Secondary phases and their role in the development of microdiscs on flux-grown ErFeO3 crystal surfaces

The results of scanning electron microscope and qualitative element analysis studies, carried out on some typical structures exhibited by flux-grown ErFeO₃ crystals grown from PbO-PbF₂-B₂O₃ flux systems, are reported. Aluminium is present as an impurity in the crystals studied. Qualitative analysis of certain structures indicates the formation of magnetoplumbite (PbO · 6Fe₂O₃) during the flux growth of ErFeO₃. Microdisc patterns are interpreted as resulting from the covering of such formations by the rapidly advancing growth fronts on the ErFeO₃ crystal surfaces. The crystallization of ErOF and ErBO₃ crystals on the ErFeO₃ crystal surfaces is also indicated by the qualitative analysis. Precipitation of the impurity phases during the flux growth of ErFeO₃ crystals and its effects on the development of the latter is discussed.     

Microstructural,etching and hardness studies on flux-grown KNiF3 crystals

Surface structures on {100} faces of flux-grown KNiF₃ crystals are reported. Etching experiments establish HNO₃ to be a dislocation etchant for the crystals. The etching behaviour of the HNO₃-KNiF₃ surface system is investigated. The results obtained on the effect of etching time and etchant concentration on lateral extension and depth of dislocation etch pits are reported. It is observed that the etchant is rendered passive after some period of initial etching. Indentation-induced hardness testing studies suggest a Vickers microhardness value in the range of (2.93 to 3.50)×10² kg mm^–2, and the response of indentation to load is in accordance with Kick's Law.     

Load and directional effects on microhardness and estimation of toughness and brittleness for flux-grown LaBO3 crystals

Results of microhardness measurements on (100) and (110) planes of flux-grown LaBO₃ crystals, in the applied load range of 10–100g, are presented. The microhardness was found to decrease with increasing load in a non-linear manner. By applying Hays and Kendall's law, the materials resistance pressure and other constants of the equation could be calculated. Hardness anisotropy, showing periodic variation of H _v with the maxima and minima repeating at every 15° change in orientation of the indentor, is described and discussed. H _max/H_min are estimated as 1.14 and 1.06 for (100) and (110) planes, respectively. The fracture toughness values, K _c, determined from measurements of crack lengths, are estimated to be 1.6, 1.7 MN m^–3/2 (for (100) planes) and 1.2, 1.5 MN m^–3/2 (for (110) planes) at 90 and 100g loads, respectively. The brittleness index, B _i, is estimated as 4.6, 4.0 m^–1/2 (for (100) planes) 6.0, 4.6 m^–1/2 (for (110) planes) at 90 and 100g, loads respectively.     

Glow curves and the emission of flux-grown BaFCl:Na crystals

The thermoluminescence glow curves and the emission spectra of flux-grown BaFCl:Na crystals were recorded. An additional TL peak at 320 K, an optical absorption band at 570 nm and an emission peak at 490 nm have been seen in X/-irradiated crystals. Bleaching, room-temperature annealing and high-temperature emission results led us to conclude that the sodium impurity is responsible for the additional glow peak, optical absorption band and emission peak.     

Characterization of flux-grown PZN-PT single crystals for high-performance piezo devices

Relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O(3-x)PbTiO(3) (PZN-PT) and Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)(PMN-PT) single crystals are the potential candidates for future high-performance piezoelectric devices due to their exceptionally high dielectric and piezoelectric properties. Characterization on flux-grown PZN-PT single crystals of different orientations revealed that PZN-(6-7)%PT single crystals show good homogeneity in dielectric and electromechanical properties and composition. When poled in [001] direction, these crystals exhibit high longitudinal-mode properties with dielectric constant (K(T)) approximately equal to 7000, piezoelectric coefficients (d(33)) approximately equal to 2800 pC/N, and electromechanical coupling factors (k(33)) > or = 0.92. For [011]-cut crystals, optimally poled PZN-7%PT single crystal exhibits very high transverse-mode dielectric and piezoelectric properties with K(T) > or = 5000, d(32) approximately equal to -3800 pC/N and k(32) > or = 0.90. [011]- poled PZN 6%PT has d(32) approximately equal to -3000 pC/N and comparable k(32) and K(T) values. In comparison with melt-grown PMNPT single crystals, flux-grown PZN-PT single crystals show good compositional homogeneity, superior and consistent dielectric and electromechanical properties, and higher depolarization temperatures (TDP).     

