Distinct effects of Al3+ substitution at Cu-site and Al2O3 addition on step-like elastic anomalies and electron–phonon coupling constant in EuBa2Cu3O7−δ superconductors

Journal of Alloys and Compounds 490 (2010) 358–365

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Distinct effects of Al3+ substitution at Cu-site and Al2 O3 addition on step-like elastic anomalies and electron–phonon coupling constant in EuBa2 Cu3 O7−ı superconductors M.F. Bakar, A.K. Yahya ∗ Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 5 August 2009 Received in revised form 29 September 2009 Accepted 30 September 2009 Available online 9 October 2009 Keywords: Al3+ substitution Al2 O3 addition Electron–phonon coupling constant Elastic property EuBa2 Cu3 O7−ı Oxygen ordering Ultrasonic velocity

a b s t r a c t Ultrasonic longitudinal and shear velocities at 9 MHz were measured in temperature ranges of 80–280 K and 80–220 K, respectively, in EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) and EuBa2 Cu3 O7−ı + y wt.% Al2 O3 (y = 0.2 and 0.4) superconductors. A step-like elastic anomaly indicating sudden lattice stiffening was observed for x = 0 at around 260 K. Partial substitution of Al3+ in place of Cu at x = 0.06 and 0.10 suppressed the step-like velocity anomaly and displayed monotonous velocity change with temperature. In contrast however, addition of Al2 O3 only shifted the anomaly to slightly lower temperatures. Interestingly, maximum Tc was observed at x = 0.06 and this coincides with enhanced value of the computed BCS electron–phonon coupling constant, . The step-like elastic anomaly was discussed in terms of oxygen ordering involving Cu–O chains. Substitution of Al3+ is suggested to go into Cu–O chain sites and effectively destroys oxygen ordering which in turn caused suppression of the elastic anomaly. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Superconductors are special materials whose electrical resistance abruptly goes to zero below a certain temperature called the critical temperature, Tc . Although numerous studies on high-temperature superconductors using different techniques to understand their superconducting and normal state behaviors have been reported since their discovery, the mechanism of hightemperature superconductivity is still not definite. While most attention has been focused in the region below the onset of superconductivity, Tc onset several intriguing phenomena such as existence of pseudogap [1–4], excess conductivity [5], superconducting fluctuation behavior [6,7] and elastic anomalies [8–10] were observed at higher temperatures. Although these phenomena do not fall in the superconducting region, they were reported in superconducting samples and therefore must be properly studied to verify if they act as vital precursors to high-temperature superconductivity taking place at much lower temperatures. Investigation of bulk properties and dynamic processes in hightemperature superconductors are often carried out using ultrasonic

∗ Corresponding author. E-mail address: [email protected] (A.K. Yahya). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.09.186

waves which produced non-destructive mechanical stress disturbance on atoms in solids. Calculation based on measured ultrasonic velocity values yields information on elastic moduli, Debye temperatures and electron–phonon coupling constants which adds to the understanding of high-temperature superconductivity [11]. Previous reports on ultrasonic velocity measurements on several REBa2 Cu3 O7−ı (RE = Y, Ho, Gd and Er) superconductors found step-like elastic anomalies at temperatures around and above 200 K indicating sudden hardening tendency during cooling [8,12–16]. These step-like anomalies have been observed in superconducting samples and are found to be oxygen related [8,14–16]. The steplike anomalies were often reported to be accompanied by large attenuation peaks which are not related to any relaxation process [17]. Our previous study on Gd1113 which is isostructural to RE123 suggests that oxygen content may not be the principal factor influencing the step-like anomalies [18]. Instead, the anomaly may be due to the phenomenon of oxygen ordering taking place in Cu–O chains [14,16,18]. However, the true nature of the anomaly is still debatable and more studies need to be carried out to gather new information on its nature. On the other hand, elemental substitution studies on RE123 have been extensively reported. Elemental substitutions in place of Cu i.e. REBa2 (Cu1−x Mx )3 O7−ı have been reported to show slightly different effects; substitution of M = Al3+ and Ga3+ was suggested to go into Cu–O chains while substitution of M = Zn2+ and Ni2+ was sug-

