Effect of Co partial substitution on the valence state of Ru in the Gd2−xCo x Ru2O7 pyrochlore

Polycrystalline samples of Gd_2−xCo_xRu₂O₇ with x = 0.0, 0.1 and 0.4 were synthesized by the molten salt method. The samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrical resistivity measurements. Rietveld refinements of the XRD patterns and XPS measurements showed that the Co²⁺ ion replaces Gd³⁺ sites. As a result, the lattice parameter a and the Ru-O bond length decrease; then, the Ru-O-Ru bond angle increases. Those changes induce a charge compensation which was detected by XPS measurements. The analysis of these results shows that the Ru 3d_5/2 core level could be fitted assuming the contribution of two different chemical states of the Ru. The Ru 3d_5/2 core level is localized at 280.7 and 281.6 eV, which corresponds to Ru⁴⁺ and Ru⁵⁺. The valence band XPS spectra show an increase in Co 3d states at the Fermi level as the Co content increases, which contribute to the decrease in the electrical resistivity.

Pyrochlore compounds can be written with general formula A23+B24+O6−2O′−2 with four nonequivalent atom sites. The space group is Fd3m (No. 227), and there are eight formula units per unit cell (Z = 8) [[1] ]. The A cations have an eight coordination (AO8), and the smaller B cations have a six coordination (BO6) which forms infinite interpenetrating networks of tetrahedrons that share the corners. Another stoichiometry emerges due to a significant capacity of these oxides to form a solid solution, for example, as A23+(B3+B5+) O6−2O′−2 and A22+B25+ O6−2O′−2, where the A cation can represent an earth rare (E.R.) or alkaline earth metals (A.E.M.), and the B cation is a transition metal (T. M.).

Pyrochlore compounds have been widely studied due to their applications, for example, in solid electrolytes [[2] -[4] ], anodes [[5] -[7] ] and cathodes [[5] , [8] -[10] ] for fuel cells and sensors, catalysis [[11] -[13] ], dielectrics [[14] -[16] ], novel conductivity [[17] , [18] ] and for their magnetic properties [[19] , [20] ].

In the last decades, pyrochlore based on Ru compounds has been extensively studied due to their intriguing electronic properties. For example, the Bi2Ru2O7 and Pb2Ru2O6.5 pyrochlore compounds show interesting electrical properties having a metallic behavior and low resistivity of 10−3 Ω cm at room temperature [[21] ], while the (R.E.)2Ru2O7 (R.E. = rare earth and Y) is semiconductors with low activation energies. In other cases, such as Tl2Ru2O7−δ [[22] ] and Pb2−x(R.E)xRu2O7 with R.E. = Nd, Gd pyrochlore compounds [[23] ], a metallic-semiconductor transition around 120 K is observed. Similar results are observed when a divalent T. M. substitutes the A site; for example, in the Y2−x3+Znx2+Ru2O7 pyrochlore compound, the electric conductivity increases with the increasing Zn2+ ion [[24] , [25] ]. To understand the origin of the metallic-semiconductor transition studies on the correlation of electric properties between crystal structures has been reported. The results show that the particular behavior can be associated with Ru-O-Ru bond angle: between 135° and 140° and between 129° and 134°, it has a metallic and semiconducting response, respectively [[26] ].

In the crystal structure (R.E.)2Ru2O7, the Ru ion has a six coordination, and due to crystal field, the Ru 4d states show a split into t2g and eg groups [[27] ]. Studies of electronic band structure have shown that the main contributions to the electronic density of states at the Fermi level N(EF) come from the anti-bonding states of Ru 4d and the O 2p states. In the Bi2Ru2O7 and Pb2Ru2O6.5 pyrochlore, the metallic behavior is associated with the significant mixing between Ru 4d band and the 6s orbital of Bi and Pb ions [[28] ]. Although the conduction mechanisms in the pyrochlore oxides based on Ru have been discussed, the electronic structure and properties of these materials are poorly understood.

