Heat Capacities of Fe${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O and Ni${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O between 1.4 and 20 °K

The heat capacities of Fe ${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O and Ni${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O have been measured between 1.4 and 20 °K. Peaks associated with long-range spin ordering are found at 3.74 °K for Fe${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O and at 14.5 °K for Ni${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O. An additional Schottky-like maximum occurs at 3 °K in the heat capacity of the nickel salt. These anomalies can be correlated with corresponding features of the observed magnetic susceptibilities. A unit cell of these salts contains two each of two inequivalent metal ions, types $A$ and $B$. Formate groups form $\mathrm{AA}$ and $\mathrm{AB}$ bridges and are thought to mediate the dominant superexchange interactions. The thermal data show that the $\mathrm{AB}$ interaction is much weaker than the $\mathrm{AA}$ interaction in Ni${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O; for Fe${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O their relative strengths could not be determined.