Sushma Bhat

Dielectric studies of lanthanum heptamolybdate crystals grown from gels

Abstract Results of dielectric measurements conducted on lanthanum heptamolybdate grown by a gel technique employing the system La(NO 3 ) 3 MoO 3 NH 4 OHHNO 3 Na 2 SiO 3 are presented. The variations of the dielectric constant e′, dielectric loss tan δ and conductivity σ with the temperature and frequency are critically analyzed. The dielectric constant increases with temperature at lower frequencies (10 3 Hz), reaching a maximum value of 320 at 125 °C, after which it abruptly decreases. The dielectric loss tan δ and conductivity σ show both a temperature as well as a frequency dependence. The sharp increase in e′ with temperature is attributed to total polarization resulting from a space charge effect. The functional relationship between the dielectric constant, conductivity and temperature is determined by fitting the observed data with the respective polymeric expressions of the type ϵ ′ = a 0 + a 1 T 2 - lnσ = a + b ( T - T 0 ) 2 The non-linearities and anomalous behaviour of the dielectric constant near the transition temperature, supported by hysteresis studies, indicate the possibility of these materials ferroelectric.

Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium

Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium is investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal behaviour suggests that the materials are unstable at lower energies and pass through various stages of decomposition, decomposing to respective rare earth oxides which remain stable on further heating. It is estimated that both pure yttrium and pure samarium tartrate crystals carry eight waters of hydration, while mixed yttrium samarium tartrate crystals carry six waters of hydration. Critical examination of TG and DSC curves shows that the initial decompositions are endothermic and the latter are exothermic. Thermal kinetics of these materials has been worked out using Horowitz-Metzger, Piloyan-Novikova and Coats-Redfern equations. Application of these equations to these materials yields values of activation energy, order of reaction and frequency factor which are in reasonably good agreement.

Growth of neodymium heptamolybdate crystals in silica gels at ambient temperatures

Abstract The growth of neodymium heptamolybdate crystals is described in the system Nd(NO3)3-MoO3-NH4OH-HNO3- Na2SiO3 using a single gel, single tube technique. The influence of pH, gel concentration and concentration of upper and lower reactants on the size, quality and nucleation density of the crystals is discussed. The techniques of concentration programming and seeded growth are used to enhance the crystal size. Factors influencing the formation of Liesegang rings during the growth are described. Both gel pH and temperature are observed to have a remarkable effect on Liesegang ring formation. The grown crystals are observed to exhibit varied morphology, for example platelets, cuboids, spherulites, crystal aggregates and coalesced crystals.

Characterization of lanthanum heptamolybdate crystals grown from silica gels

Crystals of lanthanum heptamolybdate grown from silica gel using the system La(NO3)3-MoO3-NH4OH-HNO3-Na2SiO3 were characterized. The results of energy-dispersive X-ray analysis, X-ray and electron diffraction, infrared spectroscopy, and optical and scanning electron microscopy are reported. The composition of the crystals is established as La2Mo7O24430H2O. The crystals exhibit varied morphologies, including square and octagonal platelets, cuboids, multifaceted crystals, coalesced and aggregated forms and spherulites. The spherulites are reported to have formed as a result of a large number of tiny crystallites becoming adhered in a spherical envelope.

Fracture toughness and hardness studies on flux-grown YbFeO3 single crystals

Abstract Results of Vickers microhardness measurements carried out on the (001) and (110) planes of flux-grown YbFeO3 single crystals in the applied load range of 10–100 g are presented. The microhardness for a particular load is independent of the loading time. The variation of the load is found to be nonlinear and is explained by applying Hays and Kendalls law. The work hardening index, materials resistance pressure and other constants have been calculated. The average value of the fracture toughness Kc, as determined from measurements of crack length, is estimated to be 0.1319 g μm −3 2 and 0.1076 g μm −3 2 for the (110) and (001) planes, respectively. The brittleness index is estimated as 5.9565 μm −1 2 and 9.7174 μm −1 2 for the (110) and (001) planes, respectively.

