Electronic Structure and Stability of Ordered Vacancy Phases of NbN
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ABSTRACT A recently reported metastable phase of NbN with a superconducting T,==16.4 K is characterized using full potential electronic structure methods. This new phase, which has Pm3m (cubic) symmetry, can be described as the BI (rocksalt) structure with 25% ordered vacancies on each sublattice. We compare the equation of state and electronic spectrum of this Pm3m phase with its rocksalt counterpart [1] and with Nb 4 N3 in the 14/mmm (tetragonal) phase, which allows the characterization of N vacancies without accompanying Nb vacancies. For Pm3m NbN, the calculated lattice constant is 5% smaller than reported and the energy is 1.00 eV/molecule higher than BI NbN, suggesting that the newly reported phase is something other than a stoichiometric Pm3m phase of NbN. We report on the energy surface for tetragonal distortions of this phase, from which we evaluate its structural stability and obtain Poisson's ratio.
INTRODUCTION A notable feature of niobium nitrides is their propensity to stabilize in structures with varying degrees of stoichiometry and vacancy concentrations, which results in a broad spectrum of physico-chemical properties. Depending upon stoichiometry, NbN phases range from insulating to superconducting with an upper limit T, of 17.3 K [2,3,4]. It was the recent report of a new metastable superconducting phase that motivated us to investigate the electronic properties and structural stability of ordered vacancy NbN structures. In this work we address three superconducting phases of NbN using first principles density functional techniques: (1) the rocksalt structure (space group Fm3m), (2) the tetragonally distorted rocksalt structure obtained when one-quarter of the N is removed, denoted by Nb 4N3 and (3) Nb 3 N3 , the rocksalt structure obtained when one-quarter of the Nb and N are removed. These phases will be referred to as Nb 4 N4 , Nb 4 N3 , and Nb 3 N3 , respectively. The new phase of NbN, which prompted this work, was reported as thin films synthesized on MgO (100) substrates using pulsed laser deposition [3]. According to x-ray diffraction studies, its structure may be regarded as the rocksalt structure with 25% ordered vacancies on both the metal and metalloid sublattices. Thus, each atomic site is four-fold planar coordinated with atoms of the opposite type and the vacancy sites are six-fold coordinated with atoms of one type. The structure may be envisaged as nonpenetrating Nb octahedra and nonpenetrating N octahedra, both centered at vacancy sites. The space group is Pm3m. Nb 3N 3 reportedly has a superconducting T, of 16.4 K and a lattice parameter of 4.442 A, which are both comparable to the experimental values of 16 K and 4.378 A for Nb 4N 4. Also considered in this study is a tetragonal distortion of the rocksalt structure with 25% ordered vacancies on the metalloid sites. This allows for the characterization of N vacancies without accompanying Nb vacancies. Thin films of Nb 4 N3 vapor-deposited on MgO (100) substrates have a superconducting T, near 11.5 K [2]. The space group is 14/mm
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