• S K Dhar

      Articles written in Pramana – Journal of Physics

    • Studies on the valence state of Ce and Eu in some new boron and silicon containing rare earth intermetallic compounds

      S K Malik S K Dhar R Vijayaraghavan

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      Compounds of the formula RPd3Bx (R=rare earth with 0⩽x⩽1) and RPd3Six (R=La, Ce, Eu with 0⩽x⩽0.3) can be prepared by alloying boron or silicon with parent RPd3 compounds. Addition of boron (silicon) does not change the structure but results in lattice expansion. The valence state of Ce in CePd3 and that of Eu in EuPd3 is strongly influenced by boron and silicon. Ce is known to be in a valence fluctuating state in CePd3 while Eu is trivalent (J=0) in EuPd3. The increase in the lattice parameter as a function of boron concentration is observed to be larger in CePd3Bx and EuPd3Bx compared to that in other RPd3Bx alloys giving the first indication of the change in the valence state of Ce and Eu. This is confirmed from susceptibility measurements. With the addition of boron, susceptibility increases and the effective paramagnetic moments approach the values corresponding to Ce3+ (J=5/2, μeff=2.54 μB) and Eu2+ (J=7/2, μeff=7.94 μB) in the two alloy systems CePd3Bx and EuPd3Bx respectively. In the case of europium alloys,151Eu Mössbauer studies point out the importance of near-neighbour environment effects. Further, in EuPd3B, where all the europium ions are crystallographically equivalent, a single Mössbauer line, with an isomer shift characteristic of europium ions in valence-fluctuating state, is observed at 300 K. However, at 88 K the Mössbauer absorption splits into two lines corresponding to europium ions in two valence states,e.g. divalent- and trivalent-like. Such a behaviour indicates thermally-induced charge ordering of europium ions. Addition of silicon to CePd3, like boron, results in unusual lattice expansion and changes the valency of cerium towards 3+. the valence change is further corroborated by susceptibility measurements. In EuPd3Six alloys, susceptibility and Mössbauer studies indicate that in the limiting single phase alloy EuPd3Si0.25 the europium ions are on the verge of valence instability. Susceptibility results on CeRh3Bx alloys are also presented.

    • Electromagnetic response of a single phase Y-Ba-Cu-O compound superconducting in the 90 K range

      V Nagarajan P L Paulose A K Grover S K Dhar E V Sampathkumaran

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      The results of x-ray diffraction, electrical resistance, DC and AC magnetization studies on two specimens of compositions Y2.1Ba3.9Cu6O14−δ and Y1.8Ba4.2Cu6O14−δ are presented. Our studies confirm that La3Ba3Cu6O14 type orthorhombic phase is responsible for superconductivity in the 90 K range. The heat treatment that yields high quality samples with sharp transition in electrical resistance as well as in AC magnetic susceptibility measurements is described. Magnetic response just belowTc is found to be sensitive to the measuring field.

    • Low temperature heat capacity of YBa2Cu3O7

      N Nambudripad S K Dhar

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      We have measured the heat capacity of superconducting, single phase YBa2Cu3O7 in the temperature range 2 to 18 K. An extrapolation of the data between 4 and 9 K gives aC/T (T → 0) of ∼ 25 mJ/mole K2. The Debye temperature obtained from the high temperature linear portion ofC/T vsT2 plot is 325 K.

    • Possible coexistence of superconductivity and magnetic order in NdPt2B2C

      S K Dhar A D Chinchure E Alleno C Godart L C Gupta R Nagarajan

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      Coexistence of superconductivity and magnetic order has been one of the exciting aspects of the quaternary borocarbide superconductors, So far, RNi2B2C (R=Tm, Er, Ho and Dy) are the only known magnetic superconductors in this family. Here, we present our resistivity, magnetization and heat capacity studies on NdPt2B2C (nominal composition, NdPt1.5Au0.6B2C and NdPt2.1B2.4C1.2). We find superconductivity in both samples with Tc,onset ∼ 3 K. Bulk magnetic order is found to occur below 1.7 K. We suggest that NdPt2B2C is a possible magnetic superconductor.

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