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

Heat capacity, thermal expansion and barocaloric effect in fluoride $$\hbox {K}_{2}\hbox {TaF}_{7}$$

“…Recently, studies of differential thermal analysis under pressure for some complex fluorides and oxyfluorides revealed the absence of the pressure effect, at least to 0.6 GPa, on the phase transition entropy [14][15][16]. Taking into account these results, we assumed that in the case of K 2 TaF 7 pressure also does not affect ∆S.…”

Sequence of phase transitions in (NH4)3SiF7

“…The temperature stability of the initial tetragonal phase P4/mbm of (NH 4 ) 3 SiF 7 was examined by measurements with a DSM-10M differential scanning microcalorimeter (DSM). Recently it has been shown that among fluorides (NH 4 ) 3 M 4+ F 7 there are at least two crystals undergoing a phase transition into the cubic phase Pm3m above room temperature at ambient -(NH 4 ) 3 SnF 7 13 or high -(NH 4 ) 3 TiF 7 16 pressure.…”

“…11 Moreover, some other fluoride double salts (NH 4 ) 3 M 4+ F 7 (M 4+ = Ti 4+ , Sn 4+ ) undergo phase transitions but of different sequences, and a correlation with the M 4+ ionic radius was found. [11][12][13][14][15][16] Therefore, one might expect the temperature variation would also destabilise the room temperature structure of the silicon double fluoride salt, leading to phase transitions.…”

Raman scattering and phase transitions in (NH4)3TiF7

“…An unusual succession of two structural phase transitions was observed upon heating and cooling . Maxima of the heat capacity peaks were located at the temperatures T 1 = 361 ± 2.0 K and T 2 = 293 ± 1.0 K. The sum of entropies associated with both phase transitions is rather large: ∑ΔS i = ΔS 1 + ΔS 2 = 32 ± 2 J/mol K . Individual values of entropy change in (NH 4 ) 3 TiF 7 differ from each other significantly: ΔS 1 = 9.2 ± 0.6 J/(mol K) (~Rln3), ΔS 2 = 22.7 ± 1.6 J/(mol K) (~Rln16).…”