Cerium compounds with very low magnetic heat capacities: The Casimir-du Pré method applied to polycrystalline samples

As part of a continuing search for materials useful as magnetic coolants and thermometers in the millikelvin region, the magnetic heat capacities of the cerium diglycollate compounds ${\mathrm{Na}}_3$[Ce${\left({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{O}}_5\right)}_3$] · 2NaCl${\mathrm{O}}_4$ · 6${\mathrm{H}}_2$O (CDG) and ${\mathrm{Na}}_3$[Ce${\left({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{O}}_5\right)}_3$] · 9${\mathrm{H}}_2$O (TCDG) have been investigated at helium temperatures. The Casimir-du Pré method involving adiabatic susceptibility measurements was used. As only polycrystalline samples of CDG could be prepared, the thermodynamic equations generally valid for the application of the method to such samples have been examined. The expressions obtained were checked by a comparison of the results from powder and single-crystal measurements on cerium magnesium nitrate (CMN), before the method was applied to CDG. Expressed as the coefficient $b$ in the "high" -temperature expansion $\frac{C_M}{R}=\frac{b}{T^2}+\cdots$, the resuls are 3.78(20) m${\mathrm{K}}^2$ from the powder measurmeents on CDG and 3.57(3) m${\mathrm{K}}^2$ from measurments on a single crystal of TCDG. Judged from their small magnetic heat capacities, both CDG and TCDG should be comparable to or better than CMN as refrigerants and thermometers.