Thermodynamic investigation by heat capacity measurements of ferrimagnetic A₂Mn[Mn(CN)₆] (A=K, Rb, Cs) Prussian blue compounds

Abstract

Heat capacity measurements of a new series of Prussian blue analogs of A2Mn[Mn(CN)6] (A=K, Rb, Cs) composition were performed using thermal relaxation calorimetry. The Cs compound has a face-centered cubic structure with a linear Mn-C≡N-Mn linkage, while the monoclinic Rb and K compounds have nonlinear Mn-C≡N-Mn linkages. For all of the compounds, large broad thermal anomalies associated with magnetic transitions were observed in the temperature dependence of the heat capacity. The systematic changes in the heat capacity for the three compounds under magnetic fields of up to 7 T were found to be consistent with ferrimagnetic ordering with large spontaneous magnetization. Although the peak temperatures were slightly lower than reported values obtained by magnetic susceptibility measurements, the magnetic entropy was evaluated to be 22.0 ± 2.5 J K(-1) mol(-1). This value is consistent with an entropy of Rln12 corresponding to full entropy of one low-spin and one high-spin Mn(II) ion in the formula unit, though some ambiguity remains in lattice estimation. Broadening of the peak width of the magnetic heat capacity divided by the temperature was observed as the size of the alkali ions decreased from Cs to K. This behavior is consistent with an increase in the lattice distortion produced by the bending of the C≡N-Mn angles.

Increase in the Magnetic Ordering Temperature (Tc) as a Function of the Applied Pressure for A2Mn[Mn(CN)6] (A = K, Rb, Cs) Prussian Blue Analogues

“…This broadening is reflected in the peak width of the FC curves. The upward shift of the transition temperature is consistent with the behavior produced by the chemical pressure 13,18 because the external pressure contracts the interstitial space in the crystal and bends the MnNC angle. The monotonous change of T c with pressure demonstrates that the external pressure plays a role similar to that of the chemical pressure effects arising from different sized cations in this compound (Table 1).…”

“…The thermodynamic work recently reported by Kawamoto et al revealed that the magnetic transitions occur as a 3D bulk ordering of high-spin Mn II ( S = 5 / 2 ) and low-spin Mn II ( S = 1 / 2 ) . The heat capacity peak under magnetic field shows an upward shift with an increase in the magnetic field, as is typically observed in 3D ferrimagnetic/ferromagnetic orderings.…”

Pressure Dependence of the Magnetic Ordering Temperature (Tc) for the Na2Mn[Mn(CN)6] Noncubic Prussian Blue Analogue

“…Heat capacity, C p , studies reveal a peak in C p ( T , H ) that increases with an increasing magnetic field ( H ) in accord with ferrimagnetic ordering for A 2 Mn­[Mn­(CN) 6 ] (A = K, Rb, and Cs) . This also suggests that the magnetic and thermodynamic properties are strongly affected by lattice distortion such as the ∠Mn–N–C bending angle.…”

“…15 Heat capacity, C p , studies reveal a peak in C p (T,H) that increases with an increasing magnetic field (H) in accord with ferrimagnetic ordering for A 2 Mn[Mn(CN) 6 ] (A = K, Rb, and Cs). 16 This also suggests that the magnetic and thermodynamic properties are strongly affected by lattice distortion such as the ∠Mn−N−C bending angle. To further understand this system, the pressure effect on T c , T c (P), was studied for A 2 Mn[Mn(CN) 6 ] (A = K, Rb, and Cs) 17 and herein for A = Na.…”

Mix and Match: Organic and Inorganic Ions in the Perovskite Lattice

“…In the group of Miller, a series of alkali metal A 2 Mn 2+ Mn 2+ (CN) 6 compounds with A = Na, [165] K, [166] Rb, [166] and Cs [167] were investigated starting in 2010. It is linear and well known to bind multiple metals, such as Fe in prussian blue.…”