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Cerium is a lanthanide chemical element with symbol $\mathrm{Ce}$ and atomic number $58$. Its accessible $+3$ and $+4$ oxidation states distinguish much of its chemistry from rare earths dominated by only $+3$.^[ref]

Changing the cerium oxidation state exchanges an electron with the surroundings and can couple to oxygen defects in cerium-containing oxides. Such redox flexibility helps explain why composition, defect population and chemical environment must be specified when describing a cerium material.

Cerium magnesium nitrate is a hydrated double nitrate salt, commonly written $\mathrm{C}{\mathrm{e}}_2\mathrm{M}{\mathrm{g}}_3(\mathrm{N}{\mathrm{O}}_3{)}_{12}\cdot 24{\mathrm{H}}_2\mathrm{O}$. The cerium ions give the crystal a low-temperature magnetic response useful in temperature measurement.^[ref]

In a magnetic thermometer, a measured magnetic susceptibility serves as a temperature-dependent signal. A 1968 comparison found a small offset between powdered and single-crystal thermometers, showing that sample form and calibration matter even for the same salt.^[ref]