M.F. Bakar, A.K. Yahya / Journal of Alloys and Compounds 490 (2010) 358–365

gested to occupy CuO2 planes of RE123 compounds [19–25]. The former does not affect Tc drastically for low dopant concentrations compared to the latter. Since oxygen ordering in RE123 compounds has been attributed to oxygen atoms in Cu–O chains, ultrasonic velocity studies on Cu-substituted RE123 could highlight the role of Cu–O chains in oxygen ordering and its influence on the step-like anomaly. Ultrasonic investigations on RE123 materials with substituted RE3+ and Ba2+ sites have been reported [14,16] but to our knowledge similar ultrasonic study on effects of direct substitution into Cu–O chains such as expected for M = Al+3 /Gd3+ substitution in REBa2 (Cu1−x Mx )3 O7−ı has not been previously reported. Besides elemental substitution, addition of secondary-phase compounds in high temperature superconductors has generated interest amongst researches. Previous reports of addition of nanosize SnO2 , ZrO2 and Al2 O3 in bulk superconductors showed such additions enhanced critical current densities and increase flux pinning abilities of the samples [26–32]. Knowledge on effects of the secondary phase additions on elastic properties of superconductors is also equally important especially for wider practical application of the materials. However, to our knowledge elastic studies on RE123 superconductors with any of the above additives are limited. In this paper, ultrasonic longitudinal (between 80 K and 280 K) and shear velocities (between 80 K and 220 K) at 9 MHz were measured in polycrystalline EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) superconductors and EuBa2 Cu3 O7−ı + y wt.% Al2 O3 (y = 0, 0.2 and 0.4) composites. The lattice anharmonicity model as discussed by Lakkad [32] was used as a comparison to evaluate the extent of deviation of the elastic behavior at high temperatures. Eu123 was chosen as step-like velocity anomaly has not been previously observed for the compound and also because Eu3+ has an ionic radius larger that most of the other RE3+ of RE123 materials that was reported to display step-like elastic anomalies. Besides looking into possible differences induced by the larger Eu3+ ionic size, the other aim of the study is to compare the effects of Al3+ substitution at Cu site and Al2 O3 addition on EuBa2 Cu3 O7−ı on ultrasonic velocity and the step-like elastic anomaly. Analyses of powder X-ray diffraction (XRD) diffractograms and scanning electron microscope (SEM) micrographs will also be presented and discussed.

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the carrier frequency of 9 MHz. The velocity measurement was performed in a Janis Cryostat model VNF-100T and the temperature was changed at rate of about 1 K/min during heating. For polycrystalline ceramic material, the elastic moduli for longitudinal and shear modulus can be approximated by an isotropic-elastic medium [33]. In this approximation, the longitudinal and shear modulus are given by CL = v2l and  = v2s , respectively, where  is the mass density, vl is the longitudinal velocity, vs is the shear velocity, CL is the longitudinal modulus and  is the shear modulus. The Debye temperature ( D ) can be estimated using the standard formula D =

 h   3N 1/3 4V

kB

vm

(1)

where 3

v3m

=

1

v3l

+

2

v3s

(2)

h is the Planck constant, kB is the Boltzmann constant, N is the number of mass point, V is the atomic volume, vm is the mean velocity, vl is the longitudinal velocity and vs is the shear velocity. The electron–phonon coupling constant , was calculated using the BCS formula in the weak coupling limit [34] where  is related to Tc by the following formula Tc = 1.13D e−(1/)

(3)

