Interatomic Electric Quadrupole-Quadrupole Coupling in Salts of Cerium

We have calculated the magnetic properties of a cerium salt in terms of the one-atom (crystalline field) model at temperatures greater than 1°K. Because the cerium ion is not in an S state, its electronic charge distribution has a quadrupole moment; we have determined the influence of interatomic electric quadrupole-quadrupole interactions on the magnetic moment and optical rotation at very low temperatures. In cerium ethylsulfate this coupling is antiferromagnetic with a characteristic temperature of τ=0.01〈r2〉4 degrees, where 〈r2〉½ is the rms radius of the 4f orbit in A. This interaction may be at least partly responsible for a low temperature anomaly found by Becquerel, de Haas, and van den Handel, as well as for the sharp increase in specific heat noticed during adiabatic demagnetization. In other salts τ may be of opposite sign (ferromagnetic case) and possibly much larger (∼1°K).