I15-Extreme Conditions
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Diamond Proposal Number(s):
[20310, 13678, 11768, 9873, 9696]
Abstract: The pressure-induced structural phase transitions in the lanthanide elements provide insight into changes in their electronic structures at high densities. After a series of transitions via close-packed structures, the regular trivalent lanthanides (La to Lu, excluding Ce, Eu and Yb) undergo first-order transitions to so-called collapsed phases, the structures of which have long been reported as monoclinic. However, the diffraction data from these phases are not well fitted by this monoclinic structure, and the patterns from Nd and Sm are distinctly different to those from the higher-Z lanthanides (Gd→). Here we present results from recent diffraction studies on Tb, Gd, Sm, Nd and Y to above 300 GPa, which reveal that there are two different collapsed structures, neither of which is monoclinic. High-precision equation of state studies of the same elements reveal distinct changes in compressibility once the collapsed phases are obtained. We also show that these new structures are strikingly similar to those observed in the higher-Z actinides at high pressure, greatly strengthening the structural systematics of the 4f and 5f elements.
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Sep 2022
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[11768]
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.
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Oct 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[22350]
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.
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Jun 2021
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I15-Extreme Conditions
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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.
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Apr 2021
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I15-Extreme Conditions
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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.
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Apr 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[8176, 9366]
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.
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Jan 2021
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I15-Extreme Conditions
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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
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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
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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
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I15-Extreme Conditions
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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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