Structural and magnetic effects of mechanically grinding Co3(OH)2(C4O4)2·3H2O

Abstract

The magnetic and structural parameters of the well-characterized cobalt(II) compound Co3(OH)2(C4O4)2·3H2O (2, 3) change significantly when the compound is mechanically ground. These parameters are also dependent on the storage conditions of the polycrystalline material before grinding. When the material is stored in an anhydrous (desiccators) environment (2), the structural properties of the ground sample (4) agree with the polycrystalline material although the magnetic properties differ somewhat. When storing the material under ambient conditions (3), the material undergoes a structural transformation after grinding (5) that is accompanied by changes in magnetic properties. The synthetic precursor (1) to 2 and 3 also shows magnetic properties that are not observed in 2, 3. The powder XRD of 1 suggests that the dominant species in the mixture is Co(C4O4)(H2O)2, which provides a new synthetic pathway for 2 and 3. The synthesis of 1 was performed unprotected from the atmosphere, and therefore the room temperature effective magnetic moment of 1 was well below the other materials studied. The room temperature effective magnetic moment of 1, 4, and 5 (per cobalt center) are 4.0, 5.1, and 5.0 μB. Dynamic magnetic susceptibility of 1, 4, and 5 confirm that 1 and the ground derivatives 4 and 5 are not isostructural with the polycrystalline materials. The dynamic susceptibility of 1 is too complex to analyze quantitatively. It contains two peaks and two shoulders that are within 11K of one another, and a peak at 14K dominates the rest of the features. 4 and 5 have features that can be analyzed and compared with 2, 3. The frequency independent out-of-phase peak associated with 2, 3 becomes frequency dependent in 4. The Arrhenius fit to the frequency dependence suggests spin-glass behavior based on the small pre-exponential time of 9.59×1012s and the physically unreasonable activation barrier of 99K, which is well above the activation barrier observed for known single-chain magnets. A second frequency-dependent out-of-phase peak is observed for 4 and 5 at 11K that is not present in 2, 3. This peak is analyzed with the Vogel–Fulcher function, and is attributed to cluster spin-glass interactions.