N. W. A. van Uden

Raman scattering studies on single-crystalline bulk AlN under high pressures

We report on the Raman analysis of wurtzite single-crystalline bulk AlN under hydrostatic pressures up to 10 GPa. The pressure dependence of the AlN phonon frequencies was investigated. Mode Gruneisen parameters of 1.39, 1.57, 1.71, 0.93, and 1.26 were determined for the A1 (TO), E1 (TO), E2 (high), A1 (LO), and the quasi-longitudinal optical phonons, respectively. Recent theoretical calculations underestimate the pressure-induced frequency shift of the AlN phonons by about 20%–30%. Mode Gruneisen parameters of AlN were compared to those of GaN.

Raman scattering studies on single-crystalline bulk AlN: temperature and pressure dependence of the AlN phonon modes

Abstract We report on the Raman analysis of single-crystalline bulk AlN. AlN phonon modes were investigated as a function of temperature and hydrostatic pressure. Phonon decay channels were studied via the AlN Raman linewidth. Mode Gruneisen parameters describing the low-pressure behavior of the AlN phonon modes were determined and used to estimate hydrostatic stress in amber discolored AlN substrates.

Raman spectroscopy of B12As2 under high pressure

We report a Raman spectroscopy study of B12As2 under hydrostatic pressure up to 15 GPa. Mode Gruneisen parameters were determined for the B12As2 phonon modes. Phonon modes attributed to the As–As chain have a weak pressure dependence (0.6–1.9 cm−1 GPa−1) relative to inter- and intra-icosahedral vibrations (3.4–6.6 cm−1 GPa−1). The pressure dependence of B12As2 phonon frequencies is compared to those reported for α-boron and the origin of the mode at 505 cm−1 with its weak pressure dependence is discussed.

Solvation pressure as real pressure: I. Ethanol and starch under negative pressure

The reality of the solvation pressure generated by the cohesive energy density of liquids is demonstrated by three methods. Firstly, the Raman spectrum of ethanol as a function of cohesive energy density (solvation pressure) in ethanol–water and ethanol–chloroform mixtures is compared with the Raman spectrum of pure ethanol under external hydrostatic pressure and the solvation pressure and hydrostatic pressure are found to be equivalent for some transitions. Secondly, the bond lengths of ethanol are calculated by molecular dynamics modelling for liquid ethanol under pressure and for ethanol vapour. The difference in bond lengths between vapour and liquid are found to be equivalent to the solvation pressure for the C–H3 ,C –H 2 and O–H bond lengths, with discrepancies for the C–C and C–O bond lengths. Thirdly, the pressure-induced gelation of potato starch is measured in pure water and in mixtures of water and ethanol. The phase transition pressure varies in accordance with the change in solvation pressure of the solvent. These results demonstrate the reality of ‘negative pressures’ generated by reductions in the cohesive energy density of solvent mixtures.

Zen diamond-anvil low-pressure cell

A diamond-anvil cell can be operated with only one anvil in order to generate modest pressures in relatively large volumes. We demonstrate it to pressures up to 2.5 GPa with gaskets of steel, brass, and other metals, with a sample chamber 0.25 mm in diameter by 0.25–0.9 mm depth, and with various pressure media. In this form the cell is very simple to operate and is useful for much work on biological systems and soft solids which requires pressures in the 1 GPa range.

Negative effective pressures in liquid mixtures

Ethanol was studied under hydrostatic pressure and in mixtures with water and with chloroform. High resolution Raman spectroscopy was used to monitor the change in vibrational frequency as a function of pressure and composition. Molecular dynamics (MD) simulations were carried out and were compared with experimental results. Results show that for mixtures the cohesive energy density (CED) acts like a real pressure. The CED of a mixture is different from the CEDs of the components, and the differences result in effective pressures, negative and positive, on the components of the mixture. Other strong effects overshadow the working of this pressure in some cases. Nevertheless, it is possible to generate an effective negative pressure, which will have various applications in for example protein folding.

Theory of the Anomalous Low Band-Gap Pressure Coefficients of Semiconductor Strained Layers

There is a significant and unexplained drop in the band-gap pressure coefficients of III–V ternary semiconductor alloys grown as strained layers compared with the bulk binary values. For example, the drop for InxGa1—xAs is about (50 x) meV/GPa. In the past, first order effects of pressure have been treated using linear elasticity and this fails to predict the observed pressure coefficients. Ideally strained layers would be treated using an equation of state valid for an arbitrary state of strain, however, such theory is not currently available. Here, we present work towards such an equation and an approximate analysis using non-linear elasticity theory that does account for the data observed. The analysis relies on treating the non-linear variation of the substrate and layer lattice constants and the variation of the Poissons ratio with respect to pressure. The analysis is tested against experimental data for 〈001〉-grown layers available in the literature. In addition, we present new results on layers grown in the 〈111〉 direction. This acts as a further test on the theory because it involves a different Poissons ratio. The experimental results agree well with the predictions of the non-linear theory.

A novel high pressure tool: the solvation pressure of liquids

Co-solvents were studied to determine if the change in the cohesive energy density (CED) generates an effective solvation pressure equivalent to the application of an external hydrostatic pressure. Raman modes of chloroform under hydrostatic pressure with co-solvents (chloroform–ethanol, chloroform–acetone) and in the vapour phase were recorded. In some cases the Raman frequency shifts indicate that the solvation pressure behaves as a true hydrostatic pressure. The pressure-induced gelation of starch grains was studied in aqueous media. A higher co-solvent concentration is postulated to put the grains under effective negative pressure, and indeed an increase in the external pressure needed for gelation was seen after the introduction of solvents. The quantitative agreement between the change of solvation pressure and hydrostatic pressure is very good over a wide range of solvent concentration.

Determination of the Mode Grüneisen Parameter of AlN using different Fits on Experimental High Pressure Data

To investigate the pressure dependence of the AlN phonon frequencies Raman spectra of single-crystalline bulk AlN under hydrostatic pressure up to 10 GPa were recorded. The Raman peak positions of the A 1 (TO), E 1 (TO), E 2 (high), A 1 (LO) and quasi-longitudinal optical (QLO) phonons were plotted as a function of pressure. The experimental data was fitted using the traditional parabolic fit (M. Kuball et al . (2001) Appl. Phys. Lett., 78, 724 [1]) and fits to physical models, density, volume, etc. The mode Grüneisen parameters of the different phonons were determined for each fit and significant differences are found between the various fits. Results are compared with recent theoretical calculations (J.-M. Wagner et al . (2000) Phys. Rev. B, 62, 4526 [2]).