A computational examination of the zeta and eta phases in the hafnium nitrides

Abstract

The hafnium nitrides are an interesting class of ceramics that have shown promise in a diversity of applications that include high temperature coatings, 1,2 uses in electronics, 3 solar cells, 4 and even hydrogen storage. 5 Despite decades of research and the wide applications of these materials, the stability of the phases that comprise the hafnium-nitrogen (Hf-N) system are still debated. 6-10 For the advancement of these materials in these and other emerging technologies, it is particularly important to concretely establish and understand the equilibrium phase diagram in the Hf-N system. One of the earliest phase equilibrium diagrams for the Hf-N system was described by Toth. 11 As with many ultrahigh temperature ceramics, this system has a rocksalt phase, δ-HfN, which can be found at nitrogen concentrations between 39 and 52 at.%, depending on temperature. In contrast to the transition metal carbides which also form the rocksalt structure, 12,13 δ-HfN is able to retain the rocksalt structure for a modest nonmetal-rich composition. 7,10 Thus, δ-HfN can be regarded as a strongly nonstoichiometric compound. With the depletion of nitrogen, Rudy 14 originally reported two specific subnitride phases, the-Hf 3 N 2−x (originally called the ε-Hf 3 N 2) and ζ-Hf 4 N 3−x phases. The former eta phase was identified to have a 9R metal atom stacking sequence, that is (hhc) 3 or ABABCBCAC. The latter zeta phase has a similar metal atom stacking sequence given as (hhcc) 3 or ABABCACABCBC, which is equivalent to the structure identified by Yvon and Parthe as the ζ-V 4 C 3−x phase. 15,16 The compositions of the eta and zeta phases were only identified for single samples at chemistries of HfN 0.56 and HfN 0.65 , respectively. However, Rudy was not able to identify either a composition range for the phases (and thus they were assumed to be line compounds) nor establish if or how much the nitrogen atoms were ordered in either structure. Rudy further identified the upper decomposition temperatures for these respective phases to be approximately 2000°C (eta) and 2300°C (zeta).

Ferroelectric Hafnium Nitride Thin Films Directly Formed on Si(100) Substrate

“…Hafnium nitride (HfNx) is well known as a thermally stable material, and various applications have been reported such as high temperature coating, solar cells, hydrogen storage, and so on [23]. Generally, the Hf-rich HfNx (x≤1.0) shows metallic properties, while the N-rich HfNx (x>1.0) shows insulating property [24].…”