Complete phase transition from hexagonal LnF(3) (Ln(3+) = La(3+), Ce(3+), Pr(3+)) to monodisperse ultrasmall (∼7 nm) cubic Ln(0.8)M(0.2)F(2.8) (M(2+) = Ca(2+), Sr(2+), Ba(2+)) disordered solid solution nanocubes was successfully achieved through alkaline-earth doping, which induced great intensification of the near-infrared to visible upconversion emissions of the optically active rare earth ions.