T. R. Narayanan Kutty

Photoluminescence of Sr2−xLnxCeO4+x/2(Ln = Eu, Sm or Yb) prepared by a wet chemical method

A wet chemical route is developed for the preparation of Sr2CeO4 denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO2.xH(2)O, 40<x<75) by the addition of ammonium carbonate. During calcination, CeO2.xH(2)O dehydroxylation is followed by the reaction with SrCO3 to form Sr2CeO4 with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr2CeO4 originates from the Ce4+-O2- charge-transfer (CT) transition resulting from the interaction of Ce4+ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce4+-O2- CT emission is studied by doping with Eu3+, Sm3+ or Yb3+ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce4+-O2- CT state to trivalent lanthanide ions (Ln(3+)) if the latter has CT excited states, leading to sensitizer-activator relation, through non-resonance process involving exchange interaction. Yb3+-substituted Sr2CeO4 does not show any line emission because the energy of Yb3+-O2- CT level is higher than that of the Ce4+-O2- CT level. Sr2-xEuxCeO4+x/2 shows white emission at xless than or equal to0.02 because of the dominant intensities of D-5(2)-F-7(0-3) transitions in blue-green region whereas the intensities of D-5(0)-F-7(0-3) transitions in orange-red regions dominate at concentrations xgreater than or equal to0.03 and give red emission. The appearance of all the emissions from D-5(2), D-5(1) and D-5(0) excited states to the F-7(0-3) ground multiplets of Eu3+ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu3+. White emission of Sr2-x SmxCeO4+x/2 at xless than or equal to0.02 is due the co-existence of Ce4+-O2- CT emission and (4)G(4)(5/2)-H-6(J) Sm3+ transitions whereas only the Sm3+ red emission prevails for xgreater than or equal to0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce4+. Quenching of Ce4+-O2- CT emission by other Ln(3+) is due to the ground state crossover arising out of the NNN interactions.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Coarse Bn+On/2·xH2O (10 < x < 120) gels, free of anionic contaminants react with A(OH)2 solutions under refluxing conditions at 70–100 °C giving rise to nanoparticles of multicomponent oxides (A = Ba, Sr, Ca, Mg or Pb; B = Zr, Ti, Sn, Fe, Al or Cr). These include ABO3 perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel–crystallite (G–C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A2+ ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B—(OH)—B or B—O—B, leading to the breakdown of the gel into crystallites. G–C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity.

Thermal decomposition of titanyl oxalates—I: Barium titanyl oxalate

Thermal decomposition of barium titanyl oxalate tetrahydrate (BTO) has been investigated employing TGA, DTG and DTA techniques and gas and chemical analysis. The decomposition proceeds through five steps and is not affected much by the surrounding gas atmosphere. The first step which is the dehydration of the tetrahydrate is followed by a low-temperature decomposition of the oxalate groups. In the temperature range 190–250°C half a mole of carbon monoxide is evolved with the formation of a transient intermediate containing both oxalate and carbonate groups. The oxalate groups are completely destroyed in the range 250–450°C, resulting in the formation of a carbonate which retains free carbon dioxide in the matrix. The trapped carbon dioxide is released in the temperature range of 460–600°C. The final decomposition of the carbonate takes place between 600–750°C and yields barium titanate. The i.r. spectra, surface area measurements and X-ray, powder diffraction data support entrapment of carbon dioxide in the matrix.

Thermal decomposition of titanyl oxalates IV. Strontium and calcium titanyl oxalates

Abstract Thermal decomposition of strontium titanyl oxalate tetrahydrate and calcium titanyl oxalate hexahydrate have been studied employing TG, DTA, gas and chemical analysis. The decompositions proceed through three major steps: dehydration, decomposition of the oxalate to a carbonate and the decomposition of the carbonate to yield the final products, the metatitanates. The intermediates of the oxalate decomposition are found to be Sr 2 Ti 2 O 4+x (CO 3 ) 2 -x(CO 2 ) x and Ca 2 Ti 2 O 4 (CO 3 ) 2 , respectively. The entrapment of carbon dioxide in the former and the presence of non-equivalent carbonate groups in the latter are substantiated by their i.r. spectra. The penultimate solid residues are poorly crystalline Sr 2 Ti 2 O 5 CO 3 and amorphous Ca 2 Ti 2 O 5 CO 3 . Decompositions of these carbonates are accompanied by growth in particle size of the products, SrTiO 3 and CaTiO 3 , respectively.