2. Experimental details The EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) samples were prepared by mixing appropriate amounts of Eu2 O3 , BaCO3 , CuO and Al2 O3 powders with purity ≥99.99% using conventional solid state synthesis method. The mixed powders were ground in an agate mortar and calcined in air at around 900 ◦ C for 48 h with several intermittent grindings before oven cooled. The powders were then pressed into pellets of ≈13 mm diameter and 3 mm thick under a pressure of around 6–7 tons. The pellets were then sintered at around 930 ◦ C for 24 h and slow cooled to room temperature at 40 ◦ C/h. The non-superconducting EuBa2 Cu3 O7−ı sample was prepared by reheating the x = 0 sample at around 900 ◦ C about 2 h and quenching it immediately in liquid nitrogen. To prepare EuBa2 Cu3 O7−ı + y wt.% Al2 O3 (y = 0.2, 0.4) composites, the EuBa2 Cu3 O7−ı samples were reground into powder form and added with appropriate amounts of Al2 O3 . The mixture was reground and pressed into pellets before sintering at 930 ◦ C for 24 h followed by slow cooling. Electrical measurements were carried out using the standard four-point-probe technique with silver paste contacts. The samples were examined by X-ray powder diffraction with CuK␣ radiation using Rigaku model D/MAX 2000 PC system. Ultrasonic velocity was measured using a Matec 7700 system which utilizes the pulse-echo-overlap technique. Nonaq stopcock grease was used to bond the polished sample surface with a quartz transducer. Sound velocity was propagated along the direction of pressing using X-cut (longitudinal) or Y-cut (shear) transducer with

Fig. 1. X-ray powder diffraction pattern for EuBa2 (Cu1−x Alx )3 O7−ı with x = 0, x = 0 (quenched), x = 0.06 and x = 0.10. Impurity peaks are indicated by an asterisk (*).

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Table 1 Zero-resistance transition temperature (Tc zero ), onset transition temperature (Tc onset ), room temperature resistivity, 123 lattice parameters, unit cell volume and orthorhombicity for EuBa2 (Cu1−x Alx )3 O7−ı with (a) x = 0, (b) x = 0 (quenched), (c) x = 0.06 and (d) x = 0.10 and EuBa2 Cu3 O7−ı + y wt. % Al2 O3 with (e) y = 0.2 and (f) y = 0.4. Sample

Tc zero (K) ± 0.1

Tc onset (K) ± 0.1

Resistivity (m cm) ± 0.1

a (Å) (a) x = 0 (b) x = 0 (quenched) (c) x = 0.06 (d) x = 0.10 (e) y = 0.2 (f) y = 0.4

87.5 – 89.0 38.3 83.0 65.4

94.9 – 93.0 79.7 92.7 75.5

14.0 48.6 × 102 3.5 18.5 16.3 3.7

3.860 3.899 3.863 3.867 3.857 3.856

b (Å) ± ± ± ± ± ±

0.002 0.003 0.003 0.003 0.002 0.001

As a comparison to the measured temperature dependent sound velocity curves, the anharmonicity model proposed by Lakkad [32] which uses a phenomenological model of anharmonic oscillator together with the Debye lattice vibration spectrum was plotted together with the velocity curves to illustrate the extent of deviation of the velocity behavior from the calculated curve at high temperatures. The equation proposed by Lakkad [32] to predict the temperature variation of the elastic constant E(T) of a material is given in Eq. (4):



E(T ) = E0 1 − kF

 T 

(4)

D

where E0 is the elastic constant at 0 K, k is a constant and  D is the Debye temperature. F(T/ D ) in Eq. (4) is given in Eq. (5): F

T  D

=3

 T 4  D

0

D /T

x 3 ex dx (ex − 1)

Volume, V (Å3 )

Lattice parameters

(5)

Here, x = hf/kB T where h is the Planck constant, f is the frequency of vibration and kB is the Boltzmann constant. The temperature dependence of the sound velocity curves was fitted to this model

