T.C. Huang

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 Niue5f8O 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.

X-ray depth profiling of iron oxide thin films

A nominally γ-Fe 2 O 3 thin film (oxidized from an Fe 3 O 4 film) has been structurally depth profiled using x-ray scattering in a grazing incidence, asymmetric Bragg geometry. By varying the grazing incidence angle, the x-ray penetration depth is varied from tens of Angstroms to several thousand Angstroms, slightly larger than the film thickness. At small incidence angles a diffraction pattern characteristic of α-Fe 2 O 3 is observed, while at larger angles the pattern is predominantly from γ-Fe 2 O 3 , showing that there is a surface layer of α-Fe 2 O 3 . These results are quantified and the thickness of the α phase found to be 90 A. The presence of the α phase explains a nonferromagnetic layer observed previously. These data together with magnetic and chemical data suggest that the nonferromagnetic layer forms during oxidation of the Fe 3 O 4 film due to outward diffusion of Fe ions and their subsequent oxidation to form α-Fe 2 O 3

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 .

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.

High-temperature crystal structures of orthorhombic and rhombohedral PrNiO3

Abstract Crystal structures of orthorhombic and rhombohedral PrNiO3 have been studied by high-temperature X-ray powder diffraction. In the orthorhombic PrNiO3 (space group Pbnm) Ni-O octahedra are slightly distorted, and the O-Ni-O bond angles are close to ±3.4° of the 90° angle of an undistorted octahedron. The octahedral tilt is large and decreases from 22.2° to 20.5° when the temperature increases from room temperature to 400°C. The coordination number of Pr is less than 12 and may effectively be only 8. The atoms move from orthorhombic positions toward the so-called ‘ideal cubic perovskite positions’ with increasing temperature. However, they move in such a way that the structure assumes a rhombohedral symmetry when the temperature reaches 500°C. In the rhombohedral PrNiO3 (space group R3c) the Ni-O octahedra are trigonally distorted, and the Pr atoms have a coordination number 12. The rotation of octahedra necessary to produce the rhombohedral structure decreases slightly from 11.38° to 11.00° when the temperature increases from 500° to 600°C.

Effects of deposition conditions on the superconducting properties of r.f. and d.c. magnetron sputter-deposited YBa2Cu3O7−x films

Abstract Thin films of YBa 2 Cu 3 O 7− x were r.f. and d.c. magnetron sputter deposited onto silicon, Al 2 O 3 , MgO, BaTiO 3 and SrTiO 3 substrates at up to 700°C from a stoichiometric oxide target in an argon and O 2 mixture. Stoichiometric films were obtained using a substrate-to-target configuration so that bombardment of the growing film by energetic particles is minimized. These films show an ending T c of about 87 K when deposited on SrTiO 3 (100) at 300°C and after annealing in flowing O 2 at 900°C for about 1 min, or an ending T c of 76 K when deposited on Si(100) at 650°C without further high temperature treatments. The effects of substrate, deposition temperature, and substrate bias on the superconducting properties of these films are presented.

Analysis of cobalt-doped iron oxide thin films by synchrotron radiation☆

Abstract Surface and bulk structures of as-deposited Fe 3 O 4 and post-oxidized γ-Fe 2 O 3 thin films were successfully identified by synchrotron diffraction analysis. A new synchrotron diffraction method which combines the Seemann-Bohlin and the grazing incidence techniques was used to record polycrystalline diffraction patterns for structure depth profiling analysis. The post-oxidized film was found to have an α-Fe 2 O 3 layer at the surface and γ-Fe 2 O 3 in the bulk of the film. The formation of a thermodynamically stable antiferromagnetic α-Fe 2 O 3 surface caused the magnetically dead layer previously detected by neutron reflection analysis. Synchrotron diffraction results also showed that the structure of the as-deposited film remains constant throughout the thickness of the film. The presence of a superlattice (300) peak indicates the film has been oxidized beyond Fe 3 O 4 .

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

Abstract The annealing characteristics and the superconducting properties of Tl 2 Ca 2 Ba 2 Cu 3 O 10 thin films sputter-deposited onto yttrium- stabilized ZrO 2 substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T c achieved tends to degrade with increasing substrate temperatures, a zero resistance T c of 121 and ≈90 K, respectively, being observed for the films deposited at ∼-ambient temperature and at 500°C. The formation of the highest T c phase (Tl 2 Ca 2 Ba 2 Cu 3 O 10 ) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T c phases such as Tl 2 Ca 1 Ba 2 Cu 2 O 8 .

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.

Characterization of HfBx films deposited by r.f. diode and r.f. magnetron sputtering

Abstract The electrical resistivity, microstructure and X-ray photoemission spectra of approximately 1000 A thick HfB x films deposited by r.f. diode and r.f. magnetron sputtering are reported. The resistivity of these films was found to depend strongly on the oxygen impurity but not on the boron concentration in the film. An increase in the x value from 1 to 5 was shown to increase the film resistivity by less than a factor of 2, but an increase in the oxygen impurity concentration from 0.3% to 0.6% was found to increase the film resistivity by nearly a factor of 10. The oxygen impurity concentration in the film was estimated from Auger electron spectroscopy and secondary ion mass spectrometry depth profiles of films deposited with various oxygen partial pressures. The presence of HfB 2 in the film was confirmed using X-ray diffraction and selected area electron diffraction techniques. The binding energies of Hf4f 7 2 and B 1 s transitions for HfB 2 were found to be 14.4 ± 0.2eV and 187 ± 0.2eV respectively.