What radium compound was first isolated in a pure state?

Radium chloride was the first radium compound isolated in a pure state.

Marie Curie and her collaborators separated radium from barium by exploiting the slight difference in solubility between their chlorides. Repeated fractional crystallization gradually enriched the radium-bearing fraction until sufficiently pure radium chloride could be obtained.

This compound was central to the early history of radium. It provided a workable, purified chemical form for studying radium’s properties and later served as the starting material for producing elemental radium.

The compound should not be confused with radium carbonate, bromide, or sulfate, which can also occur in radium chemistry. Radium chloride is particularly associated with the Curies’ original separation process because the chloride salts could be repeatedly crystallized to separate the radium from chemically similar barium.