3.908 3.902 3.902 3.900 3.904 3.905

Orthorhombicity (b − a)/(a + b)

c (Å) ± ± ± ± ± ±

0.006 0.011 0.008 0.008 0.005 0.001

11.728 11.767 11.736 11.735 11.746 11.747

± ± ± ± ± ±

0.004 0.013 0.005 0.007 0.003 0.003

176.9 179.0 176.9 177.0 176.9 176.9

± ± ± ± ± ±

0.4 0.8 0.6 0.6 0.4 0.1

0.0062 0.0004 0.0050 0.0043 0.0066 0.0063

± ± ± ± ± ±

0.0010 0.0011 0.0014 0.0014 0.0009 0.0003

using elastic constants at two different temperatures along the curve together with the computed Debye temperature ( D ). 3. Results and discussion 3.1. Structural and electrical characterization of EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) Powder X-ray diffraction analyses revealed all samples consist of orthorhombic 123 structures with space group Pmmm. All samples are single phased 123 with the exception of x = 0.10 which showed small amount of impurity phase. Fig. 1 shows the XRD patterns for all samples and Table 1 shows their calculated lattice parameters. The calculated orthorhombicity of the quenched x = 0 sample is very small and as such it can be considered as pseudo-tetragonal in structure (Table 1). Oxygen contents of the samples were estimated based on comparison with previous report on c-lattice parameter variation with oxygen content of Eu123 by Donnelley et al. [35]. Oxygen content of the as-prepared EuBa2 Cu3 O7−ı (x = 0) was estimated as O6.9 while for the quenched x = 0 sample oxygen content

Fig. 2. SEM of internal section of EuBa2 (Cu1−x Alx )3 O7−ı samples: (a) x = 0, (b) x = 0 (quenched), (c) x = 0.06 and (d) x = 0.10.

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361

was estimated as around O6.3 . For both the x = 0.06 and x = 0.10 samples, based on previous suggestion by Brecht et al. [36] a large difference in oxygen content compared to the as-prepared x = 0 sample is not expected as the substitution levels are low. Consequently, based on their calculated c-lattice values, oxygen contents of the x = 0.06 and x = 0.10 samples are estimated to be around O6.8 . SEM micrographs of the internal section of EuBa2 Cu3 O7−ı (x = 0) (Fig. 2(a)) showed irregular shaped grains in addition to the existence of voids and pores. The estimated average grain size for x = 0 was between 15 and 25 ␮m. The quenched x = 0 sample (Fig. 2(b)) was also porous but showed fused grains with unclear grains boundary. The microstructures of EuBa2 Cu2.94 Al0.06 O7−ı (x = 0.06) (Fig. 2(c)) and EuBa2 Cu2.90 Al0.10 O7−ı (x = 0.10) (Fig. 2(d)) both showed roughly similar microstructures which consist of fused grains. The porosity of the samples is much lower compared to EuBa2 Cu3 O7−ı (x = 0). Temperature dependent electrical resistance measurements (Fig. 3), showed the as-prepared x = 0 sample was superconducting with Tc zero of 87.5 K and Tc onset of 94.9 K and exhibits metallic normal state behavior. Substitution of small amount of Al3+ at x = 0.06 caused Tc zero to slightly increase while maintaining metallic normal state behaviors. Further substitution at x = 0.10 induced a metallic to semi-metallic normal state behavior and suppressed Tc zero and Tc onset to 38.3 K and 79.7 K, respectively. However, the quenched x = 0 sample was non-superconducting and showed semiconductor-like behavior down to 40 K. 3.2. Ultrasonic velocity measurements of EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) For x = 0, both absolute value of vl and vs and other related elastic moduli and the Debye temperature decreased as the oxygen content of the sample was reduced by quenching (Table 2). The decrease in the velocity and elastic moduli values is probably caused by oxygen O(4) removal from the Cu–O chains which weakens inter-ionic forces and caused expansion of the Eu123 unit cell along c-axis [33]. This is consistent with similar reports on longitudinal and shear velocities for oxygen-reduced REBa2 Cu3 O7−ı (RE = Gd, Er, Pr and Ho) [8,14,15,33]. Substitution of the small amount of Al3+ caused both absolute values of vl and vs , related elastic modulus and the Debye temperature to decrease. It is possible that for x = 0.06 and x = 0.10, Al3+ enters the Cu-chain and it weakens bonding strength causing the decrease in the velocity and related elastic moduli. However, the calculated coupling constant,  for x = 0, 0.06 and 0.10 samples showed highest value at x = 0.06. This indicates that low level substitution at Cu-chain can enhance

Fig. 3. The normalized resistance versus temperature for EuBa2 (Cu1−x Alx )3 O7−ı with x = 0, x = 0 (quenched), x = 0.06, x = 0.10 and EuBa2 Cu3 O7−ı + y wt. % Al2 O3 with y = 0.2 and y = 0.4.

