Site 4b Research Articles

High pressure structural evolution of cubic solid solution YbInO3

The structural evolution of cubic (c) solid solution YbInO3 (space group Ia3¯) under pressure has been investigated using synchrotron based angle dispersive x-ray diffraction measurements and density functional theory based first principles calculations. A first order irreversible structural phase transition is observed at ∼15 GPa. The ambient cubic phase coexists with the high pressure phase up to the highest pressure achieved in the experiment, i.e., 42 GPa. Based on experimental and theoretical investigations, the high pressure phase of c-YbInO3 is proposed to be an orthorhombic solid solution structure having the space group Pnma. In the high pressure Pnma phase, the coordination of Yb1/In1 (general site 4c) atoms with respect to O atoms increases from six to eight, while Yb2/In2 (special site 4b) atoms remain in six coordination. Our theoretical calculations show that distorted octahedra (YbO6 and InO6), sitting at the general position in c-YbInO3, change its geometry toward regular octahedra with increasing pressure, which gives rise to the phase transformation.

Temperature evolution of 3d- and 4f-electron magnetic ordering in the ferrimagnetic Mn self-doped perovskite (Yb0.667Mn0.333)MnO3

A high-pressure synthesis method was employed to prepare Mn-self-doped perovskites (R 0.667Mn0.333)MnO3 (R = Yb, Lu) at about 6 GPa and 1670 K. Crystal and magnetic structures of (Yb0.667Mn0.333)MnO3 have been studied by combining neutron powder diffraction, magnetic susceptibility and specific heat measurements. Within the orthorhombic space group Pnma, magnetic cations are located on site 4c (A site, occupied by two thirds of Yb3+ and one third of Mn2+) and on site 4b (B site, occupied by two thirds of Mn3+ and one third of Mn4+). The degree of structural distortion of the MnO6 octahedra follows the general trend of (R1−x Mn x )MnO3 compounds which shows a decrease with increasing amount of Jahn–Teller inactive Mn4+ cations. Mn–Mn interactions produce a collinear ferrimagnetic structure (T C,Mn = 106 K) with ferromagnetically ordered Mn moments at the B site being coupled antiferromagnetically with ordered Mn moments at the A site. Mn–Yb interactions induce a small but non-zero ferromagnetic Yb3+ moment which can explain a small decrease of the magnetic susceptibility at low temperature. Yb–Yb interactions create an antiferromagnetic structure at T N,Yb ≈ 40 K. Ordered moments of the ferrimagnetic and antiferromagnetic structures are oriented perpendicular to each other within the ac-plane and Yb3+ moments contribute to both structures. The appearance of ordered Yb3+ moments induced by Mn–Yb interactions in perovskite (Yb0.667Mn0.333)MnO3 is a result of the Mn self-doping on the A site and has not been observed in the orthorhombic perovskite modification (space group Pnma) of the undoped parent compound YbMnO3, but interestingly, it also appears in the hexagonal non-perovskite modification (space group P63 cm) of YbMnO3.

Nanostructured Crystals of Fluorite Phases Sr1–xRxF2+x (R Are Rare-Earth Elements) and Their Ordering. 16: Defect Structure of the Nonstoichiometric Phases Sr1–xRxF2+x (R = Pr, Tb–Yb) As Grown

The defect structure of as grown Sr0.72Pr0.28F2.28, Sr0.87Tb0.13F2.13, Sr0.88Dy0.12F2.12, Sr0.89Ho0.11F2.11, and Sr0.9R0.1F2.1 single crystals (R = Y, Er, Tm, Yb) prepared under identical conditions has been studied by X-ray diffraction analysis. All crystals belong to the CaF2 structure type (sp. gr. $$Fm\bar {3}m$$ ) and have vacancies in the main anion motif and interstitial fluorine anions at site 32f in the crystal with R = Pr and site 48i for the other R. Interstitial fluorine anions are found near site 4b in the crystals with R = Tb and Dy (sites 32f and 24e, respectively) and at site 4b in the crystal with R = Er. Relaxation (static displacement of some main anions to site 32f) is observed in all crystals (except for R = Tb). Displacements of some Sr2+ cations to sites 32f and 24e is simultaneously observed in crystals with R = Tb, Dy, Tm, and Yb. In crystals with R = Ho, Y, and Er, some Sr2+ cations are displaced only to site 32f; no cation displacement is observed for R = Pr. A model of the defect structure of the phases is proposed based on the structure of crystals studied, according to which interstitial fluorine anions and R3+ cations are aggregated into [Sr4–nRnF26] clusters of tetrahedral configuration for the phase with R = Pr or [Sr14–nRnF64+n] clusters of octacubic configuration for other R.

