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Neutron structural studies on the superconducting (Nd1-xCax)(Ba1.6La0.4)Cu3Oz system

We have investigated the influence of Ca ions substitution on the structural and superconducting properties of (Nd1-xCax)(Ba1.6La0.4)Cu3Oz system. Magnetization, X-ray diffraction and neutron diffraction studies have been carried out on a series of compounds with x = 0.0 to 0.6. The superconducting transition temperature Tc, determined from magnetization measurements increases with increasing Ca2+ substitution. Neutron diffraction studies reveal that these compounds crystallize in a tetragonal structure (space group P4/mmm). A detailed analysis of the neutron diffraction data reveals that Ca and La ions are intermixed at the nominal Ba and Nd sites. While a major fraction of Ca ions occupy the usual Nd site, a small fraction occupies the Ba site. Consequently, the corresponding amount of La substitutes at the nominal Nd site. The intermixing of Ca and La sites randomizes the chain site oxygens leading to a tetragonal structure despite an oxygen content close to 7.0 for all the Ca doped samples. Further increase of Ca content lead to change in its co-ordination from six-fold to eight-fold at x>= 0.4.


Introduction
The normal state transport properties of oxide superconductors are unusual and systematic studies of them are therefore important for understanding high T c superconductivity. YBa 2 Cu 3 O 7-δ (Y123) superconductor has been extensively studied; replacing Y by R (R = Rare Earth) does not affect superconductivity except when R is Ce, Tb or Pr. The hole concentration, p, is an important parameter in the p-type ceramic high T c materials. The hole concentration can be varied by changing the concentration of substituents at different crystallographic sites (i.e. Y, Ba or Cu), or by the ordering of oxygens in basal plane. It is well known that in R 1+x Ba 2-x Cu 3 O 7-δ , the light rare earth element (LR = La to Nd) can occupy the Ba site as well as the R site. For LR elements the ionic radii approach that of Ba [1,2], allowing for a large degree of substitution by R for Ba without forming second phase, which results in oxygen defects on the anti chain site. Nd has an ionic radius closest to that of Ba and, therefore, exhibits the greatest amount of solubility with Ba (x ≅ 0.8) [3].
When trivalent LR occupies the divalent Ba site, charge balance requires ½ oxygen for each R is incorporated into the structure. In the fully oxygenated orthorhombic structure the extra oxygen occupies the anti-chain site [O (5)] [4][5][6]. Since the basal plane acts as a charge reservoir, the extra oxygen results in modification of the charge distribution in the structure. Superconductivity is suppressed due to the localization of holes [6][7][8]. Accompanying the LR 3+ substitution on the Ba 2+ site is a change in the crystal structure from orthorhombic to tetragonal (O-T) and an increase in the flatness of the CuO 2 plane [9]. A comparison of substituting magnetic ions (such as Eu 3+ , Pr 3+ , Nd 3+ ) [7,9,10] and non-magnetic ions (such as La 3+ ) [10][11][12][13] at Ba site leads to the conclusion that besides magnetic pair breaking, there is a considerable contribution to the T c originating from other effects like defect scattering, carrier localization or hole filling.
By studying La substituted Nd123 compounds we have shown that increasing La concentration in Nd(Ba 2-y La y )Cu 3 O z (0 ≤ y ≤ 0.5) causes a reduction in T c due to hole filling, and a structural phase transition (O-T) is observed at y = 0.3 [11]. A similar structural transformation is observed with nonmagnetic La 3+ at Ba 2+ , with magnetic rare earth R (R = Eu, Ho, Dy) substitutions [10,12,13]. This T c is expected to revive when divalent Ca is substituted for trivalent rare earth R. When Ca is incorporated in (Nd 0.94 Ca 0.06 )Ba 2 Cu 3 O 7-δ , the sample is observed to be more metallic at 8 GPa and 523 K [14]. On the other hand, thermoelectric power measurements have shown that La doping in Nd 1-x La x Ba 2 Cu 3 O y causes a decrease in hole concentration smaller than that of Ca doping in Nd 1-x Ca x Ba 2 Cu 3 O y , which suggests that some La (~26%) would enter in to the Ba site with the generation of additional BaCuO 2+v phase [15]. Our present efforts are to understand the effect of simultaneous substitution of Ca and La at the Nd and Ba sites, respectively, on structure and superconductivity of (Nd 1-x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z system. If all the La atoms occupy the Ba site, the oxygen content would have to increase to maintain charge balance. As Ca (1.12 Å, in 8 fold coordination) is slightly larger than Nd (0.995 Å), there should be a preference for Ca to occupy the Ba-site rather than the Nd site, which is solely based on lattice strain arguments. However, this preference is not observed, and is expected to be due to some inter

Experimental details
The (Nd 1-x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z , series of compounds with 0 ≤ x ≤ 0.6, were synthesized via a solid state reaction, the details of which are given in Ref. [14]. Powder Xray diffraction patterns of the compounds were obtained in the 2θ range 20º ≤ θ ≤ 80º using Cu K α radiation.
Room temperature neutron diffraction patterns, were obtained at the University of Missouri Research Reactor Facility using a position sensitive detector (PSD) diffractometer.
This diffractometer is equipped with a Si [511] monochromatic, horizontally curved with about 11-12 m radius and vertically segmented with 1.5 m radius and with 9 asymmetrically cut blades. For collecting the diffraction data, the sample was contained in a thin-walled vanadium can of about 3-4 mm diameter and about 5 cm long, and rotated about the long axis to average out any preferred orientations. The data were obtained in a 2θ range of 5º ≤ θ ≤ 105º (at 0.05º intervals), using a PSD (with 5 detector elements) that covered a range of 20º at a time. Neutrons of wavelength 1.4875 Å were used for the diffraction experiments.
Magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design) in the temperature range 1.7-300 K and an applied field of 50 Oe.

