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First-principles molecular dynamics is employed to describe the atomicstructure of amorphous SiN, a non-stoichiometric compound belonging to theSi$_x$N$_{y}$ family. To produce the amorphous state via the cooling of theliquid, both the Car-Parrinello and the Born-Oppenheimer approaches areexploited to obtain a system featuring sizeable atomic mobility. At hightemperatures, due to the peculiar electronic structure of SiN, exhibiting gapclosing effects, the Car-Parrinello methodology could not be followed sincenon-adiabatic effects involving the ionic and electronic degrees of freedom dooccur. This shortcoming was surmounted by resorting to the Born-Oppenheimerapproach allowing to achieve significant ionic diffusion at $T$= 2500 K. Fromthis highly diffusive sample, an amorphous state at room temperature wasobtained with a quenching rate of 10 K/ps. Four different models were created,differing by their sizes and the thermal cycles. We found that the subnetworkof atoms N has the same environment than in the stoichiometric materialSi$_3$N$_4$ since N is mostly threefold coordinated with Si. Si atoms can alsobe found coordinated to four N atoms as in Si$_3$N$_4$, but a substantialfraction of them forms homopolar bonds with one, two, three and even four Si.Our results are not too dissimilar from former models available in thelitterature but they feature a higher statistical accuracy and refer moreprecisely to room temperature as the reference thermodynamical condition forthe analysis of the structure in the amorphous state.

Atomic structure of amorphous SiN: combining Car-Parrinello and Born-Oppenheimer first-principles molecular dynamics