Exploring the behavior of hydrocarbon under pressure is important for understanding its role in planetary sciences and also for exploring novel organic chemistry. In this study, we explored the high-pressure behavior of a linear-chain hydrocarbon, tricosane (C23H48), using Raman spectroscopy. We compressed tricosane up to 23 GPa and did not find any evidence for pressure-induced amorphization within the conditions explored in this study. Upon compression, we observe new modes in the low energy region 100–300 cm−1. In order to understand the appearance of these new modes at high pressures, we used complementary ab initio calculations and explored the effect of chain configurations (linear and bent) on the predicted Raman spectra. We find that these new modes observed at higher pressures are better explained by bent configuration of tricosane chains. Thus, based on high-pressure Raman spectra, it is very likely that a linear chain of tricosane is bent under pressure, i.e., it undergoes a pressure-induced trans-gauche transformation. It is also likely that such bent regions (i.e., kinks) will act as sites along which large chain hydrocarbons could dissociate into smaller chain lengths at extreme conditions relevant to the interiors of Jovian planets.

We have carried out detailed experimental investigations on polycrystalline CuO using dielectric constant, dc resistance, Raman spectroscopy and X-ray diffraction measurements at high pressures. Observation of anomalous changes both in dielectric constant and dielectric loss in the pressure range 3.7–4.4 GPa and reversal of piezoelectric current with reversal of poling field direction indicate to a change in ferroelectric order in CuO at high pressures. A sudden jump in Raman integrated intensity of Ag mode at 3.4 GPa and observation of Curie-Weiss type behaviour in dielectric constant below 3.7 GPa lends credibility to above ferroelectric transition. A slope change in the linear behaviour of the Ag mode and a minimum in the FWHM of the same indicate indirectly to a change in magnetic ordering. Since all the previous studies show a strong spin-lattice interaction in CuO, observed change in ferroic behaviour at high pressures can be related to a reentrant multiferroic ordering in the range 3.4 to 4.4 GPa, much earlier than predicted by theoretical studies. We argue that enhancement of spin frustration due to anisotropic compression that leads to change in internal lattice strain brings the multiferroic ordering to room temperature at high pressures.

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Octadecane (C18H38) is an aliphatic hydrocarbon that is abundant in carbonaceous chondritic meteorites. It is debated whether these hydrocarbons found in the meteorite are pristine or are a result of subsequent modifications as these meteorites are delivered to the Earth. It is well-known that meteorites are often subjected to extreme pressures and temperatures upon entering the Earth’s atmosphere. To explore the behavior of octadecane at high pressures, that is, how its molecular structure responds to compression, we use a diamond anvil cell in conjunction with Raman spectroscopy. We find that at room temperatures, upon compression to ∼5 GPa, a linear-chain octadecane molecule transforms into a bent-chain configuration. Similar transitions from linear to a bent configuration in other hydrocarbons have been documented. We find a linear correlation between the transition pressure from linear to bent configuration, and the chain length of the alkane, that is, longer chain lengths, is likely to be less stable in the linear configuration under compression. These kinks in the bent-chain configuration are likely sites for the dissociation of the longer chain hydrocarbons to smaller hydrocarbons. The octadecane sample examined in this study did not undergo any additional transition to the highest pressure (∼18 GPa) explored in this study.

Room temperature compression of graphitic materials leads to interesting superhard sp 3 rich phases which are sometimes transparent. In the case of graphite itself, the sp 3 rich phase is proposed to be monoclinic M-carbon, however for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using X-ray diffraction, and interpret the findings with molecular dynamics modelling. Experiment and modelling both predict a two stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at ~35 GPa to an unstable, disordered sp 3 rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated sp 3 rich material formed at high pressure reveals a non-crystalline structure with two different sp 3 bond lengths.

The pressure-dependent phonon modes of InAs nanowires have been investigated by Raman spectroscopy under high pressure up to ∼58 GPa. X-ray diffraction measurements show that InAs nanowires at 21 GPa exhibit a phase transition from a wurtzite to an orthorhombic crystal structure, with a corresponding drastic change in the first-order Raman spectra. In the low-pressure regime, a linear increase in phonon frequencies is observed, whereas splitting between longitudinal and transversal optical phonon modes decreases as a function of applied pressure. The calculated mode Grüneisen parameters and Born's transverse effective charge indicate that the wurtzite InAs nanowires exhibit a more covalent nature under compression.

High pressure Raman spectroscopy, X-ray diffraction, and dielectric measurements have been carried out in Ba1−xSrxTiO3 (x = 0.05 and 0.1). Detailed structural analysis revealed a single phase transition from tetragonal P4mm to cubic Pm3m symmetry. Increase in Sr ion concentration resulted in decrease in the phase transition pressure. The dielectric measurements showed considerable lowering of transition pressure which has been attributed to bulk behaviour of the material.

Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) is produced by weathering of continental rocks and is an important constituent of terrigenous sediment flux in subduction zone. It helps in transporting water into the Earth's interior. Kaolinite consists of a layer of silicate tetrahedral (Tet) sheet and a layer of octahedral (Oct) sheet. There are two distinct crystallographic environments for protons: an inner hydroxyl group and an inner surface hydroxyl group that holds together adjacent Tet-Oct layers by hydrogen bonds. We investigate the high-pressure behavior of these two distinct proton environments upon compression up to 9 GPa using a diamond anvil cell and Raman spectroscopy. Upon compression, the hydroxyl stretching region exhibits major changes as kaolinite transitions from the low pressure phase K-I to the intermediate pressure phase K-II at ∼2.9 GPa, and the intermediate pressure phase K-II to the high-pressure phase K-III at ∼6.1 GPa. These are associated with significant changes in the pressure dependence of the hydroxyl modes, thus reflecting the changes in the hydrogen bonding environment (O-H•••O) between the adjacent Tet-Oct••• Tet layers. The K-I phase exhibits strengthening of hydrogen bonds between the Tet-Oct•••Tet layers, i.e., < ν 0 P d d OH . The K-II phase exhibits significantly reduced hydrogen bond strength between the Tet-Oct•••Tet layers, i.e., > ν 0 P d d OH .Based on the static DAC results, we hypothesize that, owing to the reduced strength of hydrogen bonding in the interlayer region of the K-II phase, it acts as a precursor for the super-hydrated kaolinite, where water molecules are intercalated in the interlayers of the K-II phase. To test this hypothesis, we conducted high pressure (P)-temperature (T) experiments with kaolinite and water at conditions relevant to the subduction zones. We explored up to a maximum pressure of ∼4.5 GPa and temperatures up to ∼350 °C. Irrespective of the P-T path undertaken, i.e., compression followed by heating or heating followed by compression, upon cooling "kaolinite + water", we found the appearance of new vibrational modes at ∼3550 and 3650 cm −1 . These new vibrational modes are related to the intercalated water molecules in the super-hydrated kaolinite. This super-hydrated kaolinite phase is likely to subduct significantly more water than the K-I phase.