text: crystallizes in two forms (polymorphs). At room temperature, the compound is stable in the orthorhombic cotunnite (Lead(II) chloride|) structure, whereas the cubic fluorite structure (calcium fluoride|) is stable between 925 and 963 °C. Both polymorphs accommodate the preference of the large  ion for coordination numbers greater than six. The coordination of  is 8 in the fluorite structure and 9 in the cotunnite structure. When cotunnite-structure  is subjected to pressures of 7–10 GPa, it transforms to a third structure, a monoclinic post-cotunnite phase. The coordination number of  increases from 9 to 10.
  In aqueous solution  behaves as a simple salt; in water it is a 1:2 electrolyte and the solution exhibits a neutral pH. Its solutions react with sulfate ion to produce a thick white solid precipitate of barium sulfate.
  This precipitation reaction is used in chlor-alkali plants to control the sulfate concentration in the feed brine for electrolysis.
  Oxalate effects a similar reaction:
  When it is mixed with sodium hydroxide, it gives barium hydroxide, which is moderately soluble in water.
   is stable in the air at room temperature, but loses one water of crystallization above , becoming , and becomes anhydrous above .  may be formed by shaking the dihydrate with methanol.
   readily forms eutectics with alkali metal chlorides.