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Instruments / Technical Area	Publication Details	Published
I15-Extreme Conditions	Nonexistence of the s−f volume-collapse transition in solid gadolinium at pressure Qingchen Li , Hossein Ehteshami , Keith Munro , Miriam Marqués , Malcolm Mcmahon , Simon G. Macleod , Graeme J. Ackland Physical Review B 104 DOI: 10.1103/PhysRevB.104.144108 Diamond Proposal Number(s): [11768] Show Abstract ▼ Abstract: Gadolinium has long been believed to undergo a high-pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended s -orbital to the compact f -orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the f -electrons [Fabbris et al., Phys. Rev. B 88, 245103 (2013)]. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high-pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using x-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes h c p → 9 R → dhcp → fcc → d − fcc is reproduced. The so-called “volume-collapsed” high-pressure phase is shown to be an unusual stacking of close-packed planes, with Fddd symmetry and a density change of less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the f -electrons. The key result is that there is no XRD evidence for volume collapse in gadolinium. The sequence of phase transitions is well described by standard density functional theory. There is no need for special treatment of the f -electrons or evidence of f -electron bonding. Noting that in previous spectroscopic evidence there is no change in the f -electrons we conclude that high-pressure gadolinium has no complicated f -electron physics such as Mott-Hubbard, Kondo, or valence transitions.	Oct 2021
I15-Extreme Conditions	Behavior of rubidium at over eightfold static compression C. V. Storm , J. D. Mchardy , S. E. Finnegan , E. J. Pace , M. G. Stevenson , M. J. Duff , S. G. Macleod , M. I. Mcmahon Physical Review B 103 DOI: 10.1103/PhysRevB.103.224103 Diamond Proposal Number(s): [22350] Show Abstract ▼ Abstract: The high pressure phases of Rb have previously been investigated to 101 GPa, above which Rb is predicted to adopt a double-hexagonal close-packed (dhcp, Pearson hP4) structure similar to that already observed in cesium at 72 GPa. Previous ab initio structure searches have indicated that the hP4 phase should become stable in rubidium at 143 GPa . We present data from static compression experiments on Rb up to 264 ( 8 ) GPa , showing the onset of the hP4 phase at 207 ( 6 ) GPa . The V / V 0 of ∼ 0.121 measured at 264 GPa constitutes the highest compression ratio (more than eightfold) at which structural information has been obtained from a metal using x-ray diffraction methods and is second only to x-ray measurements performed on hydrogen at V / V 0 ∼ 0.094 at 190 GPa . At these extreme compression ratios, the compressive behavior of rubidium shifts from that of a free electron metal to that of a regular d -block metal.	Jun 2021
I15-Extreme Conditions	High-pressure structural systematics in neodymium up to 302 GPa S. E. Finnegan , C. V. Storm , E. J. Pace , M. I. Mcmahon , S. G. Macleod , E. Plekhanov , N. Bonini , C. Weber Physical Review B 103 DOI: 10.1103/PhysRevB.103.134117 Show Abstract ▼ Abstract: Angle-dispersive x-ray powder diffraction experiments have been performed on neodymium metal to a pressure of 302 GPa. Up to 70 GPa we observe the h P 4 → c F 4 → h R 24 → o I 16 → h P 3 transition sequence reported previously. At 71(2) GPa we find a transition to a phase which has an orthorhombic structure ( o F 8 ) with eight atoms in the unit cell, space group F d d d . This structure is the same as that recently observed in samarium above 93 GPa, and is isostructural with high-pressure structures found in the actinides Am, Cf, and Cm. We see a further phase transition at 98(1) GPa to a phase with the orthorhombic α -U ( o C 4 ) structure, which remains stable up to 302 GPa, the highest pressure reached in this study. Electronic structure calculations find the same structural sequence, with calculated transition pressures of 66 and 88 GPa, respectively, for the h P 3 → F 8 and o F 8 → o C 4 transitions. The calculations further predict that o C 4 -Nd loses its magnetism at 100 GPa, in agreement with previous experimental results, and it is the accompanying decrease in enthalpy and volume that results in the transition to this phase. Comparison calculations on the o F 8 and o C 4 phases of Sm show that they both retain their magnetism to at least 240 GPa, with the result that o C 4 -Sm is calculated to have the lowest enthalpy over a narrow pressure region near 200 GPa at 0 K.	Apr 2021
