Chemical Stability and Crystallographic Analysis of the the Sr₂HoNbO₆ Cubic Perovskite as Potential Substrate for YBa₂Cu₃O_7-δ Superconducting Films

Abstract

The Sr₂HoNbO₆ complex perovskite has been synthesized by solid-state reaction. X-ray diffraction pattern reveals that Sr₂HoNbO₆ material crystallizes in a cubic structure, Fm3̅m space group, lattice constant a=8.018 Å, which has a lattice mismatch ~3% with YBa₂Cu₃O_7-δ. The chemical stability of Sr₂HoNbO₆ with YBa₂Cu₃O_7-δ superconducting has been studied by x-ray diffraction and measurements of magnetization as a function of temperature on Sr₂HoNbO₆-YBa₂Cu₃O_7-δ composites. X-ray diffraction patterns of the composite shown that all the X-ray diffraction peaks could be indexed for either Sr₂HoNbO₆ or YBa₂Cu₃O_7-δ with no extra peak detectable. This result implies that these compounds remain as two different separate phases in the composite with no chemical interaction. Magnetic response shows that superconducting transition temperature of pure YBa₂Cu₃O_7-δ and Sr₂HoNbO₆-YBa₂Cu₃O_7-δ composites is 93,5 K. These favorable characteristics of Sr₂HoNbO₆ show that it can be used as a potential substrate material for growth of YBa₂Cu₃O_7-δ superconducting films.

Keywords
Complex perovskite; superconducting properties; substrate material; structure

1. Introduction

It is known that the complex cubic perovskites family offers a great possibility to vary the structural distribution by the modification of the chemical components, in order to produce new materials^1,2. These ceramics evidence the improvement of the coupling properties between these specimens used as substrates for high-temperature superconducting films. In last years, complex perovskite oxides have been investigated for their use as potential new substrates for the production of cuprate superconducting films^3-4. For good quality substrates, the material candidate must satisfy the follow requirements:

1. Chemical stability, e.g. absence of chemical reactions at the interface substrate-film;

2. Good lattice parameter coupling at the crystallographic plane substrate-film to guarantee the epitaxial growth;

3. Presence of the substrate material must not affect the superconducting properties of the lm and low dielectric constant for eventual application in microwaves devices⁵.

The high chemical reactivity of YBa₂Cu₃O_7-δ (YBCO) with most of conventional substrate materials at the processing temperature imposes severe restrictions on the availability of substrates for the YBCO superconducting films⁴. Moreover, the divergence of the crystallographic properties at the interface between YBCO and the commonly available substrates, as well as the difficulty in obtaining a low dielectric constant in good substrate materials, that is chemically and structurally compatible with the YBCO superconductor, constitutes a great motivation to produce new optimal materials for this application. For example, MgO, the widely utilized substrate for YBCO thin films, produces an interlayer of barium salt at the YBCO-MgO interface, when the processing temperature is above 700 ºC⁴. On the other hand, MgO which is chemically compatible with YBCO, has high dielectric constant and loss factor values. These characteristics restrict its use as a substrate for YBCO films for application at microwave frequencies. Another used substrate for the growth of YBCO films is LaAlO₃, which evidences good crystallographic coupling but is available only as twinned single-crystals¹.

There are reports which show that the non-conductor complex cubic perovskites evidence a low dielectric constant, making it suitable for microwave applications⁵. In this work, we report the synthesis and characterization of Sr₂HoNbO₆ (SHNO) as substrate for the production of YBCO high-temperature superconducting films. Pure SHNO and SHNO/YBCO (0 to 100 vol.%) were produced and characterized to study the viability to utilize SHNO as a substrate to produce YBCO superconducting thin films. It is found by the structural characterization that the SHNO has good crystallographic coupling and is chemically non-reactive with YBCO superconducting films even under extreme processing conditions. Structural and morphological characterizations were performed to evaluate the surface quality of the materials. DC magnetic susceptibility measurements were performed to analyze the incidence of SHNO on the superconducting critical temperature of YBCO in composites.