In the present work, we report the crystalline structure and electronic properties of Gd2−xCoxRu2O7 (x = 0.0, 0.1 and 0.4) pyrochlore.

Polycrystalline samples of Gd2−xCoxRu2O7 (x = 0.0, 0.1 and 0.4) were synthesized by a molten salt method using a eutectic mixture (1:1 M) of KCl (Sigma-Aldrich > 99%) and NaCl (Sigma-Aldrich 99.5%). The precursors were RuO2 (Sigma-Aldrich 99%), CoO (Sigma-Aldrich 99.999%) and Gd2O3 (Sigma-Aldrich 99.9%). In the reaction, the salts were first mixed in an agate mortar until a fine homogeneous powder was gotten. To obtain the samples, a 25.55:1 M proportion of salt to reactants was mixed stoichiometrically. The mixture was heated at 1200 °C for 12 h. The product obtained was washed and stirred in distilled water for 2 h to remove the salts. Then, it was filtered in a Nalgene® System using a 0.22-μm Millipore® filter. The X-ray diffraction patterns were measured at room temperature using a Bruker (D5000) diffractometer with Co Kα radiation with a Fe filter in 0.02° steps from 10° to 120° and refined with the Rietveld program MAUD v 1.7.7 [[29] ]. The chemical orbital and valence band were measured by X-ray photoelectron spectroscopy. The measurements were taken in an ultra-high vacuum (UHV) system Scanning XPS microprobe PHI 5000 Versa Probe II, with an Al Kα X-ray source (hν = 1486.6 eV) and an MCD analyzer. The surface of the samples was etched for 10 min with 2.0 kV Ar+ at 0.25 µA mm−2. The XPS spectra were obtained at 45° to the normal surface in the constant pass energy mode (CAE), E0 = 100 and 10 eV for survey surface and high-resolution narrow scan, respectively. The XPS spectra were analyzed using the SDP v 4.1 program [[30] ]. Electrical resistance versus temperature was measured using the four-probe technique in a Physical Properties Measurement System Model Dyna Cool (Quantum Design). The measurements were taken at a range of temperatures between 300 and 60 K.

Figure 1 displays the X-ray diffraction patterns for the polycrystalline samples of Gd2−xCoxRu2O7 with x = 0.0, 0.1 and 0.4. The main features of the patterns correspond to the Gd2Ru2O7 cubic structure cubic (JCPDS No. 28-0425). The inset shows the shift of (222) reflexion toward higher angles as a function of Co content. The X-ray diffraction patterns were Rietveld-fitted by considering the possibility that Co2+ ions can occupy Gd3+ sites. Figure 2 shows, as an example, the fitted X-ray diffraction pattern for x = 0.1. The structural parameters and R-factors are summarized in Table 1. The lattice parameter a for the undoped sample agrees with other published results [[1] ]. The inset Fig. 2 shows the lattice parameter a as a function of Co content (x). It can be observed that the lattice parameter a decreases linearly with an increasing dopant concentration following Vegard’s law. Using a least-squares fitting algorithm, a linear relationship was obtained between a0 and x. This can be described as:a=10.2365-0.0829xwhere a0 is 10.2365.X-ray diffraction patterns for Gd2−xCoxRu2O7 with 0 ≤ x ≤ 0.4 compounds. The inset shows the shift of the (222) plane reflexion with increasing Co contentThe Rietveld-fitted result for the x = 0.1 sample along with experimental (filled circle), calculated (line) and the difference between the observed and calculated patterns (bottom line) is observed. The inset shows the lattice parameter a as a function of Co content (x) fitted by Vegard’s law

Structural parameters obtained from the Rietveld fitting of the X-ray diffraction patterns of Gd2−xCoxRu2O7 at 295 K

Space group: F d 3 m (No 227). Atomic positions: Gd: 16d (1/2, 1/2, 1/2); Ru: 16c (0, 0, 0); O(1): 48f (x, 1/8, 1/8), O(2): 8b (3/8, 3/8, 3/8) position; N is the occupancy factor

This result may be explained considering the coordination number and the ionic radii of the Gd3+, Ru4+ and Co2+ ions. It is well known that the Gd3+ and Co2+ ionic radii in eight coordination are 1.053 and 0.90 Å, while Ru4+ and Ru5+ in six coordination are 0.62 and 0.565 Å, respectively [[31] ].

