Hans Ahsbahs

Single-crystal compression and crystal structure of clinopyroxene up to 10 GPa

Abstract The hydrostatic compression of synthetic single crystals of diopside, CaMgSi2O6, and hedenbergite, CaFeSi2O6, was studied at 33 pressures up to 10 GPa by X-ray diffraction. In addition, intensity data for hedenbergite were collected at 12 pressures up to 10 GPa. For determination of the elasticity two crystals were loaded together in a diamond cell. The axial compressibilities βa, βb, and βc of diopside and hedenbergite are 2.36(4), 3.17(4), and 2.50(4) × 10-3 GPa-1, and 1.93(5), 3.38(6), and 2.42(8) × 10-3 GPa-1, respectively. The bulk moduli (KT₀) and their pressure derivatives (K′T₀ ) were determined simultaneously from a weighted linear fit of a third order Birch-Murnaghan equation of state to the volume data at elevated pressures. KT₀ and K′T₀ are 104.1(9) GPa and 6.2(3) for diopside and 117(1) GPa and 4.3(4) for hedenbergite, respectively. The unit-cell parameters decrease continuously with pressure. The larger polyhedra show more compression than the smaller ones. Between 0.1 MPa and 10 GPa the polyhedral volumes of CaO8, FeO6, and SiO4 decrease by 8.4, 6.6, and 2.9%, respectively. The longest bonds of CaO8 and FeO6 show most compression. Significant compression in the two shortest Si-O1 and Si-O2 bond lengths of the SiO4 tetrahedra was observed at relatively low pressures, resulting in a tetrahedral volume compression of 1.6% between 0.1 GPa and 4 GPa and 1.3% between 4 and 10 GPa. The compression of the unit cell can be described by the volume compression of the individual CaO8 and FeO6 polyhedra, with the SiO4 tetrahedron playing a minor role. Diopside is more compressible than hedenbergite as shown by their axial and volume compressibilities because the FeO6 octahedron is significantly more rigid than MgO6 at high pressures. This observation implies that octahedrally coordinated Fe2+ behaves differently from Mg at high pressures, in contrast to their behavior at ambient conditions.

Anisotropic crystal structure distortion of the monoclinic polymorph of acetaminophen at high hydrostatic pressures

The anisotropy of structural distortion of the monoclinic polymorph of acetaminophen induced by hydrostatic pressure up to 4.0 GPa was studied by single-crystal X-ray diffraction in a Merrill-Bassett diamond anvil cell (DAC). The space group (P2(1)/n) and the general structural pattern remained unchanged with pressure. Despite the overall decrease in the molar volume with pressure, the structure expanded in particular crystallographic directions. One of the linear cell parameters (c) passed through a minimum as the pressure increased. The intramolecular bond lengths changed only slightly with pressure, but the changes in the dihedral and torsion angles were very large. The compressibility of the intermolecular hydrogen bonds NH...O and OH...O was measured. NH...O bonds were shown to be slightly more compressible than OH...O bonds. The anisotropy of structural distortion was analysed in detail in relation to the pressure-induced changes in the molecular conformations, to the compression of the hydrogen-bond network, and to the changes in the orientation of molecules with respect to each other in the pleated sheets in the structure. Dirichlet domains were calculated in order to analyse the relative shifts of the centroids of the hydrogen-bonded cycles and of the centroids of the benzene rings with pressure.

Effect of hydrostatic pressure on the γ-polymorph of glycine 1. A polymorphic transition into a new δ-form

Abstract The results of a high-resolution powder diffraction study of the effect of high hydrostatic pressure up to 8 GPa on the pure γ-polymorph of glycine (P31) are discussed. A phase transition with a jumpwise change of cell volume and cell parameters was observed. The transition starts at about 2.73 GPa and is still not complete even at 7.85 GPa. The crystal structure of the previously unknown high-pressure polymorph of glycine (δ-polymorph) could be solved and refined in the space group Pn. In this structure, glycine zwitter-ions are linked via NH…O hydrogen bonds into layers, which form double-layered bands via additional NH…O hydrogen bonds. The structure of the individual layers in the high-pressure polymorph is similar to that in the previously known α- (P21/n) and β- (P21) forms, but the packing of the layers is essentially different. The pressure-induced polymorphic transformation in the γ-glycine can be compared with a change in the secondary structure of a peptide, when a helix is transformed into a sheet.

High-pressure X-ray investigation of zincite ZnO single crystals using diamond anvils with an improved shape

A high-pressure cell for single-crystal investigations requires large opening angles. If it does not contain any beryllium but only two diamonds mounted on steel backing plates, normally only lower pressures can be attained without risk of disruption of the diamond anvils [Ahsbahs (2004). Z. Kristallogr. 219, 305–308]. In order to enable pressures up to at least 10 GPa, specially shaped anvils were designed. In a first study with such anvils, the high-pressure behaviour of zincite ZnO was investigated up to the transition pressure of about 9.5 GPa. A slight decrease of the axial ratio c/a with increasing pressure was observed. The fit of the Birch–Murnaghan equation of state gave a bulk modulus K = 146.5 (8) GPa with a fixed pressure derivative K′ = 4. During the phase transformation, the single crystals were destroyed. The pronounced preferred orientation of the obtained material, however, enabled the determination of the orientation relations between the low-pressure wurtzite-type and the high-pressure NaCl-type phase. The orientation relations are similar to those in CdS and CdSe.

