Volume 229, Issue 2

No Abstract

The high-pressure elastic behavior and structure evolution of a natural balliranoite, i.e. a mineral isotypic with cancrinite belonging to the davyne subgroup, (Na 4.47 Ca 2.86 K 0.11 )(Si 5.96 Al 6.04 O 24 )Cl 2.03 (CO 3 ) 0.78 (SO 4 ) 0.33 , a = 12.680(1) Å, c = 5.3141(5) Å and V = 739.9(1) Å 3 , S.G. P 6 3 , have been studied by means of in-situ single-crystal X-ray diffraction with a diamond anvil cell, up to 6.77(2) GPa. No evidence of phase transition, structure collapse or change of the compressional behavior have been observed within the pressure range investigated. The unit-cell volume evolution as function of pressure has been fitted with a second-order Birch-Murnaghan equation of state (BM EoS), yielding the following refined parameters: V 0 = 735.5(7) Å 3 , K V 0 = 48.1(8) GPa. Fitting of the a vs. P and c vs. P data with linearized third-order BM-EoS leads to the following elastic anisotropy at ambient conditions: K a 0 : K c 0 = 1.35:1. The P -induced structure evolution of balliranoite is mainly governed by the framework re-arrangement through tilting of quasi-rigid tetrahedra. A comparative analysis of the elastic behavior and of the structure deformation mechanisms of balliranoite and cancrinite at high-pressure are carried out.

High-pressure/high-temperature conditions of 5.5 GPa and 750 °C applied to a powder of Li 2 SiF 6 led to a pressure-supported crystal growth of Li 2 SiF 6 maintaining single crystals of sufficient quality for a single-crystal structure determination. The compound crystallizes in the space group P 321 (No. 150) with 3 formula units and the lattice parameters a = 8.219(2), c = 4.5580(9) Å, V = 266.65(8) Å 3 , R 1 = 0.0178, and wR 2 = 0.0391 (all data). This work verified the published powder diffraction data of Li 2 SiF 6 and the predicted space group. Additionally, powder-FT-IR and Raman-spectroscopic investigations on single-crystals of Li 2 SiF 6 were performed for the first time.

The characterization of pressure induced structural and chemical transformations employing the Diamond Anvil Cell (DAC) technique often requires a high spatial resolution. This is due to the small sample dimensions when the compression reaches the megabar range, or when heterogeneous materials are obtained in pressure induced chemical reactions. The characterization of these samples can be performed by using synchrotron light based techniques, which can provide optical or X-ray spots of few microns, whereas laboratory experiments are limited to Raman spectroscopy. Here we describe a cutting-edge Raman system with a transverse spatial resolution of 1–3 μm dedicated to the characterization of samples compressed in DACs. A few examples of the application of this technique to characterize different heterogeneous materials before and after high-pressure chemical reactions are reported to enlighten the performances of the system.

This article reviews the most recent results concerning second harmonic generation (SHG) experiments of non-phase matchable and phase matchable powder samples at high pressures and explains the pressure dependence of the intensity of the SHG signal by correlating it to the ratio between the average coherence length and the average particle size. The examples discussed here include pressure-induced structural changes in quartz, ZnO, ice VII and KIO 3 . It is shown that the second harmonic generation technique is a unique tool for the detection of pressure-induced structural phase transitions. It is laboratory based and allows fast measurements. It is complementary to X-ray diffraction and provides additional information about the presence of an inversion center for unknown or controversially discussed structures at high pressure.

Herein we report the characterisation of a novel salt of the “legal high” (±)-4′-methylmethcathinone hydrogen sulfate and its polymorphism under ambient and high pressure conditions. Under ambient pressure only one polymorph of the title compound was isolated but when subjected to high pressure a further two polymorphs are observed. The phase transitions at ≥0.5 GPa and ≥3.6 GPa are reversible single crystal to single crystal transitions which is possible due to the similarity of the packing in the molecules. It is proposed that these transitions are driven by the pV term of the Gibbs Free Energy equation rather than by the relief of repulsive interactions between molecules.

Density functional theory based calculations are commonly employed to complement experimental high pressure research. Here, a brief overview of the underlying theory and available codes is provided, followed by some applications. The influence of the choice of the exchange-correlation functional on predicted structural parameters and physical properties is discussed.

The hydrostatic compression of the chiral metal-organic framework [Ni(L–Asp)(H 2 O) 2 ]·H 2 O (NiAsp–I) has been studied to 6.07 GPa by single crystal X-ray diffraction. Over the course of this pressure range the compound undergoes a phase transition between 0.58 GPa and 1.26 GPa, yielding NiAsp–II. Both polymorphs are orthorhombic, P 2 1 2 1 2 1 , and feature 1D chains of Asp–Ni–Asp moieties. During the transition, the Ni(II) octahedra rotate, resulting in a dramatic shift in the position of the axial bonds with respect to the direction of polymer formation, and gives rise to an anisotropic compression of the unit cell dimensions.

The volumetric measurements in a piston-and-cylinder press have been described as a complementary method for diffraction experiments in a diamond-anvil cell. The measurements can provide precise volume compression data for liquids and solids, including the magnitudes of volume decrement on freezing and solid-solid phase transitions. Details of the experimental setup, its operation, as well as procedures for processing the data and their analysis have been presented for a series of organic compounds. The history of the method has been outlined.

A review of recently developed methods to study the structural properties of simple molecular solids under pressures up to the megabar range using synchrotron X-ray diffraction techniques is presented. This includes the growth of single-crystals at high pressure and temperature conditions and the use of He as pressure-transmitting medium. The application of these methods to solid O 2 and CO 2 is then presented as illustrative examples, showing how they enabled to solve long-standing debates on the structure of the high-pressure crystalline phases of these compounds.

We review the available information on crystal twinning at high pressures and analyze the effect of hydrostatic pressure on the pseudosymmetry of selected compounds. Twinning by merohedry, type I and II, as well as pseuomerohedral twinning at high pressures has been described. For twinning by merohedry type I (inversion twinning) a reliable characterization of the twin domains and volume fractions is difficult and largely depends on the experimental conditions, i.e. the number of measured Friedel pairs and the chosen wavelength. For twinning by merohedry, type II, and for twinning by pseuodmerohedry twin vol­ume fractions could be reliably determined for several cases. In none of these a significant influence of hydrostatic pressure on the volume fractions of the individuals was observed. Pressure-induced twinning has been observed for compounds which undergo first-order phase transitions. The twinning operation in the described cases is related to the loss of rotational symmetry elements of the higher symmetrical polymorph although the high and low pressure phases are not in group-subgroup relationship. The analysis of pseudosymmetry of several compounds as a function of pressure suggests that this parameter can be used to predict the (in)stability of compounds. In particular, a decrease in pseudosymmetry seems to be strongly correlated with the occurrence of first-order phase transitions in which the crystal breaks or amorphizes. The effect of hydrostatic and chemical pressure on the pseudosymmetry of the crystal structures can lead to different or even opposite trends, suggesting that the increase or decrease of pseudosymmetry cannot be understood as a pure volume effect.