2. Experimental

SHNO polycrystalline samples have been prepared by a solid-state reaction process. Stoichiometric ratios of the precursor powders SrCO₃ (purity 99,90%), Ho₂O₃ (99,90%) and Nb₂O₅ (99,5%) were finely ground and thoroughly mixed. The precursor powder was pressed into a disc and the material was calcined at 1100 ºC for 148 h in ambient atmosphere. The calcined mixture was again crushed, finely ground and pressed at 6 ton/cm² pressures to form a disc (10 mm diameter, 2 mm thickness). This disc was sintered at 1100 ºC for 17 h in vacuum atmosphere and furnace-cooled to room temperature. The XRD pattern of the sintered material showed a single-phase structure. For chemical stability studies, a single-phase YBCO superconductor was prepared by solid-state synthesis. 0 to 100 mass% of SHNO was mixed in YBCO superconductor powder and the mixture was pressed into circular discs (10 mm diameter, 1 mm thickness) at a pressure of 5 ton/cm² and heat treated at 960 ºC in oxygen atmosphere for 8 h. After the heat treatment, YBCO-SHNO samples were slowly cooled down to room temperature for proper oxygenation. Structural characterization was carried out by using a Panalytical X-Pert PRO MPD diffractometer (CuKα =1,540598 Å). Refinements of the experimental data were performed through the GSAS code⁶. XRD patterns of these samples were recorder for crystallographic phase characterization, the chemical stability and the crystallographic parameter coupling of the YBCO-SHNO composites. The influence of SHNO on the superconducting properties of YBCO was studied in the composite samples through DC magnetic susceptibility measurements by using a Quantum Design model MPMS SQUID system. Morphological characterization of samples was systematically effectuated from scanning electron microscopy (SEM), by using Quanta 200 SEI Electron-Optics equipment. Compositional analysis of films was performed by energy dispersive X-ray (EDX) experiments by means of a microprobe Bruker coupled to the SEM microscopy.

3. Results and discussion

Figure 1 shows the XRD pattern for the SHNO material, which consists of strong peaks. These are characteristics of a primitive cubic perovskite plus a few weak line reflections arising from the super-lattice. It is obtain that this material crystallizes in a cubic double perovskite, Fmm space group (#225).

Figure 1
Characteristic XRD pattern for the complex perovskite Sr₂HoNbO₆. Symbols represent experimental diffraction data and base line is the difference between experimental and simulated patterns (continuous line). The arrows indicate three peaks corresponding to non-double perovskite phase and belonging to a 0,8% of holmium trioxide.

No evidence for a distortion from the cubic symmetry is observed in the XRD pattern. The basic perovskite composition is ABO₃, where A is a large ion suitable to the 12-coordinated cube-octahedral sites and B is a smaller ion suitable to the 6-coordinated octahedral site⁷. Cubic complex perovskite with mixed species on a site (particularly the B site) may be represented by multiples of this formula unit and a larger unit cell, e.g., A₂BB'O₆⁸. Thus, in the SHNO composition, Sr⁺² with the largest ionic radius (1,13 Å) occupies position A of the cubic complex perovskite, Ho⁺³ (ionic radius 0,97 Å) and Nb⁺⁵ (ionic radius 0,70 Å) cations occupy the B and B' positions. The ionic radii were calculated by using the SPuDs software⁹. Due to the ordering of B and B' on the octahedral site of the ABO₃ unit cell, there is a doubling in the lattice parameter on the basic cubic-perovskite unit-cell. Thus, the whole XRD pattern of SHNO can be indexed in the A₂BB'O₆ cubic structure with the cell edge a=2a_p, where a_p represents the cell lattice of the cubic perovskite. The presence of the superstructure reflection lines (311), (531), (533) and (551) in the XRD pattern of SHNO is the signature of an ordered cubic complex perovskite structure. In a substitutional solid solution A₂BB'O₆, there is a random arrangement of B and B' on equivalent lattice positions in the crystal structure. It upon stable heat treatment, the random solid solution rearranges into a structure in which B and B' occupy the same set of positions but in a regular way, such a structure is described as superstructure¹⁰.

In the superstructure, the position occupied by B and B' are no longer equivalent and this feature is exhibited in the XRD pattern of the material by the presence of superstructure reflection lines⁸. For a double cubic perovskite of the formula A₂BB'O₆ the intensity, in particular the (111) superstructure reflection, is proportional to the difference in the scattering power of the B and B' atoms, when all the atoms are situated in the ideal position^9,11. A disordered arrangement of B and B' should result in zero intensity. Therefore Ho⁺³ and Nb⁺⁵ cation ordering in SHNO in B and B' positions is clearly distinguished by the presence of the significant intensity of (311), (531), (533) and (551) superstructural reflection lines. The lattice parameters of SHNO, calculated from the XRD data are a=8,018(0) Å while for the YBCO sample we obtained a=3,866(1) Å, b=3,874(2) Å and c=11,651(9) Å. The structural parameters experimentally obtained are shown in table 1. On the other hand, it is observed in figure 1 the presence of three peaks, which do not belong to the SHNO. From the Rietveld refinement it was concluded that the rare-earth precursor oxide Ho₂O₃ is present in the sample in a percentage of 0,8%. Arrows in figure 1 signal the respective peaks.

