Journal of Alloys and Compounds 398 (2005) 16–20 High-pressure behaviour of lanthanum mono pnictides Gitanjali Pagare a,b , Sankar P. Sanyal b,∗ , P.K. Jha c a Government M.L.B. Girls P.G. College, Bhopal 462011, India b Condensed Matter Physics Laboratory, Department of Physics, Barkatullah University, Bhopal 462026, India c Department of Physics, M.S. University of Baroda, Vadodara 390002, India Received 7 February 2005; accepted 17 February 2005 Available online 27 April 2005 Abstract Pressure induced structural phase transition of lanthanum mono pnictides (LaY; Y = P, As and Sb) has been studied theoretically by using an interionic potential method. The effective ionic charge has been modified to include the Coulomb screening effect due to the delocalisation of f electron of the rare earth (RE) ion. The anomalous structural properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of La ion with the p orbital of neighbouring pnictogen ion. All the three compounds are found to undergo from their initial NaCl (B 1 ) phase to body centered tetragonal (BCT) phase at high pressure and agree well with the experimental results. The transition pressure of LaY compounds is found to increase with decreasing lattice constant of the NaCl phase. The nature of bonds between the ions is predicted by simulating the ion–ion (La–La and La–Y) distance at high pressure. Elastic properties of these compounds have also been studied and second order elastic constants are calculated. © 2005 Elsevier B.V. All rights reserved. Keywords: High pressure; Lanthanum; Mono pnictides; Phase transition 1. Introduction In recent years, the chalcogenides and pnictides of rare earths (REs) with NaCl structure have drawn great atten- tion of condensed matter scientists because of their diversive and unusual structural, magnetic and phonon properties [1]. Most of these unusual features, however, have been corre- lated with the existence of unfilled f-electron shells of the lanthanide ions which are delocalised [2] and, therefore, in- teract strongly with the lattice. So far as the structural prop- erties are concerned, several pnictides of lanthanide group, which crystallize in NaCl (B 1 ) type crystal structure, have been investigated by using high-pressure X-ray diffraction technique [3–8] and reported to undergo either CsCl (B 2 ) or body centered tetragonal (BCT) structure. The La mono pnictides (LaY; Y=P, As and Sb) with the NaCl crystal structure have recently attracted particular interest as a proper reference material for the understanding ∗ Corresponding author. E-mail address: spsanyal@sancharnet.in (S.P. Sanyal). of various anomalous physical properties of other rare earth pnictides. One of the intriguing features of structural phase transition of RE compounds is that some of these compounds transform to CsCl (B 2 ) phase, while some others prefer BCT structure at high pressure. The X-ray diffraction of cerium mono pnictides has been studied up to 30 GPa at room temperature [8,9]. Amongst all the Ce pnictides, only CeP exhibits an isomorphous transition about 10 GPa, which is associated to an electronic transition involving the change in the valence state of Ce [9]. Furthermore, this compound transforms from NaCl (B 1 ) structure to CsCl (B 2 ) phase around 19 GPa [7]. The high-pressure phase of other pnic- tides of Ce are either BCT or CsCl (B 2 ). The high-pressure forms of the bismuthides of Ce and Pr are reported as the co-existing cubic CsCl and tetragonal phases [10]. The elec- tronic and structural properties of lanthanum pnictides (LaN, LaP, LaAs and LaSb) have been studied by Vaitheeswaran et al. [11,12] by means of self-consistent tight binding linear muffin tin orbital (TB-LMTO) method. The one electron en- ergy band structure for YSb and a series of La pnictides, i.e. LaN, LaP, LaAs and LaSb has been calculated by Hasegawa 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.044