Energetics of point defects in rocksalt structure transition metal nitrides: Thermodynamic reasons for deviations from stoichiometry
Energetics of point defects in rocksalt structure transition metal nitrides: Thermodynamic reasons for deviations from stoichiometry
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DOI：
10.1016/j.actamat.2018.07.074
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发表时间：
2018-10-15
期刊：
影响因子：
9.4
通讯作者：
Gall, Daniel
中科院分区：
文献类型：
--
作者：
Balasubramanian, Karthik;Khare, Sanjay, V;Gall, Daniel
第一原理计算的点缺陷形成能在第3-6族过渡金属（Me）氮化物MeNx被用来解释的热力学原因的大的组成范围内（通常x = 0.7-1.3）的岩盐结构。低于化学计量比（x < 1）和超过化学计量比（x > 1）的组成都是由于相对低的空位形成能，其从第3族氮化物中的氮和阳离子空位的平均2.7和4.5eV降低（ScN，YN，LaN）至-1.8和-0.8 eV的第6族氮化物（CrN，MoN，WN），表明它们分别对于第6族和第4-6族氮化物在零温度下变得热稳定。类似地，四氮杂环和111-或110-分裂构型的氮和阳离子氮化物对于第3-5族是不稳定的，但对于第6族氮化物变得化学稳定，这与后一种化合物的机械不稳定性一致。所有的反位缺陷都具有高的形成能，并且不太可能形成。在有限的温度下，氮的化学势与N-2气体的平衡是强烈影响的汽相熵，导致在缺陷自由能的位移，例如，1.2 eV在1帕N-2在800 K，导致氮空位和阳离子的可能性在升高的温度增加。此外，点缺陷的构型熵导致在800 K下对于1%空位缺陷浓度的例如0.4eV的校正。考虑到这些熵贡献导致例如TiN的200-1100 K、ZrN的500-1400 K和HfN的1200 - 1400 K的化学计量的预测温度窗口，而对于低于和高于这些范围的温度分别预期相当大的阳离子和氮空位浓度。肖特基对缺陷的预测在VN为T> 200 K和NbN，TaN，和6族氮化物在所有温度下，独立的N-2分压。总体结果表明，热力学参数（即使在动力学障碍的情况下）可以解释许多报道的岩盐结构氮化物的组成与温度和压力的关系。(C)2018 Acta Materialia Inc.由爱思唯尔有限公司出版。保留所有权利。
First principle calculations of point defect formation energies in group 3-6 transition metal (Me) nitrides MeNx are employed to explain the thermodynamic reasons for the large reported compositional range (typically x = 0.7-1.3) in the rocksalt structure. Both under-stoichiometric (x < 1) and over-stoichiometric (x > 1) compositions are due to relatively low vacancy formation energies that decrease from an average of 2.7 and 4.5 eV for nitrogen and cation vacancies in group 3 nitrides (ScN, YN, LaN) to -1.8 and -0.8 eV in group 6 nitrides (CrN, MoN, WN), indicating that they become thermodynamically stable at zero temperature for group 6 and for group 4-6 nitrides, respectively. Similarly, nitrogen and cation interstitials in tetragonal and 111- or 110-split configurations are unstable for groups 3-5 but become thermodynamically stable for group 6 nitrides, consistent with the mechanical instability of the latter compounds. All antisite defects possess high formation energies and are unlikely to form. The nitrogen chemical potential at finite temperatures and in equilibrium with a N-2 gas is strongly affected by the vapor phase entropy, leading to shifts in the defect free energy of, for example, 1.2 eV at 1 Pa N-2 at 800 K, causing an increasing likelihood for nitrogen vacancies and cation interstitials at elevated temperatures. In addition, the configurational entropy of point defects causes a correction of e.g. 0.4 eV for a 1% vacancy defect concentration at 800 K. Considering these entropy contributions leads to predicted temperature windows for stoichiometry of e.g. 200-1100 K for TiN, 500-1400 K for ZrN, and 1200 -1400 K for HfN, while considerable cation and nitrogen vacancy concentrations are expected for temperatures below and above these ranges, respectively. Schottky pair defects are predicted in VN for T> 200 K and in NbN, TaN, and group 6 nitrides at all temperatures, independent of the N-2 partial pressure. The overall results show that thermodynamic arguments (even in the absence of kinetic barriers) can explain many of the reported composition vs temperature and pressure relationships in rocksalt structure nitrides. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.