1. Introduction

Pyrochlore oxides have attracted considerable attention in many areas of Material Science during the last few decades, due to their chemical and structural flexibility as well as their wide range of properties, such as superconductivity^1,2, semi-conductivity³, ionic conductivity^4,5, ferromagnetism⁶ and luminescence ^7,8. Oxide ion-conducting pyrochlores find applications in solid oxide fuel cells as electrolytes^9,10 and gas sensors. During the last decade, there has been substantial interest in the use of pyrochlore oxides for nuclear waste disposal due to their high radiation resistance¹¹. The structure and electronic properties of pyrochlore oxides depend on the disordering of cations and anions. The stability of the pyrochlore structure is generally decided by the ratio of ionic radii of cations at A and B sites¹². These systems tend to get much more complicated when both the A and B sites are occupied by magnetic ions. These compounds show very interesting magnetic features, caused by the coupled magnetic interactions between the 4f electrons of rare earth, those between the d electrons of transition metals, and those between the d and f electrons¹³. For Nd₂Ru₂O₇, Sm₂Ru₂O₇, and Eu₂Ru₂O₇ compounds, there is evidence that has been reported for a spin-glass state below this temperature, as well as the coexistence of a weak ferromagnetic state with the spin glass state below 20 K¹⁴. In previous research^14-16, these compounds exhibit magnetic transitions for A = Pr, Nd, Sm, Eu and Y at 165, 150, 135, 120, and 145 K, respectively, suggesting that there are contributions to the magnetism from both the trivalent rare earth and Ru⁴⁺. In the rare-earth ruthenates (A₂B₂O₇) the small magnetic moment (≈1-2μB) associated with the Ru⁴⁺ ion (S = 1) has been proved to happen at a relatively high temperature between ≈70 and 160 K, depending on the ionic size of the rare-earth ion^17-18. For example, solid solutions of Bi_2-x Ln _x Ru₂O₇ (Ln = Pr - Lu) (0≤x≤2) system have been investigated previously for some lanthanides^19-21. The unsubstituted Bi₂Ru₂O₇ compound is metallic and Bi_2-x Ln_x Ru₂O₇(0≤x≤2) solid solutions have been shown to cross a metal-semiconductor transition on increasing the rare-earth content x²¹. Thus, the modification and control of their electrical properties, ranging from a good metal to an insulator material, is essential for the development of functional applications. Therefore, considering the importance and potential applications of pyrochlores-type compounds, this work was focused on the study of the structural, electrical and magnetic properties of the (Er_2-x Sr_x)Ru₂O₇ system (ESRO), for x = 0.0, 0.02, 0.05, 0.07, and 0.10. With the incorporation of Sr²⁺ ions (atomic radii of 1.18 Å) stoichiometry and if, the coordination angle 130°C²² is modified or this will have structural changes, that will be reflected in the electrical and magnetic properties of the system.

2. Experimental methods

Polycrystalline samples of the ESRO system were synthesized by solid-state reaction at ambient pressure in air. The starting materials were RuO₂ (Cerac, 99.9%), Er₂O₃ (Sigma-Aldrich, 99.9%) and SrCO₃ (Cerac, 99.9%). Structure and purity of the starting materials were determined by XDR. Prior to weighing, SrCO₃ was preheated during 10-20 minutes at 120°C in order to dehydrate it. The stoichiometric mixture of the starting materials was done in air during 30 minutes, grinded with an agate mortar, resulting in homogenous slurry. The resultant ESRO mixture was compressed into pellets (13 mm diameter, 1.0 -1.5 ± 0.05 mm thickness) by applying a pressure of 3 tons/cm² during 5 minutes under vacuum. The resulting compacted specimens were then sintered in air at 1200°C during 4 days and then cooled down to room temperature following the natural cooling of furnace to 6 h. Samples were characterized by X-ray powder diffraction (XRD) using an APD 2000 diffractometer with Cu Kα radiation (λ=1.5406 Å) and a graphite monocromator. Diffraction patterns were collected at room temperature in air, over the 2θ range 10° - 90° with a step size of 0.025° and a time per step of 15 seconds. Rietveld refinement²³ was carried out using the FullPROF program²⁴. Changes in morphology and grain size were induced in the samples by performing different heat treatments during all the process of the samples preparation and examined by scanning electron microscopy (SEM) on a Hitachi S-3400N-II System. The 15.00 K.X micrographs were taken with a voltage of 15 kV, a current intensity of 1000 pA and WD = 4.5 mm. Energy Dispersive X-Ray (EDX) was performed on the same SEM system, which is equipped with an EDAX 9900 device. Low temperature DC resistance measurements were performed using the standard four-probe method from room temperature down to 10 K. Magnetic properties were examined using a Quantum Design PPMS DynaCool-9 System with a vibrating sample magnetometer (VSM) option. Magnetic moment (M) versus temperature (T) measurements were performed at 360 to 2 K using fields up to ± 400 Oe.

