Ashis Bhattacharjee

Heat capacities and phase transitions of the molecule-based mixed-valence complex NBu4[FeIIFeIII(ox)3] and the mixed-metal complex NBu4[ZnIIFeIII(ox)3]

Heat capacities of two molecule-based materials NBu 4 [Fe II Fe III (ox) 3 ] and NBu 4 [Zn II Fe III (ox) 3 ] (Bu= n -butyl, ox=oxalate) were measured by adiabatic calorimetery. In case of ferrimag...

Heat capacity calorimetry of two Mn4 large-spin clusters: [Mn4(hmp)6R2](ClO4)2 [Hhmp=2-hydroxymethylpyridine, R=OAc− or Cl−]

Abstract Heat capacities of two large-spin (ground spin state S =9) manganese clusters, [Mn 4 (hmp) 6 (O 2 CCH 3 ) 2 ](ClO 4 ) 2 and [Mn 4 (hmp) 6 Cl 2 ](ClO 4 ) 2 , Hhmp=2-hydroxymethylpyridine (Mn 4 OAc and Mn 4 Cl for short, respectively) were studied under 0–9 T magnetic fields in the 1.8–30 K temperature range. Broad humps were observed in Mn 4 OAc and Mn 4 Cl around 7.6 and 5 K, respectively, under zero magnetic field, which were shifted to lower temperatures with increasing magnetic field. However, the calorimetric studies could not detect any long-range ordering phenomena in these materials within the working range of temperature, which complies with the earlier reported magnetic measurements. Heat capacity of Mn 4 OAc increased with increasing magnetic field at low temperatures up to 3 T, followed by a decrease, indicating a field-induced transition. In the case of Mn 4 Cl, no field dependence of heat capacity was observed below 3 K when the magnetic field is lower than 0.5 T. The zero-field magnetic entropy amounted to 22.7 and 20.1 J K −1 mol −1 for Mn 4 OAc and Mn 4 Cl, respectively, which are close to R ln (2 S +1)=24.5 J K −1 mol −1 expected for an S =9 spin system. The uniaxial single-ion anisotropy parameter D ′/ k B for Mn 4 OAc and Mn 4 Cl was determined to be −0.45 and −0.28 K, respectively (where k B is Boltzmanns constant). Comparison between the experimental and calculated magnetic heat capacities strongly suggests that both Mn 4 OAc and Mn 4 Cl possess the nature of S =9 one-dimensional antiferromagnetic chains.

AC Magnetic Susceptibility of the Assembled-Metal Complex {NBu4[FeIIFeIII(ox)3]}∞ (Bu=n-C4H9, ox=oxalato)*

AC magnetic susceptibility (χ AC =χ ′ -iχ ′′ ) of {NBu 4 [Fe II Fe III (ox) 3 ]} ∞ was measured in the 4–50 K range for different frequencies (0.1–1000 Hz). Both of the χ ′ ( T ) and χ ′′ ( T ) plots showed two peaks in two temperature regions: 4–33 K and 33–50 K. The χ ′ ( T ) and χ ′′ ( T ) peaks in the 33–50 K range can be associated with the magnetic phase transition at the Neel temperature (43.3 K). However, the χ ′ ( T ) peak in the 4–33 K range was found to be shifted towards lower temperature from 15.5 K to 13.5 K with increasing frequency, accompanied by a decrease in peak height. The fractional relative change in the peak temperature per decade change in frequency was calculated. The χ ′′ ( T ) in the 4–33 K range exhibited a broad peak around 25 K. The frequency dependence of χ ′ ( T ) and χ ′′ ( T ) in the 4–33 K range occurred in the region where the irreversibilities in the ZFC (zero-field-cooled)–FC (field cooled) susceptibilities appeared. In this temperature range, a small heat capacity a...