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Aust. J. Phys., 1975, 28, 63-8 The Debye-Waller exponents for oc-iron and sodium are calculated at different temperatures using the Behari-Tripathi (1969) modified angular-force model, which takes into account the effect of electron-ion interactions. The computation uses Blackman's sampling technique. The results are compared with existing experimental data. They are found to agree satisfactorily up to a certain temperature, above which they become progressively lower than the corresponding experimental values.

Aust. J. Phys., 1975, 28, 57-61 Phonon dispersion curves for the normal modes of vibration in chromium and tantalum are calculated along the symmetry directions [100], [110] and [111] using the five force-constant model of Behari and Tripathi (1970a). The results are compared with experimental values obtained from inelastic neutron spectroscopy and reasonably good agreement is found.

From the measurements of the intensities of thermal diffuse scattering observed in the stationary-crystal photographs taken with monochromatic Cu Ke radiation, all the nine elastic constants of orthorhombic 1,3,5-triphenylbenzene single crystals have been determined by the photographic method. Each constant is evaluated from two or more directions of thermal wave vectors. The reciprocal nodes involved for the determination of all the constants were 200, 206, 002, 015 and 006. Diffusely scattered intensities were corrected for general scattering, polarization and skew factors. Corrections for absorption, divergence and second-order diffuse scattering were found negligible. The values of elastic constants in units of 10 ~° dynes.cm -2 are C1~ = 0-31, C22 = 1"36, (733 = 2"33, 6"44= 5"02, 6"55=0"34, C66 = 3"69, C~2= -1"78, 6"13 =0"92 and C2a = 0"80. Correlation of the elastic data with the structure have been attempted.

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