${\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7 \stackrel{∆}{\to } \mathrm{X} \uparrow$

What is X?

On heating, ammonium dichromate forms green color residue and water, along with the liberation of nitrogen gas. 

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\stackrel{∆}{\to }\underset{\mathrm{X}}{{\mathrm{N}}_2}\uparrow +\underset{\mathrm{Green} \mathrm{color}}{{\mathrm{Cr}}_2{\mathrm{O}}_3}+4{\mathrm{H}}_2\mathrm{O}$