Topotaxial Reaction Research Articles

Defining the hematite topotaxial crystal growth in magnetite–hematite phase transformation

Magnetite and hematite iron oxides are minerals of great economic and scientific importance. The oxidation of magnetite to hematite is characterized as a topotaxial reaction in which the crystallographic orientations of the hematite crystals are determined by the orientation of the magnetite crystals. Thus, the transformation between these minerals is described by specific orientation relationships, called topotaxial relationships. This study presents electron-backscatter diffraction analyses conducted on natural octahedral crystals of magnetite partially transformed into hematite. Inverse pole figure maps and pole figures were used to establish the topotaxial relationships between these phases. Transformation matrices were also applied to Euler angles to assess the diffraction patterns obtained and confirm the identified relationships. A new orientation condition resulting from the magnetite–hematite transformation was characterized, defined by the parallelism between the octahedral planes {111} of magnetite and rhombohedral planes \{10\bar {1}1\} of hematite. Moreover, there was a coincidence between one of the octahedral planes of magnetite and the basal {0001} plane of hematite, and between dodecahedral planes {110} of magnetite and prismatic planes \{11\bar {2}0\} of hematite. All these three orientation conditions are necessary and define a growth model for hematite crystals from a magnetite crystal. A new topotaxial relationship is also proposed: (111)Mag || (0001)Hem and (\bar {1}\bar {1}1)_{\rm Mag} || (10\bar {1}1)_{\rm Hem}.

Topotaxial reactions during oxidation of ilmenite single crystal

The mechanism of ilmenite–rutile transformation during oxidation of natural ilmenite crystal was studied at elevated temperatures in air. The progress of oxidation with annealing time was studied in the temperature range between 600 and 900 °C. 2.5 mm cubes were cut from the single Mn-ilmenite crystal in two special orientations, [001]ILM and $$ \left[ {1\bar{1}0} \right]_{\text{ILM}} $$ , that allowed determination of crystallographic relations among the reaction products. Using X-ray diffractometry, energy-dispersive spectroscopy, and electron microscopy (SEM, TEM) techniques, we determined that the ilmenite to rutile and hematite transformation is triggered by surface oxidation of divalent cations (Fe, Mn) from the starting ilmenite and their crystallization in the form of hematite and bixbyite on the surface of the single crystal. Surface oxidation and out-diffusion of Fe2+ and Mn2+ ions opens paths for exsolution of rutile within the pseudo-hexagonal oxygen sublattice of the parent ilmenite, following simple topotaxial orientation relationship $$ {\langle{001}\rangle}_{\text{RUT}} \;\left\{ {010} \right\}_{\text{RUT}} \;||\;{\langle{210}\rangle}_{\text{ILM}} \;\left\{ {001} \right\}_{\text{ILM}} $$ . With this transformation, new channels for fast out-diffusion of divalent cations to the oxidation surface are opened along the c-axis of the rutile structure. The volume difference of the reaction products causes cracking of the single crystal, which opens additional free surfaces for accelerated oxidation. The results of this study contribute to better understanding of the recrystallization processes during pre-oxidation of ilmenite.

Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)

Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the \(\left\langle {0 10} \right\rangle_{R} \left\{ { 10 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 10} \right\}_{H}\) orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact \(\left\langle {0 10} \right\rangle_{R} \left\{ { 40 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 70} \right\}_{H}\). The intergrowth is dictated by the formation of \(\left\{ { 1 70} \right\}_{H} |\left\{ { 40 1} \right\}_{R}\) equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian–ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form \(\left\{ { 30 1} \right\}_{R} |\left\{ {0 30} \right\}_{H}\) equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian–ilmenite crystals was close to Ilm67Hem33, typical for Fe–Ti-rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian–ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.

Topotaxial formation of titanium-rich barium titanates during solid state reactions on (110) TiO 2 (rutile) and (001) BaTiO 3 single crystals

The orientation relationships of Ti-rich barium titanate phases formed by solid state reactions at high temperatures were studied using (110) TiO 2 (rutile) and (001) BaTiO 3 single-crystal substrates. Well-oriented Ba 6Ti 17O 40 islands were observed after a vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 1 nm with the TiO 2 substrate, whereas a thin film consisting of well-oriented BaTiO 3 and Ba 6Ti 17O 40 grains was formed after vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 50 nm with the rutile substrate. A topotaxial orientation relationship between Ba 6Ti 17O 40 and TiO 2 was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO 2 and Ba 6Ti 17O 40. The mechanism of the reaction occurring between BaO vapor and the TiO 2 surface at high temperature is discussed. On the other hand, several well-oriented Ba 4Ti 13O 30, Ba 6Ti 17O 40 and Ba 2Ti 5O 12 phases were observed to be embedded in the mainly forming Ba 2TiSi 2O 8 phase after a solid–solid reaction of amorphous SiO 2 thin films with (001) BaTiO 3 substrates at temperatures above 1000 °C. They were formed by a topotaxial reaction involving the transformation of (111) planes of BaTiO 3 into (001) planes of the Ti-rich phases by removal of BaO and insertion of TiO 2. Cross-sections of the interfaces between the substrates and the various reaction products are studied by (high-resolution) transmission electron microscopy.

