  Strong nonradiative recombination has been observed in quasi-one-dimensional
antimony selenide, which runs counter to the simple intuition that claims high
defect tolerance exists in semiconductors with antibonding state in the valence
band and bonding state in the conduction band. Here we reveal such a defect
intolerance actually stems from the richness of structural metastability and
vibrational anharmonicity owing to the low-symmetry atomic structure. Taking
the deep defect V$_{\rm Se}$ as a benchmark, we show the defect with its
ground-state configuration alone does not act as a recombination center.
Instead, we identify three different configurations with different formation
energies, such richness of metastability offers a higher probability to
accomplish a rapid recombination cycle. Another contributing factor is the
anharmonicity in the potential energy surfaces that is caused by the large
atomic relaxation, which elevates the total capture coefficient by 2-3 orders
of magnitude compared with harmonic approximation. Therefore, the unique
properties from both crystals and phonons in quasi-one-dimensional system
enhance the nonradiative recombination, making the traditional intuition of
defect tolerance invalid. These results highlight the importance of the correct
identification of metastable defects and phonon anharmonicity in the
nonradiative recombination in low-symmetry semiconductors.