3. Results and discussions

Figure 1 shows the XRD patterns of the sintered Er_2-x Sr_x Ru₂O₇(0≤x≤0.10) compounds. All samples crystallize in the cubic unit cell with Fd3¯m (# 227) space group and form a continuous solid solution. For the ESRO samples, it detected two phases that are identified as

Figure 1 XDR Patterns evolution of (Er_2-x Sr_x)Ru₂O₇ system, with x = 0.0, 0.02, 0.05, 0.07, and 0.10. 

Er₂Ru₂O₇ and Er₂O₃ compounds. The solid line corresponds to a cubic phase with Fd3¯m (# 227) space group and it is identified as Er₂Ru₂O₇ compound with PDF (72-7620). The phase marked with an asterisk (*) corresponds to the Er₂O₃ compound, PDF (8-0050). The presence of this very small amount of Er₂O₃ compound was detected in the ESRO system then indicated that occur an overload of the reagent in all the system. This is presumably a consequence of loss of volatile ruthenium oxide¹³. The Rietveld analysis also indicates that there is no intermix between the Er³⁺ and the Ru⁴⁺ ions¹³. The individual bond lengths, x values and angle for the ESRO system are listed in Table I. These structural features are consistent with previous reports on rare-earth ruthenate pyrochlores²⁵. The initial parameters of the pyrochlore phase Er₂Ru₂O₇ and the second phase Er₂O₃ reagent were taken from the results reported in^25-26. The cubic lattice parameter a increases monotonously with strontium content x, in agreement with previous data (see below). The structural parameters with Fd3¯m space group were refined using a structural model, Er, 16c (0, 0, 0); Sr, 16c (0, 0, 0); Ru, 16d, (1/2, 1/2, 1/2); O(1), 48f, (x, 1/8, 1/8), x = 0.4286; O(2), 8a, (1/8, 1/8, 1/8). It is worth to mention that samples with x = 0.02 - 0.10 have not been previously reported in the literature. They show very weak reflections of a secondary phase, identified as Er₂O₃ compound PDF (8-0050). The other lattice parameters of (Er _2-x Sr _x )Ru₂O₇ system, x = 0.0, 0.02, 0.05, 0.07, and 0.10, also vary with the inclusion of the Sr-ion content and Ru-ion coordination. They are consistent with the small difference between the ionic radii of Er³⁺ at the Sr²⁺ (Er³⁺ = 0.89 Åand Sr²⁺ = 1.18 Å)²⁷. We conclude that the Er³⁺ ions are substituted, partially (randomly, and intertitially) by Sr²⁺ ions with the observed in the structural parameters of each of the compounds.

Figure 2 shows the SEM image of the sample with x = 0.10. The image shows the effect of heat treatments and processing route on the grain morphology of the sample. Considerable variations in sizes, very few phases and shapes of polycrystals can be observed from the micrograph. The grain size varies between 0.27 µm to 0.62 µm. The micrographs were taken on the surface of the respective pellets of the ESRO samples with a magnification of 15.00 K.X. Also, in some regions a semi-fusion can be observed and may be attributed to the ruthenium content. We performed an EDX analysis on all samples to verify the chemical composition. The results are present in Table II. The error range of the analysis is between 1 and 6wt% ²⁸; therefore it can said that the experimental and theoretical atomic percentages of the elements resemble each other.

