Indian Journal of Pure & Applied Physics Vol. 43, February 2005, pp. 109-114 Lattice dynamics of thorium pnictides and chalcogenides Mahendra Aynyas 1 , P K Jha 3 & Sankar P Sanyal 2 1 Department of Physics, S V P G College, Bairagarh, Bhopal 462 030 2 Department of Physics, Condensed Matter Physics Laboratory, University of Bhopal, Bhopal 462 026 3 Department of Physics, Faculty of Science, The M. S. University of Baroda, Vadodara 390 002 E-mail address: spsanyal@sancharnet.in Received 19 August 2004; accepted 7 December 2004 The phonon spectra of thorium pnictides and chalcogenides (ThX) have been analysed using rigid ion and breathing shell models, in view of their structural properties at high pressure. The significance of these two approaches in predicting the dynamical properties of ThX compounds has been discussed and the role of f-electron has been examined. The dominant ionic nature of bonding has been predicted for ThX compound from the large LO-TO phonon splitting at zone centre. The low temperature specific heat for these compounds has been reported. [Keywords: Lattice dynamics, Thorium pnictides, Chalcogenides, Phonon spectra] IPC Code: B01D 9/00 1 Introduction The uranium monopnictides and monochalco- genides crystallize in simple NaCl structure at normal pressure and room temperature and have been extensively studied in recent years, both experi- mentally and theoretically 1-7 . The corresponding thorium compounds having the same structure except for ThTe which crystallizes in the CsCl structure, are comparatively less explored so far. In the case of uranium chalcogenides and UP and UN, the lattice parameters, elastic and phonon properties are strongly related to the localized state of the 5f electrons of the uranium ion 3 . On the other hand, thorium has essentially no 5f electrons, therefore, a comparison of the elastic and phonon properties of the uranium and thorium compounds should give some information on the influence of the 5f electrons and the type of bonding in these compounds. The high pressure structural behaviour, equation of state and electronic structure calculation of thorium compounds have received some importance in the recent past 7-8 . It has been observed from high pressure X-ray diffraction studies that thorium nitride does not show any phase change up to 47 GPa unlike its uranium counterpart 9 . Another anomaly is also observed in the case of thorium telluride (ThTe) which crystallize in CsCl (B2) structure at ambient condition, while uranium telluride (UTe) undergoes a B1(NaCl) to B2(CsCl) structural transition at high pressure. A remarkable feature in the case of bulk modulus scaling shows that the B 0 is inversely proportional to the power -1.85 of V 0 for thorium monochalcogenides while this value lies between –5/3 for the uranium monopnictides and –2 for the uranium monochalcogenides 9-10 . Electronic structures of ThAs and ThSb at high pressure calculated from first principle tight binding (TB-LMTO) have also recently been reported 8 . No other first principle or model calculations to understand the physical properties of these compounds are available so far. Motivated by the above facts and some of our earlier work on the phonon dynamics and high pressure behaviour of several uranium and rare-earth compounds 2,11 , we have recently, reported the high pressure structural and elastic properties of thorium compounds by using simple interatomic potential approach 12-13 . In the present paper, we report for the first time the results on phonon properties of thorium compounds (ThX; X = N, P, As, Sb, S, Se) using theoretical models relevant to the present system, namely, breathing shell 14 model (BSM) and two body rigid ion model (RIM) 14 which have been found to explain the phonon properties in uranium and rare- earth compounds 2,9 successfully. Since these two models have different approaches as far as the interactions between ions are concerned. The present study will also be helpful in making qualitative understanding of phonons in this group of solids and draw some conclusion regarding the role of f- electrons. We closely follow the method outlined in Ref. 14.