K. K. Raina

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 Kicks 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 [Metallography6 (1973) 275] is discussed.

Growth of lanthanum tartrate crystals in silica gel

The spherulitic, dendritic and single-crystal growth of hydrated lanthanum tartrate by controlled diffusion in silica gels is reported. The influence of growth parameters, e.g. reactant concentrations, gel pH, gel ageing, on the size and nucleation density of crystals has been studied. Operative mechanisms of crystallization, results of growth kinetics and morphology of crystals are discussed. The adsorption property of the gel is found to play a vital role during the crystallization of lanthanum tartrate crystals, Parabolic kinetics, characteristic of a one-dimensional diffusion-controlled process, for single crystals is observed to be obeyed in case of variation of upper reactant concentration.

Characterization and thermal behaviour of gel grown mixed rare-earth (Didymium) tartrate crystals

Sherulites, crystal aggregates and platelets of Di2 (C4H4O6)3 · 5H2O mixed crystals were grown in gel using the single tube diffusion method. The material was characterized by using different techniques such as chemical analysis, EDAX, X-ray and electron diffraction, infrared and mass spectroscopy. The thermal behaviour of the material was studied using differential thermal analysis, DTG, thermogravimetric analysis and differential scanning calorimetry. The material is thermally unstable and starts decomposing at 50‡ C. Thermal analysis results indicate the application of the contracting cylindrical kinetic model for the solid state reactions involved in the decomposition process. The values of kinetic parameters, e.g. order of reaction, activation energy and frequency factor are worked out. The implications are discussed.

Characterization and thermal behaviour of gel-grown gadolinium tartrate crystals

Gadolinium tartrate crystals in the form of spheru!itoe were synthesized by using a controlled diffusion system in sillica gel. Characterization of the material was performed by utilizing the techniques of chemical analysis, X-ray and electron diffraction, infrared and mass spectroscopy, and by As thermal behaviour. The material turns out to be a dihvdrate and the chemical composition vvoethus established as Gd2(C4H4O6)3 · 2H2O. The data obtained from the thermal analysis show the tendency of the material to decompose, and this is further confirmed by mass spectroscopy The decomposition process is completed in four steps until gadolinium oxide is obtained at 840* C. The energetics of the reactions at each stage of decomposition have been examined and mechanisms for the decomposition reactions are proposed.

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.

Growth of praseodymium tartrate crystals in silica gel

The growth of praseodymium tartrate crystals in the system Pr(NO3)3-Na2 SiO3-C4H6O6, using a single-tube-single-gel technique is described. The growth conditions are delineated and a spherulitic morphology is reported. The spikes attached to the spherulites are single crystals of praseodymium tartrate. The mechanisms of crystallization for various types of spherulites are described. The information presented contributes to the understanding of spherulitic growth in general, and that of praseodymium tartrate in particular.

Characterization and thermal behaviour of lanthanum tartrate crystals grown from silica gels

Results obtained on characterization of lanthanum tartrate crystals, grown by the gel method, using chemical analysis, x-ray and electron diffraction, infra-red and mass spectroscopy are reported. The thermal behaviour is studied using DTA, TGA and DTG. The decomposition pattern is reported to be typical of a hydrated metal tartrate. Kinetic parameters like order of reaction, frequency factor and activation energy are evaluated. Contracting cylinder kinetic model is found to be the best fit for the decomposition processes involved. Magnetic susceptibility measurements indicate the material to be diamagnetic.

Characterization and thermal behaviour of praseodymium tartrate crystals grown by the silica gel technique

Results of EDAX, XRD, IR, TG, DSC and SEM carried out on crystalline materials obtained by diffusion of praseodymium ions through silica gel impregnated with tartaric acid are reported. The crystallized material assumed spherulitic morphology which was established to be Pr2(C4H4O6)35H2O. EDAX confirmed the presence of praseodymium. X-ray diffraction data giving 2θ, intensity andd-values is also reported for the first time. Infrared spectrum in the range of 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis (TG and DSC) indicated that the material is thermally unstable; the decomposition only occurs at 40–560 °C, after which it reduces to Pr2O3. SEM results suggest spherulitic growth arising from diverging crystal fibres which originate from multiple nuclei dispersed within a centrally-located spherical region.