Non-stoichiometry and Defects in the Weyl Semimetals TaAs, TaP, NbP, and NbAs

TL;DR

This study investigates the structural defects and non-stoichiometry in Weyl semimetals TaAs, TaP, NbP, and NbAs, revealing varying degrees of defects and compositions, with NbAs being closest to ideal stoichiometry.

Contribution

It provides detailed structural analysis of defects and non-stoichiometry in four Weyl semimetals, identifying NbAs as the most stoichiometric among them.

Findings

TaP shows significant pnictide deficiency and stacking faults.

TaAs exhibits transition metal deficiency and high stacking fault density.

NbAs is nearly stoichiometric, ideal as a model compound.

Abstract

We report a structural study of the Weyl semimetals TaAs, TaP, NbP, and NbAs, utilizing diffraction techniques (single crystal x-ray diffraction and energy dispersive spectroscopy) and imaging techniques (transmission electron microscopy/scanning transmission electron microscopy). We observe defects of various degrees, leading to non-stoichiometric single crystals of all four semimetals. While TaP displays a large pnictide deficiency with composition TaP$_{0.83(3)}$, and stacking faults accompanied by anti-site disorder and site vacancies, TaAs displays transition metal deficiency with composition Ta$_{0.92(2)}$As and a high density of stacking faults. NbP also displays pnictide deficiency, yielding composition NbP$_{0.95(2)}$, and lastly, NbAs display very little deviation from a 1:1 composition, NbAs$_{1.00(3)}$, and is therefore recommended to serve as the model compound for these semimetals.