152 research outputs found
Revisiting spin state crossover in (MgFe)O by means of high resolution X-ray diffraction from a single crystal
(MgFe)O is a solid solution with ferrous iron undergoing the high to low spin
state (HS-LS) crossover under high pressure. The exact state of the material in
the region of the crossover is still a mystery, as domains with different spin
states may coexist over a wide pressure range without changing the crystal
structure neither from the symmetry nor from the atomic positions point of
view. At the conditions of the crossover, (MgFe)O is a special type of
microscopic disorder system. We explore the influences of (a) stress-strain
relations in a diamond anvil cell, (b) time relaxation processes, and (c) the
crossover itself on the characteristic features of a single crystal (111) Bragg
spot before, during and after the transformation. Using high resolution X-ray
diffraction as a novel method for studies of unconventional processes at the
conditions of suppressed diffusion, we detect and discuss subtle changes of the
(111) Bragg spot projections which we measure and analyze as a function of
pressure. We report changes of the spot shape which can be correlated with the
HS-LS relative abundance. In addition, we report the formation of structural
defects as an intrinsic material response. These static defects are accumulated
during transformation of the material from HS to LS.Comment: 28 pages, 11 Figure
Isostructural phase transition in Tb2Ti2O7 under pressure and temperature: Insights from synchrotron X-ray diffraction
Tb2Ti2O7, a pyrochlore system, has garnered significant interest due to its
intriguing structural and physical properties and their dependence on external
physical parameters. In this study, utilizing high-brilliance synchrotron X-ray
diffraction, we conducted a comprehensive investigation of structural evolution
of Tb2Ti2O7 under external pressure and temperature. We have conclusively
confirmed the occurrence of an isostructural phase transition beyond the
pressure of 10 GPa. The transition exhibits a distinct signature in the
variation of lattice parameters under pressure and leads to changes in
mechanical properties. The underlying physics driving this transition can be
understood in terms of localized rearrangement of atoms while retaining the
overall cubic symmetry of the crystal. Notably, the observed transition remains
almost independent of temperature. Our findings provide insights into the
distinctive behaviour of the isostructural phase transition in Tb2Ti2O7
Compression experiments to 126 GPa and 2500 K and thermal equation of state of Fe3S: Implications for sulphur in the Earth’s core
Pressure-volume-temperature (P-V-T) experiments on tetragonal Fe3S were conducted to 126 GPa and 2500 K in laser-heated diamond anvil cells (DAC) with in-situ X-ray diffraction (XRD). Seventy nine high-T data as well as four 300-K data were collected, based on which new thermal equations of state (EoS) for Fe3S were established. The room-T data together with existing data were fitted to the third order Birch-Murnaghan EoS, which yielded, GPa and with fixed at 377.0 Å3. A constant term in the thermal pressure equation, Pth = , fitted the high-T data well to the highest temperature, which implies that the contributions from the anharmonic and electronic terms should be minor in the thermal pressure term. The high-T data were also fitted to the Mie-Grüneisen-Debye model; with and q fixed at 417 K and 1 respectively. Calculations from the EoS show that crystalline Fe3S at 4000-5500 K is denser than the Earth's outer core and less dense than the inner core. Assuming a density reduction due to melting, liquid Fe3S would meet the outer core density profile, which however suggests that no less than 16 wt%S is needed to reconcile the observed outer core density deficit. The S-rich B2 phase, which was suggested to be a potential liquidus phase of an Fe3S-outer core above 250 GPa, namely the main constituent of its solid inner core, would likely be less dense than the Earth's inner core. As such, while the outer core density requires as much sulphur as 16 wt%, the resulting liquidus phase cannot meet the density of the inner core. Any sulphur-rich composition should therefore be rejected for the Earth's core
Novel hydrogen clathrate hydrate
We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K
documented by single-crystal X-ray diffraction, Raman spectroscopy, and
first-principles calculations. The oxygen sublattice of the new clathrate
hydrate matches that of ice II, while hydrogen molecules are in the ring
cavities, which results in the trigonal R3c or R-3c space group (proton ordered
or disordered, respectively) and the composition of (H2O)6H2. Raman
spectroscopy and theoretical calculations reveal a hydrogen disordered nature
of the new phase C1', distinct from the well-known ordered C1 clathrate, to
which this new structure transforms upon compression and/or cooling. This new
clathrate phase can be viewed as a realization of a disordered ice II,
unobserved before, in contrast to all other ordered ice structures.Comment: 9 pages, 4 figures, 1 table; Supplementary materials: Materials and
Methods, Supplementary Figures S1-S8, Tables S1-S3, and Bibliography with 18
Reference
- …