Earth science: Crystallography's journey to the deep Earth

Duffy, Thomas S.

doi:10.1038/506427a

Cited by 13 publications

(12 citation statements)

References 11 publications

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“…In transitions driven by changing the temperature at ambient pressure, strain effects are usually small enough to warrant a harmonic treatment of the elastic contributions to the Landau potential (LP), and this approach also works for transitions driven by moderate external stress. However, the last two decades have seen a rapid improvement of the experimental capabilities for studying materials at extreme pressures [9]. It has become routine to control stresses that frequently represent sizable fractions of the elastic constants of the materials investigated.…”

Section: Introductionmentioning

confidence: 99%

Finite-strain Landau theory applied to the high-pressure phase transition of lead titanate

et al. 2017

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Standard Landau theory coupled to infinitesimal strain allows a concise description of the temperature-driven ferroelectric tetragonal-to-cubic phase transition in PbTiO 3 at ambient pressure. Unfortunately, it fails to cover its high-pressure counterpart at ambient temperature. For example, the experimental transition pressure is vastly underestimated, and neither the change from first to second order with increasing pressure nor the unusual pressure dependence of the tetragonal unit cell parameters observed in experiment are reproduced. Here we demonstrate that a combination of density functional theory and a recently constructed finite-strain extension of Landau theory provides a natural mechanism for resolving these discrepancies between theory and experiment. Our approach also allows us to determine the full tetragonal-cubic phase boundary in the (P ,T) plane including an estimate of the tricritical point. We show that a careful analysis of the thermal elastic baseline is an essential ingredient to the success of this theory.

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Section: Introductionmentioning

confidence: 99%

Finite-strain Landau theory applied to the high-pressure phase transition of lead titanate

et al. 2017

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“…With the development of new synchrotron techniques, it is now possible to more routinely carry out compression experiments at 300 K using single-crystal diffraction on low-symmetry silicates (Dera et al 2013b;Duffy 2014). At these low temperatures, equilibrium reconstructive transitions are kinetically inhibited.…”

Section: Resultsmentioning

confidence: 99%

High-pressure phases of cordierite from single-crystal X-ray diffraction to 15 GPa

Finkelstein¹,

Dera²,

Duffy³

2015

American Mineralogist

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High-pressure single-crystal X-ray diffraction experiments were conducted on natural cordierite crystals with composition Mg 1.907(18) Fe 0.127(6) Al 4.01(2) Si 4.96(3) Na 0.026(3) O 18.12(9) using a synchrotron X-ray source. The samples were compressed at 300 K in a diamond-anvil cell to a maximum pressure of 15.22(15) GPa with a neon pressure-transmitting medium and a gold pressure calibrant. We observed a recently described orthorhombic to phase transition, as well as a further transition to a second triclinic phase. We solved and refined both new triclinic phases in space group P1, and designate them cordierite II and III. The structures of cordierite II and III were refined at 7.52 (3) and 15.22(15) GPa, respectively. The lattice parameters at these pressures are a = 15.567(3), b = 9.6235(4), c = 9.0658(6) Å, a = 89.963(5)°, b = 86.252(10)°, and g = 90.974(8)° for cordierite II, and a = 8.5191(19), b = 8.2448(3), c = 9.1627(4) Å, a = 85.672(4)°, b = 85.986(7)°, and g = 70.839(10)° for cordierite III.Across the phase transitions there is a significant reduction in the length of the a-axis (~2 Å per phase transition), whereas both the b-and c-axis remain largely unchanged. Cordierite II has fourfold-and fivefold-coordinated Si and Al, while cordierite III has fourfold-, fivefold-, and sixfold-coordinated Si, fourfold-and fivefold-coordinated Al, and fivefold-and sixfold-coordinated Mg. The sequence of high-pressure phases shows increasing polymerization of coordination polyhedra. These results, together with other recent studies, suggest that mixed four-, five-, and sixfold coordination states may occur more commonly in silicate structures compressed at 300 K than previously recognized.

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“…5,7-9]. As recently emphasized by Duffy [10], perovskites have also received considerable attention in Earth Sciences, since it was discovered that the main minerals in the upper mantle (i.e. olivine, pyroxene, and garnet) undergo a series of phase transitions to the denser (Mg,Fe)SiO 3 perovskite structure (s.g. Pbnm), at upper/lower mantle interface pressure and temperature conditions [11,12].…”

Section: Introductionmentioning

confidence: 98%

Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI)

Ardit

2015

Journal of Physics and Chemistry of Solids

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Earth science: Crystallography's journey to the deep Earth

Cited by 13 publications

References 11 publications

Finite-strain Landau theory applied to the high-pressure phase transition of lead titanate

Finite-strain Landau theory applied to the high-pressure phase transition of lead titanate

High-pressure phases of cordierite from single-crystal X-ray diffraction to 15 GPa

Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI)

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