From these results, the decrease in the lattice parameter of a value can be related to the substitution of Co2+ ions into the Gd3+ sites. This behavior is consistent with the observed changes in Y2−xZnxRu2O7 when Zn2+ occupies the Y3+ sites [[25] ].

From the structural refinement results, it could be concluded that cobalt atoms mostly occupy the Gd sites, leading to significant changes on the Ru-O(1) bond length, rather than the Gd/Co-O(1) or Gd/Co-O(2) bond lengths. From our structural refinement, we can conclude that the majority of cobalt atoms go to the Gd3+ sites, which give place to significant changes on the Ru-O(1) bond length, rather than in the Gd/Co-O(1) and Gd/Co-O(2) bond length. Furthermore, the Gd/Co-O(1)-Gd/Co bond angle decreases and the Ru-O(1)-Ru bond angle associated with the rotation of RuO6 octahedral around the c-axis increases from 128.12° to 130.23° as a function of Co content. According to earlier studies on the correlation between the Ru-O-Ru bond angle and the electrical properties [[26] ], our compounds have a behavior semiconductor. On the other hand, the proposal to substituting Co2+ ions in the Ru4+ sites does not induce significant changes in the quality fitting value of χ2; therefore, we cannot discard this possibility.

The stability of the structure of Gd2−xCoxRu2O7 was investigated considering the rA/rB ratio where rA and rB are the A- and B-site ionic radii, which determines the stability field for the formation of a pyrochlore structure. Subramanian et al. [[1] ] had shown that the range of stability for A23+B24+O7 pyrochlore compound is localized between 1.46 < rA/rB < 1.80. In this work, the values obtained decrease from 1. 70 to 1.65, which shows that the Gd2−xCoxRu2O7 structure is stable.

X-ray photoelectron spectroscopy (XPS) was used to identify and analyze the chemical state and electronic state of the pyrochlore-type compounds. Figure 3 shows the XPS survey spectra after Ar+ etching the surface of the Gd2−xCoxRu2O7 samples with 0.0 ≤ x ≤ 0.4 compositions.XPS survey spectra after Ar+ etching of the Gd2−xCoxRu2O7 polycrystalline samples

Figure 3 shows the XPS survey spectra after Ar+ etching the surface of the Gd2−xCoxRu2O7 samples with 0.0 ≤ x ≤ 0.4 compositions.

Figure 4 shows the high-resolution spectra for the Ru 3d as a function of Co content. The spectrum of Gd2Ru207 comprises a simple spin-orbit doublet with narrow, symmetric components in which splitting is about 4.2 eV. The Ru 3d5/2 and Ru 3d3/2 core levels are localized at 280.70 and 284.9 eV, respectively. The first value is consistent with the reported for RuO2 in the NIST XPS database [[32] ]: Ru 3d5/2: 280.30-280.90 eV, average: 280.60 eV. This value shows that the ion is in valence Ru4+. For x > 0.0, the Co substitution leads to a larger broadening of the core level through the addition of a second spin-orbit doublet on the high binding-energy side. This second doublet is localized at 281.6 and 285.8 eV, which are associated with the Ru5+ as is reported in the Y1-xZnxRu2O7 [[25] ], Ca2R2uO7 [[33] ] and Hg2Ru2O7 [[34] ] pyrochlore compounds. From Rietveld analyses, we found that the remarkable decreases in the average Ru-O bond length support the mixed valence Ru4+ (0.64 Å)/Ru+5(0.565 Å). This fact should have a notable effect on the electronic properties. To investigate this behavior, we had obtained and analyzed the spectrum of the valence band by XPS.Deconvolution of the high-resolution XPS spectra of the Ru 3d core levels of the studied samples. The notation Sat corresponds to satellites associated with Ru core level

