High-pressure behaviour of lanthanum mono pnictides

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 (B1) 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.