⁵⁷Fe Mössbauer spectroscopy and high-pressure structural analysis for the mechanism of pressure-induced unique magnetic behaviour in (cation)[Fe^IIFe^III(dto)₃] (cation = Ph₄P and ⁿPrPh₃P; dto = 1,2-dithiooxalato)

A mixed-valence iron(ii,iii) coordination polymer, (Ph₄P)[Fe^IIFe^III(dto)₃] (2; Ph₄P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), exhibits a thermal hysteresis loop and a low temperature shift of the ferromagnetic phase transition temperature, with increasing pressure. The latter magnetic behaviour can also be observed in a novel compound (ⁿPrPh₃P)[Fe^IIFe^III(dto)₃] (3; ⁿPrPh₃P = n-propyltriphenylphosphonium). To understand the structural information under pressure, we performed high-pressure powder X-ray diffraction, and the result suggests that there was no structural phase transition for either compound. Considering the ⁵⁷Fe Mössbauer spectroscopy studies, both 2 and 3 may have a high transition entropy, and this finding is caused by pressure-induced unique magnetic behaviours.