeotypes

X-ray absorption spectroscopy in mineralogy: Theory and experiment in the XANES region

Berlinites under pressure

Combined x-ray absorption spectroscopy (XAS) and x-ray diffraction experiments (EDX) on powders, both in the dispersive mode, may enable the determination of structural parameters as a function of pressure. After phase transitions, it may even be possible to determine the structure of the high pressure phases. The EDX enables the determination of the cell parameters, and eventually the space group from the systematic extinctions. On the other hand, the XAS provides information on the local environment of an atomic species, i.e., the coordination number and the first (eventually even second) neighbour distances with a high precision. The combination of these two techniques has been successfully used to determine the high pressure structure of ZnTe and HgTe. We present here recent results obtained on the high pressure properties of chalcopyrites and of berlinites. Finally, recent developments and future possibilities with the new synchrotron sources are presented.

Combined energy dispersive x-ray absorption and diffraction under high pressure

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Berlinites under pressure

Combined energy dispersive x-ray absorption and diffraction under high pressure