Thermal decomposition of titanyl oxalates—II: Kinetics of decomposition of barium titanyl oxalate

Abstract Kinetics of the thermal decomposition of barium titanyl oxalate have been studied. Decomposition of the anhydrous oxalate is complex and deceleratory throughout. Kinetics of decomposition of the intermediate carbonate Ba2Ti2O5CO3 is greatly influenced by the thermal effects during its formation. The sigmoidal (α, t) curves obey a power law equation followed by first order decay. Presence of carbon in the vacuum prepared carbonate has a strong deactivating effect. Decomposition of the carbonate is accompanied by growth in particle size of the product, barium titanate.

Thermal decomposition of titanyl oxalates—III: Lead titanyl oxalate

Abstract Lead titanyl oxalate tetrahydrate (LTO) has been prepared using a chloride-free medium. The thermal decomposition of LTO has been investigated employing thermoanalytical and gas analysis techniques. The decomposition in air or oxygen is straight forward and proceeds through three main steps: dehydration, decomposition of the oxalate to a carbonate and decomposition of the carbonate to give lead metatitanate as the final product. This is complicated in a vacuum or non-oxidising atmosphere, by the partial reduction of Pb(II) to Pb(0) at the oxalate decomposition step. The formation of free metallic lead affects the stoichiometry of the intermediate carbonate, inhibits the evolution of carbon dioxide after the decomposition of the carbonate and yields a mixture of lead metatitanate and titanium rich PbTi3O7 as the final products. For the preparation of pure lead metatitanate by decomposing LTO, oxidising conditions should be maintained during the decomposition.

Thermal decomposition of rare-earth trioxalatocobaltates

The thermal decomposition of rare-earth trioxalatocobaltates LnCo(C2O4)3 · x H2O, where Ln = La, Pr, Nd, has been studied in flowing atmospheres of air/oxygen, argon/ nitrogen, carbon dioxide and a vacuum. The compounds decompose through three major steps, viz. dehydration, decomposition of the oxalate to an intermediate carbonate, which further decomposes to yield rare-earth cobaltite as the final product. The formation of the final product is influenced by the surrounding gas atmosphere. Studies on the thermal decomposition of photodecomposed lanthanum trioxalatocobaltate and a mechanical mixture of lanthanum oxalate and cobalt oxalate in 1 : 2 molar ratio reveal that the decomposition behaviour of the two samples is different. The drawbacks of the decomposition scheme proposed earlier have been pointed out, and logical schemes based on results obtained by TG, DTA, DTG, supplemented by various physico-chemical techniques such as gas and chemical analyses, IR and mass spectroscopy, surface area and magnetic susceptibility measurements and X-ray powder diffraction methods, have been proposed for the decomposition in air of rare-earth trioxalatocobaltates as well as for the photoreduced lanthanum salt and a mechanical mixture of lanthanum and cobalt oxalates.

