• Volume 43, Issue 1

      July 1994,   pages  1-115

    • Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

      M S Somayazulu Nandini Garg Surinder M Sharma S K Sikka

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      Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2.

    • Effect of Fe substitution on the thermal expansion of the highTc superconductor Y1Ba2Cu3Oy

      Ashok Rao Om Prakash S N Das P N Dheer

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      Thermal expansion measurements have been carried out on Fe substituted superconducting compounds Y1Ba2(Cu1−xFex)3Oy (0<x<4%) using a high resolution dilatometer employing the three terminal capacitance technique. The experimental set up is sensitive enough to detect changes in α of less than 10−8K−1. Results show that the jump Δα in the coefficient of linear thermal expansion at the superconducting transition temperature,Tc, increases almost linearly with Fe concentration. The normal state thermal expansion coefficient α first decreases, attains a minimum value aroundx=1% and then increases for higher Fe concentrations. The oxygen content per unit formula is almost constant up tox=1% and then increases rapidly withx. It has also been observed that the anomalous behaviour of α around 260 K observed by Meingastet al [22] for Fe concentrationx=5% is due to inadequate annealing of the sample.

    • Magnetism, exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

      Nirmala Chandrasekharan Sukumaran Vasudevan

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      FePS3 is a layered antiferromagnet (TN=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate5T2g ground state of the Fe2+ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σi [δ(Liz2−2)+|λ|Li.Si]−ΣijJijSi.Sj. The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,Jij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;Jnnk=27.7 K; andJnnnk=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aTN=122 K for FePS3, which is remarkably close to the observed value of theTN. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS3, for which the high temperature series expansion susceptibility is available.

    • X-ray diffraction studies of RbBr-RbI mixed crystals

      T Kumara Swamy K G Subhadra D B Sirdeshmukh

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      Mixed crystals of RbBr and RbI have been prepared from melt. Using an X-ray diffractometer powder patterns have been recorded. The lattice constants show slightly positive deviations from Vegard’s law. From the integrated intensities, the mean Debye-Waller factor has been determined. The Debye-Waller factor shows a highly non-linear composition dependence with positive deviations from linearity, the values for intermediate compositions exceeding those for the end members. The Debye temperatures calculated from the Debye-Waller factors show a non-linear composition dependence with negative deviations from linearity.

    • Interplay of superconductivity and antiferromagnetism in SmRh4B4

      T C Loya S L Kakani

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      A microscopic theory of interplay of superconductivity and antiferromagnetism in rare earth ternary systems is developed from first principles for less than half filledf atomic shells. Self consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ, are derived using a Green’s function technique and equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor SmRh4B4. The present model explains true coexistence of superconductivity and antiferromagnetism and the suppression of superconductivity by antiferromagnetism. The behaviour of superconducting order parameter (Δ), magnetic order parameter (Γ), the specific heat, the density of states, free energy and critical field (Hc) is also studied for the system SmRh4B4.

    • Photoconductivity and dark conductivity studies of solution-gas interface grown Ag2S films

      P S Nikam C B Shinde

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      Ag2S films of thicknesses ranging between 500–4000 Å were formed on glass substrate, employing solution-gas interface technique. Dark conductivity of Ag2S films has been studied at different temperatures and Bi(III) concentrations. Photoconductivity of these films has been investigated at different temperatures, illumination levels, excitation energies and Bi(III) concentrations. Dark conductivity is explained on Slater and Neugebauer models, while photoconductivity is explained on Miccoci and Rose models.

    • Enhanced bistability with the line-narrowed modes in a nonlinear modulated index Fabry-Perot cavity

      S Dutta Gupta

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      We exploit the recently reported line narrowed modes [1] for observation of bistability in a nonlinear Fabry-Perot cavity with intracavity index modulation. We show that the frequency response of the structure exhibits larger bistability loops for the line narrowed modes.

    • New band system of MnSe molecule

      K N Uttam R Gopal M M Joshi

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      Thermal emission spectrum of MnSe molecule, excited in high temperature graphite furnace, has been photographed in the spectral region λλ4150–5800 Å at a reciprocal linear dispersion of 7.3 Å/mm. The study reveals the presence of two band systems viz.A–X andB–X in whichB–X is entirely new. While the systemA–X consists of 29 bands, the new systemB–X comprises of 24 single headed and red degraded bands. Vibrational analysis performed suggested that both the systems involve ground state6Σ with a vibrational frequencyωe=361.0 cm−1. Transition responsible for MnSe spectrum appears to be of the type6Σ–6Σ.

      An estimation of the ground state vibrational frequency for MnTe molecule gives rise to its value as 280 cm−1.

    • Thermal diffusion factors of hydrogenic trace mixtures with helium by column calibration factor

      G Dasgupta A K Datta S Acharyya

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      Using column calibration factor (CCF)Fs for a given column, the temperature dependence of experimental thermal diffusion factors αT of hydrogenic trace mixtures in helium are accurately determined. This study, however, observes the inelastic collision effect in these trace mixtures when αT by our CCF method are compared with those by the existing methods and theoretical ones respectively.

    • Construction of broadband homonuclear TOCSY sequences via numerical optimization

      N Sambasiva Rao Narsimha Reddy

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      Some new isotropic mixing sequences have been designed via numerical optimization procedure. The performance of the isotropic mixing sequences generated here, has been assessed and compared with other existing sequences by computer simulation. The global quality factors for some of the present sequences are better, when compared to the sequences that are routinely used in the literature for TOCSY experiments.

    • Design and fabrication of a high temperature X-ray powder diffraction camera

      Mohd Ateequddin K A Hussain K G Subhadra D B Sirdeshmukh

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      A high temperature X-ray powder diffraction camera has been designed and fabricated with indigenous material. The camera design is similar to the Unicam model but overcomes some inconvenient features of the latter. The camera incorporates a miniheater which is inexpensive and easily replaceable. The camera enables determination of lattice parameters with an accuracy of ±0.0002 Å up to 800°C.

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