PACS: 82.33.Pt; 61.05.C-; 75.50.Lk
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 superconductivity1,2, semi-conductivity3, ionic conductivity4,5, ferromagnetism6 and luminescence 7,8. Oxide ion-conducting pyrochlores find applications in
solid oxide fuel cells as electrolytes9,10 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 research14-16, 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
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
3. Results and discussions
Figure 1 shows the XRD patterns of the sintered
Er2-x Srx
Ru2O7
Er2Ru2O7 and Er2O3 compounds. The solid line corresponds to a cubic phase with
x | a (Å) | Ru-O(1) | Er-O(2) | Er-O(1) | Ru-O(1)-Ru |
0 | 10.124(8) | 1.930(3) | 2.192(1) | 2.544(3) | 136.01(2) |
0.02 | 10.131(4) | 1.931(5) | 2.193(5) | 2.545(9) | 136.01(2) |
0.05 | 10.141(8) | 1.933(5) | 2.195(7) | 2.548(5) | 136.01(2) |
0.07 | 10.141(9) | 1.933(5) | 2.195(7) | 2.548(5) | 136.01(2) |
0.10 | 10.143(1) | 1.933(8) | 2.196(0) | 2.548(8) | 136.01(2) |
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.
ELEMENTS | ||||
Samples | Er | Sr | Ru | O |
x = 0:0 | 25.01 (± 0.21) | 0.0 | 18.97 (±0.32) | 56.01 (± 0.41) |
x = 0:02 | 24.73 (± 0.64) | 1.04 (±0.18) | 18.22 (± 0.53) | 56.02 (± 0.67) |
x = 0:05 | 22.56 (± 0.48) | 2.71 (± 0.42) | 17.45 (± 0.37) | 57.28 (± 0.45) |
x = 0:07 | 24.22 (± 0.62) | 2.41 (± 0.43) | 15.03 (± 0.94) | 58.34 (± 0.41) |
x = 0:10 | 21.43 (± 0.39) | 3.89 (±0.27) | 17.60 (± 0.30) | 57.07 (± 0.71) |
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
Figure 4 demonstrates the correlation between the cubic lattice parameter
Figure 5 demonstrate the temperature dependence
of magnetization for (Er2-x
Srx)Ru2O7
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 Er1.95Sr0.05Ru2O6.975 compound. A magnetic transition at Tc
Figure 5 (e)-(d) illustrate the temperature dependence of magnetic moments for the Er1.93Sr0.07Ru2O6
In Fig. 5 (f) we observed the behavior of the Er1.90Sr0.10Ru2O6.95 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 Er1.90Sr0.10Ru2O6.95 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 Er1.90Sr0.10Ru2O6.95 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 Er1.90Sr0.10Ru2O6.95 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).