Common microscopic origin of the phase transitions in Ta2NiS5 and the excitonic insulator candidate Ta2NiSe5

• Published in: npj computational materials Vol. 7; no. 1; pp. 1 - 14
• Main Authors: Windgätter, Lukas, Rösner, Malte, Mazza, Giacomo, Hübener, Hannes, Georges, Antoine, Millis, Andrew J., Latini, Simone, Rubio, Angel
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 20.12.2021; Nature Publishing Group; Springer Nature; Nature Portfolio
• Subjects: 639/301/1034/1038; 639/301/119/2795; 639/301/119/995; 639/766/119/1000; Broken symmetry; Characterization and Evaluation of Materials; Chemistry and Materials Science; Computational Intelligence; Crystal structure; Crystals; First principles; Materials Science; Mathematical and Computational Engineering; Mathematical and Computational Physics; Mathematical Modeling and Industrial Mathematics; Phase transitions; Physics; Stability analysis; Theoretical
• ISSN: 2057-3960; 2057-3960
• DOI: 10.1038/s41524-021-00675-6

The structural phase transition in Ta 2 NiSe 5 has been envisioned as driven by the formation of an excitonic insulating phase. However, the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the phase transition in its complementary material Ta 2 NiS 5 does not show any experimental hints of an excitonic insulating phase. We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta 2 NiSe 5 and Ta 2 NiS 5 using extensive first-principles calculations. In both materials the crystal symmetries are broken by phonon instabilities, which in turn lead to changes in the electronic bandstructure also observed in the experiment. A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap. We conclude that an excitonic instability is not needed to explain the phase transition in both Ta 2 NiSe 5 and Ta 2 NiS 5 .