Five di4erent phases of Pb 5 Al 3 F 19 ([I] to [V]) occur on cooling. The ferroelastic one [III] was stabilized at room temperature by a low rate of Cr+Al substitution. The structure was determined by X-ray di4raction analysis on a single-domain crystal. The symmetry is triclinic, P1 , with cell parameters a ؍ 10.72(3) A > , b ؍ 10.67(3) A > , c ؍ 7.24(2) A > , ؍ 109.27(5)3, ؍ 110.14(5)3, ؍ 83.33(5)3. The crystal network, which is closely related to those of the other varieties, is composed of both isolated and trans-corner-linked AlF 6 octahedra developing parallel to the c-axis. The structure is closely derived from that of the paraelastic phase [II], mainly by the inclination of the previous tetragonal c-axis on the a+b plane. The loss of the mirror symmetry of the high-temperature phase leads to a break in the Pb 2# ion alignment along the 4-fold axis. Polydomain crystals were observed under a polarizing microscope; the ferroelastic [III]+paraelastic [II] phase transition occurs at 326 K on heating.