The structure of Mg 2SnO 4/MgO topotaxial reaction fronts during gas–solid and solid–solid reactions

Thin spinel films were grown on MgO(001) substrates by a surface reaction between the MgO substrate and (i) a SnO 2 vapour, or (ii) a solid SnO 2 film at temperatures around 1200°C. Regime (i) is called a “gas–solid reaction’’, regime (ii) a “solid–solid reaction’’. Investigations by SEM, XRD, TEM/SAED, and HRTEM revealed the films obtained by the gas–solid reaction to grow in almost [001] orientation and to develop a specific morphology. They are composed of domains, the crystal lattices of which are tilted by less than 1° off the overall orientation around two different 〈110〉 axes. A network of interfacial dislocations with Burgers vectors a/2 [011] and a/2 [101] accommodates the Mg 2SnO 4/MgO lattice misfit of +2.5%. The films obtained by the solid–solid reaction grow in the very [001] orientation and do not consist of domains. Here, a network is proven of interfacial dislocations with Burgers vectors a/2 [110] and a/2 [1 1 0], which are parallel to the reaction front. The observations are discussed in terms of the interplay between reaction kinetics, the properties of the misfit-accommodating interfacial dislocations persisting at the moving reaction front, and the differences between gas–solid and solid–solid reactions concerning the starting conditions of the growing spinel phase.

Topotaxial Reaction Fronts in Complex Ba-Ti-Si Oxide Systems Studied by Transmission Electron Microscopy

Processes and interface structures occurring during BaTiO 3 sintering in the presence of the sintering aid SiO 2 are studied by model experiments. The formation of fresnoite Ba 2 TiSi 2 O 8 and different Ti-rich barium titanates by solid state reactions proceeding at the (001)BaTiO 3 /SiO 2 interface is followed by XRD and TEM. High resolution transmission electron microscopy (HRTEM) permits the structure to be studied of the various solid-solid reaction fronts involved. The observations indicate, e.g., a certain topotaxial reaction mechanism at the Ba 4 Ti 13 O 30 /BaTiO 3 (001) and Ba 6 Ti 17 O 40 / BaTiO 3 (001) reaction fronts, which comprises the outdiffusion of barium, the indiffusion of titanium and oxygen, and a restacking of the BaTiO 3 {111 }planes.

Formation of ceramic thin films by solid-state interface reactions

The formation of single-crystal Mg2TiO4 films and large-grained, multiply positioned MgTiO3 films by solid-state interface reactions is investigated by transmission electron microscopy, selected area electron diffraction, electron probe microanalysis, Rutherford backscattering spectrometry, and other methods. Stoichiometric single-crystal Mg2TiO4 spinel films grow by a topotaxial reaction between the MgO substrate and a TiO2 vapor. A submicron-marker experiment revealed the cations to be the diffusing species. The films are free of observable lattice defects, except for a network of cation antiphase boundaries. Two new preparation methods are presented that enable edge-on solid–solid reaction interfaces to be investigated by in situ observation in the high-voltage electron microscope. Periodic lines of contrast at the semicoherent Mg2TiO4/MgTiO3 interface are interpreted in terms of dislocations, which probably restack the oxygen sublattice of MgTiO3 to that of spinel. The single-phase MgTiO3 films obtained show a regular eightfold-positioned structure, partly due to grain growth and reorientation processes. Interface strain is assumed to determine whether Mg2TiO4 or MgTiO3 is formed as the final thin-film phase during the solid–solid reaction.

Structure investigations of the Ge‐GeO2 system by high‐resolution electron microscopy

AbstractThe crystal structure of thermally oxidized Ge was investigated by high‐resolution electron microscopy (HREM), mainly the interface Ge/oxide. Under special conditions the reaction Ge + O2 → GeO2 which takes place at (111) surface planes leads to suitable thin crystal regions. The GeO2 occurs normally as amorphous films on the crystal surface. Furthermore, hexagonal GeO2 can grow at the interface Ge/oxide by a topotaxial reaction; the orientation relation between these two lattices was ascertained. Intensive electron irradiation was used to initiate and to observe structure changes in boundary regions.

Magneto-optic Kerr effect in CoFe2−xMxO4 (M=Mn, Cr, Al) and BaFe12−2x–COxTixO19 with tetrahedrally coordinated Co2+

Polar Kerr effect has been observed at λ=0.4–2.0 μm on hot-pressed ceramics of Co2+Fe3+2−xM3+xO4 (M=Mn, Cr, Al, 0≤x≤1.2) and BaFe3+12−2x–Co2+xTi4+xO19 (0≤x≤1.5), in which tetrahedrally coordinated Co2+ ions [Co2+(Td)] increase with x. To avoid effect of birefringence in the hexagonal ferrite, ceramics with prefered orientation along the c axis have been prepared by a topotaxial reaction technique. Enhancement in the Kerr effect associated with the crystal field transitions of Co2+ (Td) is observed at λ≊0.7 and 1.7 μm in both CoFe2−xMxO4 and BaFe12−2x–CoxTixO19, though the magnitude in the latter compound is several times smaller than in the former. The large normal remanence observed by surface Kerr effect in CoFe2−xMxO4 can not be attributed to the residual stress induced by hot-pressing as has been pointed out previously.

Polycrystalline β-Alumina with Preferred Orientated Grains Prepared by Topotaxial Reaction of Gibbsite and Sodium Carbonate

Polycrystalline β-Alumina with Preferred Orientated Grains Prepared by Topotaxial Reaction of Gibbsite and Sodium Carbonate