G. W. Stinton

The crystal structures of δ and δ ∗ nitrogen

The crystal structures of the high-pressure δ and δ ∗ phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the δ phase is very similar to isostructural γ-O2 and comprises spherically disordered molecules, with a preference for avoiding pointing along the cubic ⟨100⟩ directions, and disklike molecules with a uniform distribution of orientations. The structure of the δ ∗ phase is tetragonal and the space group is identified unambiguously as P42/ncm with unit cell parameters of a=8.603(5) A and c=5.685(5) A at 14.5 GPa. The orientations of the partially disordered molecules have been experimentally determined for the first time and are similar to those predicted on the basis of molecular dynamics simulations.

High-pressure, high-temperature single-crystal study of Bi-IV

Bi-IV is the stable high-pressure, high-temperature phase of bismuth at ∼4 GPa and ∼500 K. It was first identified in 1958, but its structure has remained uncertain. An X-ray powder-diffraction study of Bi-IV reported the structure as monoclinic, but a subsequent reinterpretation of the same data concluded that the structure was C-centred orthorhombic (oC16), with the same atomic arrangement as in Cs-V and Si-VI. To resolve the uncertainty over the structure of Bi-IV, we investigated this phase at 3.2 GPa and 465 K by single-crystal synchrotron X-ray diffraction. All of the observed reflections could be indexed on the orthorhombic oC16 structure proposed by Degtyareva, with a=11.191(5) Å, b=6.622(1) Å and c=6.608(1) Å. The spacegroup was confirmed as Cmce. Refinement of the data resulted in an excellent fit (R=2.8% ), and gave atomic coordinates very similar to those of the oC16 structures in Cs-V and Si-VI.

Pressure-induced dehydration and the structure of ammonia hemihydrate-II

The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition. The structure is monoclinic with spacegroup P2(1)/c and lattice parameters a = 3.3584(5) Å, b = 9.215(1) Å, c = 8.933(1) Å and β = 94.331(8)° at 3.5(1) GPa. The atomic arrangement has a crowned hexagonal arrangement and is a layered structure with long N-D···N hydrogen bonds linking the layers. The existence of pressure-induced dehydration of AMH may have important consequences for the behaviour and differentiation of icy planets and satellites.

Determining complex crystal structures from high pressure single-crystal diffraction data collected on synchrotron sources

As part of a Long Term Project, single-crystal diffraction techniques have been developed for use at the high pressure beamlines ID09 and ID27 at the European Synchrotron Radiation Facility, and have been utilised to determine the crystal structures of various high pressure phases, including those with incommensurate structures, at both high and low temperatures. The same techniques have also been used to determine the structures of high pressure phases at the SRS, Diamond and Petra-III synchrotron sources. In this paper, we describe technical details of the methods developed, and describe some of the considerations necessary for planning experiments and collecting and processing the data. We then illustrate the quality of data that can be obtained, and the complexity of the structures that can be refined, using recent results obtained from complex high pressure phases of N2 and Ba.

The Structure of Eu-III

Previous x-ray diffraction studies have reported Eu to transform from the hcp structure to a new phase, Eu-III, at 18 GPa. Using x-ray powder diffraction we have determined that Eu remains hcp up to 33 GPa, and that the extra peaks that appear at 18 GPa are from an impurity phase with space group Rc . Above 33 GPa the diffraction pattern becomes very much more complex, signalling a transition to a phase with a distorted hcp structure.

On the stability of the disordered molecular alloy phase of ammonia hemihydrate

The disordered-molecular-alloy phase (DMA) of ammonia hydrates [J. S. Loveday and R. J. Nelmes, Phys. Rev. Lett. 83, 4329 (1999)] is unique in that it has substitutional disorder of ammonia and water over the molecular sites of a body centred cubic lattice. Whilst this structure has been observed in ammonia di- and mono-hydrate compositions, it has not been conclusively observed in the ammonia hemihydrate system. This work presents investigations of the structural behaviour of ammonia hemihydrate as a function of P and T. The indications of earlier studies [Ma et al. RSC Adv. 2, 4290 (2012)] that the DMA structure could be produced by compression of ammonia hemihydrate above 20 GPa at ambient temperature are confirmed. In addition, the DMA structure was found to form reversibly both from the melt, and on warming of ammonia hemihydrate phase-II, in the pressure range between 4 and 8 GPa. The route used to make the DMA structure from ammonia mono- and di-hydrates--compression at 170 K to 6 GPa followed by warming to ambient temperature--was found not to produce the DMA structure for ammonia hemihydrate. These results provide the first strong evidence that DMA is a thermodynamically stable form. A high-pressure phase diagram for ammonia hemihydrate is proposed which has importance for planetary modelling.

Phase transitions in europium at high pressures

For several decades, x-ray diffraction studies on europium (Eu) metal at high pressure were complicated by the presence of a rhombo-hedral contaminant phase, hR6, which has been recognised as such only recently. Using x-ray powder diffraction, we have determined the hR6 contaminant to undergo a phase transition to a cubic phase, cI12, at 34.8(12) GPa. Consideration of the volume per Eu atom of the two contaminant phases at the same pressure suggests that they have different stoichiometries or chemical compositions. We also report a phase transition in pure Eu from the incommensurately-modulated Eu-IV phase to Eu-V between 38 and 42 GPa.

Melting of potassium to 22 GPa

Using in-situ x-ray diffraction, the melting curve of potassium was determined to 22 GPa and was found to be remarkably similar to that of sodium, and strikingly different to that reported previously. The existence of a maximum in the bcc phase was determined at 5.8(5) GPa and 530(10) K; the melting temperature was then observed to decrease over several GPa, flattening out at the bcc-fcc-liquid triple point at 13.6(3) GPa and 466(10) K, before further decreasing from 15.6(3) GPa to a minimum at 19(1) GPa and 390(10) K. It then regained a positive slope and increased rapidly at a rate of 65(5) K/GPa In the tI19 phase it was observed that the guest chains melted before the host structure at 20.3(3) GPa and 420(10) K, but were solid at 22.5(3) GPa and 350(10) K.