Table 1
Atomic positions of cations and anions on the unit cell of Sr₂HoNbO₆, obtained from the Rietveld refinement.

Atom	Wyckoff Site	x	y	z
Sr2+	8c	0.2500	0.2500	0.2500
Ho3+	4a	0.0000	0.0000	0.0000
Nb5+	4b	0.5000	0.0000	0.0000
O2-	24e	0.2678	0.0000	0.0000

Figure 2 shows the structure of the SHNO double perovskite constructed from the data of Rietveld refinement.

Figure 2
Crystalline structure of the Sr₂HoNbO₆ cubic complex perovskite.

As expected from the discussion above, the cationic ordering of the Ho³⁺ and Nb⁵⁺ in the B and B' sites of the cubic double perovskite is clearly observed. On the other hand, no tilting or rotation of the Ho-O₆ and Nb-O₆ octahedra is observed in the perfect cubic structure.

The magnetic response of SHNO has been investigated by measuring the DC magnetic susceptibility in the temperature range 50 to 300 K and at an applied magnetic field of 500 Oe. Figure 3 shows the temperature dependence of the DC magnetic susceptibility as a function of temperature for SHNO.

Figure 3
DC magnetic susceptibility as a function of temperature for SHNO (data points) and Curie fitting of the paramagnetic characteristic (line).

The magnetic susceptibility data of figure 3 can be fitted well with the Curie-Weiss law χ=χ₀+C/(T-θ_c), where C=Nµ²_eff/3k_B is the Curie constant, N is Avogadros's number, µ_eff is the effective magnetic moment (µ_eff=P_effµ_B), P_eff represent the effective Bohr magneton number, µ_B is the Bohr magneton, K_B is the Boltzmann constant, θ_c is the paramagnetic Curie temperature and χ₀ is the temperature independent susceptibility term. The value of the temperature independent susceptibility term is χ_o = 0.00945 emu/mol.Oe. The Curie constant, estimated from the fitting in figure 3 is C=12.55892 emu.K/mol.Oe. The effective magnetic moment due the Ho³⁺ ion, calculated from the Curie constant is µ_eff=10,01 µ_B. This value corresponds 94,4% with the expected P_eff=g[J(J+1)]^1/2=10,60 for an isolated Ho⁺³ cation with configuration 4f¹¹5d⁰6s², calculated by the Hund's rules, where g represents the Landé factor and J is the quantic number. This difference may be attributed to the crystal field effects of the trivalent Ho⁺³ cation, which explain the magnetic susceptibility in SHNO.

In order to verify the possibility of using SHNO as a substrate material for thin films of YBCO superconductor, we have studied the chemical reactivity of SHNO with YBCO. Approximate amounts of SHNO (0 to 100 mass%) were mixed with YBCO, as described in section 2 of this paper. XRD patterns of these composites are shown in figure 4.

Figure 4
XRD pattern for 0 to 100 mass% of SHNO in an YBCO matrix.

As observed in figure 4, all the peaks in the XRD could be indexed either for YBCO or for SHNO and no extra XRD peak. Within the accuracy of XRD technique, these results show that YBCO and SHNO remain as two distinct separate phases in the YBCO-SHNO composites and SHNO is chemically stable with YBCO superconductor for all mass% addition of SHNO. Concern to the crystallographic coupling, we notice that there is a matching ~3 % between the SHNO lattice parameter a/2=4.009 Å and the ab-plane crystallographic constants of YBCO a≈b≈3,87 Å. This matching value between the lattice constant of possible substrate materials and YBCO ab-plane crystallographic parameter is not very far from the matching reported for the SrTiO₃ single crystal [12], which is the substrate most widely used for the growth of the YBCO superconductor (2.14%).

The superconductivity in YBCO-SHNO composite samples was studied by measuring the dc magnetic susceptibility with an applied field of 5 kOe and in the temperature range 50 to 300 K. Figure 5 shows curves of the dc magnetic susceptibility as a function of temperature for the YBCO-SHNO composites with 90, 70 and 50 mass% of SHNO addition in YBCO superconductor.