Table 2 Density, porosity, longitudinal velocity (vl ), shear velocity (vs ), longitudinal modulus (CL ), shear modulus (), bulk modulus (B), Young’s modulus (Y), Debye temperature ( D ) measured at 80 K, and calculated electron–phonon coupling constant for EuBa2 (Cu1−x Alx )3 O7−ı with (a) x = 0, (b) x = 0 (quenched), (c) x = 0.06 and (d) x = 0.10 and EuBa2 Cu3 O7−ı + y wt.% Al2 O3 with (e) y = 0.2 and (f) y = 0.4. The values in brackets are not corrected for porosity. Sample

Density (g/cm3 )

Porosity (%)

vl (km s−1 )

vs (km s−1 )

CL (GPa)

 (GPa)

B (GPa)

Y (GPa)

 D (K)



(a) x = 0

5.89 ± 0.04

13.6

5.59 ± 0.08 (5.20 ± 0.08)

3.10 ± 0.04 (2.88 ± 0.04)

214 ± 2 (159 ± 2)

65.7 ± 0.5 (49.0 ± 0.5)

126 ± 2 (94 ± 2)

168 ± 6 (125 ± 6)

430 ± 1 (400 ± 1)

0.58 ± 0.01 (0.61 ± 0.01)

(b) x = 0 (quenched)

5.77 ± 0.04

13.5

4.70 ± 0.07 (4.38 ± 0.07)

2.66 ± 0.04 (2.48 ± 0.04)

147 ± 1 (111 ± 1)

47.2 ± 0.4 (35.5 ± 0.4)

84 ± 1 (63 ± 1)

119 ± 4 (90 ± 4)

367 ± 2 (336 ± 2)

–

(e) x = 0.06

6.17 ± 0.05

9.5

4.48 ± 0.07 (4.26 ± 0.07)

2.80 ± 0.04 (2.67 ± 0.04)

137 ± 2 (112 ± 2)

53.6 ± 0.6 (43.9 ± 0.6)

65 ± 2 (54 ± 2)

126 ± 7 (104 ± 7)

385 ± 2 (366 ± 2)

0.63 ± 0.01 (0.65 ± 0.01)

(f) x = 0.10

6.19 ± 0.05

8.7

4.66 ± 0.08 (4.46 ± 0.08)

2.88 ± 0.05 (2.75 ± 0.05)

147 ± 2 (123 ± 2)

56.2 ± 0.7 (46.9 ± 0.7)

72 ± 2 (60 ± 2)

134 ± 8 (112 ± 8)

395 ± 2 (377 ± 2)

0.41 ± 0.01 (0.41 ± 0.01)

(c) y = 0.2

6.05 ± 0.04

11.4

4.83 ± 0.07 (4.55 ± 0.07)

3.00 ± 0.04 (2.83 ± 0.04)

160 ± 1 (125 ± 1)

61.7 ± 0.5 (48.4 ± 0.5)

77 ± 1 (61 ± 1)

146 ± 5 (115 ± 5)

412 ± 2 (387 ± 2)

0.58 ± 0.01 (0.60 ± 0.01)

(d) y = 0.4

5.64 ± 0.05

17.2

4.51 ± 0.06 (4.10 ± 0.06)

2.61 ± 0.03 (2.38 ± 0.03)

138 ± 1 (95 ± 1)

46.4 ± 0.4 (31.8 ± 0.4)

76 ± 1 (52 ± 1)

116 ± 4 (79 ± 4)

360 ± 2 (328 ± 2)

0.54 ± 0.01 (0.57 ± 0.01)

