Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC Magnetic Shielding and Irreversibility Line in High-Tc Superconductors

We report here the preparation, X-ray diffraction with Rietveld refinement, AC magnetic susceptibility (χac = χ′ + iχ′′), resistivity, iodometric measurements and effect of heat treatments in (Y1−xSmx)SrBaCu3O6+z. Each sample has undergone two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. For each x, the [AO] heat treatment increases the orthorhombicity ε = (b − a)/(b + a) (for 0 ≤ x ≤ 1), Tc (for x ≥ 0.4) and reduced the linear resistivity parameters with a diminution of the interaction of holes with phonons. At all T < Tc and for any applied field Hdc, we observed an enhancement of AC magnetic shielding and the irreversibility line in the samples [AO] for x > 0.5, revealing an improvement in the pinning properties. Remarkable correlations were found. In the [AO] samples, the measured data are explained by the increase in phase purity, in cationic and chain oxygen ordering, psh and the decrease in d[Cu(1)─(Sr/Ba)].


Introduction
YBa 2 Cu 3 O 6.95 is superconducting below 92 K and characterized by double Cu(2)O 2 layers (oriented along the a-b plane) responsible for carrying the supercurrent and Cu(1)O chains (along the b direction) that provide a charge reservoir for these planes [1,2].
The four distinct crystallographic sites Y, Ba, Cu plane, and Cu chain can be substituted with different elements. Single-phase LnBa 2 Cu 3 O 6+z (Ln = rare earth) can be synthesized with T c = 92 K. All these compounds show an orthorhombically distorted oxygen-deficient tripleperovskite structure and both the orthorhombic distortion and T c depend sensitively on the oxygen content (6 + z) [3]. Wada et al. [4], Izumi et al. [5] studied the structural and superconducting properties of La 1+x Ba 2−x Cu 3 O y (with 0 ≤ x ≤ 0.5). They concluded that in order to have T c maximal, this structure must have an ordered arrangement of La and Ba along c axis with an occupation factor of 0 and 1 for the oxygen at (1/2, 0, 0) and (0, 1/2, 0), respectively.
We want to see if an isovalent substitution of Ba +2 by Sr +2 with smaller ionic radius can modify the results discussed above when Y +3 is replaced by the rare earth Sm +3 with bigger ionic radius. Understanding the effect of the Y and Ba atomic plans on the superconductivity in these compounds, we have studied the structural, superconducting and magnetic properties of (Y 1−x Sm x )SrBaCu 3 O 6+z . We found that the effect of heat treatments on these properties depended on the content of Sm.

Experimental techniques
We prepared the polycrystalline samples by solid-state sintering of oxides (Y 2 O 3 , Sm 2 O 3 , CuO) with a purity of 99.999% and carbonates (SrCO 3 99.999% pure, BaCO 3 with a purity of 99.99%). All these chemicals were thoroughly mixed in desired proportions and calcined at 950°C in air for 12-18 h. The obtained ceramic was ground, mixed, pelletized and heated in air at 980°C for 16-24 h. This was repeated twice. For each sample, the circular pellets were subjected to heat treatment in oxygen at 450°C for 60-72 h and furnace cooled. This was denoted as sample [O].
X-ray diffraction spectra of the samples were measured with Philips diffractometer fitted with a secondary beam graphite monochromator and using Cu Kα (40 kV/20 mA) radiation. The angle 2θ was varied from 20° to 120° in steps of 0.025°and the counting time per step was 10 s. The XRD spectra were resolved with Rietveld refinement.
A detailed description of the basic arrangement of the experiment of the AC magnetic susceptibility can be found in [6]. The sample in the form of a slab is placed in the magnetic field H ext = H dc + H ac cos(ωt) with the static component H dc and the AC component with the amplitude H ac and the frequency f = ω/2π. The sample's magnetic response was detected by a pick-up coil surrounding the sample. Superconducting transitions were determined by the measure of the real (χ′) and the imaginary (χ″) parts of the AC magnetic susceptibility as a function of temperature in H ac = 0.11 Oe and at f = 1500 Hz. Also, χ′ and χ″ were measured in 0 < H dc < 150 Oe with applied H ac .
We used the Van Der Pauw method [7] for measuring resistivity ρ(T). The sample was attached to a cane in a cryostat with closed helium circuit with a cryogenic pump, a regulator of temperature (1 μA-10 mA) and 1 μV resolution digital voltmeter controlled with a computer. T c was determined by both the measured χ′(T) and ρ(T).
For each x, the same sample [O] was then heated in argon at 850°C for about 12 h, cooled to 20°C and oxygen was allowed to flow instead of argon and the sample was annealed at 450°C for about 72 h. This sample is denoted as [AO]. XRD, resistivity and AC susceptibility measurements were done on a part of this sample. We measured 6 + z by iodometry technique on a part of each sample.

Crystalline structure
The X-ray diffraction spectra of all the samples are shown in Figure 1  In Figure 2 we show, respectively, the variation of the parameters a, b, c and the volume V of the unit cell obtained with Rietveld refinement [9] as a function of x and heat treatment. When x increases, the lattice parameter a (c and the volume V of the unit cell) increased but b is constant leading to a decrease of the orthorhombicity (ε = (b − a)/(b + a)) ε [O] in Figure 3. The substitution of Y +3 (0.893 Å) by the rare earth Sm +3 (0.965 Å), with a superior ionic radius, leads to a linear increase of c and V. The orthorhombicity depends strongly on the Sm content x. When x increases from 0 to 1, ε decreases quickly from 8.24 × 10 −3 to 1.5 × 10 −3 in the samples [O] in Figure 3. This indicated a structural phase transition from orthorhombic to tetragonal. ε decreases slowly from 9.9 × 10 −3 to 5.24 × 10 −3 with an orthorhombic symmetry in the samples [AO]. We found also that the orthorhombicity depends strongly on the heat treatment [AO]. For each x, the latter increased the orthorhombicity (for 0 ≤ x ≤1). The increase was maximum, from 1.5 × 10 −3 to 5.24 × 10 −3 for x = 1 in [12].