Etching of (0 0 0 1) habit faces and matched cleavages of flux grown strontium hexaferrite crystals

Results of etching (0 0 0 1) planes of flux grown strontium hexaferrite crystals in 85% H3P04 at 120 °C and 37% HCl at 100 °C are presented. Fractography reveals one-to-one correspondence of cleavage patterns on the two matched (0 0 0 1) cleaved planes. Etch patterns including hexagonal, point-bottomed pits with smooth sloping planes, hexagonal but flat-bottomed pits, geometrically centred hexagonal pits with regularly spaced terracing, eccentric hexagonal pits with irregularly spaced terracing, a large flat-bottomed hexagonal pit with a smaller point-bottomed hexagonal pit within it but having different geometrical centres and flat-bottomed pits with a beak at their centres are illustrated. It is explained that they are indicative of normal, inclined, stepped and bending dislocations in strontium hexaferrite crystals. Pits due to impurity inclusions are also explained. The explanations are supported by the results of mismatchings of etch patterns on matched cleavages.

Growth of neodymium lanthanum heptamolybdate mixed crystals in silica gels

Abstract The optimum conditions for the growth of mixed crystals of neodymium lanthanum heptamolybdates in silica gel are found by investigating different growth parameters. A two-gel-single-tube technique coupled with seeded growth and concentration programming yields the maximum size crystals. The crystals assume spherulitic, square and octagonal platelet, and cuboid morphologies in the gel medium. Employing energy dispersive X-ray analysis, it is shown that the mixed crystals are those of neodymium lanthanum heptamolybdate in which the two rare earths combine in 1:1 ratio.

Characterization of gel-grown neodymium heptamolybdate crystals

Neodymium heptamolybdate crystals grown from silica gels [1] are characterized em ploying energy dispersine X-ray analysis, X-ray and electron diffraction, infra-red spectroscopy, optical microscopy and scanning electron microscopy techniques and the results obtained are presented. The crystals are established to have the composition Nd2Mo7O2427H2O. The results of electron diffraction studies indicate the material to be thermally unstable. The crystals growing within the gel medium exhibit varied morphologies including square and octagonal platelets, cuboids, multifaceted crystals, coalesced and aggregated forms and spherulites. The spherulitic morphology is shown to arise due to crystallites adhering in a spherical envelope and are not as a result of crystal fibres radiating out from a centrally located common nucleus. The morphology of the building blocks of the crust at the gel reactant interface is discussed.

Evaluation of SrGaxInyFe12−(x+y)O19 single crystals

Abstract Scanning of (0001) habit faces of flux-grown crystals of the type SrGa x In y Fe 12−( x+y ) O 19 (where x = 5, 7, 9 and y = 0.8, 1.3, 1) reveals that most of them contain structures. The structures include pyramids, hillocks, microdiscs, depressions and stepped structures. The stepped structures show the incorporation of more bismuth as an impurity. Fractography reveals (0001) cleavage in substituted hexaferrites. It is shown that the variation in the concentration of gallium and indium in the substituted hexaferrites affects the cleavage in these crystals. 85% H 3 PO 4 at 120 °C and 37% HCl at 100 °C are established as the dislocation etchants for the substituted hexaferrites. Etching of grain boundaries and twinned structures is illustrated. Through etching of matched pairs, it is shown that the linear defects and the planar defects such as grain boundaries and twinning penetrate deep into the body of the crystal. The suitability of substituted crystals as substrates for the growth of good-quality hexaferrite films is discussed.

Spectral characterization of lanthanum heptamolybdate single crystals

Abstract Using sodium metasilicate gel, lanthanum heptamolybdate (La 2 Mo 7 O 24 · n H 2 O) single crystals are grown by the diffusion of lanthanum nitrate into the set gel embedded with a mixture of molybdenum trioxide, ammonium hydroxide and concentrated nitric acid. The infrared (IR) absorption spectrum of lanthanum heptamolybdate (LHM) single crystals identifies the presence of molybdate (MoO 4 ) 2− groups and water of crystallization, while energy dispersive X-ray analysis (EDAX) confirms the presence of heavy elements (La and Mo) in LHM. X-ray photoelectron spectroscopic (XPS) studies of LHM re-establish the presence of lanthanum and molybdenum besides their chemical states as their respective oxides. It is shown that the oxygens in the sample are present in three different environments as terminal oxygen (MoO), bridging oxygen (MoOLa) and oxygen of lattice water (HOH). The implications are discussed.