Ashok K. Razdan

Microhardness of flux grown pure doped and mixed rare earth aluminates and orthochromites

The results of microhardness measurements on flux-grown crystals of (i) single (pure) rare earth aluminates RAlO3 (R = Eu, Gd, Dy, Er) and rare earth orthochromites RCrO3 (R = Y, Gd, Yb), (ii) rare earth aluminates doped with neodymium, erbium, ytterbium and holmium, and (iii) mixed rare earth aluminate crystals of the type (La1−x) Pr(x)AlO3 (x=0, 0.25, 0.75 and 1.00) are presented. The variations in the microhardness value with load are non-linear in all cases. Kicks law fails to explain the observed variations. Instead, they are best explained by the application of the idea of materials resistance pressure in the modified law proposed by Hays and Kendall. The results indicate that the doping does not increase the hardness value of crystals in all cases. The hardness instead depends on the composition of the parent material as well as the dopant entering into the crystal lattice. Mixed rare earth aluminate crystals are shown to be harder than those of single rare earth aluminates.

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 Kendalls law, the materials resistance pressure and other constants of the equation could be calculated. Hardness anisotropy, showing periodic variation of Hv with the maxima and minima repeating at every 15° change in orientation of the indentor, is described and discussed. Hmax/Hmin are estimated as 1.14 and 1.06 for (100) and (110) planes, respectively. The fracture toughness values, Kc, 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, Bi, is estimated as 4.6, 4.0 μm−1/2 (for (100) planes) 6.0, 4.6 μm−1/2 (for (110) planes) at 90 and 100g, loads respectively.

Optical microscopic studies on grown and etched surfaces of flux grown LaAlO3 crystals

Surface structures on as-obtained flux-grown crystals of LaAlO3 have been investigated. Strictly oriented square, circular and rhombus shaped pointed, as well as flat-bottomed etch pits are observed. Etch pits along lineage boundaries, intersecting low-angle tilt boundaries and helical dislocations are illustrated and described. Different orientation of etch pits reveal twinning in LaAlO3 crystals. Microdisc patterns and flux inclusions are also observed. The etch patterns on the as-obtained LaAlO3 crystals are explained to be as a result of the flux cleaning operation of crystals in HNO3. Experiments on etching established HNO3 to be a dislocation etchant for LaAlO3 crystals. Dislocation etching kinetics of the HNO3-LaAlO3 surface system are investigated for the freshly identified sites as well as for sites having a previous history of etching. Data obtained on the effects of etching time, etchant concentration and temperature on the dislocation etch rates, are analysed. The results obtained are presented and discussed.

Crystal growth of yttrium and samarium tartrates from silica gels

Abstract The results on the growth of hydrated crystals of rare earth tartrates R 2 (C 4 H 4 O 6 ) 3 , where RY, Sm from silica gels, in two different temperature ranges (25–30°C and 35–40°C), employing single-and double- diffusion techniques, are presented. The effects of various parameters such as gel pH, gel aging and concentration of reactants have been investigated for each system. We report a new kind of periodic zone, generally known as Liesegang rings, for yttrium tartrate crystals when grown in the temperature range 35–40°C. The periodicity of nucleation zones, however, is broken for samarium tartrate crystals. An attempt has been made to improve the size of spherulites by using the method of seeded growth.

Spherulitic crystal growth of Y1 − xSmx rare earth tartrates in silica gels

Abstract Pure (Y,Sm) and mixed rare earth (Y 1 − x Sm x ) tartrates grow as spherulites in silica gel by reaction of the corresponding rare earth nitrate ions with tartaric acid. The morphology of the spherulites grown is illustrated. The spherulites exhibit different morphologies even within the same variety of crystals. Different types of structures of spherulitic formations are described. The internal structure of the spherulites is shown to be fibrous. It is shown that the internal structures by and large support the mechanism proposed by Kotru et al.

Dislocation etchants for flux grown YFeO3 single crystals

Abstract Etching studies have been carried out on two different crystallographic planes, i.e., 110 and 001, of flux grown YFeO3 single crystals. We report two dislocation etchants: (i) H3PO4 and (ii) H3PO4 in combination with HNO3. The etch pit density is estimated to be in the range of (4–5) x 104 cm-2.

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.

Elemental compositional changes in surface microstructures of flux-grown YCrO3 single crystals

Abstract Elemental analyses have been carried out on a few interesting surfaces of single crystals of flux-grown YCrO 3 by energy-dispersive spectroscopic techniques. We report on the formation of various microstructures on host surfaces which have suffered elemental compositional changes in an intriguing manner during the flux growth of YCrO 3 crystals.

Defect characterization of flux-grown GdCrO3 single crystals by scanning electron microscopy and energy dispersive spectroscopy

Results of scanning electron microscopic and energy dispersive X-ray spectroscopic studies conducted on the defect structures exhibited by the virgin surfaces (habit faces) of single crystals of GdCrO3 and on the microstructures of the polished surfaces of these crystals grown from a PbO-PbF2 flux, are described. Some typical surface microstructures observed on the virgin surfaces are elliptical networks, ridges and furrows, microcrystals, inclusions and irregular microelevations. The stoichiometry of the elements of GdCrO3 across some microareas is disturbed. Energy dispersive analyses of virgin and polished surfaces of GdCrO3 crystals reveal the precipitation of lead compounds as secondary phases. The presence of other contaminants like Al and Si also appears in the energy spectrum. Crystallization and/or precipitation of secondary phases, including Pb2CrO5 and Pb2OF2 (undissolved flux), during the flux growth of the major phase GdCrO3, is discussed.

Chemical etching of (100) and (110) faces of flux-grown LaBO3 crystals

Abstract Experiments on the etching of (100) and (110) faces of LaBO 3 crystal surfaces are offered. The results reveal HNO 3 to be a dislocation etchant for both LaBO 3 crystal faces. It is shown that the shape of the etch pits due to HNO 3 is different for different habit faces. The dependence of the etch rates for (100) and (110) faces (lateral as well as vertical) on the concentrations and temperature of the etchant, are described and discussed. It is shown that the faces resist attack of the etchant in the direction of the normal to the surface after 2 h of etching irrespective of the concentration of the etchant used at different temperatures. It is further shown that till the time the passivity sets in, the variation of depth with etching time is linear in all the cases.