DFT study of Li2NiTiO4 and vanadium-doped Li2NiTiO4

Density functional theory study of Li2NiTiO4 and vanadium-doped Li2NiTiO4 are performed for understanding their structural and electrochemical phenomena, viz., structural geometry like lattice parameters, change in lattice volume with Li+ extraction, Li+ de-intercalation voltage, electrochemical capacity etc. Li2NiTiO4 has cubic symmetry with space group Fm3m (space group number 225). De-intercalated structure of LiNiTiO4 is obtained by removing one Li atom from Li2NiTiO4 unit cell while shifting another Li atom from octahedral site 4b to tetrahedral lattice site 8c. Due to Li+ extraction, the change in unit cell volume is ~ 5.9%. Li+ de-intercalation voltage is calculated by subtracting total energy of the unit cell of LiNiTiO4 and bcc Li from Li2NiTiO4. The voltage comes out to be 4.84 V which is nearly at the threshold for electrochemical stability of used electrolytes. The redox couples in this case are Ni+2/Ni2 + δ and O−2/O−2+γ. The presence of redox couple O−2/O−2+γ leads to the probability of evolution of oxygen during charging. However, partial (50% in this case) vanadium doping at Ti site reduces the redox voltage to 4.64 V considering the de-intercalation reaction forming LiNiTi0.5V0.5O4 from Li2NiTi0.5V0.5O4, by activating the redox couple V+3/V+4 in Li2NiTi0.5V0.5O4. This also reduces the possibility of evolution of oxygen during de-intercalation reaction by shifting the main redox couple from O−2/O−2+γ to V+3/V+4, which leads to greater structural stability of electrode materials during charge-discharge cycles.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 13: Crystal Structure of SrF2 and Concentration Dependence of the Defect Structure of Nonstoichiometric Phase Sr1 – xLaxF2 + x As Grown (x = 0.11, 0.20, 0.32, 0.37, 0.47)

The defect structure of as-grown SrF2 and nonstoichiometric phases Sr1 – xLaxF2 + x (x = 0.11, 0.20, 0.32, 0.37, 0.47) single crystals, grown from a melt under identical conditions, has been studied by X-ray diffraction analysis. All crystals belong to the CaF2-type structure, sp. gr. $$Fm\bar {3}m$$ . Deficit of fluorine anions is found in the 8c site in SrF2. Interstitial anions are not visualized in SrF2 in difference electron-density maps. The Sr1 – xLaxF2 + x phases exhibit the presence of vacancies in the main anion motif and interstitial fluorine ions of three types: in two sites 32f (w, w, w) with different coordinates w and in one site 4b. A model of the defect structure of Sr1 – xLaxF2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La3+ are grouped into clusters of the [Sr1 – nLanF26] tetrahedral configuration. Calculations based on structural data revealed that the average number of La3+ ions per cluster linearly increases from 2.6 to 3.13 with an increase in the LaF3 concentration. The average crystal volume corresponding to one cluster decreases from 1170.6(3) to 336.1(5) A3. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) A3, passing through a minimum in the composition with x = 0.32, which is similar to that of congruently melting phase, and then increases to 2.44(9) A3 at х = 0.47. Dynamic thermal displacement of matrix anions in Sr1 – xLaxF2 + x is observed in the [111] direction towards the cubic void center in the anion sublattice. Therefore, according to the mechanism of electrical conductivity, anion jumps are most likely in this direction.

