Crystal-field interaction and oxygen stoichiometry effects in strontium-doped rare-earth cobaltates

Inelastic neutron scattering was employed to study the crystal-field interaction in the strontium-doped rare-earth compounds $R_x{\mathrm{Sr}}_{1-x}{\mathrm{CoO}}_{3-z}$ ($R=\mathrm{Pr}$, Nd, Ho, and Er). Particular emphasis is laid on the effect of oxygen deficiencies that naturally occur in the synthesis of these compounds. The observed energy spectra are found to be the result of a superposition of crystal fields with different nearest-neighbor oxygen coordination at the $R$ sites. The experimental data are interpreted in terms of crystal-field parameters, which behave in a consistent manner through the rare-earth series, thereby allowing a reliable extrapolation for rare-earth ions not considered in the present work.

Figure 1

Fraction of different oxygen coordinations around the $R^{3+}$ ions in $R_x{\mathrm{Sr}}_{1-x}{\mathrm{CoO}}_{3-z}$ as a function of the oxygen deficiency $z$. The results were obtained from a Monte Carlo simulation in an $N$×$N$×$N$ lattice of unit cells with $N =15$.

Figure 2

Crystal-field level sequence at the $\mathrm{Er}{}^{3+}$ sites in ${\mathrm{Er}}_x{\mathrm{Sr}}_{1-x}{\mathrm{CoO}}_{3-z}$ for 12, 11, and 10 coordinating $\mathrm{O}{}^{2-}$ ions, calculated from Eqs. (1, 2, 3, 4, 5) with $A_4 =0.306$ meV and $A_6 =0.0151$ meV. The arrows mark selected ground-state transitions, and the corresponding numbers denote the transition probability ${\left|〈{\Gamma }_n\left|J_{\perp }\right|{\Gamma }_m〉\right|}^2$.

Figure 3

Energy spectra of neutrons scattered from ${\mathrm{Er}}_{0.15}{\mathrm{Sr}}_{0.85}{\mathrm{CoO}}_{2.71}$ and ${\mathrm{La}}_{0.15}{\mathrm{Sr}}_{0.85}{\mathrm{CoO}}_{2.71}$ at $T =10$ K. The data were taken with the use of the TOF spectrometer KDSOG for moduli of the scattering vector 1.6 < $Q$ < 3.3 Å${}^{-1}$. The lines denote Gaussian fits as explained in the text. The arrows mark the crystal-field transitions according to Fig. 2.

Figure 4

Energy spectra of neutrons scattered from ${\mathrm{Er}}_{0.10}{\mathrm{Sr}}_{0.90}{\mathrm{CoO}}_{2.70}$. The data were taken with the use of the TOF spectrometer FOCUS for moduli of the scattering vector 0.4 < $Q$ < 2.2 Å${}^{-1}$. The incoming neutron energy was 3.3 meV. The arrow marks the transition according to Fig. 2. The inset shows the temperature dependence of the intensities integrated in the range 0.2 ⩽ Δ$E$ ⩽ 0.8 meV (spheres) and scaled to the calculated Boltzmann population factor for the ground-state doublet (line).

Figure 5

Energy spectra of neutrons scattered from ${\mathrm{Ho}}_{0.10}{\mathrm{Sr}}_{0.90}{\mathrm{CoO}}_{2.73}$. The lines denote Gaussian fits as explained in the text. The top displays the crystal-field level sequence derived from the data, and the arrows mark the observed transitions. (a) Data taken at $T =1.5$ K with the use of the TOF spectrometer FOCUS for moduli of the scattering vector 0.5 < $Q$ < 4.9 Å${}^{-1}$. The incoming neutron energy was 15.5 meV. (b) Data taken at $T =10$ K with the use of the TOF spectrometer KDSOG for moduli of the scattering vector 2.4 < $Q$ < 3.7 Å${}^{-1}$.

Figure 6

Energy spectra of neutrons scattered from ${\mathrm{Ho}}_{0.10}{\mathrm{Sr}}_{0.90}{\mathrm{CoO}}_{2.73}$. The data were taken with the use of the TOF spectrometer CNCS for moduli of the scattering vector 0.8 < $Q$ < 1.8 Å${}^{-1}$. The incoming neutron energy was 3.31 meV. The arrows mark the transitions according to Fig. 5. The inset shows the temperature dependence of the intensity integrated in the range 0.8 ⩽ |Δ$E$| ⩽ 1.2 meV (spheres) and scaled to the calculated Boltzmann population factor for the ground-state triplet ${\Gamma }_5$⁽¹⁾ (full line) and the first excited doublet ${\Gamma }_3$⁽¹⁾ (broken line).

Figure 7

Energy spectrum of neutrons scattered from ${\mathrm{Pr}}_{0.10}{\mathrm{Sr}}_{0.90}{\mathrm{CoO}}_{2.77}$ at $T =3$ K. The data were taken with the use of the TOF spectrometer CNCS for moduli of the scattering vector 1.5 < $Q$ < 2.5 Å${}^{-1}$. The incoming neutron energy was 20 meV. The lines denote Gaussian fits on top of a power-law background as explained in the text. The arrows mark the crystal-field transitions according to Fig. 8.

Figure 8

Crystal-field level sequence at the $\mathrm{Pr}{}^{3+}$ sites in ${\mathrm{Pr}}_x{\mathrm{Sr}}_{1-x}{\mathrm{CoO}}_{3-z}$ for 12 and 11 coordinating oxygen ions, calculated from Eqs. (1, 2, 3, 4, 5) with $A_4$ = −14.2 meV and $A_6 =1.79$ meV. The rectangle in (a) is displayed as an enlarged figure in (b). The arrows mark selected ground-state transitions, and the corresponding numbers denote the transition probability ${\left|〈{\Gamma }_n\left|J_{\perp }\right|{\Gamma }_m〉\right|}^2$.

Figure 9

Energy spectrum of neutrons scattered from ${\mathrm{Nd}}_{0.10}{\mathrm{Sr}}_{0.90}{\mathrm{CoO}}_{2.74}$ at $T =1.5$ K. The data (filled circles) were taken with the use of the TOF spectrometer CNCS for moduli of the scattering vector 0.8 < $Q$ < 1.8 Å${}^{-1}$. The incoming neutron energy was 3 meV. The inset shows the temperature dependence of the intensity integrated in the range 0.35 ⩽ Δ$E$ ⩽ 1.15 meV (open circles; data taken with the use of the TOF spectrometer FOCUS) and scaled to the calculated Boltzmann population factor for the ground-state quartet ${\Gamma }_8$⁽¹⁾ with a doublet-doublet splitting of 0.6 meV (line).