Browsing by Author "Scarlett, NVY"

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    Crystal chemistry and formation mechanism of non-stoichiometric monoclinic K-jarosites
    (Mineralogical Society, 2013-04-01) Grey, IE; Scarlett, NVY; Brand, HEA
    Syntheses in acidified hydrothermal (HT) solutions (1 N H2SO4 or stronger) produce monoclinic non-stoichiometric K-jarosites which contain Fe-site vacancies with long-range order. Syntheses in non-acidified HT solutions produce rhombohedral K-jarosites which contain relatively large numbers of Fe-site vacancies with no long-range order. Increasing the [Fe]/[K] ratio, reaction temperature and reaction time in non-acidified solutions promotes the formation of monoclinic jarosites which contain Fe-site vacancies with short-range order. A structural model including details of the ordering of the Fe-site vacancies was obtained by refinement of single-crystal synchrotron data from one of the HT synthesis products; this model was used to refine synchrotron powder X-ray diffraction data from products synthesized at different reaction times, temperatures and [Fe]/[K] ratios. Thermal and chemical analyses are consistent with a model for non-stoichiometry in which domains of stoichiometric jarosite are intergrown with butlerite-like iron-deficient domains with a composition [Fe-2(SO4)(2)(OH)(2)(H2O)(4)]. It was found that heterogeneous nucleation of monoclinic jarosite on Si disks is preceded by the formation of an oriented film of Maus's Salt, K5Fe3O(SO4)(6)center dot 10H(2)O, as a precursor phase, and that this transforms topotactically into oriented jarosite, which contains butlerite-like layers parallel to the disk surface. Structural models for the transformation of Maus's Salt into jarosite are proposed.© 2013, Mineralogical Society.
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    Evidence of continuous pottery production during the late Byzantine period in the Studenica Monastery, a UNESCO World Heritage Site
    (Elsevier, 2019-05) Stojanović, S; Bikić, V; Miličić, L; Evans, IR; Scarlett, NVY; Brand, HEA; Damjanović-Vasilić, L
    A collection of 63 pottery shards excavated at the Studenica Monastery, Serbia, originating from two distinct cultural strata (late 12th until the late 13th century, and the 14th and the first half of 15th century) was subject of this work. Mineralogical and chemical composition of body and glaze and production technology of investigated pottery were determined combining optical microscopy, inductively coupled plasma-optical emission and wavelength dispersive X-ray fluorescence spectrometry, Fourier transform infrared and micro-Raman spectroscopy, high-resolution synchrotron powder X-ray diffraction and multivariate statistical analysis. In addition, clay rod with traces of glaze from the kiln found within the Monastery complex was investigated. The firing temperature was estimated at 600–700 °C for the most of cookware and at 800–1000 °C for tableware. Pottery, made of non-calcareous clay, was covered with transparent lead based glaze and copper and iron were used as colorants. Combining results of all used techniques no significant differences in mineralogical and chemical composition among samples from two cultural strata were identified indicating continuous pottery production process from 13th until 15th century in Studenica. © 2019 Elsevier B.V
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    Formation of jarosite minerals in the presence of seed material
    (Universities Space Research Association, 2015-03-17) Brand, HEA; Scarlett, NVY; Grey, IE
    Introduction: There has been a resurgence in interest in jarosite, (K,Na)Fe3(SO4)2(OH)6, and related minerals since their detection on Mars by the MER rover Opportunity [1]. In this context, the presence of jarosite has been recognised as a likely indicator of water at the surface of Mars in the past and it is hoped that study of their formation mechanisms will provide insight into the environmental history of Mars [2]. Jarosites are also of great importance to a range of mineral processing and research applications. For example: they are used in the removal of iron species from smelting processes; they form detrimentally in biometallurgical systems and they are present in acid mine drainage environments. Jarosites are also of considerable theoretical interest as model compounds for spin frustration in Kagomé-Heisenberg antiferromagnetic materials [3]. Knowledge of the formation mechanisms of jarosites is an indispensable prerequisite for understanding their occurrences, stabilities and potential environmental impacts both on Mars and Earth. We are engaged in a program of research to study the nucleation and crystal growth of jarosites under a variety of conditions. Here we report the results of in situ synchrotron powder diffraction experiments designed to follow the crystallisation and growth kinetics of jarosite minerals in the presence of seed materials. Figure 1 shows the structure of jarosite. Layers of blue corner sharing Fe octahedra are connected by yellow sulphate tetrahedra. Cations such as potassium or sodium can also be found in these interstitial layers (red). Jarosite is traditionally reported with rhombohedral symmetry [3]. However, we have synthesised monoclinic jarosite under certain preparative conditions [4].
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    In situ SAXS studies of the formation of sodium jarosite
    (Wiley-Blackwell, 2013-07-01) Brand, HEA; Scarlett, NVY; Grey, IE; Knott, RB; Kirby, N
    This paper reports the results of time-resolved synchrotron small-angle scattering and powder diffraction experiments where natrojarosites were synthesized in situ in order to observe the species produced at the earliest stages of nucleation. The sample temperatures were 333, 353 and 368 K. These compounds were synthesized by co-precipitation from solution on the Small and Wide Angle Scattering and Powder Diffraction beamlines at the Australian Synchrotron. Scattering data were collected continuously throughout the syntheses. The results presented here show that the first particles to form in solution appear to be amorphous and nucleate on the walls of the reaction vessel. Crucially, there is a single nucleation event which forms particles with an elliptical disc morphology which then grow uniformly before natrojarosite crystallization is observed in complementary powder diffraction data. This nucleation event may represent the key to controlling the growth of jarosites in industrial and environmental settings. © 2013, Wiley-Blackwell.