I15-Extreme Conditions	Pressure-induced bcc-rhombohedral phase transition in vanadium metal M. G. Stevenson , E. J. Pace , C. V. Storm , S. E. Finnegan , G. Garbarino , C. W. Wilson , D. Mcgonegle , S. G. Macleod , M. I. Mcmahon Physical Review B 103 DOI: 10.1103/PhysRevB.103.134103 Show Abstract ▼ Abstract: Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30–70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated.	Apr 2021
I15-Extreme Conditions	The phase diagram of Ti-6Al-4V at high-pressures and high-temperatures Simon Macleod , Daniel Errandonea , Geoffrey Adam Cox , Hyunchae Cynn , Dominik Daisenberger , Sarah Finnegan , Malcolm Mcmahon , Keith Munro , Catalin Popescu , Christian Storm Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/abdffa Diamond Proposal Number(s): [8176, 9366] Show Abstract ▼ Abstract: We report results from a series of diamond-anvil-cell synchrotron X-ray diffraction and largevolume- press experiments, and calculations, to investigate the phase diagram of commercial polycrystalline high-strength Ti-6Al-4V alloy in pressure-temperature space. Up to ~30 GPa and 886 K, Ti- 6Al-4V is found to be stable in the hexagonal-close-packed, or alpha phase. The effect of temperature on the volume expansion and compressibility of alpha-Ti-6Al-4V is modest. The martensitic alpha→omega (hexagonal) transition occurs at ~30 GPa, with both phases coexisting until further compression to ~38-40 GPa completes the transition to the omega phase. Between 300 K and 844 K the alpha→omega transition appears to be independent of temperature. Omega-Ti-6Al-4V is stable to ~91 GPa and 844 K, the highest combined pressure and temperature reached in these experiments. Pressure-volume-temperature equations-of-state for the alpha and omega phases of Ti- 6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in the omega-Ti-6Al-4V on decompression, with the hexagonal structure reverting back to the  alpha phase at pressures below ~9 GPa at room temperature, and at a higher pressure at elevated temperatures. Based on our data, we estimate the Ti-6Al-4V alpha-beta-omega triple point to occur at ~900 K and 30 GPa, in good agreement with our calculations.	Jan 2021
I15-Extreme Conditions	Structural phase transitions in yttrium up to 183 GPa E. J. Pace , S. E. Finnegan , C. V. Storm , M. Stevenson , M. I. Mcmahon , S. G. Macleod , E. Plekhanov , N. Bonini , C. Weber Physical Review B 102 DOI: 10.1103/PhysRevB.102.094104 Show Abstract ▼ Abstract: Angle-dispersive x-ray powder diffraction experiments have been performed on yttrium metal up to 183 GPa. We find that the recently discovered o F 16 structure observed in the high- Z trivalent lanthanides is also adopted by yttrium above 106 GPa, pressures where it has a superconducting temperature of ∼ 20 K. We have also refined both tetragonal and rhombohedral structures against the diffraction data from the preceding “distorted-fcc” phase and we are unable to state categorically which of these is the true structure of this phase. Finally, analysis of yttrium's equation of state reveals a marked change in the compressibility upon adoption of the o F 16 structure, after which the compression is that of a “regular” metal. Electronic structure calculations of o F 16 -Y confirm its stability over o F 8 structure seen in Nd and Sm, and provide insight into the nature of the shift of orbital character from s to d under compression.	Sep 2020
I15-Extreme Conditions	High-pressure structural systematics in samarium up to 222 GPa S. E. Finnegan , E. J. Pace , C. V. Storm , M. I. Mcmahon , S. G. Macleod , H.-P. Liermann , K. Glazyrin Physical Review B 101 DOI: 10.1103/PhysRevB.101.174109 Diamond Proposal Number(s): [22350, 20311] Show Abstract ▼ Abstract: Angle-dispersive x-ray powder diffraction experiments have been performed on samarium metal up to 222 GPa. Up to 50 GPa we observe the Sm type (hR9) → dhcp (hP4) → fcc (cF4) → distorted fcc (hR24) → hP3 transition sequence reported previously. The structure of the high-pressure phase above 93 GPa, previously reported as having a monoclinic structure with space group C2/m, is found to be orthorhombic, space group Fddd, with eight atoms per unit cell (oF8 in Pearson notation). This structure is the same as that found in Am, Cm, and Cf at high pressures. Analysis of samarium's equation of state reveals marked changes in compressibility in the hP3 and oF8 phases, with the compressibility of the oF8 phase being that of a “regular” metal.	May 2020