Results and discussion
X-ray powder diffraction studies were performed on series of compounds (Nd 1- x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z with x = 0.0 to 0.6, showed them to be single phase. A fit of a typical neutron diffraction pattern is shown in Figure 1. Figure 2 shows the typical zero-field-cooled (ZFC) and field cooled (FC) magnetization data of (Nd 1-x Ca x )(Ba 1.6 La 0.4 )Cu 3 Oz with x = 0.0, 0.1, 0.2 and 0.4 with an applied filed of 50 Oe. In both ZFC and FC states data were acquired during warming of the sample. The superconducting transition temperature (T c ) is defined as the onset of diamagnetic response. It is seen from figure 2 and figure 5 that T c increases with increasing Ca concentration, which is in agreement with an earlier report [11]. For the sample with x = 0.1, the diamagnetic signal is small, suggesting that superconductivity is not a bulk property of the sample. It is evident from the figure that the Meissner fraction increases with increasing Ca content, which may indicate that a superconducting phase is forming while more Nd 3+ is replaced by Ca 2+ . However, neither XRD nor neutron diffraction shows any evidence of another phase at room temperature.
The neutron diffraction data were analyzed by Rietveld refinement procedure using the Generalized Structural Analysis System (GSAS) program. The structure refinement carried out with orthrorhombic structure, space group Pmmm. It was realized that lattice parameter 'a' and 'b' are nearly the same, exhibit possible formation of tetragonal phase.
Therefore, neutron diffraction data on all samples were refined on the basis of a tetragonal structure (space group P4/mmm). Neutron scattering factors used were (in units of fm) 8  Initially structure refinement model assumed with Nd/R and Ba/La ratios fixed by initial stoichiometry, but site occupancies were allowed to vary. In the subsequent analysis of neutron diffraction data, it was assumed that Nd and Ba fully occupy their normal sites. After fixing Nd and Ba occupancies, the Ca and La occupancies were allowed to vary freely both at the actual sites (Ca at Nd site and La at Ba site) and the nominal sites (La at Nd site and Ca at Ba site) with the condition that combined fractional and nominal at R site (Ca at Nd + La at Nd site) and Ba site (La at Ba + Ca at Ba) add up to unity (i.e. full occupancy). In order to maintain a meaningful and reliable variation of Ca and La occupancies at the two sites this procedure was adopted uniformly for all the samples. The variation of occupancies for La and Ca at different sites is shown in Fig. 4(a) and 4(b) respectively. The intermixing behaviour of La and Ca occupancy variation with R (R=Nd, Dy, and Y) through neutron diffraction has also been observed in La-1113, La-2125 and R-123 systems [20][21][22][23].
It is well known that Ca substitution at the R-site decreases the oxygen content of R-123 compounds; however, La substitution at the Ba-site increases the oxygen content [12,13].
Analysis of the neutron diffraction data reveals that some Ca atoms occupy the actual Nd-site while some occupy the Ba site. Consequently, corresponding amount of La substitutes at the Nd site. The oxygen content, obtained from refinement of neutron data, is found to be close shortened [16,17]. The effective Cu valence, estimated from the overall oxygen content (z) obtained from neutron diffraction data are listed in Table-1, which increases with increasing Ca concentration. Simultaneous increase in T c with Cu valence is observed, and showed maximum T c of 84 K at x = 0.4 ( Figure 5). The introduction of Ca into (Nd 1- x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z alters the normal and superconducting properties substantially [11].
The ionic radii of Ca 2+ in six-fold and eight-fold coordination are significantly different. The ionic size of Nd 3+ in eight-fold coordination is (0.995 Å), which matches more closely with that of Ca 2+ in six-fold (0.99 Å) than that of eight-fold coordination (1.12 Å). The decrease in lattice parameter a and cell volume up to x = 0.4, indicates that Ca at the Nd site would have preferred a six fold coordination. The lattice parameter 'c' remains nearly unchanged which is in good agreement with earlier reports of the possibility of six-fold coordination in those samples [18,19] with Ca 2+ substitution at R site in fully oxygen annealed Y-123 systems. As Ca content increases in the regime x > 0.4, some Ca 2+ might change its coordination from sixfold to eight-fold, which occupy Ba site and corresponding amount of La occupy at Nd site ( Fig. 4(a) and 4(b); Table 1). Therefore, lattice parameter 'a' and cell volume increases due to the fact that La 3+ and Ca 2+ in eight-fold coordination are bigger than Nd, which indicate an increase of the interplane Cu (2) K (x = 0.6) due to excess hole doping from Ca.

Conclusions
We conclude that Ca doping enhances superconductivity for doping level up to x = 0.4; thereafter it shows a decrease in T c with increasing x, due to excess hole doping. Neutron diffraction data suggest that there is intermixing of Ca and La occupancies. At higher doping level x > 0.4 indicates possibility of eight-fold coordination of Ca. Parameter