Probing the electronic and local structure of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ using near-edge and extended x-ray absorption fine structures

We report the electronic and local structural investigation of double pervoskites ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) using x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$ edges. The ab initio simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of $1s$ electrons to the $p\text{−}d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the ${\mathrm{CoO}}_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive ${\mathrm{Co}}^{2+}$ ions with $x$. On the other hand, the $z$-in distortion in ${\mathrm{NbO}}_6$ octahedra remains unaltered up to $x=0.4$ and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x⩾$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white-line feature in the XANES spectra and possible reasons for this are discussed. More interestingly, we observe the signature of ${\mathrm{KN}}_1$ double-electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z+1$ approximation. Further, the Co $L_{2,3}$ edge shows the reduction in the crystal-field strength and hence an increase in the charge-transfer energy (${\mathrm{\Delta }}_{\text{ct}}$) with the La substitution.

Figure 1

(a) The normalized Nb $K$-edge XANES spectra of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) samples where 1, 2, 3 represent the main absorption features, and enlarged views of features 2 and 3 are shown in the insets. (b) The first derivative of the experimental absorption spectra of the Nb $K$ edge along the simulated spectrum for the $x=0$ and 1 samples and their corresponding ${\mathrm{Nb}}_{\mathrm{abs}}d$-LDOS in the XANES region. The red dashed ellipse highlights the presence of pre-edge feature in both experimental and simulated spectra, and sharp ${\mathrm{Nb}}_{\mathrm{abs}}d$-LDOS at this energy. All the experimental spectra have been vertically shifted by fixed 0.025 units for clarity. (c) The comparison of first-order derivative of the simulated absorption coefficient of Nb $K$ edge for dipole, and dipole plus quadrupole transitions for the $x=0$ sample. (d), (e) The experimental and simulated Nb $K$-edge absorption spectra along the calculated $p$-LDOS of absorbing Nb atoms for $x=0$ and 1 samples. The experimental spectra have been vertically shifted by 0.3 units for clarity. The black dashed line represents the Fermi level and red dashed lines indicate the shift in the ${\mathrm{Nb}}_{\mathrm{abs}}p$-LDOS peak corresponding to the feature 3 (shaded region).

Figure 2

(a) The simulated Nb $K$-edge XANES spectra for the $x=0$ sample by varying the number of first-nearest Sr atoms. Insets (a1) and (a2) represent the enlarged view of the spectra in different energy regimes. (b) The ${\mathrm{Nb}}_{\mathrm{abs}}p$-LDOS corresponding to the XANES spectra presented in (a), and the inset shows the enlarged view of the highlighted region. (c) The comparison of simulated Nb $K$-edge spectra of the $x=0$ sample for all first-neighbor $A$-site atoms as Sr and La (on the left axis) and corresponding ${\mathrm{Nb}}_{\mathrm{abs}}p$-LDOS (on the right axis), and the inset shows the enlarged view of the highlighted region. (d) The Nb $K$-edge spectra for the $x=1$ sample with all eight first-neighbor $A$-site atoms as Sr, four each of La and Sr as well as eight La. The insets (d1) and (d2) represent the enlarged view of the highlighted region and the corresponding ${\mathrm{Nb}}_{\mathrm{abs}}p$-LDOS, respectively.

Figure 3

(a) The normalized Co $K$-edge XANES spectra of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) samples. The letters A–G represent the absorption features resulting from various electronic transitions (see text for the detail). The red arrow indicates the shift in the rising edge towards the lower energy with the La substitution. The solid black arrow and dashed red line represent the shift and invariance in the white line for $\mathrm{x}⩽0.4$ and $x⩾0.6$ samples, respectively. Inset shows the enlarged view of the A and B (arrow) pre-edge features for clarity. (b) The average oxidation state of Co (open circles) on left axis and oxygen concentration (open triangles) on the right axis, as a function of $x$. Dashed red and blue lines represent the ideal behavior for the full transformation of Co from $3+$ to $2+$ valence state (considering the linear relationship between shift in edge position and oxidation state) and six oxygen atoms per formula unit, respectively. (c) The Co $K$-edge XANES spectra for the intermediate compositions, i.e., $x=0.2–0.8$ samples, where the solid black lines represent the Co valence state weighted linear combination of the $x=0$ (${\mathrm{Co}}^{3.0+}$) and $x=1$ (${\mathrm{Co}}^{2.25+}$) spectra. Each spectrum is vertically shifted cumulatively by 0.5 units for clarity.

