Effect of the size of the central atom on the stability of crystalline phases in solid solutions (NH4)3TixSn1-xF7

https://doi.org/10.1016/j.jssc.2023.124373

The effect of a change in internal pressure as a result of partial substitution of the central atom on the realization and stability of the initial and distorted crystalline phases in (NH₄)₃Ti_xSn_1-xF₇ solid solutions has been studied. It was found that at a Ti concentration in the range of x = 0.15–0.40, the reconstructive transition Pm-3m ↔ Pa-3 is transformed into a sequence of phase transitions Pm-3m ↔ P4/mbm ↔ P4/mnc ↔ Pa-3. In the (NH₄)₃Ti_0.15Sn_0.85F₇ solid solution, the first-order phase transition between two cubic phases at T₀ = 352 K is characterized by a significant volume jump δ(ΔV/V₀) ≈ 1 %, comparable with that in (NH₄)₃SnF₇. An increase of the Ti concentration leads to a strong decrease in the stability of the Pm-3m cubic phase: the first tetragonal P4/mbm phase appears in (NH₄)₃Ti_0.4Sn_0.6F₇ at T₀ = 400 K, which proves the existence of the predicted high-temperature cubic phase in (NH₄)₃TiF₇. In solid solutions, a decrease in birefringence and entropy of phase transitions was observed in comparison with the initial compounds with Sn and Ti as central atoms. The role of critical parameters (unit cell volume, temperature, external pressure) in the formation of cubic and distorted phases is discussed.