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We have calculated the structural and elastic properties of thorium mono-pnictides at high pressure, for the first time, using a suitable interionic potential. This method has been found quite satisfactory in the case of uranium compounds, and describes the crystal properties in the framework of ionic model. The calculated equation of state, phase transition pressures for B1 to B2 transition and bulk modulus etc. in the case of all four ThX (X = N, P, As, Sb) compounds agree well with the experimental results. We have also reported the higher order elastic constants and discuss the dominance of various types of forces in them.

We have studied the pressure–volume relationship and pressure induced structural phase transformation in ytterbium pnictide compounds (YbN, YbP and YbAs) for the first time by using an interatomic potential approach based on the rigid ion model which considers only Coulomb and Born-Mayer type short-range interactions. The theoretically predicted pressure–volume relationship curves show that YbN, YbP and YbAs exhibit normal compression and undergo a NaCl to CsCl type structural transition at 164, 115 and 95 GPa, respectively, similar to the europium chalcogenides but contrary to their counterparts Yb chalcogenides for which the compression curve shows an anomalous behaviour due to the change in the valence state. The present study also confirms that these compounds behave similar to the simple rare earth europium chalcogenide compounds rather than the rare earth mixed valence compounds. A similar conclusion is also drawn from our earlier studies on the phonon properties for these compounds. The nature of the force constants at equilibrium and high pressure is discussed.

We have studied the structural phase transition due to high pressure in UAs and UTe for the first time by using an interatomic potential approach based on rigid ion model formulation. The theoretically predicted phase transition pressure and other structural properties for these compounds agree reasonably well with the measured values. The relative volume change (V/V01) for both compounds show that it is for UTe twice that for UAs and hence confirms that there may be more f–d hybridization mixed valence states in UTe similar to mixed valence compounds, where there is volume reduction due to f–d hybridization. We did not observe any high pressure structural phase transition for other uranium compounds such as USb, US, USe etc. up to 100 GPa. The nature of the force constants at equilibrium and high pressure is discussed.