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arXiv 1007.4574cond-mat.mtrl-sci
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BaHfN $_2$电子和振动性质的第一性原理研究

First-Principles Study of Electronic and Vibrational Properties of BaHfN$_2$

Amandeep Kaur, Erik R. Ylvisaker, Yan Li, Giulia Galli, Warren E. Pickett

摘要:

过渡金属氮化物BaHfN $_2$由闭合壳层离子的弱键合的中性板组成,与当电子掺杂时具有令人印象深刻的超导T $_c$的其它层状氮化物具有结构和化学相似性:A $_x$ HfNCl、A $_x$ ZrNCl、A $_x$ TiNCl,对于适当的供体(A)浓度$x$,分别具有$T_c= 25.5$、$15.2$和$16.5$ K。这些相似性表明BaHfN $_2$在掺杂时是另一种相对高T $_c$氮化物的可能性,其结构的影响和特定过渡金属离子的作用尚待理解。我们报告的第一性原理电子结构计算化学计量比BaHfN $_2$使用密度泛函理论与平面波基组和可分离的双空间高斯赝势。获得了0.8eV的间接带隙,并且最低导带主要是Hf 5 $d_{xy}$特征,类似于$β$ -ZrNCl和$α$ -TiNCl。两个N位,一个在Hf层中,另一个在Ba层中,被发现具有非常各向异性的玻恩有效电荷(BEC):偏离形式电荷(-3)对于两个位置是相反的,并且对于两个取向(面内,面外)是相反的。讨论了LO-TO分裂,并与相关化合物的BEC和介电常数张量进行了比较,报道了电子掺杂对带心声子的影响。

英文摘要:

The transition metal nitride BaHfN$_2$, which consists of weakly bonded neutral slabs of closed shell ions, has structural and chemical similarities to other layered nitrides which have impressive superconducting T$_c$ when electron doped: A$_x$HfNCl, A$_x$ZrNCl, A$_x$TiNCl, with $T_c= 25.5$, $15.2$ and $16.5$ K, respectively for appropriate donor (A) concentrations $x$. These similarities suggest the possibility of BaHfN$_2$ being another relatively high T$_c$ nitride upon doping, with effects of structure and the role of specific transition metal ions yet to be understood. We report first-principles electronic structure calculations for stoichiometric BaHfN$_2$ using density functional theory with plane-wave basis sets and separable dual-space Gaussian pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest conduction band is primarily of Hf 5$d_{xy}$ character, similar to $β$-ZrNCl and $α$-TiNCl. The two N sites, one in the Hf layer and another one in the Ba layer, were found to have very anisotropic Born effective charges (BEC):deviations from the formal charge (-3) are opposite for the two sites, and opposite for the two orientations (in-plane, out of plane). LO-TO splittings and comparison of BECs and dielectric constant tensors to those of related compounds are discussed, and the effect of electron doping on the zone-center phonons is reported.

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