Figure 5 shows the XPS valence band spectra of Gd1−xCoxRu2O7. From band structure calculations in pyrochlore ruthenates [[27] , [28] ] and ruthenium oxides [[35] ], it is known that the contributions to the electronic density of states, DOS, from − 8 up to − 2 eV can be associated with the O 2p states. Near the Fermi energy, EF, the Ru 4d states with t2g character are predominant, which are hybridized with O 2p states in an anti-bonding way. Moreover, as Co content increases, the Co 3d contribution increases near Fermi.XPS valence band for Gd1−xCoxRu2O7 polycrystalline samples. The inset shows the valence band from − 0.5 to 0.5 eV

The bands close to EF are of importance since they manage the transport properties. The XPS valence band spectra show an increase in the valence band intensity at the Fermi energy as Co content increases, see inset of Fig. 5. This increase indicates that the charge carrier’s density at the Fermi level increases, which suggests changes to the transport properties due to Co content on the Gd2Ru2O7 structure. It is well known that in the pyrochlore compounds, the transport properties are affected by the T. M substitution. Here, to investigate the effect of the Co substitution on the transport properties, we analyze the electrical resistance below room temperature.

Figure 6 shows the normalized resistance as a function of the temperature of the investigated samples. The Arrhenius equation was used to describe the electrical conduction of our samples, R = R0 exp(Eg/2kBT), where the constant R0 is proportional to the carrier concentration, Eg is the energy gap energy, and kB is the Boltzmann’s constant. The samples show a semiconductor behavior; however, the resistance decreases as the Co content increases. The inset in Fig. 6 shows the curves resistance fitted by the Arrhenius equation in the region of 200-300 K. The nonlinear behavior displayed in the inset indicates that the Co-doped samples do not follow an ideal Arrhenius-type law. The energy gap values obtained from 200 to 300 K were 0.195, 0.169, 0.164 eV and the activation energy 0.097, 0.084 and 0.082, for x = 0.0, 0.1 and 0.4, respectively. The activation energy of 0.097 eV found for the Gd2Ru2O7 in this study is comparable to the activation energy of 0.10 eV reported by R. J Bouchard [[36] ]. The decreases in gap energy suggest that a charge transfer mechanism occurs due to the presence of mixed valence of Ru4 +/Ru5 +. To explain this behavior consider the next reaction: Ru4+ + h → Ru5+, where h is a hole, and this reaction indicates that the major carriers are holes. This process is responsible for the sizable decrease in the resistivity of the Gd2Ru2O7 concerning the undoped one, as shown in Fig. 6.Normalized resistance versus temperature for Gd1−xCoxRu2O7 polycrystalline samples. The inset shows the variation of ln R with the reciprocal of temperature (T−1)

We presented a systematic study on the effect of Co substitution in the Gd sites of the Gd2Ru2O7 pyrochlore on the crystal structure and electronic properties. X-ray diffraction and X-ray photoelectron spectroscopy studies show that Co2+ ion occupies the Gd3+ sites, which induces a mixed valence in the ruthenium ion (Ru4+ and Ru5+) and a decrease in the lattice parameter a and, as a consequence, a decrease in the Ru-O(2) bond length and increases in the Ru-O-Ru bond angle. This substitution has notable effects on the electrical properties. It is found that the electrical resistivity shows a drastic decreasing for x ≤ 0.4 of Co content. This fact is confirmed by the increase in the intensity of the valence band near to the Fermi energy as is observed by XPS.

A.A will like to thank CONACYT and R.E to the Project DGAPA-UNAM IN109718 and IA106117. They will also like to thank L. Huerta, Adriana Tejeda-Cruz, M.M.S. Alberto Lopez-Vivas, A. Pompa and C. Gonzalez for providing technical help.

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