The phase transition of Pb(8)F(14)I(2)

ABSTRACT: The reversible phase transition of Pb(8)F(14)I(2) is of continuous type and takes place at about 107 °C as monitored by temperature-dependent single crystal and powder X-ray diffraction measurements, optical microscopy, and differential scanning calorimetry. The low-temperature ferroelastic phase crystallizes in the orthorhombic crystal system (23 °C, Bmmb, Z = 2, a = 6.0699(6) Å, b = 6.0165(6) Å, c = 25.077(2) Å, 1487 structure factors, 41 parameter, R(F (2)) = 0.0346, wR(F (2)) = 0.0771) and changes its symmetry to the tetragonal crystal system into the high-temperature paraelastic phase (130 °C, I4/mmm, Z = 1, a = 4.2667(12) Å, c = 25.388(7) Å, 430 structure factors, 303 parameter, R(F (2)) = 0.0575, wR(F (2)) = 0.1564). Group–subgroup relationships between the two structures and a hypothetical intermediate structure are presented. GRAPHICAL ABSTRACT: [Image: see text]