Dass, Lachhan

Kachhava, C M

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Letters to the EditorIndian J .  Phya. 50, 104S-1045 (1976)O n the evaluation  o f  e ffective  charge o f  ion ic  crystalsLachhman D a s s  a n d  0. M. K a u h h a v a  Department of Physios, University of Rajastltaii, Jaipm {Received 25 May 1976)Tlxo couoopt of offocitivo charge occupies a j)osjtion of central importance in the (lielecl.ric tlieoiies ol ionic crystals. It is, therefore, ot practical value to calculate it from tliooretical formulae. Tho shell model d\ic to Dick h Overhausei (1968) lias proved itself to bo tlio best upto-duto available theory. However, the involvc- meiit; of so manj’^ parameters makes the theory more of a qualitative nature. Hence it is desirable to work oii simx>lo foimulations for tho computation of ofloctive ohaigo. This communicnlion presents such results on some alkali halides which arc the xiioto-typc of ionic crystalsTieatmg the deformation dipole moment, due to oveilap between nearest neighbours, in terms of exchange charges, Hardy (1961) show(*d that tho offettivo charge, .v, obeys the relation,= 2d(r„'“—r+=S)(p-i-2ro“ i)A exp(-rop-^). (1)The validity cf this equation, in the original work, was examined by plotting tlie experimental values of left hand (quantity against the light hand quantities minus the factor 2d. Tho reliability so concluded is of rather approximate nature and the test is rather qualitaiive. However, the calculation of s from eq. (1 ) not only loads to theoretical values but their comparison w'itph the experimental data provides a direct and iirecisc check of the appropriatoness of the features of the theory also Those residts are cited in the tabic. The values of the overlap parameters were obtained through the usual thermodynamical conditions (Bom & Mayer 1932). The experimental values are those obtained from Bzigrti (1960) relation.The formulation due to Woods, Cochran & Brookhouse (1960) is of more simplified nature owing to the assumption that only negative ion is polarizable as a oonsequenoo of the shell-shell interaction. The final result is3v 47T "Cq—1 0^+ 2 * (2)Hero we have employed and e„ data of Burstein (1965) excepip for Lil which1043

On The Evaluation of Effective Charge of Ionic Crystals

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On The Evaluation of Effective Charge of Ionic Crystals

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