M. S. THESIS COLLOQUIUM
Name: Mr. Harshvardhan Singh Deora
Title: “Exploration of Lanthanum Tungsten Nitride: Ferroelectricity and Polar Metallicity”
Date & Time : Tuesday, 18th March 2025 at 04:00 P.M
Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
The introduction of charge carriers in ferroelectric materials typically screens the long-range electrostatic interactions that drive ferroelectric structural distortions, thus inhibiting polarization switching [1, 2]. However, certain materials, known as “polar metals,” can exhibit a coexistence of polarity and metallicity [3]. Materials that sustain robust polarization in the presence of charge carriers can be well-suited for engineering tunable metal-insulator transitions [4] and may be relevant to phenomena such as superconductivity and topological states [5, 6].
While oxide materials with the perovskite structure have been extensively studied for decades, perovskite nitrides are now emerging as a new class of materials with properties distinct from their oxide counterparts. Among the theoretically predicted thermodynamically stable nitride perovskites [7], the recently synthesized lanthanum tungsten nitride, LaWN3, has been identified as a semiconducting ferroelectric material with potentially intriguing properties worthy of in-depth investigation [8].
In this thesis, we employ first-principles density functional computations to study the intricate relationship between ferroelectricity and metallicity in the ferroelectric nitride LaWN3 [9]. We systematically assess the impact of electron and hole doping via the background charge method, revealing that both types of charge carriers diminish the propensity for ferroelectricity in LaWN3, although to remarkably different extents. We present the underlying mechanisms behind these findings, noting that adding electrons fills the valence orbitals of both W and N atoms. This filling leads to a screening effect on the long-range interactions, thereby reducing the off-centering of W. On the other hand, hole doping does not significantly alter the W-N bonding nature, resulting in a more robust off-centering. Furthermore, we introduce explicit impurity atoms and analyze the influence of the different factors contributing to the change in polarization, namely, the size of the dopant, charge carriers introduced by the dopant, and changes in lattice constants. We propose that doped LaWN3 can be a promising polar metal, especially with acceptor dopants. Our study comprehensively shows the interplay between polarity and metallicity in this prototypical perovskite nitride.
References:
[1] M. E. Lines and A. M. Glass, Oxford University Press, (2001).
[2] V.F. Michel, T. Esswein and N.A. Spaldin, J. Mater. Chem. C, 9, 8640 (2021).
[3] Y Shi et al., Nature Materials, 12, 1024–1027 (2013).
[4] D. Puggioni, et al., Phys. Rev. Lett. 115, 087202 (2015).
[5] C.W. Rischau, et al., Nature Physics 13, 643–648 (2017).
[6] W.C. Yu et al., Phy. Rev. Materials 2, 051201 (2018).
[7] R Sarmiento-Perez et al., Chem. Mater. 27, 5957 (2015).
[8] Talley et al., Science 374, 1488–1491 (2021).
[9] H. S. Deora and A. Narayan, Phys. Rev. Materials 8, 124409 (2024).