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coupling between electrons and phonons and caused Tc to increase. On the contrary, previous reports of ionic substitution at other sites such as in (Er,, Pr)Ba2 Cu3 O7−ı [16], (Dy,Pr)BaSrCu3 O7−ı [37] and GdBaSr(Cu, Zn)3 O7−ı [38,39], caused lowering of  indicating weakening of electron–phonon coupling. Fig. 4(a) and (b) shows the temperature dependencies of the longitudinal (80–280 K) and shear (80–220 K) relative velocity changes, respectively, for superconducting and quenched EuBa2 Cu3 O7−ı (x = 0) together with the lattice anharmonicity curve calculated using the Lakkad model given in ref. [32]. Superconducting EuBa2 Cu3 O7−ı (x = 0) showed a step-like anomaly indicating pronounced lattice stiffening at around 260 K with an overall change in velocity of 1.9% (Fig. 4(a)). However upon quenching to reduce the oxygen content, no anomaly was observed and only a monotonous change in velocity with increasing temperature was seen (Fig. 4(b)). Quenching also caused the overall change in velocity to increase to 2.6%. Overall quenching also reduced the deviation of the longitudinal and shear velocity curves from the respective calculated anharmonicity curves. In contrast to a previous report

Fig. 5. Temperature dependence of (a) longitudinal and (b) shear velocity in EuBa2 (Cu1−x Alx )3 O7−ı with x = 0.06 and x = 0.10.

Fig. 4. Temperature dependence of (a) longitudinal and (b) shear velocity in EuBa2 (Cu1−x Alx )3 O7−ı with x = 0 and x = 0 (quenched).

on shear velocity measurements on Eu123 [40], no shear velocity anomalies were observed for both as-prepared EuBa2 Cu3 O7−ı (x = 0) and quenched x = 0 samples in this work. Fig. 5(a) and (b) shows the temperature dependencies of the longitudinal (80–280 K) and shear (80–220 K) relative velocity changes, respectively, for EuBa2 (Cu1−x Alx )3 O7−ı (x = 0.06 and 0.10) together with the lattice anharmonicity curve calculated using the Lakkad model [32]. No step-like anomaly was observed for both x = 0.06 and x = 0.10 samples and only a monotonous change in velocity with increasing temperature was seen for both modes. The overall change in longitudinal velocity was 1.2% and 1.5% for x = 0.06 and x = 0.10, respectively. The overall change in shear velocity was 0.9% and 1.2% for x = 0.06 and x = 0.10, respectively. For EuBa2 (Cu1−x Alx )3 O7−ı (x = 0.06 and 0.10), although the overall change in velocity increases with Al3+ , the deviation of the velocity curves from the respective anharmonicity curves

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Fig. 6. X-ray powder diffraction pattern for EuBa2 Cu3 O7−ı + y wt.% Al2 O3 with (a) y = 0.2 and (b) y = 0.4.

decreased. 3.3. Structure and electrical measurements of EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2, 0.4) Powder X-ray diffraction patterns (Fig. 6) revealed orthorhombic 123 phase structures for both EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2 and 0.4) composites. No Al2 O3 peaks were detected in the XRD patterns probably due to the small amount of Al2 O3 used and formation of 123 phase does not appear to be affected by the addition. The lattice parameters for both samples are shown in Table 1. Based on calculated c-lattice values, the oxygen content of the 123 phase in the y = 0.2 and y = 0.4 samples was estimated to be O6.8 . SEM micrographs of the internal section of EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2, 0.4) composites (Fig. 7(a)) showed irregular shaped grains in addition to the existence of voids and pores. The microstructures were very different between the two samples. For y = 0.2, elongated grains with sizes as large as 60 ␮m were observed. However, for y = 0.4, the average grains size was only around 7–10 ␮m while some parts of the sample showed fused grains. Temperature dependent electrical resistance measurements (Fig. 3(c)), showed normal state behaviors for both y = 0.2 and 0.4 samples are metallic. Addition of Al2 O3 at y = 0.2 to EuBa2 Cu3 O7−ı only slightly suppressed Tc zero from 87 K (y = 0) to 83 K (y = 0.2). At y = 0.4 there is a larger drop of Tc zero to 65.4 K. 3.4. Ultrasonic velocity measurements of EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2, 0.4) Addition of Al2 O3 in EuBa2 Cu3 O7−ı caused the absolute values of longitudinal and shear velocities, related elastic moduli and the calculated Debye temperature to decrease with increasing Al2 O3 . The additions not only reduced elasticity of the sample but also caused deterioration of superconducting behavior as electron–phonon coupling constant,  decreased (Table 1). Fig. 8(a) and (b) shows the temperature dependencies of the longitudinal (80–280 K) and shear (80–220 K) relative velocity

