X-ray and Electron Diffraction Study of a Precipitation Phase in Nd_1.85Ce_0.15CuO_4+y Single Crystal

Abstract

X-ray diffraction measurements have been performed to investigate a crystal structure of a precipitation phase in oxygen-reduced Nd 1:85 Ce 0:15 CuO 4þy (NCCO). A detailed structure analysis of the precipitation phase as well as the parent NCCO phase revealed that the precipitation phase is a cubic (Nd,Ce) 2 O 3 , which is distorted tetragonally by the surrounding NCCO phase. The averaged grain size of the precipitation phase exceeds 200A parallel to the CuO 2 plane of the parent NCCO, while the size along layered direction is short-ranged ($30 A). The grain thickness along layered direction is strongly correlated with the homogeneity of superconducting state. The volume fraction of the (Nd,Ce) 2 O 3 phase is about 2%. We also performed electron diffraction as well as microscopy experiment, which confirms that the (Nd,Ce) 2 O 3 phase segregate from the parent NCCO phase and exists as an epitaxial layer along the c-axis.

Microscopic annealing process and its impact on superconductivity in T′-structure electron-doped copper oxides

“…To test this possibility, single crystals of PLCCO (space group I4/mmm, a = 3.98 Å, c = 12.3 Å) were grown using the traveling-solvent floating-zone technique. These included four samples prepared from the same as-grown batch: as-grown nonsuperconducting (ag-NSC), reduced superconducting (r-SC), oxygenated nonsuperconducting (o-NSC), and re-reduced superconducting (r2-SC) crystals.As shown in previous work [18][19][20][21] , annealing necessary to produce superconductivity also induces epitaxial intergrowths of the cubic R 2 O 3 impurity phase.The R 2 O 3 has a cubic structure with space group Ia3 and lattice parameter a c = 2√2a ~ c/1.1, where the "c" subscript indicates "cubic". The Miller indices of the T' and impurity phases are related as ( , , ) [ To test the reversibility of the impurity phase, we carried out x-ray powder diffraction measurements on portions of the ag-NSC, r-SC, and o-NSC single crystal samples of PLCCO that were crushed into fine powders.…”

“…5%) entirely suppress AF order and superconductivity appears over a wide range of hole concentrations (from 6% to 30%). In the case of T' structured electron-doped superconductors, doping alone by substituting the trivalent ions R 3+ in R 2 CuO 4 with tetravalent Ce 4+ is insufficient to induce superconductivity and the as-grown materials such as Nd 2-x Ce x CuO 4±δ (NCCO), Pr 2-x Ce x CuO 4±δ (PCCO), and Pr 0.88 LaCe 0.12 CuO 4 (PLCCO) are semiconducting with static long-range AF order [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] .In these materials, superconductivity (maximum T c ~25 K) is achieved only when the samples within a relatively narrow dopant range (0.1≤x≤0.18) are annealed in a reducing atmosphere . The reduction process suppresses the AF order most severely at higher Ce-doping levels above x≥0.1 (ref.…”

“…31) have recently determined the Fermi surface topology and the superconducting gap function in electron-doped materials. However, the quality of the sample surfaces studied has not been conclusively established, and one must now consider the possible effect of an exposed NSC R 2 O 3 surface in future ARPES and scanning tunneling microscopy experiments.Furthermore, it should also be possible to grow higher-quality single-crystal T' structured copper oxides by adding excess Cu in the starting materials to eliminate Cu deficiencies and subsequent R 2 O 3 contamination.Because of the availability of single crystals of the T' structured copper oxides, the vast majority of transport, structural, magnetic, and electronic-property measurements of electron-doped superconductors have been obtained from these materials [9][10][11][12][13][14][15][16][17][18][19][20][21][22]25,[27][28][29][30][31] . For other electron-doped high-T c superconductors with different crystal structure types, such as the infinite-layer Sr 1-x La x CuO 2 family 32-34 , the microscopic process of inducing superconductivity is still unclear 33,34 .…”

“…), electron-doping alone is insufficient, and annealing the as-grown sample in a low oxygen environment to remove a tiny amount of oxygen is necessary to induce superconductivity 2,3 . Previous work [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] suggests that oxygen reduction may influence mobile carrier concentrations 7 , decrease disorder/impurity scattering 8,10,11,23 , or suppress the long-range AF order 16,17,22 . However, the microscopic process of oxygen reduction, its effect on the large electron-hole phase diagram asymmetry and mechanism of superconductivity 2,3 are still unknown.…”

“…Experimentally, it was found that about 1%~2% of the impurity phase, absent in the as-grown materials, appears with superconducting phase after the oxygen reduction process [18][19][20][21] . Although not explicitly demonstrated, the R 2 O 3 impurity phase may disappear again when annealed samples are oxygenated to eliminate superconductivity 15,21 . Therefore, superconductivity in electron-doped materials might be intimately related to the appearance of the impurity phase.…”