Volume 40, Issue 5
May 1993, pages 1-407
pp 1- May 1993 Comment
pp 335-343 May 1993
Using paracrystalline statistics, we have simulated one-dimensional and three-dimensional X-ray diffraction patterns from natural silk fibres for various values of crystal size and lattice distortion parameters. This is in agreement with the experimental observation of X-ray pattern reported earlier.
pp 345-356 May 1993
The infrared absorption in isotopically disordered anharmonic crystals is theoretically investigated using double time thermal Green’s function technique. The Hamiltonian used in the study includes the cubic and quartic anharmonicities alongwith the terms associated with the crystal defects. The total i.r. absorption in impurity induced anharmonic crystals is obtained as the sum of diagonal and non-diagonal terms. The diagonal terms can be further separated into defect dependent, anharmonic and impurity-anharmonicity cross term contributions, the non-diagonal terms chiefly depend on mass change parameters and vanishes in the absence of impurities. The impurity-anharmonicity interactions involve defect and anharmonic parameters simultaneously and contribute significantly to the absorption coefficient. A brief description of impurity modes, phonon life times and strength of absorption is given with special reference to impurity-anharmonicity interactions to study the effects of temperature, frequency and impurity concentration on optical absorption.
pp 357-365 May 1993
High pressure Raman spectroscopic studies on Gd2(MoO4)3(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO4 tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.
pp 367-376 May 1993
We report the diamond anvil cell (DAC) high pressure powder X-ray diffraction studies on amorphous selenium (a-Se) under truly hydrostatic pressure condition up to 20 GPa. Amorphous selenium exhibits a sharp and irreversible transition to a hexagonal structure at 10.6 ± 0.1 GPa. It is also known that metallization occurs in a-Se around this pressure. Some plausible arguments are provided to suggest that the amorphous to crystalline transition may be driven by metallization.
pp 377-389 May 1993
Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As2S3 are reported. The a.c. conductivity of pure As2S3 obeys a well-known relationship: σac ∝ωs. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As2S3. But a different behaviour of a.c. conductivity has been observed for the nickel doped As2S3. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σac ∝ωs) for nickeldoped As2S3 is similar to pure As2S3 at lower temperatures. But at higher temperatures, ln σac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D0′) in the band gap. Single polaron hopping is expected to occur between theseD0‘ andD+ states. Furthermore, allD+,D0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As2S3. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.
pp 391-398 May 1993
Time differential perturbed angular correlation measurements of the 133–482 keVγ-γ cascade of181Ta in Hf-doped YBa2Cu3O7−x are presented. The181Hf precurser nuclei are incorporated into the sample by thermal neutron irradiation. Two quadrupole interaction frequencies are observed in the as-irradiated sample:vQ1=161±10 MHz with intensityf1=75%, asymmetry parameterη1=0.32 and damping parameter Λ1=0.42, andvQ2=1108±40 MHz withf2=25%,η2=0.62, and Λ2=0.60. On annealing the sample in air at various temperaturesTa and quenching to room temperature,f1 remained nearly constant forTa<600°C andvQ1 for all annealing temperatures indicating that these are insensitive to oxygen stoichiometry. This frequency is interpreted to be due to181Hf substitutingY sites. BeyondTa=600°C,f1 increased and reached a constant value of 90% forTa=800°C. The value ofvQ2 showed a slight variation between 1086 and 1160 MHz, whilef2 remained nearly constant at 25% forTa<600°C. This component is identified to be due to181Hf substituting Cu 1 sites in the Cu-O chains of YBCO. Above 600°CvQ2 decreased and reached a value of 808 MHz beyond 750°C.
pp 399-407 May 1993
In this paper we report the experimental Compton profile of polycrystalline molybdenum. The measurements have been made by scattering 59.54 keVγ-rays and are compared with the recent band structure calculation of Janiet al. . These results have also been compared with our calculation based on the renormalized-free-atom model for different 4d-5s configurations. It is found that the present experimental data are relatively in good agreement with the band structure calculation.
Volume 93 | Issue 6
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