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

Heat capacities of two large-spin (ground spin state S=9) manganese clusters. [Mn-4(hmp)(6)(O2CCH3)(2)](ClO4)(2) and [Mn-4(hmp)(6)Cl-2](ClO4)(2). Hhmp = 2-hydroxymethylpyridine (Mn4OAc and Mn4Cl for short: respectively) were studied under 0-9 T magnetic fields in the 1.8-30 K temperature range. Broad humps were observed in Mn4OAc and Mn4Cl 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 Mn4OAc 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 Mn4Cl, 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 Mn4OAc and Mn4Cl, respectively, which are close to R In (2S + 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 Mn4OAc and Mn4Cl was determined to be -0.45 and -0.28 K, respectively (where k(B) is Boltzmann's constant). Comparison between the experimental and calculated magnetic heat capacities strongly suggests that both Mn4OAc and Mn4Cl possess the nature of S = 9 one-dimensional antiferromagnetic chains. (C) 2001 Elsevier Science Ltd. All rights reserved.