EPR study of local symmetry sites of ${\text{Ce}}^{3+}$ in ${\text{Pb}}_{1-x}{\text{Ce}}_{x}A$ ($A=\text{S}$, Se, and Te)

The local site symmetry of ${\text{Ce}}^{3+}$ ions in the diluted magnetic semiconductors ${\text{Pb}}_{1-x}{\text{Ce}}_{x}A$ ($A=\text{S}$, Se, and Te) has been investigated by electron-paramagnetic resonance (EPR). The experiments were carried out on single crystals with cerium concentration $x$ ranging from 0.001 to 0.035. The isotropic line due to ${\text{Ce}}^{3+}$ ions located at the substitutional Pb cation site with octahedral symmetry was observed for all the studied samples. We determined the effective Landé factors to be $g=1.333$, 1.364, and 1.402 for $A=\text{S}$, Se, and Te, respectively. The small difference with the predicted Landé factor $g$ of 10/7 for the ${\Gamma }_7$ $\left(J=5/2\right)$ ground state was attributed to crystal-field admixture. In addition, EPR lines from ${\text{Ce}}^{3+}$ ions located at sites with small distortion from the original octahedral symmetry were also observed. Two distinct sites with axial distortion along the $⟨001⟩$ crystallographic direction were identified and a third signal in the spectrum was attributed to sites with the cubic symmetry distorted along the $⟨110⟩$ direction. The distortion at these distinct Ce sites is attributed to Pb lattice vacancies near the cerium ions that compensate for its donor activity.

Figure 1

EPR spectrum of ${\text{Pb}}_{1-x}{\text{Ce}}_x\text{Te}$ crystal with $x=0.004$. Signal I is due to ${\text{Ce}}^{3+}$ ions with cubic $\left(O_h\right)$ site symmetry. Its intensity was divided by 4 to emphasize the presence of the other lines. Signals II and III are due to ${\text{Ce}}^{3+}$ ions with the original cubic site symmetry distorted along the $⟨001⟩$ axis. Signal IV is attributed to cubic Ce sites distorted along the $⟨110⟩$ direction.

Figure 2

EPR spectrum of ${\text{Pb}}_{1-x}{\text{Ce}}_x\text{Se}$ $\left(x=0.013\right)$ crystal at $T=4.2\text{K}$. The EPR signals from ${\text{Ce}}^{3+}$ ions located into weakly distorted cubic site symmetry (sites II and III) are manifested around the central signal due to ${\text{Ce}}^{3+}$ ions in an octahedral cubic environment (site I). The intensity of the central line (site I) was divided by 4 to emphasize the presence of the other lines.

Figure 3

Evolution of the EPR spectrum of ${\text{Pb}}_{1-x}{\text{Ce}}_x\text{Se}$ as a function of cerium concentration. The height (peak to peak) of the central signal due to ${\text{Ce}}^{3+}$ ions in site I is normalized to the same value.

Figure 4

Ratio of the population of site III to the population of site I for the three DMSs. The data were obtained from the EPR intensity analysis of signals.

Figure 5

(a) Carrier concentration of ${\text{Pb}}_{1-x}{\text{Ce}}_{x}A$ samples as a function of the Ce concentration determined at $T=300\text{K}$. The data represented by a circle in gray color for ${\text{Pb}}_{1-x}{\text{Ce}}_x\text{Se}$ is from previous work (Ref. 27). (b) Concentration of lead vacancy as a function of $x$ calculated from the relation $N_{V_{{\text{Pb}}^{2+}}}=(N_{{\text{Ce}}^{3+}}-n/2)$.