| Sumario: | Abstract: The magnetic properties and magnetic structures from neutron diffraction of two synthetic natrochalcites, NaM[supII][sub2](H[sub3]O[sub2])(MoO[sub4])[sub2], M = Co (1Co) or Ni (2Ni), are reported. They are isostructural (monoclinic C2/m) and consist of chains of edge-shared MO[sub6] octahedra connected by μ-O from H[sub3]O[sub2] and MoO[sub4][sup2-]. These chains form a three-dimensional network with O-H-O, O-Mo-O, and O-Na--O bridging 4, 3, and 4 metal ions, respectively. Both compounds behave as canted antiferromagnets but differ in their behaviors, 1Co showing a broad maximum (28 K) above the Néel transition (21 K) and the canting taking place at 13K, some 8 K below T[subN], while for 2Ni the canting takes place at T[subN] (28 K). Analyses of the neutron powder diffraction data shed some light on the geometry of D[sub3]O[sub2][sup-] and suggest antiferromagnetism with a propagation vector k = (0,0,0) with the moments within each chain being parallel but antiparallel to those in neighboring chains. The difference between 1 Co and 2Ni is in the orientation of the moments; they are parallel to the chain axis (b-axis) for iCo and perpendicular to it for 2Ni with a major component along the c-axis and a small one along the a-axis. The heat capacity data peak at 20.9(3) K (1 Co) and 25.1(1) K (2Ni). The derived magnetic entropies, following correction of the lattice contribution using the measured data for the nonmagnetic Zn analogue, suggest S = 1/2 for 1Co but is lower than that expected for 2Ni (S = 1). In both cases, only Ca. 60% of the entropy is found below the magnetic ordering temperature, suggesting considerable short-range correlations at higher temperatures. While the temperature at which the magnetic diffraction becomes observable coincides with that of at the peak in heat capacity, it is lower than T[subN] observed by magnetization measurements in both cases, and there is evidence of short- range ordering in a narrow range of temperature (T[subN] ± 5 K).
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