Phase transitions in mixed-valence potassium manganese hexacyanoferrate Prussian blue analogue: Heat capacity calorimetric study

Phase transitions in K0.2Mn0.66IIMn1.44III[Fe0.2IIFe0.8III(CN)6]O0.66(CH3COO)1.32]·7.6H2O—a mixed valence PB analogue have been investigated with the help of heat capacity calorimetry under 0–90 kOe magnetic fields in the 1.8–300 K temperature range. Two heat capacity anomalies at 2.1 and 7.5 K have been detected, which correspond to ferromagnetic and ferrimagnetic transitions, respectively. The magnitudes as well as the position of these thermal anomalies have varied with changing magnetic field. The estimated zero-field magnetic entropy amounted to 29.2JK-1mol-1, which is close to the expected value (33.7JK-1mol-1). The uniaxial zero-field splitting parameter is estimated to be D/kB=14.7K. A comparatively stronger antiferromagnetic interaction seems to lead to the magnetic transition around 7.3 K, whereas a weaker ferromagnetic interaction gives rise to the thermal anomaly at 2.1 K. The present results on magnetic phase transitions are in complete agreement with the earlier observations made through magnetic studies. Additionally, a glass transition at 194 K has been observed in the heat capacity study presumably due to freezing of the orientational motion of the H2OH2O molecules present in the material. This phenomenon is reflected into the temperature dependence of the estimated FeIIIFeIII and FeIIFeII concentrations in the present material obtained earlier through Mössbauer spectroscopy.