Organic molecular evidence of seafloor hydrocarbon seepage in sedimentary intervals down a core in the northern South China Sea

The hydrocarbons in sedimentary organic matter (OM) at Site 4B in the Pearl River Mouth Basin (PRMB), northern South China Sea (SCS) were analyzed. The odd/even predominance (OEP) of long chain n-alkanes (>n-C24) with CPI (carbon preference index) values from 1.62 to 3.80 and n-C29/n-C31 being the two most abundant in most of the samples are strongly indicative of a terrestrial higher plant source, and the even/odd predominance (EOP) distribution of mid-chain n-alkanes (n-C16–22) suggests a marine bacterial input, probably chemical autotrophic bacteria. However, the biomarker distributions at the 65–70 cm, 80–85 cm and 85–90 cm intervals show a similar to those of crude oils, which were characterized by a dominance of an unresolved complex mixture (UCM) in the n-C20+ region, low CPI values (<1.5) of long chain n-alkanes, low values of ββ/(αβ + ββ + βα)-C30 hopane (<0.15), and high values (>9) of total tricyclic terpanes/αβ-C30 hopane. These features are typical characteristics of mature OM that has experienced catagenesis and metagenesis, suggesting that outside hydrocarbons migrated into these three intervals. In terms of petroleum systems developed in the PRMB, hydrocarbon seepage from deep source rocks via a diaper structure and fault system is proposed to be responsible for the occurrence of this mature OM.

Local food environment and fruit and vegetable consumption: An ecological study.

Ecological studies are essential for understanding the environment-diet relationship. The purpose of this study was to describe environmental conditions and their relationship with fruit and vegetable (FV) consumption among Brazilian public health service users in the city of Belo Horizonte. We evaluated food stores contained within 1600 m buffer zones at 18 Health Academy Programme sites, from 2013 to 2014. Variables at the community (density, proximity and type) and the consumer (sectional location of FV; availability, quality, variety, price and advertising of FV and ultra-processed foods) nutrition environment were measured by direct observation, while aggregate data from users (income and FV consumption) were obtained by interview. Data were analysed using the Kernel intensity estimator, average nearest neighbour value and Local Moran's Index for local spatial autocorrelation. We interviewed 3414 users and analysed 336 food stores. Major geographical variations in the FV consumption were identified. Average consumption was higher (site 2A: 410.5 ± 185.7 g vs. site 4B: 311.2 ± 159.9 g) in neighbourhoods with higher income and concentration of food stores, and better index of access to healthy foods. Sites with poor FV consumption had the most stores with poor access to healthy foods (index in the first tertile, ≤ 10). In conclusion, negative characteristics of the food environment, as seen in the present study, may contribute to low FV consumption, suggesting the need for the development and consolidation of public policies aimed at creating healthy environments through built environment interventions that increase access to and consumption of healthy foods like FV.

Sulfur speciation in marine sediments impacted by gas emissions in the northern part of the South China Sea

Three cores of marine sediments from the Shenhu area in the northern part of the South China Sea were analyzed by XANES analysis for sulfur speciation. The area has been investigated for the presence of hydrocarbons and potential gas hydrate formations. Cored samples of site 4B showed a specific profile of sulfur speciation with sharp and frequent variations in relative contents of sulfate and sulfide, which differed greatly from the profiles obtained for the sediments taken at sites 5B and 6A. The upper part of core 4B (of 0–95 cm) was soft and rich in pore water, containing mainly coarse silt sand. The lower part of the core (i.e., depth > 95 cm) was relatively dryer and darker in color, and dominated by silts and clay resembling sediments from mud volcanoes. The sulfur speciation results revealed that sulfate makes up almost 100 percent of all sulfur species in the upper part of the core 4B, which indicates strong oxidizing conditions, whereas the lower part of the same core has high relative contents of sulfide, sometimes close to 100% S2−. In the lower part of the core, the relative content of sulfide and sulfate changes rapidly and frequently, indicating rapid changes of oxidizing and reducing conditions. On the other hand, the vertical profiles of sulfur species for the cores from sites 5B and 6A are relatively consistent with lower sulfide contents indicative of stable and weaker reducing conditions. We hypothesize that the frequent and sharp variations in the ratios of sulfide to sulfate at site 4B may indicate some intermittent eruption of methane with clay from petroleum reservoirs underneath the sea floor over a relatively short period of time.