I15-Extreme Conditions	Structural ordering in liquid gallium under extreme conditions James W. E. Drewitt , Francesco Turci , Benedict J. Heinen , Simon G. Macleod , Fei Qin , Annette K. Kleppe , Oliver T. Lord Physical Review Letters 124 DOI: 10.1103/PhysRevLett.124.145501 Diamond Proposal Number(s): [18961] Open Access Show Abstract ▼ Abstract: The atomic-scale structure, melting curve, and equation of state of liquid gallium has been measured to high pressure ( p ) and high temperature ( T ) up to 26 GPa and 900 K by in situ synchrotron x-ray diffraction. Ab initio molecular dynamics simulations up to 33.4 GPa and 1000 K are in excellent agreement with the experimental measurements, providing detailed insight at the level of pair distribution functions. The results reveal an absence of dimeric bonding in the liquid state and a continuous increase in average coordination number ¯ n Ga Ga from 10.4(2) at 0.1 GPa approaching ∼ 12 by 25 GPa. Topological cluster analysis of the simulation trajectories finds increasing fractions of fivefold symmetric and crystalline motifs at high p − T . Although the liquid progressively resembles a hard-sphere structure towards the melting curve, the deviation from this simple description remains large ( ≥ 40 % ) across all p − T space, with specific motifs of different geometries strongly correlating with low local two-body excess entropy at high p − T .	Apr 2020
I15-Extreme Conditions	The high-pressure, high-temperature phase diagram of cerium Keith Munro , Dominik Daisenberger , Simon Macleod , Steve Mcguire , Ingo Loa , Catalin Popescu , Pablo Botella , Daniel Errandonea , Malcolm Mcmahon Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/ab7f02 Diamond Proposal Number(s): [9548, 7533] Open Access Show Abstract ▼ Abstract: We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to $\sim$22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic \textit{oC}4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the $\alpha$-\textit{cF}4 -- \textit{oC}4 -- \textit{tI}2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the \textit{oC}4 phase, and 1-2 GPa lower than previously reported. We find the $\alpha$-\textit{cF}4 $\rightarrow$ \textit{tI}2 phase boundary to have a positive gradient of 280 K/GPa, less steep than the 670 K/GPa reported previously, and find the \textit{oC}4 $\rightarrow$ \textit{tI}2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2-3 GPa in the transition pressures at which the \textit{oC}4 $\rightarrow$ \textit{tI}2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the $\alpha$-\textit{cF}4 $\rightarrow$ \textit{tI}2 is not second order at all temperatures up to 820 K.	Mar 2020
I15-Extreme Conditions	Thermal equation of state of ruthenium characterized by resistively heated diamond anvil cell Simone Anzellini , Daniel Errandonea , Claudio Cazorla , Simon Macleod , Virginia Monteseguro , Silvia Boccato , Enrico Bandiello , Daniel Diaz Anichtchenko , Catalin Popescu , Christine M. Beavers Scientific Reports 9 DOI: 10.1038/s41598-019-51037-8 Diamond Proposal Number(s): [23122] Open Access Show Abstract ▼ Abstract: The high-pressure and high-temperature structural and chemical stability of ruthenium has been investigated via synchrotron X-ray diffraction using a resistively heated diamond anvil cell. In the present experiment, ruthenium remains stable in the hcp phase up to 150 GPa and 960 K. The thermal equation of state has been determined based upon the data collected following four different isotherms. A quasi-hydrostatic equation of state at ambient temperature has also been characterized up to 150 GPa. The measured equation of state and structural parameters have been compared to the results of ab initio simulations performed with several exchange-correlation functionals. The agreement between theory and experiments is generally quite good. Phonon calculations were also carried out to show that hcp ruthenium is not only structurally but also dynamically stable up to extreme pressures. These calculations also allow the pressure dependence of the Raman-active E2g mode and the silent B1g mode of Ru to be determined.	Oct 2019

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