Fig. 7. SEM micrograph of internal sections for EuBa2 Cu3 O7−ı + y wt.% Al2 O3 with (a) y = 0.2 and (b) y = 0.4.

changes, respectively, for EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2, 0.4) together with the calculated lattice anharmonicity curve calculated using the Lakkad model [32]. Addition of Al2 O3 in Eu123 for y = 0.2 and 0.4 caused the step-like anomaly to be shifted down to around 210 K and 200 K, respectively compared to the x = 0 sample (Fig. 4(a)). However, additionally, at y = 0.4, a softening tendency was also be observed at around 240 K. The overall change in longitudinal velocity for both samples was around 1.7%. To our knowledge, this is the first observation of a softening tendency which is situated close to a step-like anomaly. Because the 240 K anomaly was not observed for the y = 0.2 sample it is possibly related to the amount of Al2 O3 used. The temperature dependence of shear velocity for the samples between 80 K and 220 K are shown in Fig. 8(b). The y = 0.2 and 0.4 samples also showed step-like anomalies around 190 K and 180 K, respectively. The overall change in shear velocity for y = 0.2 and y = 0.4 samples was 1.5% and 0.9%, respectively. The strong deviation of velocity curves from the respective anharmonicity curves for the Al2 O3 added samples may be due to the influence of the elastic anomalies. 3.5. Discussion The step-like anomaly observed for EuBa2 Cu3 O7−ı at around 260 K in this work was higher than most previously observed step-like anomalies which were usually observed between 200 K and 230 K [12,14–16,32]. The only 123 material with a step-like anomaly at much higher temperatures was Gd123 which was reported at around 250 K during warming [8]. Considering Gd3+ and Eu3+ as having not only comparable ionic radius which are also larger than many other RE3+ of 123 materials previously studied using ultrasound, the position of the anomaly at significantly

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because Al3+ partial substitution in EuBa2 (Cu1−x Alx )3 O7−ı (x = 0.06, 0.10) substitutes directly into Cu–O chains instead of CuO2 planes [36,20], the substitution disturbs oxygen ordering in the chains and subsequently caused disappearance of the anomaly. Existence of step-like anomaly around 200 K for both EuBa2 Cu3 O7-␦ + y wt. % Al2 O3 (y = 0.2 and 0.4) composites also strengthened the above suggestion of involvement of intrinsic oxygen ordering in Cu–O chains of Er123. In the composite, Al2 O3 exists as a separate phase and is not expected to enter the Eu123 lattice structure. This is in agreement with previous report on elastic behavior of (BiPb)2 Sr2 Ca2 Cu3 O10 /AgCa2 CuO3 composite [42]. In the present work, it is suggested that Al2 O3 resides along Eu123 grain boundaries and alters bonding strength between grains which in turn affects elasticity of the composite material. This is reflected by the change in sound velocity as shown in Table 2 but is extrinsic to Eu123 unit cells. Since the addition does not go into the Cu sites of Er123 lattice it does not affect oxygen ordering and this explains why the step-like anomaly was not suppressed for y = 0.2 and y = 0.4. The difference in position of the anomaly for y = 0.2 and y = 0.4 compared to x = 0 is probably due to the slight differences in oxygen content. The origin of the step-like anomaly may not be due to microstructure as although the x = 0 (Fig. 2(a)) and y = 0.2 (Fig. 7(a)) samples showed large differences in average grains sizes both samples showed step-like anomalies. However, the sudden softening observed in the longitudinal mode at around 250 K upon cooling for the y = 0.4 sample (Fig. 8(b)) is intriguing as to our knowledge this is the first time such an anomaly was observed in RE123. The reason for the sudden softening tendency is not clear but it maybe due to some form of lattice instability as a result of the Al2 O3 addition. 4. Conclusion

