Heat capacity, entropy, and magnetic properties of jarosite-group compounds |
| |
| Authors: | Juraj Majzlan Peter Glasnák Robert A. Fisher Mary Anne White Michel B. Johnson Brian Woodfield Juliana Boerio-Goates |
| |
| Affiliation: | 1. Institute of Geosciences, Friedrich-Schiller University, Burgweg 11, 07749, Jena, Germany
2. Institute of Geosciences, Albert-Ludwig University, Albertstra?e 23b, 79104, Freiburg, Germany
4. Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, CA, 94720, USA
5. Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, NS, B3H 1W5, Canada
3. Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT, 84602, USA
|
| |
| Abstract: | ![]()
Jarosite phases are common minerals in acidic, sulfate-rich environments. Here, we report heat capacities (C p) and standard entropies (S°) for a number of jarosite samples. Most samples are close to the nominal composition AFe3(SO4)2(OH)6, where A = K, Na, Rb, and NH4. One of the samples has a significant number of defects on the Fe sites and is called the defect jarosite; others are referred to as A-jarosite. The samples, their compositions, and the entropies at T = 298.15 K are: | K-jarosite | K0.92(H3O)0.08Fe2.97(SO4)2(OH)5.90(H2O)0.10 | 427.4 ± 0.7 | | Na-jarosite | Na0.95(H3O)0.05Fe3.00(SO4)2(OH)6.00 | 436.4 ± 4.4 | | Rb-jarosite | RbFe2.98(SO4)2(OH)5.95(H2O)0.05 | 411.9 ± 4.1 | | NH4-jarosite | (NH4)0.87(H3O)0.13Fe3.00(SO4)2(OH)6.00 | 447.2 ± 4.5 | | Defect jarosite | K0.94(H3O)0.06Fe2.34(SO4)2(OH)4.01(H2O)1.99 | 412.7 ± 4.1 | There are additional configurational entropies of 13.14 and 8.23 J mol−1 K−1 in defect and NH4-jarosite, respectively. A detailed analysis of the synchrotron X-ray diffraction patterns showed a large anisotropic peak broadening for defect and NH4-jarosite. The fits to the low-temperature (approx. <12 K) C p data showed that our samples can be divided into two groups. The first group is populated by the K-, Na-, Rb-, and NH4-jarosite samples, antiferromagnetic at low temperatures. The second group contains the H3O-jarosite (studied previously) and the defect jarosite. H3O- and defect jarosite are spin glasses and their low-T C p was fit with the expression C p = γT + ΣB j T j , where j = (3, 5, 7, 9). The linear term is typical for spin glasses and the sum represents the lattice contribution to C p. Surprisingly, the C p of the K-, Na-, Rb-, and NH4-jarosite samples, which are usually considered to be antiferromagnetic at low temperatures, also contains a large linear term. This finding suggests that even these phases do not order completely, but have a partial spin-glass character below their Néel transition temperature. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
