Charge-transfer phase transition and zero thermal expansion in caesium manganese hexacyanoferrates

Abstract

A series of caesium manganese hexacyanoferrates is prepared; Cs^I_1.78Mn^II[Fe^II(CN)₆]_0.78[Fe^III(CN)₆]_0.22 (1), Cs^I_1.57Mn^II[Fe^II(CN)₆]_0.57[Fe^III(CN)₆]_0.43 (2), Cs^I_1.51Mn^II[Fe^II(CN)₆]_0.51[Fe^III(CN)₆]_0.49 (3), and Cs^I_0.94Mn^II[Fe^II(CN)₆]_0.21[Fe^III(CN)₆]_0.70·0.8H₂O (4). 1–3 show charge-transfer phase transitions between the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T_1/2↓, T_1/2↑) of (207 K, 225 K) (1), (190 K, 231 K) (2), and (175 K, 233 K) (3) at a cooling and warming rates of 0.5 K min⁻¹. Variable temperature IR spectra indicate that the valence states of the LT phases of 1–3 are Cs^I_1.78Mn^II_0.78Mn^III_0.22[Fe^II(CN)₆], Cs^I_1.57Mn^II_0.57Mn^III_0.43[Fe^II(CN)₆], and Cs^I_1.51Mn^II_0.51Mn^III_0.49 [Fe^II(CN)₆], respectively. The XRD measurements for 1–3 show that crystal structures of the HT and LT phases are cubic structures (Fmm), but the lattice constants decrease from the HT phase to the LT phase; a = 10.5446(17) → 10.4280(7) Å (1), 10.5589(17) → 10.3421(24) Å (2), and 10.5627(11) → 10.3268(23) Å (3). The magnetization vs. temperature curves and the magnetization vs. external magnetic field curves show that the LT phases are ferromagnetic with Curie temperatures of 4.3 (1), 5.0 (2), and 5.6 K (3). At a cooling rate of −0.5 K min⁻¹, 4 does not show the charge-transfer phase transition, but does show a behavior of zero thermal expansion with a thermal expansivity of +0.2 × 10⁻⁶ K⁻¹ throughout the temperature range 300 and 20 K.