A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa

Abstract The anisotropy of lattice strain in the crystals of L-serine (P212121, at ambient conditions a = 5.615(1) Å, b = 8.589(2) Å, c = 9.346(2) Å) on cooling down to 100 K and with increasing hydrostatic pressure up to 4.4 GPa was compared with each other and also with the results previously obtained for the polymorphs of glycine. On cooling, the structure expanded slightly along the crystallographic a-direction, compression along the crystallographic b- and c-directions (normal to the chains of the serine zwitter-ions) was very similar. With increasing pressure, the same structure compressed in all the crystallographic directions, linear strain along c-axis was the largest, linear strain along a-axis — the smallest, linear compression along the b-axis with increasing pressure was slightly larger than that along the a-axis. The different anisotropy of lattice strain of the same structure on cooling and under pressure could be correlated with different response of intermolecular hydrogen bonds to these two scalar actions.

A comparative study of the anisotropy of lattice strain induced in the crystals of DL-serine by cooling down to 100 K, or by increasing pressure up to 8.6 GPa. A comparison with L-serine

Summary The anisotropy of lattice strain in the crystals of DL-serine (P21/n) on cooling down to 100 K and with increasing hydrostatic pressure up to 8.6 GPa was studied by single-crystal X-ray diffraction. In contrast to L-serine undergoing pressure-induced phase transitions at about 5 and 8 GPa, no phase transitions were observed in DL-serine at least up to 8.6 GPa (the highest pressure reached in the experiment). The anisotropy of strain in DL-serine on cooling was shown to be radically different from that with increasing pressure. The response of the crystal structure of DL-serine to cooling and to increasing pressure was considerably different from that of L-serine.

A comparative study of pressure-induced lattice strain of α- and γ-polymorphs of glycine

Abstract The effect of hydrostatic pressure up to 4 GPa on the two polymorphs of glycine α — s.g. P21/n, and γ — s.g. P31 (P32)) was studied by X-ray powder diffraction using a synchrotron radiation source (λ = 0.7 Å, a MAR345 image plate detector). No polymorphic transformations were detected. Relative volume changes and the aniso tropy of structural distortion of the two polymorphs were compared. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures.

New pressure cell for single-crystal X-ray investigations on diffractometers with area detectors

Abstract The development of a pressure cell using Merrill-Bassett geometry is described, that avoids the use of beryllium. Nevertheless, an opening angle of 90° was obtained for the X-rays using a new mounting technique for the diamonds. The cell was specifically designed for comfortable use on modern single-crystal diffractometers with area detectors. It has the additional advantage that no absorption correction is necessary for the cell. It is intended for pressures up to 10 GPa, the range in which liquid pressure-transmitting media are available. The first results are presented, including comparative investigations with a single-counter diffractometer equipped with an edge-collimator.

Structure solution and refinement from powder or single-crystal diffraction data? Pros and cons: An example of the high-pressure Β′-polymorph of glycine

The structure of a high-pressure polymorph of glycine (the Β′ -polymorph formed reversibly at 0.8 GPa from the Β -polymorph) was determined from high-resolution X-ray powder diffraction data collected in situ in a diamond anvil cell at nine pressure points up to 2.6 GPa. X-ray powder diffraction study gave a structural model of at least the same quality as that obtained from a single-crystal diffraction experiment. The difference between the powder-diffraction and the single-crystal models is related to the orientation of the N H3 -tails and the structure of the hydrogen-bonds network. The phase transition between the Β - and Β′ -polymorphs is reversible and preserves a single crystal intact. No transformations were observed between the Β -, α -, and Β′ -polymorphs on compression and decompression, although the α - and Β′ -polymorphs belong to the same space group (P21/c). The instability of the Β - and γ -forms with pressure can be predicted easily when considering the densities of their structures versus pressure. The direction of the transformation (i.e., which of the high-pressure polymorphs is formed) is determined by structural filiation between the parent and the high-pressure phases because of the kinetic control of the transformations.

57 Fe nuclear forward scattering of synchrotron radiation in hedenbergite CaFeSi 2 O 6 at hydrostatic pressures up to 68 GPa

Abstract The 57Fe nuclear forward scattering (NFS) of synchrotron radiation and the use of diamond anvils with helium as pressure medium allowed study of the electronic state of Fe2+ in the chain silicate hedenbergite CaFeSi2O6 at pressures up to 68 GPa. Characteristics of NFS time spectra were compared with those of conventional Mössbauer spectra. NFS time spectra of 57Fe in hedenbergite revealed a reversible phase transition between 53 and 68 GPa at room temperature, which is probably a transition from the paramagnetic phase at low pressures to a magnetic phase at high pressures. If this interpretation is correct, the Néel temperature TN of hedenbergite depends critically on pressure (TN = 45 K at I atm).