Figure 5
DC magnetic susceptibility for 100, 90, 70, 60, 50, 30, 10 and 0 mass% of SHNO in SHNO-YBCO polycrystalline composites.

As seen from figure 5, all the YBCO-SHNO composites have the same superconducting transition temperature T_c=93,5 K as expected for the pure YBCO superconductor. This shows that even up 90 mass% of SHNO, an insulating ceramic oxide, addition in YBCO did not have any deteriorating effect on the transition critical temperature of YBCO superconductor. Thus as discussed earlier SHNO is chemically stable with YBCO superconductor and at same time it did not have any deteriorating effect on the superconducting property characterized by the transition temperature T_c.

Surface morphology of sintered YBCO-SHNO composites was investigated by scanning electron microscopy (SEM). The results are shown in the micrograph of figure 6. These indicate that the surface of the samples present a crystalline character, which is typical of a polycrystalline ceramic material. The SEM micrograph of figure 6(a), for 30 mass% YBCO and 70 mass% SHNO, shows homogeneous surface morphology and particle size distribution with grain average size estimated to be 1-2 µm for both, YBCO (darkness regions) and SHNO (blank regions) compounds. The mean grain size was determined by using the intercept procedure¹³. Figures 6(b) and 6(c) reveal the increase of black regions when proportion of YBCO is added up to 90 mass%.

Figure 6
Scanning electron microscopy for (a) 30 mass% YBCO, 70 mass% SHNO, (b) and (c) 90 mass% YBCO, 10 mass% SHNO in SHNO-YBCO composites.

Notice that there is no detectable interface interaction between SHNO and YBCO grain interfaces and SHNO particles are distinguishably distributed in the YBCO matrix. Energy dispersive X-ray (EDX) spectrum showed in figure 7(a) reveals the YBCO single phase for the darkness grains of figure 6 and figure 7(b) evidences the pure SHNO for the blank regions of figure 6¹. The semi-quantitative results presented in table 2 show a good accordance between the experimental and the expected percentages obtained from the calculated stoichiometric compositions of YBCO and SHNO. Based on the foregoing, we can say that the SHNO and YBCO compounds remain unreacted within the composite material.

Figure 7
Energy dispersive X-ray spectra for (a) blank regions of the SEM images, which correspond to the SHNO double perovskite, and (b) darkness regions of the SEM images, which correspond to the YBCO compound.

Table 2
EDX experimental and expected weight percentages for each atom in (left) SHNO double perovskite, and in (right) the YBCO material, by considering 7 oxygen atoms in the unit cell.

Atom	Exp (w%)	Theor. (w%)	Atom	Exp. (w%)	Theor. (w%)
Sr	33.89	33.12	Y	14.20	13.34
Ho	33.15	31.17	Ba	42.15	41.62
Nb	19.08	17.56	Cu	28.98	28.62
O	13.88	18.14	O	14.67	16.82

4. Conclusions

It is found by the structural characterization that SHNO has an excellent crystallographic coupling and is chemically non-reacting with YBCO superconducting films even under extreme processing conditions. It is observe that there is no chemical reaction between these compounds into the SHNO-YBCO composites. Morphological characterizations were carrying out to evaluate the chemical reaction between the insulating SHNO and the metallic YBCO materials. In the SHNO-YBCO composites experimental analysis the formation of separate single-phase grains of SHNO and YBCO was observed by SEM images and XRD characterization¹. Energy dispersive X-ray (EDX) analysis show that there is no evidence of impurity traces in the samples.

DC magnetic susceptibility measurements reveal the paramagnetic characteristic of SHNO samples, which follow a Curie behavior and permit to report the corresponding magnetic constants. These results show that the presence of SHNO does not affect the superconducting critical temperature of YBCO. This systematical work permitted to corroborate our hypothesis that the SHNO cubic complex perovskite can be utilized as substrate material for the fabrication of YBCO thin films as well as other double perovskites which have been recently reported¹⁴.

5. Acknowledgments

This work was partially financed by DIB (National University of Colombia - Bogotá).

6. References

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Publication Dates

• Publication in this collection
  07 July 2016
• Date of issue
  Jul-Aug 2016

History

• Received
  20 Aug 2015
• Reviewed
  06 Apr 2016
• Accepted
  03 June 2016