Preparation and thermal stability of ammonium alkaline earth trioxalatocobaltate(III) hydrates: NH 4 + M2+¦Co(C2O4)3¦·xH2O

Trioxalatocobaltates of bivalent metals NH4M2+[Co(C2O2)3] ·xH2O, with M2+=Ba, Sr and Ca, have been prepared and characterized and their thermal behaviour investigated. The compounds decompose in a complex manner to yield bivalent metal carbonate or oxide and cobalt oxide as final products. The formation of the final products is influenced by the surrounding atmosphere during decomposition. Bivalent metal cobaltite M2+CoO3−x is not identified among the final products of decomposition. This study demonstrates the importance of the decomposition mode of the precursor in producing the desired end-product.ZusammenfassungTrioxalatokobaltate zweiwertiger Metalle der allgemeinen Formel NH4M2+ [Co(C2O4)3]· xH2O (M2+=Ba, Sr, Ca) wurden dargestellt, charakterisiert und hinsichtlich ihres thermischen Verhaltens untersucht. Die Verbindungen zersetzen sich in komplexer Weise unter Bildung von Carbonaten und Oxiden der zweiwertigen Metalle und Kobaltoxid als Endprodukte. Die Bildung der Endprodukte wird durch die die Proben während der Zersetzung umgebende Atmosphäre beeinflußt. Kobaltite der zweiwertigen Metalle, M2+CoO3−x, treten nicht als Endprodukte auf. Die Untersuchung zeigt, wie wichtig die Art der Zersetzung des Präkursors für die Darstellung erwünschter Endprodukte ist.РезюмеИсследовано термиче ское поведение синтезированных три оксалатокобальтато в двухвалентных метал лов общей формулы NH4M2+[Co(C2O4)3]·xH2O, где М2+ — барий, стронций и кальций. Со единения разлагаютс я сложным путем, давая карбонат двухвалентного металла или же в качес тве конечного продук та окись этого металла и окись кобальта. Образование конечны х продуктов разложен ия зависит от окружающей атмосфер ы в процессе разложения. Кобальти т двухвалентного мет алла M2+CoO3-x не установлен ср еди конечных продуктов разложени я. Проведенное исслед ование свидетельствует о ва жности типа разложения предшест вующего соединения п ри получении желаемого соединени я и образующегося.

Geochemical Comparison of Archaean Granulites in India with Proterozoic Granulites in Canada

Abstract The charnockitic gneisses of North Arcot, Tamil Nadu, have a bimodal compositional distribution. The basic granulites show a tholeiitic Fe-rich trend, while the intermediate and acid charnockites show a calc-alkaline one, A similar compositional variation is apparent in amphibolite facies gneisses and asscciated mafic rocks and in the low-grade schist belts of Kolar and Holenarasipur of southern Karnataka. However there are notable differences in their respective trace element compositions. The charnockitic rocks are low in largeion lithophile (LIL) elements (i. e. K, Rb, Th, U, Pb) but not in cther normally incompatible alements (i. e. Ba, Sr, Zr), and have higher K/Rb and lower Rb/Sr ratios compared with the lower grade gneisses. The geochemical data suggest that the amphibolite faciss gneisses of South Karnataka are not retrogressed charnockites; on the other hand, the charnockites may be derived from the highgrade gneisses. There is no major chemical distinction between Archaean and Proterozoic granulites. As such, the hypothesis that granulites are derived from an igneous parent magma that contained low levels of LIL trace elements does not seem to be valid. The basic members of the charnockitic suite compare better with the mafic enclaves within the high-grade gneisses than with the mafic components of the schist belts.

Hydrothermal phase equilibria studies in Ln2O3H2O systems and synthesis of cubic lanthanide oxides

Phase diagrams for the systems Ln2O3H2O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) studied at 5000 to 10,000 psi and temperature range of 200–900°C, show that Ln(OH)3 hexagonal and LnOOH monoclinic are the only stable phases from Nd to Ho. The cubic oxide phase (CLn2O3) is stable for systems of Er, Tm, Yb and Lu, with no evidence of its equilibrium in the systems of lighter lanthanides. Using strong acids, HNO3 and HCOOH, as mineralisers the cubic oxides could be stabilised from Eu down to Lu. Solid solution phases of CeO2Y2O3 and Eu2O3Y2O3 have also been synthesised with HNO3 as mineraliser, since these compounds have promising use as solid electrolyte and phosphor materials respectively.