Unusual sequence of phase transitions in (NH4)3TiF7 detected by optic and calorimetric studies

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

“…1b that the structural transformation is accompanied by absorption of latent heat, as follows from the very small values of the derivative dT/ dt 10 À3 K/min observed at the phase transition point over a certain period of time. Such a behaviour of temperature against time near T 2 points to the ''spread'' absorption of heat, which agrees well with the ''smeared'' restoration of birefringence upon heating from the G3 phase to G2 [10]. Both experimental facts points out to a reconstructive transformation.…”

“…G2 ! G3 found in optic and DSC measurements [10], two C p (T) anomalies were observed with maximum values at T 1 = 357.8 AE 0.5 K and T 2 = 291.6 AE 0.5 K.…”

“…Recently such a supposition was confirmed by optical and differential scanning calorimetric (DSC) investigations of the stability of the room temperature P4/mnc phase in a (NH 4 ) 3 TiF 7 crystal [10]. An unusual succession of two structural phase transitions was observed upon heating and cooling (cubic-G3) (T 2" = 291 K) $ (tetragonal1-G2) (T 1" = 360 K) $ (tetragonal2-G1).…”

“…Because optical investigations showed rather pronounced temperature dependence of birefringence, it was worthwhile to perform more careful studies of heat capacity compared to DSC measurements [10] in order to obtain precise information on the total heat capacity as well as excess enthalpy and entropy associated with these phase transitions. 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 .…”

“…All experiments were carried out on samples prepared from crystals grown and given a preliminary examination in [10]. To reach final conclusion about the absence of noncentrosymmetrical phases in titanate, the permittivity and tangent of the dielectric losses were also studied.…”