Figure 2 SEM of the Er_1.90 Sr_0.10 Ru₂O_6.95 sample. 

The electrical resistance as a function of temperature is presented in Fig. 3. All samples show a Mott insulating behavior. At a fixed, arbitrary temperature, the normalized resistance decreases with x, but seems saturated near x = 0.10, suggesting the solubility limit of the Sr²⁺ ion a substitution²⁹. At a fixed temperature, the slope of the temperature-dependent resistance profiles systematically decreases with increasing x, indicating that the carrier concentration increases with increasing the Sr²⁺ ion content. These results show that the substituted Sr²⁺ ions act as a donor to supply electrons to the system. No metal transition was observed, however we attribute the spin-glass behavior to the substitution of Sr²⁺ at the Er³⁺ sites (Sr²⁺ = 1.18 Åand Er³⁺ = 0.89 Å)³⁰, changing the local electronic structure of the Ru - O(1), Er - O(2) and Er - O(1) bonds lengths. Additionally is compensated with the oxygen content in Ru ion

Figure 3 The normalized electrical resistance as a function of temperature of (Er_2-x Sr_x)Ru₂O₇ system, with x = 0.0, 0.02, 0.05, 0.07, and 0.10. 

Figure 4 demonstrates the correlation between the cubic lattice parameter a and the residual resistance radio (RRR), defined here by the ratio of the resistance at 290 K and the resistance at 150 K. The parameter a increases monotonously with strontium content x, while the residual resistance radio (RRR) decreases exponentially with Sr²⁺ ions incoorporations. These effects occur in the lattice parameters and are related to the bond lengths Ru-O(1), Er-O(2) and Er-O(1). The changes in the distances Ru-O(1), Er-O(2) and Er-O(1) modify the electrical properties of our system, like can saw in the graphic.

Figure 4 Resistance normalized variation and lattice parameter a of (Er_2-x Sr_x)Ru₂O₇ x = 0.0 - 0.10 system. 

Figure 5 demonstrate the temperature dependence of magnetization for (Er_2-x Sr_x)Ru₂O₇(0≤x≤0.10) system measured at 400 Oe. Then the temperature dependence of magnetic moments for the Er₂ Ru₂O₇, and Er_1.98Sr_0.02 Ru₂O_6.99 compounds wereexhibited in the Fig. 5 (a)-(b).

Figure 5 Temperature dependence of moment magnetic (emu/g) for (Er_2-x Sr_x)Ru₂O₇ with x = 0.0, 0.02, 0.05, 0.07, and 0.10. Theapplied field for all the samples was 400 Oe. 

In these compounds, both the magnetic moments of the Er³⁺ and Ru⁴⁺ ions are in a magnetically ordered state. The present Er₂Ru₂O₇ compound don’t affect the measurements. No significant dfference between the ZFC and FC magnetic moments was observed in the whole temperature range. The origin for these magnetic moments between 2 and 360 K is that the paramagnetic moment for the Er³⁺ ions are induced by the Ru⁴⁺ ions when has a magnetic ordering. Moreover, the interaction between the Ru⁴⁺ and Er³⁺ ions may explain the long-range magnetic ordering of Er³⁺ ions at relatively higher temperature¹³.

Figure 5 (c) confirms the temperature dependence of the magnetic moment for Er_1.95Sr_0.05Ru₂O_6.975 compound. A magnetic transition at Tc≈165 K (Curie Temperature) is observed, below this transition temperature, there is slight divergence between the ZFC and FC magnetic moments. This inflection point is associated with a paramagnetic ordering below 165 K and above 165 K changes to a ferromagnetic ordering. This argument is justified by the substitution of Sr²⁺ ions for Er³⁺ ions, which modifies the network parameters and consequently modifies their magnetic properties by applying intense magnetic fields to lower temperatures.

