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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	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	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	Structure and magnetism of collapsed lanthanide elements M. I. Mcmahon , S. Finnegan , R. J. Husband , K. A. Munro , E. Plekhanov , N. Bonini , C. Weber , M. Hanfland , U. Schwarz , S. G. Macleod Physical Review B 100 DOI: 10.1103/PhysRevB.100.024107 Show Abstract ▼ Abstract: Using synchrotron x-ray diffraction, we show that the long-accepted monoclinic structure of the “collapsed” high-pressure phases reported in seven lanthanide elements [Nd, Tb, Gd, Dy, Ho, Er, and (probably) Tm] is incorrect. In Tb, Gd, Dy, Ho, Er, and Tm we show that the collapsed phases have a 16-atom orthorhombic structure ( o F 16 ) not previously seen in the elements, whereas in Nd we show that it has an eight-atom orthorhombic structure ( o F 8 ) previously reported in several actinide elements. o F 16 and o F 8 are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides, and group-III elements (Sc and Y) are much more related than previously imagined. Electronic structure calculations of Tb, combined with quantum many-body corrections, confirm the experimental observation, and calculate that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an antiferromagnetic state between 60 and 80 GPa. We find that the magnetic properties of Tb survive to the highest pressures obtained in our experiments (110 GPa). Further calculations of the collapsed phases of Gd and Dy, again using the correct crystal structure, show the former to be a type-A antiferromagnet, whereas the latter is ferromagnetic.	Jul 2019
I15-Extreme Conditions	Phase diagram of calcium at high pressure and high temperature S. Anzellini , D. Errandonea , Simon Macleod , P. Botella , D. Daisenberger , J. M. De’ath , J. Gonzalez-Platas , J. Ibáñez , M. I. Mcmahon , K. A. Munro , C. Popescu , J. Ruiz-Fuertes , C. W. Wilson Physical Review Materials 2 DOI: 10.1103/PhysRevMaterials.2.083608 Diamond Proposal Number(s): [15864] Show Abstract ▼ Abstract: Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model.	Aug 2018
I15-Extreme Conditions	One-dimensional chain melting in incommensurate potassium E. E. Mcbride , K. A. Munro , G. W. Stinton , R. J. Husband , R. Briggs , H.-P. Liermann , M. I. 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	Incommensurate-to-incommensurate phase transition in Eu metal at high pressures R. J. Husband , I. Loa , K. A. Munro , E. E. Mcbride , S. R. Evans , H. P Liermann , M. I. Mcmahon Physical Review B 90 (21), 10-15 DOI: 10.1103/PhysRevB.90.214105 Show Abstract ▼ Abstract: High pressure x-ray powder-diffraction experiments were performed on europium metal up to about 70GPa. Above 38 GPa, europium transforms from the incommensurately modulated Eu-IV phase to a second phase with an incommensurately modulated crystal structure, Eu-V. This is a previously unseen incommensurately modulated to incommensurately modulated transition in the elements at high pressure. High-pressure high-temperature experiments were also performed up to 449 K in order to make an initial estimate of the positions of the phase boundaries of the incommensurate phases.	Dec 2014
I15-Extreme Conditions	The distorted-fcc phase of samarium Rachel Husband , Ingo Loa , Keith Munro , Malcolm Mcmahon Journal Of Physics: Conference Series 500 18th APS-SCCM and 24th AIRAPT DOI: 10.1088/1742-6596/500/3/032009 Open Access Show Abstract ▼ Abstract: Angle-dispersive x-ray powder diffraction experiments were performed on samarium metal up to 50 GPa. We report that the high-pressure distorted-fcc phase has the same hR24 structure observed in praseodymium, but, unlike praseodymium, samarium does not undergo a transition to a second distorted-fcc phase. We also report a path-dependent transition to a new phase, the structure of which has not yet been determined, which occurs on annealing followed by pressure decrease.	May 2014

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