I15-Extreme Conditions
|
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.
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Sep 2020
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I15-Extreme Conditions
|
Diamond Proposal Number(s):
[22350, 20311]
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.
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May 2020
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|
I15-Extreme Conditions
|
Diamond Proposal Number(s):
[18961]
Open Access
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
|
Diamond Proposal Number(s):
[9548, 7533]
Open Access
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.
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Mar 2020
|
|
I15-Extreme Conditions
|
Diamond Proposal Number(s):
[23122]
Open Access
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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|
I15-Extreme Conditions
|
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.
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Jul 2019
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|
I15-Extreme Conditions
|
Remo N.
Widmer
,
Giulio I.
Lampronti
,
Siwar
Chibani
,
Craig
Wilson
,
Simone
Anzellini
,
Stefan
Farsang
,
Annette K.
Kleppe
,
Nicola P. M.
Casati
,
Simon
Macleod
,
Simon A. T.
Redfern
,
François-xavier
Coudert
,
Thomas D.
Bennett
Diamond Proposal Number(s):
[19046]
Abstract: We present an in-situ powder X-ray diffraction study on the phase stability and polymorphism of the metal-organic framework ZIF-4, Zn(Imidazolate)2, at simultaneous high-pressure and high-temperature, up to 8 GPa and 600 °C. The resulting pressure-temperature phase diagram reveals four, previously unknown, high-pressure-temperature ZIF phases. The crystal structures of two new phases – ZIF-4-cp-II and ZIF-hPT-II – were solved by powder diffraction methods. The total energy of ZIF-4-cp-II was evaluated using density functional theory calculations and was found to lie in between that of ZIF-4 and the most thermodynamically stable polymorph, ZIF-zni. ZIF-hPT-II was found to possess a doubly-interpenetrated diamondoid-topology and is isostructural with previously reported Cd(Imidazolate)2 and Hg(Imidazolate)2 phases. This phase exhibited extreme resistance to both temperature and pressure. The other two new phases could be assigned with a unit cell and space group, though their structures remain unknown. The pressure-temperature phase diagram of ZIF-4 is strikingly complicated when compared with that of the previously investigated, closely related ZIF-62, and demonstrates the ability to traverse complex energy landscapes of metal-organic systems using the combined application of pressure and temperature.
|
May 2019
|
|
I15-Extreme Conditions
|
Remo N.
Widmer
,
Giulio I.
Lampronti
,
Simone
Anzellini
,
Romain
Gaillac
,
Stefan
Farsang
,
Chao
Zhou
,
Ana M.
Belenguer
,
Craig
Wilson
,
Hannah
Palmer
,
Annette K.
Kleppe
,
Michael T.
Wharmby
,
Xiao
Yu
,
Seth M.
Cohen
,
Shane G.
Telfer
,
Simon A. T.
Redfern
,
François-xavier
Coudert
,
Simon
Macleod
,
Thomas
Bennett
Diamond Proposal Number(s):
[16133, 19046]
Abstract: Metal–organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure–temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure.
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Mar 2019
|
|
I15-Extreme Conditions
|
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
Diamond Proposal Number(s):
[15864]
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
|
Diamond Proposal Number(s):
[19846]
Open Access
Abstract: The phase diagram of zinc (Zn) has been explored up to 140 GPa and 6000 K, by combining optical observations, x-ray diffraction, and ab-initio calculations. In the pressure range covered by this study, Zn is found to retain a hexagonal close-packed (hcp) crystal symmetry up to the melting temperature. The known decrease of the axial ratio (c/a) of the hcp phase of Zn under compression is observed in x-ray diffraction experiments from 300 K up to the melting temperature. The pressure at which c/a reaches √3 (≈10 GPa) is not affected by temperature. When this ideal axial ratio is reached, we observed that single crystals of Zn, formed at high temperature, break into multiple poly-crystals. In addition, a noticeable change in the pressure dependence of c/a takes place at the same pressure. Both phenomena can be caused by an isomorphic second-order phase transition induced by pressure in Zn. The reported melt curve extends previous results from 24 to 135 GPa. The pressure dependence obtained for the melting temperature is accurately described by using a Simon–Glatzel equation. The determined melt curve agrees with previous low-pressure studies and with shock-wave experiments, with a melting temperature of 5060(30) K at 135 GPa. Finally, a thermal equation of state is reported, which at room-temperature agrees with the literature.
|
Jun 2018
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