Optical and scanning electron microscopic and energy-dispersive X-ray analytical studies on some typical surface structures of flux-grown DyFeO3 crystals

The results of optical, scanning electron microscope and qualitative analysis studies conducted on surface structures displayed by flux-grown DyFeO₃ crystals are reported. The crystals were grown from PbO-PbF₂ B₂O₃ flux under various conditions. Magnetoplumbite (PbO · 6Fe₂O₃) is the most favoured secondary phase. Crystallization of DyOF and DyBO₃ on the DyFeO₃ crystal surfaces also takes place, almost at the end of DyFeO₃ crystal growth. Macro- and micro-disc patterns on DyFeO₃ and a rare observation of an elliptical disc of material containing lead are illustrated. Metallic platinum deposited on flux-grown DyFeO₃ is reported when the crystals were finally cooled in contact with the flux. Additional secondary phases (material containing lead, platinum, PbO · 6Fe₂O₃, DyBO₃) and other imperfections (inclusions, cavities, microcrystals) also occur. The addition of V₂O₅ to the flux leads to incorporation of traces of vanadium in the DyFeO₃ crystals and DYVO₄ as a secondary phase.     

Typical hillocks and spirals on flux-grown DyFeO3 crystal surfaces

Multi-sided and almost rectangular growth hillocks on (1 1 0) faces of flux-grown DyFeO₃ crystals are illustrated. Some such hillocks exhibit spiral growth layers originating from the summits of hillocks. Spiral growth layers originating from more than one initiation centre interact, giving rise to closed loop, interlocked and interlaced or other complex growth formations. The origin of the multi-sided hillocks is attributed to preferential growth at the sites of screw dislocations in DyFeO₃ crystals. Defects other than screw dislocations also stimulate growth on (1 1 0) faces of DyFeO₃ crystals. An example of this is offered. The mechanism of independent growth on (1 1 0) faces of these crystals is discussed.     

Formation and identification of secondary oxide phases in co-sputtered ZnO:Cr films

The secondary oxide phases in ZnO:Cr films have been observed to evolve from an initial Cr₂O₃ phase near the film surface to a final ZnCr₂O₄ phase throughout the film as the Cr content is increased. Two absorption bands, corresponding to the ⁴A₂→⁴ T₁ and ⁴A₂→⁴ T₂ transitions of Cr³⁺, can be observed in the Cr₂O₃-rich sample, whereas the ZnCr₂O₄-rich sample is opaque at wavelengths ≤ 500 nm. Our results suggest that the formation of secondary oxide phases in ZnO:Cr films is inevitable, leading to a much lower solubility of Cr.     

Surface structural investigations on (100) and (110) faces of flux-grown lanthanum borate crystals

Results of topographical studies carried out on (100) and (110) faces of lanthanum borate crystals grown from the PbO-B₂O₃ flux system are illustrated and discussed. The habit faces display the formation of cavities, microcrystals, elliptical etch pits, elliptical hillocks, circular hillocks and irregular structures. Also described are a various number of elevated structures identified as impurity phases. Energy dispersive X-ray (EDAX) studies have confirmed that these impurity phases in the growth of lanthanum borate (LaBO₃) crystals are enriched by lead (Pb). The habit faces also exhibit some elevated regions which are reported to be more imperfect in comparison with others. It is inferred that independent growth on the habit faces has taken place during the last stage of crystal growth by a two-dimensional nucleation mechanism.     

