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METALS. SUPERCONDUCTORS Relation Between Tc and the Defect METALS. SUPERCONDUCTORS Relation between Tc and the defect structure of neodymium cerium cuprite Nd22xCexCuO42y V. I. Voronin, A. E. Kar’kin, and B. N. Goshchitski Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia A. Yu. Zuev, T. P. Rodionova, and A. N. Petrov Urals State University, 620083 Ekaterinburg, Russia ~Submitted June 27, 1997! Fiz. Tverd. Tela ~St. Petersburg! 40, 177–183 ~February 1998! Neutron powder diffraction has been used to study the defect structure of neodymium cerium cuprite Nd22xCexCuO46y (x50.15). It has been shown that in addition to oxygen vacancies, O2 sites in superconducting samples may also contain a small quantity of implanted oxygen atoms positioned between copper ions and neodymium/cerium, which control the electrical charge in the Cu–O planes. The oxygen distribution among crystal lattice sites in Nd22xCexCuO46y (x50.15) was determined, the average charge of the copper ions was calculated by the method of valence sums, and a correlation was established between the charge of the copper-oxygen plane and Tc .©1998 American Institute of Physics. @S1063-7834~98!00102-6# Neodymium cerium cuprite, Nd22xCexCuO46y, occu- merely by varying the cerium concentration or merely by pies a special position among the various copper-containing annealing in oxygen.9 high-temperature superconductors ~HTS! for various rea- However, thermogravimetry and iodometric titration can sons. First, unlike other HTS cuprites, the charge carriers are only provide data on the total oxygen content in samples, electrons.1–3 Second, superconductivity is observed in a whereas neutron diffraction can be used to determine the fairly narrow range of Ce concentrations (0.14,x,0.17). distribution of oxygen and oxygen defects in the lattice. For Finally, even within this range of cerium concentrations, the instance, neutron powder diffraction was used to demon- strate the existence of oxygen vacancies in the lattice in su- superconduction transition temperature (Tc) is extremely sensitive to the oxygen stoichiometry. This compound has a perconducting and nonsuperconducting Nd22xCexCuO42y 10 tetragonal (T8) crystal structure and the atomic configuration (x50.155) samples. However, it is unclear how the sto- 1 ichiometrically excess oxygen, observed by thermogravimet- is similar to the La22xSrxCuO42y (x.0.10) structure. The ric analysis6,11 and by iodometric titration,7 is distributed in T8 structure of Nd22xCexCuO42y compounds ~Fig. 1! con- the crystal lattice. The authors of Ref. 12 used neutron dif- sists of alternating copper-oxygen planes of CuO2 in which the Cu ions have a square oxygen-ion neighborhood ~O1 fraction analysis to solve this problem for polycrystalline Nd Ce CuO samples and showed that the oxygen at- sites! and a ‘‘sandwich’’ formed by layers of oxygen ions 22x x 42y oms may be partially distributed at position O3 ~Fig. 1!, ~O2 sites! interspersed with neodymium-cerium ions. which is completely filled in the La Sr CuO lattice by Questions involving the relation between the supercon- 22x x 42y so-called ‘‘apical’’ oxygen ~T-structure!. These conclusions ducting transition temperature, the oxygen nonstoichiometry, were then confirmed by neutron diffraction analyses for and the structure of Nd Ce CuO have attracted the at- 22x x 46y single crystals of pure Nd CuO ~Ref. 13!, cerium-doped tention of many researchers. However, information on the 2 42y Nd1.85Ce0.15CuO42y ~Ref. 14!, and for Nd1.9Ce0.1CuO42y oxygen content as a function of the thermodynamic condi- polycrystals.15 Studies have indicated that vacancies are tions in neodymium cerium cuprite, obtained by thermo- present at the O1 and O2 sites together with a small quantity gravimetry and chemical analysis ~iodometry! are contradic- of apical oxygen at positions O3. The authors of Ref. 15 tory. Some authors4,5 assert that, even after treatment in pure proposed a model for defect formation in Nd1.9Ce0.1CuO42y oxygen, the oxygen index does not exceed four. Other which assumes that both oxygen vacancies and ‘‘interstitial 6–8 authors take the view that, after annealing in oxygen, ~O3! oxygen ions are present at crystallographic positions in samples contain excess oxygen in interstitial sites in the cop- the structure of neodymium cerium cuprite. Calculations of per oxide structure. It is postulated that in a the cation valence state in Nd1.9Ce0.1CuO42y , using the Nd1.85Ce0.15CuO42y sample annealed in a rare gas atmo- method of valence sums from an analysis of cation-oxygen sphere, the total oxygen content decreases (y.0), which bond lengths,16 provided further evidence to support the leads to an additional increase in the charge carrier concen- presence of both vacancies and interstitial oxygen in the lat- tration in the lattice with the onset of superconductivity. tice. Note that the optimum doping level cannot be achieved Thus, the properties of Nd22xCexCuO42y , including su- 157 Phys. Solid State 40 (2), February 1998 1063-7834/98/020157-06$15.00 © 1998 American Institute of Physics 157 ture range 1223–1423 K. The process comprised three stages with intermediate grinding of the reaction mixture. The samples were preannealed at 1000 °C for 24 h under differ- ent oxygen pressures: PO250.0015 ~sample No. 1!, 0.0026 ~No. 2!, 0.03 ~No. 3!, and 1 atm ~No. 4!, followed by rapid quenching. 