Characteristics of authigenic pyrites in shallow core sediments in the Shenhu area of the northern South China Sea: Implications for a possible mud volcano environment

Distinct pyrites have been recovered from a shallow sediment core from Site 4B in the Shenhu area of the northern South China Sea. Based on the lithology, texture and structure of sediments, the stable sulfur isotope of pyrite and the total organic carbon (TOC) concentration of the sediments, a distinctive sediment interface is identified at a depth of about 1 m below the seafloor in the core sediments. The pyrites only accumulate in the lower part of the core as rods and foraminifera-infillings, and mainly within three intervals marked by high pyrite concentrations. The amount of pyrite in the sediments shows no remarkable correlation with TOC in the Site 4B core sediments. The stable sulfur isotopes of the pyrite have extremely negative values ranging from −41.69‰ to −49.16‰. They are considered to be the mutual product of sulfate bacterial reduction and sulfur bacterial disproportionation. Our research proposes that Site 4B might be located in or near a possible mud volcano sedimentary environment; a large amount of methane could migrate from deep to shallow sediments in an active mud volcano and thereby play a key role in the intensity of sulfate bacterial reduction and the amount of pyrite formed at Site 4B. Further, the variation in flux of deep methane fluid by intermittent mud volcanic eruptions might result in the deposition of authigenic pyrite intervals.

The Origin of Arctic Precipitation as Deduced from its Deuterium Excess (Abstract)

Continuous profiles of deuterium excess, d = δD − 8δ18O, in the snow-pack have been measured at continental, high-elevation stations: Dye 3/4B (2480 m a.s.l.) in southern Greenland, and Site G (3110 m a.s.l.; 70 km south-east of Crête) in central Greenland.Figure 1 shows the δ18O and the d profiles along the upper 4.3 m of firn at Site G. These profiles represent the snow accumulation from late 1974 to mid-1985, judging from the clear seasonality in the δ18O record. There is also a seasonality in the d values, which generally range from +5 to +12 ‰. In contrast to the findings at low-elevation stations, the two seasonal cycles are not in antiphase; the d curve lags the δ18O curve by only approximately 90°. These same features appear when plotting similar data from Dye 3.Modelling of the data proceeds in three steps. First, the isotopic composition of vapour evaporating from an oceanic source area is calculated on the basis of Merlivat and Jouzel’s (1979) model, which takes kinetic effects into account. Secondly, the vapour is advected to the ice sheet, without mixing with vapour from other sources. Any liquid precipitation is assumed to be formed by a Rayleigh condensation process down to −5°C. Thirdly, the snow formation at lower temperatures is modelled, taking account of kinetic effects due to supersaturation of vapour with respect to ice (Jouzel and Merlivat 1984).The result of this modelling is that the initial mixing ratio (wso; g H2O/kg dry air; data from IAEA/WMO) largely determines the change of d in the precipitate as δ18O decreases during cooling. Furthermore, the absolute magnitude of the deuterium excess is mainly determined by the sea-surface temperature (Ts) in the source area.The combined result of these effects is shown in Figure 2, the upper part of which applies to a high-latitude source region (Weathership A at 62 °N, 34 °W), and the lower part to a mid-latitude source region (Weathership E at 35 ° N, 48 ° W). dvo is the deuterium excess of the parent vapour as it leaves the source area; dpo is that of first-stage precipitation predicted by the model, and dp is the predicted deuterium excess in the precipitate at Site G.In the high-latitude case, dvo and dpo have pronounced summer minima and winter maxima. As the cooling proceeds (until the observed δ18O values at Site G are reached), this cycle is inverted so that at Site G the model predicts maximum dp in August and minimum dp in January, and deuterium excesses on the ice sheet which are far too low (cf. Fig. 1). This is true regardless of the choice of parameters for the model.In contrast, starting with vapour from a mid-latitude source (such as the Weathership E area), the model predicts for Site G a maximum in September and a minimum in March, with values of 14 and 4‰ respectively.The latter results are in good agreement with the observations at Site G (Fig. 1), and also with those at Site 4B, in respect of the annual mean as well as the seasonal amplitude and the phase.We conclude that, in contrast to polar low-elevation stations, the ice sheet receives precipitation from moisture which originates predominantly from much lower latitudes. The moisture has probably been carried northward at considerable altitudes by the cyclonic conveyer belts, and is thus more or less separated from the ocean surface at an early stage.A more complete description of the model will be submitted to Tellus in the near future.