Fig. 8. Temperature dependence of (a) longitudinal and (b) shear velocity in EuBa2 Cu3 O7−ı + y wt.% Al2 O3 with y = 0.2 and y = 0.4.

higher temperature for Eu123 may be related to its larger ionic size. The origin of the step-like anomaly, however, can be found by a comparison between Fig. 4(a) and Fig. 4(b) which clearly shows that 260 K step-like anomaly in EuBa2 Cu3 O7−ı (x = 0) is oxygen-content related. Similar observations were reported for other REBa2 Cu3 O7−ı superconducting samples where step-like anomaly above 200 K was suppressed after reduction of oxygen content [8,14,15,32]. On the other hand, however, our results, for EuBa2 (Cu1−x Alx )3 O7−ı (x = 0.06, 0.10) indicate that the steplike anomaly may not be merely related to oxygen content in each 123 unit cell but instead it is linked to the Cu-O chains. In fact, highly oxygenated REBa2 Cu3 O7−ı materials are known to exist in different orthorhombic superstructures namely the ortho-I, ortho-II and ortho-III superstructures which represents different oxygen ordering pattern involving Cu–O chains [13]. According to the phase diagram of Y123 suggested by Ceder et al. [41], a phase transition from ortho-I phase to the mixed ortho-I and ortho-III phase occurs around 200 K. This may be the reason for the sudden hardening as indicated by a step-like elastic anomaly around the same temperature during cooling. In the present work,

In conclusions, we measured longitudinal and shear ultrasonic velocities in EuBa2 (Cu1−x Alx )3 O7−ı (x = 0, 0.06 and 0.10) and EuBa2 Cu3 O7−ı + y wt. % Al2 O3 (y = 0.2, 0.4) superconducting ceramics. A step-like anomaly indicating sudden lattice stiffening with decreasing temperature was observed for superconducting EuBa2 Cu3 O7−ı (x = 0) at 260 K in the longitudinal mode. Disappearance of the step-like anomaly in the x = 0 sample after quenching confirms that the anomaly is oxygen related. Al3+ substitution at the Cu site of EuBa2 (Cu1−x Alx )3 O7−ı (x = 0.06 and 0.10) strongly suppresses the step-like anomaly and strongly indicates involvement of Cu–O chains. On the other hand, addition of Al2 O3 at y = 0.2 and 0.4, which is not expected to substitute into Eu123 unit cell, did not suppress the anomaly but only shifted it to slightly lower temperatures. Based on the results, the step-like elastic anomaly was suggested to be due to some form of oxygen ordering process taking place in Cu–O chains of Eu123. Substitution of Al3+ enters Cu–O chains and interferes with oxygen ordering and this is the reason for suppression of the step-like anomaly. Observation of enhancement of electron–phonon coupling constant,  together with suppression of the step-like anomaly for the x = 0.06 sample, indicates that the anomaly may not be a precursor for high-temperature superconductivity. References [1] H. Alloul, Phys. Rev. Lett. 63 (1989) 1700. [2] J. Rossat–Mignod, L.P. Regnault, P. Bourgers, P. Burlet, C. Vettier, J.Y. Henry, Physica B 186–188 (1993) 1. [3] T. Timusk, B. Statt, Rep. Prog. Phys. 62 (1999) 61. [4] L. Benfatto, S. Caprara, A. Perali, Physica A 280 (2000) 185–192. [5] S.H. Han, P. Lundqvist, O. Rapp, Physica C 282–287 (1997) 1571. [6] L. Navarette, A. Sanchez, A. Marino, Physica C 235–240 (1994) 1943–1944. [7] M. Irfan, N. Hassan, N.A. Khan, Physica C 2–3 (2009) 86–90. [8] D.P. Almond, Q. Wang, J. Freestone, E.F. Lambson, B. Chapman, G.A. Saunders, J. Phys. Condens. Matter. 1 (1989) 6853–6864.

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