2. RESULTS OF MEASUREMENTS The x-ray diffraction analyses showed that all the Nd1.85Ce0.15CuO42y samples are single-phase with a T8 te- tragonal crystal structure. The structural parameters ~atomic coordinates, site occupation numbers, thermal vibration am- plitudes, and static atomic displacements! were obtained from the neutron diffraction patterns by successive compari- sons of the calculated and experimental data. A typical neu- tron diffraction pattern is shown in Fig. 2. To a first approximation, we assumed that Nd1.85Ce0.15CuO42y has a pure T8 structure in which all the oxygen atoms are only occupy at O1 and O2 sites and the individual temperature factors for each atom are isotropic. In an earlier study18 we showed that the incorporation of aniso- tropic thermal vibrations and static atomic displacements in calculations of La22xSrxCuO42y improves the convergence with the experiment, reflecting an anisotropic variation in the crystal lattice parameters under strontium doping or fast neu- FIG. 1. T8 crystal structure: 1—Cu, 2—Nd/Ce, 3—O1, 4—O2, and 5—O3. tron irradiation. Thus, we introduced anisotropic Debye– Waller factors for all the atoms to achieve further refinement. Finally, the best fit to the experiment ~minimum values of the perconductivity, are determined by both the cerium and the R factors! was achieved by introducing an additional oxygen oxygen content. Oxygen stoichiometry is established follow- position—‘‘interstitial’’ oxygen atoms in O3 positions ~Fig. ing synthesis of the compound by the sample annealing con- 1!. Since there are a very small number of these positions, ditions; this is difficult to monitor, and frequently cannot be the occupation of the O3 positions n~O3! and their position determined by the researchers. Unfortunately, very few sys- (z coordinate! were determined by successively fixing n~O3! tematic studies have been made of the distribution of oxygen and z in small steps and then analyzing the convergence ions in the Nd22xCexCuO42y defect lattice and its influence factors, to achieve their minimum values. on the superconductivity of the samples. Similar procedures for refinement of the structure pa- The aim of the present study is to investigate the actual rameters of Nd1.85Ce0.15CuO42y were performed for all the distribution of oxygen in the lattice of Nd1.85Ce0.15CuO42y samples. These refined structure parameters are presented in compounds having a fixed cerium content (x50.15), which Table I. The unit cell parameters for sample No. 4 differed influences the valence of the copper, and to observe a corre- fairly appreciably from those of the previous three. An accu- lation between the charge of the copper atoms and the super- rate determination of the occupancy of the oxygen sites in conducting transition temperature. the cell of this sample revealed that their occupancies dif- fered from the first three, with position O2 being completely filled, whereas vacancies were identified at positions O1. 1. EXPERIMENTAL METHOD The values of Tc , determined from the onset of a super- The crystal structure was studied by x-ray diffraction conducting transition, on the temperature dependences of the ~Dron-UMI1 diffractometer using copper radiation! and neu- ac susceptibility are also presented in Table I. tron diffraction (l51.515 Å). The neutron diffraction pat- We shall examine the structural characteristics of terns at room temperature were obtained at the IVV-2M Nd22xCexCuO42y samples with different dopant concentra- atomic reactor using a D7a diffractometer. A double mono- tions. Published data12,19,20 and our own data on the crystal chromator: pyrolytic graphite ~002 reflecting plane!– lattice parameters of Nd22xCexCuO42y as a function of the germanium ~333!, was used to improve the angular resolu- cerium ion concentration, have shown that the reduction in tion (Dd/d50.3%). The structural parameters were refined the cell volume with increasing cerium concentration is by means of a Rietveld full-profile analysis using the ‘‘Full- mainly caused by a decrease in the parameter c from 12.15 prof’’ program.17 The superconducting transition tempera- (x50) to 12.07 Å (x50.15), whereas the parameter a re- ture was determined by the ac susceptibility method ~fre- mains unchanged to two decimal places (3.9460.01 Å). quency 1 kHz!. This behavior is attributable to characteristic features of the Neodymium cerium cuprite, Nd1.85Ce0.15CuO42y, was crystal structure, since it is well known that the symmetry, prepared using standard ceramic technology, in the tempera- type, and dimensions of ionic compounds are determined to 158 Phys. Solid State 40 (2), February 1998 Voronin et al. 158 FIG.
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