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Instruments / Technical Area	Publication Details	Published
I15-Extreme Conditions	One-dimensional chain melting in incommensurate potassium Emma Mcbride , Keith Munro , Graham Stinton , Rachel Husband , Richard Briggs , H P Liermann , Malcolm Mcmahon Physical Review B 91 (14) DOI: 10.1103/PhysRevB.91.144111 Diamond Proposal Number(s): [8258] Show Abstract ▼ Abstract: Between 19 and 54 GPa, potassium has a complex composite incommensurate host-guest structure which undergoes two intraphase transitions over this pressure range. The temperature dependence of these host-guest phases is further complicated by the onset of an order-disorder transition in their guest chains. Here, we report single-crystal, quasi-single-crystal, and powder synchrotron x-ray diffraction measurements of this order-disorder phenomenon in incommensurate potassium to 47 GPa and 750 K. The so-called chain melting transition is clearly visible over a 22 GPa pressure range, and there are significant changes in the slope of the phase boundary which divides the ordered and disordered phases, one of which results from the intraphase transitions in the guest structure.	Apr 2015
I15-Extreme Conditions	On the stability of the disordered molecular alloy phase of ammonia hemihydrate Craig Wilson , C. L. Bull , Graham Stinton , Daniel Amos , Mary-ellen Donnelly , John Loveday The Journal Of Chemical Physics 142 (9), 56 DOI: 10.1063/1.4913684 PMID: 25747099 Show Abstract ▼ Abstract: 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.	Mar 2015
I15-Extreme Conditions	Equation of state and high-pressure/high-temperature phase diagram of magnesium Graham Stinton , Simon Macleod , H. Cynn , Daniel Errandonea , W. J. Evans , John Proctor , Y Meng , Malcolm Mcmahon Physical Review B 90 (13) DOI: 10.1103/PhysRevB.90.134105 Diamond Proposal Number(s): [4215] Show Abstract ▼ Abstract: The phase diagram of magnesium has been investigated to 211 GPa at 300 K, and to 105 GPa at 4500 K, by using a combination of x-ray diffraction and resistive and laser heating. The ambient pressure hcp structure is found to start transforming to the bcc structure at about 45 GPa, with a large region of phase-coexistence that becomes smaller at higher temperatures. The bcc phase is stable to the highest pressures reached. The hcp-bcc phase boundary has been studied on both compression and decompression, and its slope is found to be negative and steeper than calculations have previously predicted. The laser-heating studies extend the melting curve of magnesium to 105 GPa and suggest that, at the highest pressures, the melting temperature increases more rapidly with pressure than previously reported. Finally, we observe some evidence of a new phase in the region of 10 GPa and 1200 K, where previous studies have reported a double-hexagonal-close-packed (dhcp) phase. However, the additional diffraction peaks we observe cannot be accounted for by the dhcp phase alone.	Oct 2014
I15-Extreme Conditions I19-Small Molecule Single Crystal Diffraction	Determining complex crystal structures from high pressure single-crystal diffraction data collected on synchrotron sources Malcolm Mcmahon , Ingo Loa , Graham Stinton , L. F. Lundegaard High Pressure Research 33 (3), 485 - 500 DOI: 10.1080/08957959.2013.831087 Show Abstract ▼ Abstract: 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.	Sep 2013
I15-Extreme Conditions	Phase transitions in europium at high pressures Rachel Husband , Ingo Loa , Graham Stinton , G. J. Ackland , Malcolm Mcmahon High Pressure Research DOI: 10.1080/08957959.2013.775280 Show Abstract ▼ Abstract: 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.	Mar 2013
I15-Extreme Conditions	Melting curve of potassium to 22 GPa Olga Narygina , Emma Mcbride , Graham Stinton , Malcolm Mcmahon Physical Review B 84 (5) DOI: 10.1103/PhysRevB.84.054111 Show Abstract ▼ Abstract: The melting curve of potassium has been measured experimentally to 22 GPa using in situ X-ray diffraction and gas-membrane diamond anvil cells equipped with external resistive heating. The evolution of the melting temperature with pressure is similar to that of sodium, but it is different to that reported previously. A melting maximum is found in the stability field of the bcc-phase, at 5.8(5) GPa, followed by a decrease in the melting temperature down to a melting minimum at the fcc-host-guest (tI19) transition at 19(1) GPa. The bcc-fcc-liquid and fcc-tI19-liquid triple points are found to be at 466(10) K and 13.6(3) GPa and at 390(10) K and 19.0(3) GPa, respectively.	Aug 2011
I15-Extreme Conditions	Observation of the oP8 crystal structure in potassium at high pressure L. F. Lundegaard , M. Marques , G. Stinton , G. J. Ackland , R. J. Nelmes , M. I. Mcmahon Physical Review B 80 (2) DOI: 10.1103/PhysRevB.80.020101 Show Abstract ▼ Abstract: Ab initio electronic structure calculations on potassium at high pressure reveal that the orthorhombic oP8 structure, found recently in sodium, has a lower enthalpy in potassium than do other candidate crystal structures recently reported to be stable at high pressure [Y. Ma, A. R. Oganov, and Y. Xie, Phys. Rev. B 78, 014102 (2008)]. Subsequent powder-diffraction studies to above 100 GPa confirm oP8 to be the stable phase of potassium from 54(2) to 90(2) GPa, where it transforms to the tetragonal tI4 structure, which in turn transforms to the orthorhombic oC16 structure at 96(3) GPa.	Jul 2009

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