Structural, vibrational, elastic and topological properties of PaN under pressure

Abstract

The electronic, structural, vibrational and elastic properties of PaN have been studied at both ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. The high pressure behavior of the acoustic phonon branch along the [1, 0, 0] and [1, 1, 0] directions and the C44 elastic constant are anomalous, which signals a structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition, which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, here it is found that the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). This constitutes an example of GGA and LDA leading to qualitatively different predictions, and therefore it is of great interest to examine experimentally whether this topological transition occurs.

Role of 5f electrons in the structural stability of light actinide (Th–U) mononitrides under pressure

“…It is to be noted that COHP analysis is done using LDA exchange-correlations. In our earlier studies [18][19][20] we have shown that although LDA underestimates the transition pressure (by 10 GPa for ThN and 14 GPa for UN), the structural sequence is correctly reproduced. So we expect LDA based COHP will give the correct bonding changes across the transition.…”

“…It is to be noted that in our earlier studies we have shown that the 5f-electrons are nearly itinerant in ThN, PaN and UN and the GGA exchangecorrelations well describe their bulk properties at ambient and high pressure. [18][19][20] Among all these nitrides only UN may possess on site Coulomb correlations at ambient pressure. However, recent angle-resolved photoelectron spectroscopy measurements 7 and many other earlier experiments [30][31][32][33][34][35][36] on UN confirmed the itinerant nature of the uranium 5f-electrons.…”

“…In our previous theoretical work on ThN, PaN and UN we have shown that under pressure they behave very differently: ThN undergoes a transition from NaCl (B1) to CsCl (B2), similar to ionic solids but PaN and UN undergo a transition from B1 to R% 3m under pressure. [18][19][20] From detailed electronic structure analysis it was found that a band Jahn-Teller distortion mechanism can explain the B1-R% 3m phase transitions in UN but not for PaN, though both systems have appreciable 5f-state occupations. Therefore, it will be interesting to study the high pressure structural sequence as a function of 5f-electron number for these nitride systems to unveil the role of 5f-electrons and to get a clear understanding about the underlying mechanism.…”