Mercury(II) oxide , also called mercuric oxide or simple mercury oxide , has the formula HgO . Its color is red or orange. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroidite is rarely found.
In 1774, Joseph Priestley discovered that heating mercuric oxide produced oxygen, although he did not identify the gas as oxygen (rather, Priestley called it “deflaslated air”, as this was the paradigm under which he was at the time). was working).
Montroidite structure (red atoms are oxygen)Sinabar structure
The red form of Hgo can be made by heating Hg in oxygen at roughly 350 °C, or by pyrolysis of Hg(NO 3) 2.  The yellow form can be obtained by precipitation of aqueous Hg 2+ with alkali .  The difference in color is due to particle size, with both forms having the same structure consisting of nearly linear O-Hg-O units connected in zigzag chains with an Hg-O-Hg angle of 108°.
Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite (orthorhombic, 2/m 2/m 2/m, Pnma), and the other is analogous to the sulfide mineral cinnabar (hexagonal, HP6, P3221); Both are characterized by Hg-O chains.  At pressures above 10 GPa, both structures converge to a tetragonal form. 
HgO is sometimes used in the production of mercury because it decomposes quite easily. When it decomposes, oxygen gas is produced.
It is also used as a material for the cathode for mercury batteries. 
Label on the bottle of HGO powder.
Mercury oxide is a highly toxic substance that can be absorbed into the body by inhalation of its aerosols, through the skin, and by ingestion. The substance is irritating to the eyes, skin and respiratory tract and can have an effect on the kidneys, resulting in renal impairment. Bioaccumulation occurs in the food chain important to humans, especially in aquatic organisms. The substance is banned in the European Union as a pesticide. 
Evaporation is negligible at 20 °C. HGO decomposes upon exposure to light or heating above 500 °C. Heating produces highly toxic mercury fumes and oxygen, which increases the risk of fire. Mercury(II) oxide reacts violently with reducing agents, chlorine, hydrogen peroxide, magnesium (when heated), disulfur dichloride, and hydrogen trisulfide. Shock-sensitive compounds are formed from metals and elements such as sulfur and phosphorus.