  The high-pressure and high-temperature behaviors of sodium chloride (NaCl)
have the fundamental and application significance to the high pressure physics
and chemistry. To explore the reactivity of NaCl at high pressure and high
temperature, we compressed the NaCl samples to pressures up to 85 GPa and
heated them to a temperature of 1800 by using the diamond anvil cell and the
double-sided laser heating technology. Our investigation of Raman spectroscopy
shows that NaCl decomposes to NaCl3 with orthorhombic Pnma structure plus
possible Na3Cl with tetragonal P4/mmm structure above 31 GPa. It shows that the
competition of the unusual Cl-Cl covalent bond and Na-Na metallic bond with the
Na-Cl ionic bond at high pressure and high temperature may prefer the
occurrence of the NaCl3 and Na3Cl combination. We speculate that the large
temperature gradient in the NaCl sample layers in the diamond anvil cell
experiments, which indicate a non-equilibrium state, is critical to the
decomposition reaction of NaCl observed in this study. The catalytic effect of
the used laser absorbing materials of transition metal oxides could be another
factor to promote the chemical reaction of NaCl. The results of this
investigation provide a new pathway to investigate the distinctive non-1:1
stoichiometric alkali metal-halogen compounds that may have exotic properties.