Figure 4

(a) The normalized Sr $K$-edge XANES spectra of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) samples. The inset (a1) shows the first derivative of the absorption coefficient, where the vertical dashed line represents the invariance in the rising edge position. These curves are vertically shifted for better presentation. The inset (a2) presents the enlarged view of feature II, where the arrow shows the shift in the peak position with $x$. (b) The normalized Sr $K$-edge (on top and right axis) and La $L_3$-edge (on bottom and left axis) spectra for the $x=1$ sample in 150-eV range in the vicinity of the white line. The dashed line represents the presence of the absorption feature II at the same position with respect to the white line in both the spectra.

Figure 5

(a) The magnitude of the Fourier-transformed $k^3\chi \left(k\right)$ spectra for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) in 0–6 $\AA$ spectral range, derived from the EXAFS region of the Nb $K$ edge. The inset shows the comparison of simulated spectra of the $x=0$ sample for two different Nb-${\mathrm{O}}_e\text{−}\mathrm{Co}\text{/}\mathrm{Nb}$ angles (Nb-${\mathrm{O}}_a\text{−}\mathrm{Co}/\mathrm{Nb}={180}^{\circ }$). (b)–(g) The enlarged view of (a) between 3.75 to 5.75 $\AA$ with cumulative increment in $x$ to clearly show the evolution of an additional scattering path resulting from the Co/Nb atoms, located diagonally to the absorber, as indicated by the arrows (refer text for more details). (h)–(m) The fitting of the $\left|\chi \right(R\left)\right|$ and corresponding $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectra in the window shown by the dotted green line, each $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectrum is shifted downward for clarity in presentation.

Figure 6

(a) The magnitude of the Fourier-transformed $k^3\chi \left(k\right)$ spectra for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) in 0–6 $\AA$ spectral range, derived from the EXAFS spectra of Co $K$ edge. (b) The evolution of the intensity of the white line (left axis) and amplitude of the first coordination shell (right axis) with the La substitution for the Co $K$-edge and (c) Nb $K$-edge EXAFS spectra. (d)–(i) The fitting of the $\left|\chi \right(R\left)\right|$ and corresponding $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectra in the window shown by the dotted green line, where the $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectrum in each panel is shifted downward for clarity in the presentation.

Figure 7

(a) The Nb $K$-edge XANES spectra simulated for the $x=0$ sample by varying the degree of distortion in ${\mathrm{NbO}}_6$ octahedra. The insets (a1) and (a2) represent the enlarged view of the white line and variation of the white-line intensity with the distortion parameter ${\mathrm{\Delta }}_{\mathrm{Nb}}$ defined in Eq. (3), respectively. The error bars in the white-line intensity in inset (a2) are accounting for the finite step size of the simulated spectra. (b) The corresponding ${\mathrm{Nb}}_{\mathrm{abs}}p$-LDOS simulated at different values of ${\mathrm{\Delta }}_{\mathrm{Nb}}$ for the $x=0$ sample.

Figure 8

(a) The magnitude of the Fourier-transformed $k^3\chi \left(k\right)$ spectra of Sr $K$ edge for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) samples. (b) The enlarged view of the normalized Sr $K$-edge EXAFS spectra for the $x=0$ sample, which shows the ${\mathrm{KN}}_1$ double-electron excitation feature and the resulting spectra after the manual removal of this feature. (c) The $k^3$ weighted $\chi \left(k\right)$ spectra and (d) its Fourier transformation for the $x=0$ sample before and after the subtraction of the ${\mathrm{KN}}_1$ excitation feature, where arrows highlight the change in the corresponding features. (e) The ratio of the magnitude of the Sr–Sr/La and Sr–Co/Nb scattering paths as a function of $x$. (f)–(k) The fitting of the $\left|\chi \right(R\left)\right|$ and corresponding $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectra in the window shown by the dotted green line, where the $\mathrm{Im}\left[\chi \right(R\left)\right]$ spectra in each panel are shifted downward for the clarity in the presentation. The asterisk symbol in (f) represents the peak arising from the ${\mathrm{KN}}_1$ intra-atomic transition.

Figure 9

The Co $L_{2,3}$-edge spectra (vertically offset for clarity) for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ samples. The atomic multiplet based simulated spectra for HS ${\mathrm{Co}}^{2+}$ and HS ${\mathrm{Co}}^{3+}$ are compared.