Eu²⁺-Eu³⁺ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies

The Eu atoms in Pb_1-xEu_xSe have long been assumed to be divalent. We show that p-type doping of this magnetic semiconductor alloy with Na can modify the effective Eu valence: a mixed, Eu²⁺-Eu³⁺ state appears in Pb_1-x-yEu_xNa_ySe at particular values of y. Magnetization, carrier concentration, resistivity, and thermopower of Pb_1-x-yEu_xNa_ySe are reported for a number of samples with different x and y. A pronounced increase in thermopower at a given carrier concentration was identified and attributed to the presence of enhanced ionized impurity scattering. A strong decrease in the hole concentration is observed in Pb_1-yNa_ySe when Eu is added to the system, which we attribute to a Eu²⁺-Eu³⁺ self-ionization process. This is evidenced by magnetization measurements, which reveal a significant reduction of the magnetic moment of Pb_1-xEu_xSe upon alloying with Na. Further, a deviation of magnetization from a purely paramagnetic state, described by a Brillouin function, identifies antiferromagnetic interactions between the nearest-neighbor Eu atoms: a value of J_ex/k_B = -0.35 K was found for the exchange coupling parameter. The conclusion of a Eu²⁺-Eu³⁺ self-ionization process being in effect is supported further by the electronic structure calculations, which show that an instability of the 4f⁷ configuration of the Eu²⁺ ion appears with Na doping. Schematically, it was found that the Eu 4f levels form states near enough to the Fermi energy that hole doping can lower the Fermi energy and trigger a reconfiguration of a 4f electronic shell.