Real part of the AC magnetic susceptibility and T c
The critical temperature T c of the transition from the superconductor to the normal state depends strongly on the effect of [AO] heat treatment as seen in the real part of AC susceptibility χ′(T) in Figure 4. The imaginary part of AC susceptibility χ″(T) in Figure 4 shows a single peak T p . This defined clearly the value of T c for all the samples. We can see in Figure 5 that   For each x, the [AO] heat treatment increases ε (for 0 ≤ x ≤ 1) in Figure 3 and T c (for x ≥ 0.4) in Figure 5. The [AO] heat treatment makes the coupling of the superconducting grains by Josephson junctions took place at higher temperature. This effect is revealed by the net displacement of T p to higher temperature for x ≥ 0.4.    Table 1 shows the exact measured values of the structural parameters a, b, c, V and ε of each sample as a function of the heat treatment.

Real part of the AC magnetic susceptibility and the shielding effect
The effect of [AO] heat treatment on T c was remarkable. The temperature at which the diamagnetism sets in is taken as T c and it was found to be dependent on both x and the heat treatment employed. Since the same sample was used for both heat treatments, one can compare the diamagnetic response and note that screening current of the [AO] sample increased considerably compared to that of the [O] sample for each x (see, for example, the case x = 0.8 in Figure 6(a)). Table 1 shows the exact measured values of the superconducting parameters T c , T p , ΔT c and ΔT p of each sample as a function of the heat treatment.
We can see in Figure 7 the shielding effect S which is the amplitude of the real part of the AC susceptibility [10][11][12]. S represents the exclusion of the magnetic flux by the sample in alternative dynamic mode. S was set arbitrarily equal to 0.89, 0.97 and 1, respectively, for x = 0.5, 0.

Imaginary part of the AC magnetic susceptibility and irreversibility line
Looking to the imaginary part of the AC susceptibility χ″, of the sample Y 0.2 Sm 0.8 SrBaCu 3 O 6+z in Figure 6(b) for example, we can see that the width ΔT p at half maximum of the transition in χ″(T) (see Table 1) was smaller in the samples [AO] at all H dc and the peak T p shifted less than in the sample [O]. Figure 8 shows the field H dc as a function of t = T p /T c with an enhancement of the irreversibility line due to argon treatment for x ≥ 0.5 [14]. The data can be analyzed with the help of following relation H = K′ (1 − t) n [15]. Straight line plots were obtained when ln(H) was plotted against ln(1 -t) in Figure 9. For example, the value of K′ was estimated to be 1677 . K′ may be interpreted as the field necessary to reduce the intergranular critical current to zero in the limit of T p = 0 K. We note that the argon treatment considerably increases the value of K′ and n, in Table 1 and Figure 10, indicating an improvement in the pinning properties. The dashed line indicates the value n = 1.5 for the cuprites given by Miller et al. [15].

Discussions
We saw that the [AO] heat treatment increases the orthorhombic cleaving and eliminated some weak unidentified impurity peaks in Figure 1 Figure 12. When x increases, the parameter b is constant but a (and c) increase  Our results can be explained by the disorder of the oxygen in the basal plane, on the 0(4) and 0(5) sites along b and a axis, respectively, in optimum superconducting properties and could account for the observed increase in T c [AO] in agreement with the model of transfer of charges. This is justified by the fact that, when x increases, the parameter b is constant but a (and c) increase leading to an increase of the number of oxygen atoms by chain (NOC) along a axis with a decrease of ε (T c [O]) from orthorhombic toward tetragonal structure in Figure 12.
When Sm ion occupies Ba (or Sr) site, the same amount of Ba (or Sr) cation is pushed into Y site. Sm is a three-valence ion. It increases the positive charge density around Ba (or Sr) site and the attractive force with oxygen anion. As a result, oxygen vacancies O(5) along the a-axis in the basal plane have higher chance to be filled. On the other hand, Ba +2 (or Sr +2 ) in Y +3 (or Sm +3 ) site decrease the attractive force with oxygen anion in Cu (2) plane. This increases the buckling angle Cu(2)─O(3)─Cu(2) along the a axis. When x increased from 0 to 1, the two changes of cation sites increase the parameter a. For each x, the [AO] heat treatment decreases the parameter a and increases b as shown in Figure 2. This increases the number of oxygen atoms by chain (NOC) along b axis leading to an increase of T c with a decrease of the orthorhombicity ε for x ≥ 0.2 as seen in Figure 12.
In the normal state, the heat treatment [AO] reduced considerably the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. T c (χ′) and T c (ρ = 0) were in good agreement.
For each x > 0. 5 The two arguments (cationic and anionic disorders) are justified here by the four remarkable correlations observed between T c (x), the volume of the unit cell V(x) in Figure 14 and    the number p sh (x) of holes by Cu(2)─O 2 superconducting planes in Figure 15 (deduced from the undersaturation zone of the universal relation T c /T cmax (p sh ) [17]), and on the other hand, between δT c (x) = T c [AO] − T c [O] and δε(x) in Figure 16 and between δT c (x) and δK′(x) in Figure 17. So the structural, electrical and superconducting properties are correlated with the effect of argon heat treatment.
The increase or decrease in T c must be related to the ionic size of the rare earth Sm, the variation of the Cu(1)-apical oxygen distance, hole density, anionic and cationic disorders, etc.

Conclusions
These