Microhardness measurements on single crystals of flux-grown rare earth perovskites (orthoferrites,orthochromites and aluminates)

The results of indentation-induced microhardness testing studies of flux-grown single crystals of rare earth orthoferrites, RFeO₃ (R=Gd to Er and Yb), rare earth orthochromites RCrO₃ (R=La, Eu and Dy), and rare earth aluminates RAlO₃ (R=La, Sm, Gd, Eu and Ho) are presented. The variation in the value of microhardness with load is observed to be non-linear in the case of all these materials. It is found that the results are not in accordance with Kick's law. The results have been analysed and the applicability of the idea of materials resistance pressure in the modified law as proposed by Hays and Kendall [Metallography 6 (1973) 275] is discussed.     

Grain boundary analysis and secondary phases in LaCoO3-based perovskites

Secondary phases and grain boundaries in LaCoO₃ and La_0.7Sr_0.3CoO_3−δ ceramics have been studied by transmission electron microscopy. Both materials contained small amounts of grains of a secondary phase; the face centered cubic spinel structure Co₃O₄, located at triple junctions. These grains were agglomerates of several smaller grains. High resolution electron microscopy combined with annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy were used to analyze the grain boundaries. In LaCoO₃, the grain boundaries were sharp with stable compositions of La, Co and O across the boundaries. In La_0.7Sr_0.3CoO_3−δ, a 1–2 nm thick intergranular layer between the grains was observed. This layer was rich in O and Co and deficient in Sr and La, compared to the nominal composition of the material.     

Magnetically recoverable catalyst based on porous nanocrystalline HoFeO3 for processes of n-hexane conversion

Nowadays, a wide class of rare earth orthoferrites is of great interest among acid-base catalysts due to their high activity and the possibility of magnetic recovery. However, the synthesis of chemically and phase-pure rare-earth orthoferrites in the form of nanocrystals with developed surface and microstructure is still a difficult task. In this work, we report for the first time a successful synthesis of phase-pure holmium orthoferrite (HoFeO₃) nanocrystals with a porous microstructure and a branched surface via simple glycine-nitrate combustion approach. Holmium orthoferrite nanocrystals were characterized by using energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), ⁵⁷Fe Mössbauer spectroscopy and low-temperature sorption-desorption of nitrogen. Acid-base properties of the surface and catalytic activity of porous HoFeO₃ in process of n-hexane conversion were analyzed as well. Finally, a magnetic recovery procedure was performed for the spent catalyst to analyze its efficiency for synthesized HoFeO₃. The elemental composition of the sample corresponds to holmium orthoferrite (48.8 and 51.2 at. %, for Ho and Fe, respectively) with the absence of any impurities as it was shown by EDX. The PXRD results show that the synthesized HoFeO₃ nanocrystals have an orthorhombic structure (space group Pbnm) and an average crystallite size of 62 ± 5 nm. The Rietveld method was used to refine the unit cell parameters as follows: a = 5.254(1) Å, b = 5.575(3) Å, c = 7.598(4) Å; R_wp = 2.45%. The Mössbauer spectrum of HoFeO₃ evidenced the magnetically ordered state of nanocrystals and it is represented by a sextet with quadrupole splitting (QS), of 0 mm/s, an isomeric shift (IS) of 0.36 mm/s, and an effective magnetic field (H_eff) of 497 kOe. Sorption-desorption of nitrogen shows that the holmium orthoferrite has a predominantly macro- and mesoporous structure, also confirmed by SEM, with specific surface area and total porosity values of 31 m²/g and 0.071 cm³/g, respectively. The analysis of the acid-base properties of the surface of the porous HoFeO₃ evidences a high concentration of the aprotic (Lewis) acidic (pKa = 14.2) and basic (pKa = -4.4) sites, as well as of the Brønsted weak acid (pKa = 6.4) sites. The catalytic activity of holmium nanocrystalline in process of n-hexane conversion was analyzed and compared with commercial dehydrogenation (CD-1) and cracking (ZSM-5) catalysts. The n-hexane conversion and cracking products selectivity displayed by HoFeO₃ are comparable to those of the reference catalysts, and the selectivity to dehydrogenation and especially isomerization products significantly exceeds the values of CD-1 and ZSM-5; low n-hexane oligomerization product selectivity of HoFeO₃ catalyst was observed as well. A high recovery rate (~97%) achieved as a result of the magnetic separation of the spent o-HoFeO₃-based catalyst from the reaction mixture, followed by heat treatment at 600 °C for 1 h. These results allow us to conclude that synthesized porous HoFeO₃ can be considered as a promising basis for new catalytic materials for cracking, dehydrogenation and isomerization processes with a possibility of highly efficient magnetic recovery.     

