Dongzhou Zhang

Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites

Significance The emergence of organic–inorganic hybrid lead triiodide perovskite materials promises a low-cost and high-efficiency photovoltaic technology. Although the high-power conversion efficiency of this technology has been successfully demonstrated, further improvement appears to be limited without further narrowing the band gap while also retaining or even synergistically prolonging the carrier lifetime. We report a synergistic enhancement in both band gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) of organic–inorganic hybrid lead triiodide perovskite materials under mild pressures below ∼0.3 GPa. This work could open new territory in materials science, and new materials could be invented using the experimental and theoretical guidelines we have established herein. The organic–inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley–Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon–electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.

Dehydrogenation of goethite in Earth’s deep lower mantle

Significance We found at high pressure–temperature (P-T) that the goethite FeO2H transforms to P-phase FeO2 via a two-step dehydrogenation process. First it releases some hydrogen to form P-phase FeO2Hx, and then it continuously releases the remaining hydrogen through prolonged heating. This work provides an important example that the dehydration reaction changes to dehydrogenation of FeO2H at the lower mantle conditions and the cycles of hydrogen and water become separated. The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.

High Pressure Single Crystal Diffraction at PX^2

In this report we describe detailed procedures for carrying out single crystal X-ray diffraction experiments with a diamond anvil cell (DAC) at the GSECARS 13-BM-C beamline at the Advanced Photon Source. The DAC program at 13-BM-C is part of the Partnership for Extreme Xtallography (PX^2) project. BX-90 type DACs with conical-type diamond anvils and backing plates are recommended for these experiments. The sample chamber should be loaded with noble gas to maintain a hydrostatic pressure environment. The sample is aligned to the rotation center of the diffraction goniometer. The MARCCD area detector is calibrated with a powder diffraction pattern from LaB6. The sample diffraction peaks are analyzed with the ATREX software program, and are then indexed with the RSV software program. RSV is used to refine the UB matrix of the single crystal, and with this information and the peak prediction function, more diffraction peaks can be located. Representative single crystal diffraction data from an omphacite (Ca0.51Na0.48)(Mg0.44Al0.44Fe2+0.14Fe3+0.02)Si2O6 sample were collected. Analysis of the data gave a monoclinic lattice with P2/n space group at 0.35 GPa, and the lattice parameters were found to be: a = 9.496 ±0.006 Å, b = 8.761 ±0.004 Å, c = 5.248 ±0.001 Å, β = 105.06 ±0.03º, α = γ = 90º.

Ice-VII inclusions in Diamonds: Evidence for Aqueous Fluid in Earth’s Deep Mantle

Encapsulating Earths deep water filter Small inclusions in diamonds brought up from the mantle provide valuable clues to the mineralogy and chemistry of parts of Earth that we cannot otherwise sample. Tschauner et al. found inclusions of the high-pressure form of water called ice-VII in diamonds sourced from between 410 and 660 km depth, the part of the mantle known as the transition zone. The transition zone is a region where the stable minerals have high water storage capacity. The inclusions suggest that local aqueous pockets form at the transition zone boundary owing to the release of chemically bound water as rock cycles in and out of this region. Science, this issue p. 1136 The presence of ice-VII in diamond inclusions requires regions of the mantle with a free aqueous phase. Water-rich regions in Earth’s deeper mantle are suspected to play a key role in the global water budget and the mobility of heat-generating elements. We show that ice-VII occurs as inclusions in natural diamond and serves as an indicator for such water-rich regions. Ice-VII, the residue of aqueous fluid present during growth of diamond, crystallizes upon ascent of the host diamonds but remains at pressures as high as 24 gigapascals; it is now recognized as a mineral by the International Mineralogical Association. In particular, ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and around the 660-kilometer boundary.

Isosymmetric pressure-induced bonding increase changes compression behavior of clinopyroxenes across jadeite-aegirine solid solution in subduction zones

National Science Foundation-Geosciences [EAR-1128799]; Department of Energy-Geosciences [DE-FG02-94ER14466]; COMPRES under NSF [EAR 11-57758]; GSECARS; National Science Foundation [EAR-1344942]; National Natural Science Foundation of China [41374107]; NSF [EAR-1440005]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Chinese Academy of Sciences [XDB 18010401]; Graduate Student Joint Training Program of the Institute of Geochemistry, Chinese Academy of Sciences

Single-crystal equations of state of magnesiowüstite at high pressures

Abstract Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg0.22Fe0.78)O magnesiowüstite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mössbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally coordinated ferric iron, resulting in a renormalized composition of (Mg0.215Fe0.762□0.023)O. Both high-pressure diffraction data sets are well described by a third-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K0T = 148(3) GPa, K′0T = 4.09(12), and V0 = 78.87(6) Å3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equations of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the data sets using a helium pressure medium: K0T decreases as a function of increasing iron content from periclase to wüstite and K′0T is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to the iron concentration measured here. In combination with previously reported thermal parameters, we compute the density of magnesiowüstite with this composition at core-mantle boundary conditions and discuss the implications.

Valence and spin states of iron are invisible in Earth’s lower mantle

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg that Fe3+ in the octahedral site undergoes a spin transition between 43 and 53u2009GPa at 300u2009K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.Bridgmanite is the most abundant mineral in the lower mantle and therefore is crucial to interpreting geophysical observations and models. Here, the authors show that ferric-iron-only bridgmanite Fe3+ undergoes a spin transition at 43–53u2009GPa at 300u2009K and therefore has implications for mantle structure and dynamics.

High-Pressure Geophysical Properties of Fcc Phase FeHX

Face centered cubic (fcc) FeHX was synthesized at pressures of 18–68 GPa and temperatures exceeding 1,500 K. Thermally quenched samples were evaluated using synchrotron X-ray diffraction (XRD) and nuclear resonant inelastic X-ray scattering (NRIXS) to determine sample composition and sound velocities to 82 GPa. To aid in the interpretation of nonideal (X 61⁄4 1) stoichiometries, two equations of state for fcc FeHX were developed, combining an empirical equation of state for iron with two distinct synthetic compression curves for interstitial hydrogen. Matching the density deficit of the Earth’s core using these equations of state requires 0.8–1.1 wt % hydrogen at the core-mantle boundary and 0.2–0.3 wt % hydrogen at the interface of the inner and outer cores. Furthermore, a comparison of Preliminary Reference Earth Model (PREM) to a Birch’s law extrapolation of our experimental results suggests that an iron alloy containing 0.8–1.3 wt % hydrogen could reproduce both the density and compressional velocity (VP) of the Earth’s outer core.

Experimental evidence for the survival of augite to transition zone depths, and implications for subduction zone dynamics

Abstract (Ca, Mg)-rich clinopyroxenes are abundant in Earth’s upper mantle and subduction zones. Experimental studies on the thermoelastic properties of these minerals at simultaneous high pressure and high temperature are important for constraining of the composition and structure of the Earth. Here, we present a synchrotron-based single-crystal X-ray diffraction study of natural diopside-dominated augite [(Ca0.89Na0.05Mg0.06)(Mg0.74Fe0.11Al0.14Ti0.01)(Si1.88Al0.12) O6.00] at P and T to ~27 GPa and 700 K. The experiment simulates conditions in cold subducting slabs, and results indicate that augite is stable over this pressure and temperature range. A third-order high-temperature Birch-Murnaghan equation was fit with the pressure-volume-temperature data, yielding the following thermoelastic parameters: KT0 = 111(1) GPa, KT0′

Compressional behavior of natural eclogitic zoisite by synchrotron X-ray single-crystal diffraction to 34 GPa

K_{text T0}