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

摘要

A series of caesium manganese hexacyanoferrates is prepared; (Cs1.78MnII)-Mn-I[Fe-II(CN)(6)](0.78)[Fe-III(CN)(6)](0.22) ( 1), Cs-I Mn-1.57(II)[Fe-II(CN)(6)](0.57)[Fe-III(CN)(6)](0.43) ( 2), Cs-I Mn-1.51(II)[Fe-II(CN)(6)](0.51)[Fe-III(CN)(6)](0.49) (3), and Cs-I Mn-0.94(II)[Fe-II(CN)(6)](0.21)[Fe-III(CN)(6)](0.70) center dot 0.8H(2)O ( 4). 1 - 3 show charge-transfer phase transitions between the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T-1/2 down arrow, T-1/2 up arrow) 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 Mn-1.78(II) Mn-0.78(III) (0.22)[Fe-II(CN)(6)], Cs-I Mn-1.57(II) Mn-0.57(III) (0.43)[Fe-II(CN)(6)], and Cs-I Mn-1.51(II) Mn-0.51(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 (Fm (3) over barm), but the lattice constants decrease from the HT phase to the LT phase; a = 10.5446( 17) --> 10.4280(7) angstrom ( 1), 10.5589(17) --> 10.3421(24) angstrom (2), and 10.5627(11) --> 10.3268(23) angstrom ( 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 x 10(-6) K-1 throughout the temperature range 300 and 20 K.