In this work we described the synthesis of {111} twins and taaffeites in BeO-doped spinel (MgAl2O4). The results are the first proof of our hypothesis that twinning in spinel is chemically induced and not a consequence of accidental attachment of crystals in the nucleation stage. This hypothesis was based on our previous analyses of natural {111} twins from Mogok (Myanmar), where we indirectly showed that beryllium is the most likely dopant responsible for twinning in MgAl2O4, however the presence of beryllium at the twin boundary was not proven: › Daneu N, Rečnik A, Yamazaki T, Dolenec T. Structure and chemistry of (111) twin boundaries in MgAl2O4 spinel crystals from Mogok (Burma). Phys. Chem. Minerals 34 (2007) 233-247. IF=2.238 [COBISS.SI-ID 615262]. In order to prove our hypothesis we decided to grow spinel twins in flux or in the presence of liquid phase (PbF2) from initial oxides (MgO, Al2O3, BeO). While no twins were observed in the binary MgO-Al2O3 system, twinning was abundant in the samples with BeO addition. The presence of Be in the initial stage of crystal growth leads to the formation of hcp stacking on the surface of octahedral MgAl2O4 crystals (ccp) leading to twin formation. The growth and development of composite twinned crystals is described in detail in the paper. In the first stage, i.e. until beryllium is available for the formation of the hcp stacking, such grain grows very fast (exaggeratedly) in the direction of the twin boundary. This leads to the formation of plate-like composite grains. When the twin-forming dopant is no longer present, the growth in this direction is stopped and the crystal starts to thicken according to the Ostwald ripening law. Electron microscopy analyses have shown that the samples contain simple twins, complex twins and complex modulated taaffeite BexMgyAl2(x+y)O4(x+y) phases, where the ccp and hcp sequences are interchanging. The taaffeite phases may form separate grains or epitaxial layers with the spinel. XRD analyses have revealed also that the spinel unit cell is significalntly smaller in syntheses in the presence of BeO indicating that beryllium might be incorporated in the spinel crystal structure in the form of soil solubility. This was not confirmed yet however, if the assumption is correct, then this will be the first system, where we observe that the same dopant can form tropochemical defects and at the same time also solid solution with the main phase. One of the most important conclusions of this research work is the fact that BeO doping can be used to influence microstructure development in MgAl2O4 ceramics by influencing the grain growth rate and especially their shape because otherwise cubic (isometric) grains start to grow as platelets. The described principle can be applied to other materials with the cubic spinel structure, where anisotropic grain growth (twinning) could be triggered by suitable doping and enables processing of textured microstructures for special applications (thermoelectric, platelike magnetic particles, materials for batteries,...).
COBISS.SI-ID: 26530343
This paper is the result of our cooperation with Hungarian partners from the Pannonian University in Veszprem. We have studied growth defects and epitaxial layers in nanocrystalline magnetite (Fe3O4) and its oxidation product, maghemite (gamma-Fe2O3). In magnetite, two types of planar defects are identified, (111) spinel-law twin boundaries and (110) stacking faults (SF). We have found that in contrast to our theory on chemically induced twinning, twinning in magnetite it is related to simple magnetic-field-assisted self-assembly and the growth of octahedral nanocrystals throughout their crystallization period. Simple contact twins of crystals sharing common octahedral faces, or even platelike twins develop when two crystals are joined in twinned orientation in the beginning of grain growth and continue their growth as a unit. Crystallographically, twinned domains are related by 180° rotation about the [111]-axis and with the (111) plane as the interface, producing local hcp stacking in the oxygen sub-lattice. SFs are present in both single and twinned magnetite crystals, where they are pinned to (111) twin boundaries and are present only in one domain. The displacement vector corresponding to the observed translation was determined. After the thermal treatment at 250°C both types of planar defects are retained. In addition to both types of planar defects, originating from magnetite, we identified a new formation of few nanometers- thick epitaxial layers, of a hexagonal Fe(III)-oxide–hydroxide, feroxyhyte (delta-FeOOH), covering the octahedral faces of the maghemite crystals. In the paper we set the atomic structural models of twins and stacking faults in magnetite and also of the magnetite- feroxyhyte epitaxial contact.
COBISS.SI-ID: 26941991
Previous studies of {110} twin boundaries in natural pyrite (FeS2) indicated that twinning is triggered by the presence of Cu during crystal growth. In this work, the formation sequence of Fe-sulphides produced by the chemical vapor transport (CVT) method in an evacuated quartz tube using Fe- and Cu-halides (Cu was added as the twinning triggering element) and elementary S as the reaction precursors was investigated. Depending on the mobility of elements different Fe-sulphides crystallized through gradually decreasing temperature zones. At the higher temperature zones, where metal ions are present in abundance, the main reaction product is FeS in form of pyrrhotite-3T. The main characteristic of this pyrrhotite is the presence of {111} layers cubic FeS coherently intergrown with the {0001} layers of the hosting hexagonal FeS. While the hosting structure corresponds to distorted troilite (nickeline-type structure) with octahedrally coordinated Fe2+ ions, iron in cubic FeS either remains in octahedral coordination (rocksalt-type structure) or is translated to tetrahedral interstices (sphalerite-type structure). HRTEM analysis suggests that Fe2+ ions in the cubic sequences remain in the octahedral sites, suggesting a new, cubic close-packed, rocksalt-type structure of FeS. Cubic FeS shows a high density of twins and SFs in the {111} planes. Pyrite crystallized in the temperature zone between 500-450°C, where the concentration of metal ions is depleted. With a decreasing temperature its morphology changes from octahedrons {111} pentagon dodecahedrons {210} cubes {100}. Doping with copper did not result in twinning of pyrite, suggesting that different thermodynamic conditions are present in the natural environment. Using the natural seed crystal we showed that the twin boundaries are continued across the interface into the CVT grown epitaxial pyrite, whereas the marcasite-type SFs terminate at the interface.
COBISS.SI-ID: 26523175