A.I. Nazzal

Synthesis, crystal structure, and properties of metallic PrNiO3: Comparison with metallic NdNiO3 and semiconducting SmNiO3

Abstract The compound PrNiO3 has been prepared for the first time. Using only moderate oxygen pressure, PrNiO3, NdNiO3, and SmNiO3 were made and found to crystallize in the GdFeO3—type orthorhombically distored perovskite structure. Structural refinements for all three compounds reveal NiO6 octahedra with an average NiO distance of 1.94–1.95 A, about the same as in the more distorted HoNiO3. The electrical conductivity of PrNiO3 is metallic at room temperature, but undergoes a transition at 130 K to an insulating state. Examination of the conductivities of the corresponding Sm, Nd, and La compounds reveals a monotonic pattern of behavior: as the rare earth radius is increased, the compounds become more conducting because the metal-insulator transition temperature decreases from 400 K(Sm) to 200 K(Nd) to 130 K(Pr) to none for La. From DSC and lattice constant measurements, this transition is shown to be first order, with a ∼0.2% contraction upon heating into the metallic state. Low temperature neutron measurements in the insulating phase reveal new diffraction peaks, probably related to magnetic ordering of Ni and/or RE moments.

Extra oxygen in electron superconductors: Ce and Th doped Nd2CuO4+δ and Gd2CuO4+δ

Abstract Using iodometric titration techniques, we have systematically studied the oxygen content of the title compounds. Our oxidized and reduced samples both show an excess of oxygen above the nominal value of 4. This extra oxygen tends to partially compensate the effect of the tetravalent cation doping and must be considered for a complete understanding of the doping of electron superconductors. The partial removal of some of this extra oxygen is presumably the reason for the necessity of a reduction step in the preparation of electron superconductors. In the case of our Ce doped Gd 2 CuO 4 samples, there is an especially high concentration of extra oxygen and we are unable to make them superconducting.

Synthesis and x-ray characterization of superconducting and related Ln2−xCexCuO4−y (Ln=Nd and Gd) compounds

Abstract In view of the recent attention focused on electron superconductivity, we have prepared and analyzed the crystalline structures of Ce substituted Ln2−xCexCuO4−y (Ln=Nd and Gd) compounds. X-ray powder diffraction analysis shows that the Ce ions have been succesfully doped into the Nd sites of the tetragonal Nd2CuO4 lattice and the maximum Ce solubility limit is xc = 0.20. Linear decreases in both the c lattice dimension and the unit cell volume, V, with increasing x up to the solubility limit indicate the smaller Ce4+ ions, not the larger Ce3+ ions are present. The tetragonal unit cell dimensions of the 21 K superconducting Nd1.85Ce0.15CuO4−y compound refined from a high quality powder diffraction pattern with a figure of merit F30=267.3 (0.003, 35) are: a = 3.9463 (1) A , c = 12.0809 (2) A and V = 188.14 (1) A . X-ray results also show that the maximum Ce solubility limit in Gd2CuO4 is significantly lower, with xc=0.15. Linear decreases in c and V with increasing x in Gd2-xCex CuO4−y have also been obtained indicating the presence of Ce4+ ions. The unit cell dimensions of the semiconducting Gd1.85Ce0.15CuO4−y compound are a = 3.9023 (1) A , c = 11.8310 (2) A and V = 180.16 (1) A 3 .

Disappearance of high temperature superconductivity induced by high carrier concentrations

Abstract It is shown that samples of La2−xSrxCuO4 prepared under 100 bar oxygen pressure exhibit a metallic conductivity which increases as x increases up to x=0.34. In spite of this behavior, superconductivity disappears with Tc decreasing from x=0.15 to x=0.25. This decrease in Tc is shown to be continuous (and not dominated by phase separation). Such decreases in Tc are shown to be a common feature of high temperature superconductors, being observed in YBa2Cu3Oy-like systems as well as in doped Bi 2212 and Tl 1201 compounds.

Determination of the average ionic radius and effective valence of Ce in the Nd2−xCexCuO4 electron superconductor system by X-ray diffraction

Abstract The X-ray powder diffraction technique has been used to determine the average ionic radius of Cen+ present in the Nd2−xCexCuO4 electron superconductor system. The effect of the ionic size of the dopant M on the c-lattice dimension of a lanthanide-doped Nd2−xMxCuO4 system has been determined from a similar Nd2−xThxCuO4 electron superconductor system in which the ionic radius of the Th4+ ions is known. Values of the c-lattice dimension obtained from the experimental data of Nd2−xCexCuO4 (x=0.0−0.20) samples are within the upper and lower bounds of the c-lattice dimensional calculated assuming pure Ce3+ ( r=1.143 A ) and pure Ce4+ (0.97 A) ions, indicating an intermediate ionic radius and hence an effective valence between +3 and +4. Values of the average ionic radius and the effective valence obtained from our Nd2−xCexCuO4 samples are 0.998 A and +3.84, respectively.

Search for superconductivity in analogues of La2−xSrxCuO4: Sr2−xLnxVO4 (Ln=La, Ce, Pr, Nd, Eu)

Abstract We report the preparation, unit cell parameters, and electrical conductivity of ceramic samples of Sr 2− x Ln x VO 4 , in which there are formally (1+ x ) d-electrons per transition metal, in analogy with the (1+ x ) d-holes in the high temperature superconducting system La 2− x Sr x CuO 4 . For Ln=La, Ce, Nd and Eu, we find a wide range of solid solution for 0.4≤ x ≤1.. However, a complete solid solution for 0≤ x ≤1 is found only in the case of Pr. All compounds are found to be semiconducting below room temperature, with no evidence of superconductivity.

X-ray powder diffraction analysis of the electron superconductor Nd2−xThxCuO4 system

Abstract The polycrystalline structure of the superconducting Nd 2−x Th x CuO 4 system is studied using the x-ray diffraction technique. High quality x-ray powder diffraction data are obtained, and analysis shows that the Nd 2−x Th x CuO 4 compounds synthesized by solid state reaction are well-crystallized and have a body-centered tetragonal T′-structure. A minor amount of Cu 2 O is detected suggesting a possible Cu deficiency in the x≥0.10 compounds. A significant amount (in the 10% range) of unreacted ThO 2 is also found in the x=0.25 sample which confirms the Th solubility limit in Nd 2−x Th x CuO 4 is 0.20. New x-ray powder diffraction data with excellent figures of merit of F 30 >210 and M 20 >260 of the superconductor Nd 1.85 Th 0.15 CuO 4 and its parent compound Nd 2 CuO 4 are reported. Values of the lattice dimensions with an estimated precision of 2.5×10 −5 by least-squares refinements from 32 to 33 reflections in the front reflection region are obtained. Decreases in the c lattice dimension and the c/a ratio with increasing x are caused by the substitution of smaller Th 4+ ions for larger Nd 3+ ions.