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    In situ studies into the formation kinetics of potassium jarosite
    (Wiley Blackwell, 2012-06-01) Brand, HEA; Scarlett, NVY; Grey, IE
    This paper reports the results of time-resolved synchrotron powder diffraction and small-angle scattering experiments designed to investigate the kinetics of formation of potassium jarosite by co-precipitation at 353, 368 and 393 K. Only jarosite was produced in these syntheses, and the particles that formed nucleated on the walls of the capillary reaction vessels with a disc-like shape. Relative Rietveld scale factors indicating jarosite abundance have been used as the basis for kinetic modelling of the nucleation and growth stages using a modified form of the Avrami kinetic equation. The results show that jarosite forms by a single nucleation event followed by two distinct stages of growth, each characterized by a different Avrami exponent. The value of this exponent is initially between 1 and 2, and then reduces to around 1. This suggests that jarosite growth after nucleation is controlled by effects at the solution-boundary interface, with the first stage best described by two-dimensional growth which transitions to one-dimensional growth later in the reaction. An activation energy of 89 kJ mol(-1) was estimated for the first stage of growth, in good agreement with previous work. © 2012, Wiley-Blackwell.
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    In situ synchrotron diffraction studies on the formation kinetics of jarosites
    (Wiley Blackwell, 2013-03-01) Scarlett, NVY; Grey, IE; Brand, HEA
    This paper reports the results of time-resolved synchrotron powder diffraction experiments where jarosites with different K/H3O, K/Na and Na/H3O ratios were synthesized in situ at temperatures of 353, 368 and 393 K in order to observe the effect on kinetics and species produced. The Na/H3O sample formed monoclinic jarosite at all three temperatures, whereas the K/H3O and K/Na samples formed as rhombohedral jarosites at 353 K, and as mixtures of rhombohedral and monoclinic jarosites at the higher temperatures. The relative amount of the monoclinic phase increased with increase in temperature. Unit-cell parameter changes with reaction time could be explained by changes in iron stoichiometry (samples become more stoichiometric with time) together with changes in K/H3O and Na/H3O ratios. The reaction kinetics have been fitted using a two-stage Avrami model, with two different Avrami exponents corresponding to initial two-dimensional growth followed by one-dimensional diffusion-controlled growth. Activation energies for the initial growth stage were calculated to be in the range 90-126 kJ mol(-1). © 2013, Wiley-Blackwell.
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    Thermal expansion of deuterated monoclinic natrojarosite; a combined neutron–synchrotron powder diffraction study
    (John Wiley & Sons, Inc, 2017-04) Brand, HEA; Scarlett, NVY; Knight, KS
    A combination of time-of-flight neutron diffraction and synchrotron X-ray powder diffraction has been used to investigate the thermal expansion of a synthetic deuterated natrojarosite from 80 to 440 K under ambient-pressure conditions. The variation in unit-cell volume for monoclinic jarosite over this temperature range can be well represented by an Einstein expression of the form V = 515.308 (5) + 8.5 (4)/{exp[319 (4)/T] − 1}. Analysis of the behaviour of the polyhedra and hydrogen-bond network suggests that the strength of the hydrogen bonds connected to the sulfate tetrahedra is instrumental in determining the expansion of the structure, which manifests primarily in the c-axis direction. © International Union of Crystallography
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    Understanding solvothermal crystallization of mesoporous anatase beads by in situ synchrotron PXRD and SAXS
    (American Chemical Society, 2014-07-07) Xia, F; Chen, DH; Scarlett, NVY; Madsen, IC; Lau, D; Leoni, M; Ilavsky, J; Brand, HEA; Caruso, RA
    Submicrometer-sized mesoporous anatase (TiO2) beads have shown high efficiency as electrodes for dye-sensitized solar cells and are recoverable photocatalysts for the degradation of organic pollutants. The detailed mechanism for crystallization of the amorphous TiO2/hexadecylamine (HDA) hybrid beads occurring during the solvothermal process needs to be understood so that reaction parameters can be rationally refined for optimizing the synthesis. In this work, the solvothermal crystallization was monitored by in situ synchrotron powder X-ray diffraction (PXRD) and synchrotron small-angle X-ray scattering (SAXS) techniques. In situ PXRD provided crystallization curves, as well as the time evolution of anatase crystallite mean size and size distribution, and in situ SAXS provided complementary information regarding the evolution of the internal bead structure and the formation of pores during the course of the solvothermal process. By exploring the effects of temperature (140-180 °C), bead diameter (300 and 1150 nm), bead internal structure, and solvent composition (ethanol and ammonia concentrations) on this process, the crystallization was observed to progress 3-dimensionally throughout the entire bead due to solvent entrance after an initial fast partial dissolution of HDA from the nonporous precursor bead. On the basis of the kinetic and size evolution results, a 4-step crystallization process was proposed: (1) an induction period for precursor partial dissolution and anatase nucleation; (2) continued precursor dissolution accompanied by anatase nucleation and crystal growth; (3) continued precursor dissolution accompanied by only anatase crystal growth; and (4) complete crystallization with no significant Ostwald ripening. © 2014 American Chemical Society.