Ammonium dichromate is an inorganic compound with the formula (NH 4 ) 2 Cr 2 O 7 . In this compound, as in all chromates and dichromates, chromium is in the +6 oxidation state, commonly known as hexavalent chromium . It is a salt consisting of ammonium ions and dichromate ions.
Ammonium dichromate is sometimes referred to as Vesuvian fire because of its use in tabletop “volcano” displays.  However, this exposure has become unpopular in schools due to the carcinogenic nature of the compound. It has also been used in the early days of pyrotechnics and photography.
At room temperature and pressure, the compound exists as orange, acidic crystals soluble in water and alcohol. It is formed by the action of chromic acid on ammonium hydroxide with subsequent crystallization. 
(NH 4 ) 2 Cr 2 O 7 crystals (C 2/C, Z = 4) have the same type of ammonium ion at the sites of symmetry C 1 (2, 3). Each NH 4 + center is randomly surrounded by eight oxygen atoms at a distance of ca to NO. 2.83 to ca. 3.17, typical of hydrogen bonds. 
It has been used in the early days of pyrotechnics and photography, as well as in lithography, as a source of pure nitrogen in the laboratory, and as a catalyst.  It is also used as a mordant for coloring pigments in the manufacture of alizarin, chrome alum, leather tanning and oil refining. 
Photosensitive films containing PVA, ammonium dichromate, and a phosphor are spin-coated as aqueous solutions in the production of phosphor rasters of television screens and other devices. Ammonium dichromate serves as the photoactive site.
Tabletop Volcanoes and Thermal Decomposition
The volcano’s performance involves igniting a salt mass, which initiates the following exothermic transformation:
(NH4)2Cr2O7 (s) → Cr2O3 (s) + N2 (g) + 4 H2O (g) (ΔH = −429.1 ± 3 kcal/mol)
Like ammonium nitrate, it is thermodynamically unstable. Its decomposition reaction proceeds to completion once started, producing dark green powdered chromium(III) oxide. Not all ammonium dichromate decomposes in this reaction. When the green powder is brought into water, a yellow/orange solution is obtained from the remaining ammonium dichromate.
Observations obtained using relatively high magnification microscopy during a kinetic study of the thermal decomposition of ammonium dichromate provided evidence that salt breakdown proceeds with the intervention of an intermediate liquid phase rather than a solid phase. The characteristic darkening of (NH4)2Cr2O7 crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to Cr3O2−10, Cr4O2−13, etc., ultimately yielding CrO3. The CrO3 has been identified as a possible molten intermediate participating in (NH4)2Cr2O7 decomposition
Ammonium dichromate is a strong oxidizing agent and often reacts violently with any reducing agent. The stronger the reducing agent, the more violent the reaction.  It has also been used to promote the oxidation of alcohols and thiols. Ammonium dichromate, Mg( 4 ) 2 in the presence of HSO and wet SiO 2 can act as a very efficient reagent for the coupling of thiols under oxidative solvent-free conditions. The reactions give reasonably good yields under relatively mild conditions.  The compound is used in the oxidation of aliphatic alcohols to their corresponding aldehydes and ketones with ZrCl 4 / wet SiO 2in solvent-free conditions, again with relatively high yields.  
Ammonium dichromate, like all chromium(VI) compounds, is highly toxic and a proven carcinogen.  It is also a strong stimulant.
In sealed containers, ammonium dichromate has the potential to explode when heated.  In 1986, two workers were killed and 14 others injured at Diamond Shamrock Chemicals in Ashtabula, Ohio, when 2,000 pounds of ammonium dichromate exploded as it was drying in a heater.