where Xn− is the intercalating anion (or anions).
Most commonly, = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+, and is another trivalent cation, possibly of the same element as M. Fixed-composition phases have been shown to exist over the range 0.2 ≤ x ≤ 0.33.