In Situ X-Ray Powder Diffraction: The Reactions of Pb(IV)-Fluoride with Ammonia and NH₄F

Heat capacity, entropy, dielectric properties and T–p phase diagram of (NH 4 ) 3 TiF 7

“…This hypothesis is corroborated by the cubic symmetry with the PmÀ3 m (Z = 1) space group found at room temperature in some other double salt crystals with the formula A 3 MeX 7 : (NH 4 ) 3 SnF 7 [7] and (NH 4 ) 3 PbF 7 [8]. Taking the results of [7,8,10] into account, one can reason that the third boundary at triple point 1 divides phases G1 and G0 B PmÀ3m.…”

“…Analysis of complex twinning pictures in tetragonal phases G1 and G2 has allowed the existence of a high temperature cubic parent phase with m3m symmetry in (NH 4 ) 3 TiF 7 to be assumed [10]. This hypothesis is corroborated by the cubic symmetry with the PmÀ3 m (Z = 1) space group found at room temperature in some other double salt crystals with the formula A 3 MeX 7 : (NH 4 ) 3 SnF 7 [7] and (NH 4 ) 3 PbF 7 [8]. Taking the results of [7,8,10] into account, one can reason that the third boundary at triple point 1 divides phases G1 and G0 B PmÀ3m.…”

“…In accordance with the important role of the size of the central atom in the formation of structural symmetry in double salts such as A 2 MeF 6 ÁAF [4][5][6][7][8][9], it was possible to suppose a great effect of hydrostatic pressure on the stability of crystal phases in (NH 4 ) 3 TiF 7 .…”

“…Double fluoride salts A 3 MeF 7 = A 2 MeF 6 ÁAF have been known for more than 70 years and are characterized by different room temperature symmetries of the crystal lattice depending on the size of the central atom [4][5][6][7][8][9]. Such a situation could be considered as strong evidence for the possible existence of phase transitions in these compounds with temperature and/or pressure variations.…”

(NH 4 ) 3 HfF 7 : Crystallooptical and calorimetric studies of a number of successive phase transitions

“…(NH4)3SnF7 [9], (NH4)3PbF7 [10] . In a number of these crystals, phase transitions with an unusual sequence of symmetry changes have been observed [11].…”