Figure 5 (e)-(d) illustrate the temperature dependence of magnetic moments for the Er_1.93Sr_0.07Ru₂O₆.965 and Er_1.90Sr_0.10Ru₂O_6.95 compounds measured at 400 Oe. Again a magnetic transition is observed at 165 K, like in Fig. 5 (c) below this transition temperature, a remarkable divergence between the ZFC and FC magnetic moments is found. This inflection point is associated with a paramagnetic ordering below 165 K and above 165 K changes to a ferromagnetic ordering. However at a temperature of 157 K a new inflection point is presented which is related to an antiferromagnetic arrangement and is associated with TN (Néel Temperature). This magnetic behavior below 157 K indicates the onset of magnetic ordering between Ru⁴⁺ ions at short distances increasing for incorporation of Sr²⁺ ions. These arguments are again justified by the substitution partially (randomly, and intertitially) and increase of Sr²⁺ ions by Er³⁺ ions, which modifies the lattice, grain and shape size parameters (observed by SEM) and consequently modifies their magnetic moments to lower temperatures.

In Fig. 5 (f) we observed the behavior of the Er_1.90Sr_0.10Ru₂O_6.95 compound in the temperature range of 142 to 180 K between the ZFC and FC magnetic moments. This graph presents at FC an inflection point associated to paramagnetic ordering below 165 K and above 165 K changes to a ferromagnetic ordering, while for the same Er_1.90Sr_0.10Ru₂O_6.95 compound (Fig. 5 (e)) but with ZFC presents a different behavior, this displays an antiferromagnetic ordering. It is well know that the magnetic transition in 157 K corresponds to the antiferromagnetic ordering of ruthenium moments associated to TN (Néel Temperature). The high transition temperatures and the very large differences between the ZFC and FC magnetic moment indicate the existence of a very strong interaction between Ru⁴⁺ ions for incorporation of Sr²⁺ ions and the accommodation of the packing structural lattices.

Figure 5 (f) we can observed the behavior of the Er_1.90Sr_0.10Ru₂O_6.95 composition in the temperature range of 142 to 180 K and the very large divergence between the ZFC and FC magnetic moment indicate the existence of a strong interaction between ruthenium ions at short distances increasing for incorporation of Sr²⁺ ions. This graph presents at FC an inflection point associated to paramagnetic ordering below 165 K and above 165 K changes to a ferromagnetic ordering, while for the same Er_1.90Sr_0.10Ru₂O_6.95 composition but with ZFC presents a different behavior, this displays an antiferromagnetic ordering. It is well know that the magnetic transition in 157 K corresponds to the antiferromagnetic ordering of ruthenium moments associated to TN (Néel Temperature). A mixture of phenomena, the magnetic behavior with the packing of the lattice in the final structure measured.

4. Conclusion

In this work, we obtained polycrystalline samples of ESRO system by solid-state reaction method in air at atmospheric pressure, in which a solubility up to x = 0.10 was observed. SEM micrographs exhibit an almost-spherical grain size distribution from 0.27 and 0.62 µm. The changes in the distances Ru-O(1), Er-O(2), and Er-O(1) modify the electrical properties of our system. The magnetic behavior of the ESRO system of each of the compounds was measured between ZFC and FC detecting the magnetic moments. The x value for x = 0.0 and x = 0.02 present a paramagnetic order and don’t show any magnetic transition. However, when x = 0.05 has a very weak magnetic transition at 165 K, that temperature is identified as a TC (Curie Temperature). Finally, for the compounds with x = 0.07 and x = 0.10 both present a remarkable divergence between ZFC and FC.

In these two compounds is possible to observe the different arrangements that occurs throughout the temperature range. The first ordering corresponds to paramagnetic and is present below 165 K, whereas for the second order is present above 165 K and corresponds to a ferromagnetic order and third order corresponds to an antiferromagnetic order and is at a temperature of 157 K associated to TN (Néel Temperature).