PRESSURE-VOLUME RELATION AND PRESSURE-INDUCED STRUCTURAL PHASE-TRANSFORMATION IN YTTERBIUM PNICTIDES

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 app roach based on the rigid ion model which considers only Coulomb and Bo rn-Mayer type short-range interactions. The theoretically predicted pr essure-volume relationship curves show that YbN, YbP and YbAs exhibit normal compression and undergo a NaCl to CsCl type structural transiti on at 164, 115 and 95 GPa, respectively, similar to the europium chalc ogenides 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 compo unds behave similar to the simple rare earth europium chalcogenide com pounds rather than the rare earth mixed valence compounds. A similar c onclusion is also drawn from our earlier studies on the phonon propert ies for these compounds. The nature of the force constants at equilibr ium and high pressure is discussed.