Heat capacity calorimetry of two Mn-4 large-spin clusters: [Mn-4(hmp)(6)R-2](ClO4)(2) [Hhmp=2-hydroxymethylpyridine, R = OAc- or Cl-]

Heat capacities of two large-spin (ground spin state S=9) manganese cluster s. [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. Broa d 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 i ncreasing magnetic field. However, the calorimetric studies could not detec t any long-range ordering phenomena in these materials within the working r ange of temperature, which complies with the earlier reported magnetic meas urements. Heat capacity of Mn4OAc increased with increasing magnetic field at low temperatures up to 3 T, followed by a decrease, indicating a field-i nduced transition. In the case of Mn4Cl, no field dependence of heat capaci ty 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 Mn4 OAc and Mn4Cl, respectively, which are close to R In (2S + 1)= 24.5 J K-1 m ol(-1) expected for an S = 9 spin system. The uniaxial single-ion anisotrop y 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 ant iferromagnetic chains. (C) 2001 Elsevier Science Ltd. All rights reserved.