On the importance of hexagonal phases in TM (TM = Ti, Zr, and Hf) mono-nitrides

Abstract

Phase transitions are an important topic in the understanding of the properties of IVb group mono-nitrides (IVbTMNs) and their applications. Until now, only structural phase transitions concerning cubic phases that are far away from the ground state have been reported. However, previous reports have suggested a possible transformation to the Bk(BN-type) hexagonal phase as well. With this in mind, this work aims to study the stability of phase transitions involving hexagonal phases. Here, we perform a complete theoretical study based on the density functional theory approach of the crystal and vibrational and electronic structures of several phases for IVbTMN (TM = Ti, Zr, and Hf) compounds. Among these, the hexagonal phases are known to be stable in the V−VITMN homologous systems. Our study reveals the existence of a new pressure-induced phase transformation at the compression regime from B1 (Nacl-type) to Bi (AsTi-type) for ZrN and from B1 to Bk at the expansion regime for all IVbTMN. In addition, we have found that hexagonal Bi, Bk, and trigonal LiSnS2-type phases are close to the ground state and are dynamically stable. However, the metal weight plays a crucial role in the stability of hexagonal phases. The stability of the Bi and Bk hexagonal phases is important since they might represent intermediate states in the cubic B1 to B4 phase transition of the alloyed IVbTMN.