Inequivalent Magnetic Ions in Dihydrated Formates of Fe+ + and Ni+ +

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Abstract
Several dihydrated formates including Ni(HCOO)2·2H2O and Fe(HCOO)2·2H2O crystallize in a monoclinic structure whose unit cell contains two each of two inequivalent types of metal ions. The temperature variation of the powder susceptibility, χp, of the Ni+ + salt suggests that those ions on one type of site become cooperatively ordered near 15.7°K while the rest remain essentially independent even at 1.3°K. χp for the Fe+ + salt exhibits a huge peak at 3.68°K associated apparently with the long‐range ordering of moments on sites of both types. Mössbauer experiments at 300°K on the Fe+ + salt clearly distinguish ions on the two sites, the ratio of the effective electric field gradients being ∼5 to 1. The smaller field gradient is assumed to characterize the site (type 1) approximately octahedrally coordinated by oxygens from six formate groups. The Mössbauer spectrum near the transition point differs from those measured at 300°, 77°, 20°, and 4°K. The results show that the internal magnetic field is larger at the type 1 sites. Preliminary calculations using the observed nuclear hyperfine structure indicate that the ratio of magnetic fields at the two sites is about four to one.