The growth of single crystals of the intermediate phases NiAl and Ni3Al

Single crystals of stoichiometric NiAl, nickel-rich Ni₃Al, and aluminium-rich Ni₃(Al, Ti) have been grown by a modified Bridgman technique; a strain-anneal method has also been used to produce small, single crystals of non-stoichiometric NiAl.     

Formation of oxide phases in the system Fe2O3-NiO

Mixed metal oxides in the system Fe₂O₃-NiO were prepared by coprecipitation of Fe(OH)₃/Ni(OH)₂ and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe₂O₃, NiO and NiFe₂O₄ in samples prepared at 800°C. The oxide phases -Fe₂O₃, NiO, NiFe₂O₄ and a phase with structure similar to NiFe₂O₄ were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe₂O₃-NiO are discussed. Two very strong infrared bands at 553 and 475 cm^–1, a weak intensity infrared band at 383 cm^–1 and two shoulders at 626 and 441 cm^–1 were observed for -Fe₂O₃ prepared at 1100°C. NiFe₂O₄, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm^–1, while NiO showed a broad infrared band at 466 cm^–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.     

Formation of microtubes in CdI2 crystals doped with BiI3

The formation of microtubes (MTs) with rectangular cross section in the volume of CdI₂ layered crystals doped with Bi³⁺ trivalent impurity ions has been observed for the first time by scanning electron microscopy. The MTs grow predominantly in pores that appear in the volume of CdI₂-BiI₃ crystals as a result of a difference in the valence of host Cd²⁺ ions and guest Bi³⁺ ions substituted into regular crystal lattice sites. The formation of MT structures is related to the rolling of I-Cd-I trilayers containing impurity ions, which takes place as a result of the presence of uncompensated charges at the edges. Deviation of the MT cross section from circular is explained by nonuniformity of the elastic properties of I-Cd-I trilayers with Bi impurity, which is related to the appearance of vacancies in the cation subsystem upon replacement of Cd²⁺ ions by Bi³⁺ ions.     

Lattice disorder and electrical conductivity of flux-grown Ca(WO4) x (MoO4)1−x crystals

The growth of solid solutions of Ca(WO₄) _x (MoO₄)_1–x in the rangeX=0.072 to 0.86 has been successfully accomplished for the first time, employing a flux technique. The fact that proper mixing occurs is established by observing the variation in unit cell parameters and density of the crystals. The electrical conductivity of the pelletized samples is interpreted in terms of Schottky and Frenkel defect concentrations. The role of oxygen ion vacancies is stressed for the intrinsic region of conductivity.     

Electron microscopy of second phases in manganese-zinc ferrite crystals

The surface and bulk microstructure of Mn-Zn ferrite single crystals oxidized at 0.7% _{o 2} have been investigated using SEM and TEM methods, assisted by energy- dispersive X-ray micro analysis. In samples heat-treated at 1000° C, haematite formed as an irregular surface layer and as laths which grew throughout the bulk, parallel with the {1 1 1 } planes (where F = ferrite). The haematite laths were related epitaxially to the ferrite host through the relationships (0001)_H | {1 1 1 }_F (where H = haematite) and . However, it was found that the latter parallelism was not exact because the haematite and ferrite lattices were rotated by 1.1 ± 0.2° about the axis perpendicular to the interface between them (i.e. [0001]H, <1 1 1 >_F). The sites for the nucleation of haematite second phase were also observed in highly oxidized ferrite. The formation of small planar defects (< O.1 m in length) in the ferrite, lying parallel to {1 10} planes, is interpreted as the combination of a local shear with the ordering of cation vacancies resulting from the oxidation. Small volumes of other second phases were also recognized, by virtue of both compositional and microstructural differences from the host ferrite.