The Origin of Arctic Precipitation as Deduced from its Deuterium Excess (Abstract)

Continuous profiles of deuterium excess, d = δD − 8δ18O, in the snow-pack have been measured at continental, high-elevation stations: Dye 3/4B (2480 m a.s.l.) in southern Greenland, and Site G (3110 m a.s.l.; 70 km south-east of Crête) in central Greenland. Figure 1 shows the δ18O and the d profiles along the upper 4.3 m of firn at Site G. These profiles represent the snow accumulation from late 1974 to mid-1985, judging from the clear seasonality in the δ18O record. There is also a seasonality in the d values, which generally range from +5 to +12 ‰. In contrast to the findings at low-elevation stations, the two seasonal cycles are not in antiphase; the d curve lags the δ18O curve by only approximately 90°. These same features appear when plotting similar data from Dye 3. Modelling of the data proceeds in three steps. First, the isotopic composition of vapour evaporating from an oceanic source area is calculated on the basis of Merlivat and Jouzel’s (1979) model, which takes kinetic effects into account. Secondly, the vapour is advected to the ice sheet, without mixing with vapour from other sources. Any liquid precipitation is assumed to be formed by a Rayleigh condensation process down to −5°C. Thirdly, the snow formation at lower temperatures is modelled, taking account of kinetic effects due to supersaturation of vapour with respect to ice (Jouzel and Merlivat 1984). The result of this modelling is that the initial mixing ratio (wso; g H2O/kg dry air; data from IAEA/WMO) largely determines the change of d in the precipitate as δ18O decreases during cooling. Furthermore, the absolute magnitude of the deuterium excess is mainly determined by the sea-surface temperature (Ts) in the source area. The combined result of these effects is shown in Figure 2, the upper part of which applies to a high-latitude source region (Weathership A at 62 °N, 34 °W), and the lower part to a mid-latitude source region (Weathership E at 35 ° N, 48 ° W). dvo is the deuterium excess of the parent vapour as it leaves the source area; dpo is that of first-stage precipitation predicted by the model, and dp is the predicted deuterium excess in the precipitate at Site G. In the high-latitude case, dvo and dpo have pronounced summer minima and winter maxima. As the cooling proceeds (until the observed δ18O values at Site G are reached), this cycle is inverted so that at Site G the model predicts maximum dp in August and minimum dp in January, and deuterium excesses on the ice sheet which are far too low (cf. Fig. 1). This is true regardless of the choice of parameters for the model. In contrast, starting with vapour from a mid-latitude source (such as the Weathership E area), the model predicts for Site G a maximum in September and a minimum in March, with values of 14 and 4‰ respectively. The latter results are in good agreement with the observations at Site G (Fig. 1), and also with those at Site 4B, in respect of the annual mean as well as the seasonal amplitude and the phase. We conclude that, in contrast to polar low-elevation stations, the ice sheet receives precipitation from moisture which originates predominantly from much lower latitudes. The moisture has probably been carried northward at considerable altitudes by the cyclonic conveyer belts, and is thus more or less separated from the ocean surface at an early stage. A more complete description of the model will be submitted to Tellus in the near future.

A neutron powder diffraction study of (V,Cr) 3C 2− x

The crystal structure of (V,Cr) 3C 2− x (space group Cmcm (No. 63), Z = 4, Re 3B-type structure) was refined from neutron powder diffraction data using the Rietveld profile analysis method. Partial ordering was found among the metal positions according to the formula (V 0.46Cr 0.54)(V 0.22Cr 0.78) 2C 0.75C 0.69 · The carbon atoms were found to occupy the centers of trigonal prisms (occupation of site 4c, 75(1)%) and of octahedra (occupation of site 4b, 69(1)%). The structural chemistry of Re 3B-type carbides is discussed.

Proprietes cristallographiques et magnetiques de perovskites de formule Ba 3Fe 2−xLu xUO 9

The crystallographic study of compounds with formula Ba 3Fe 2−xLu xUO 9 (0 ⩽ x ⩽ 1, 2) shows that they are cubic ordered perovskite type compounds with probable space group Fm3m. Lutecium is equally distributed between the site of coordination 12 (8c) and the site 4b. On the magnetic point of view, these compounds have a behaviour of ferrimagnetic substance. The Néel theory is not applicable to these compounds. The model suggested by Geller et al. for the static magnetic behaviour of substituted garnets is most likely to account for the results obtained for these compounds.