Nonlinear Lattice Dynamics and Discrete Breathers in B2 Crystals: A Comparative Study of CsCl, LiPb, and NiTi

Discrete breathers (DBs) are nonlinear vibrational excitations localized on small groups of atoms in perfect crystal lattices. While theoretically proven, a systematic understanding of DB formation in binary crystals with the B2 structure remains limited. We employ molecular dynamics simulations using the LAMMPS package to investigate the nonlinear dynamics of three representative B2 crystals: ionic CsCl, and intermetallic LiPb and NiTi. We calculate the amplitude-frequency dependencies of delocalized nonlinear vibrational modes (DNVMs) and analyze DB existence conditions based on phonon spectrum features and anharmonicity type. Our analysis reveals that a significant atomic mass difference creates a phonon band gap, enabling gap DBs in CsCl and LiPb, whereas NiTi, with similar atomic masses, exhibits no gap. A simplified model assuming identical bond stiffnesses accurately predicts frequency ratios in CsCl and LiPb but fails for NiTi due to strong bond stiffness asymmetry. We demonstrate the successful excitation of long-lived gap DBs in LiPb by initializing atomic displacements based on the G1 DNVM pattern on heavy Pb atoms. These gap DBs remain stable for over 20 ps with negligible energy dissipation. In contrast, DBs with frequencies above the phonon spectrum (excited on light Li atoms) exhibit shorter lifetimes (~2 ps). The study establishes that both atomic mass ratio and interatomic bond stiffness asymmetry are critical parameters governing nonlinear dynamics in B2 crystals. The predicted long-lived gap DBs in LiPb provide a target for future experimental detection via inelastic neutron or X-ray scattering, offering new insights into energy localization and transport in biatomic alloys.