H. Shaked

Pressure-induced structural changes in superconducting HgBa2Can−1CunO2n+2+δ (n = 1, 2, 3) compounds

Abstract The crystal structures of superconducting HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ have been investigated with a pressure up to 0.6 GPa and HgBa 2 Ca 2 Cu 3 O 8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c -axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a -axis. The one-layer compound, HgBa 2 CuO 4+δ , shows the largest a -axis compressibility, while HgBa 2 Ca 2 Cu 3 O 4+δ shows the smallest compressibility. The bond compressibilities of HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ are significantly different from HgBa 2 Ca 2 Cu 3 O 8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

Oxygen vacancy ordering and superconductivity in YBa2Cu3O7−x

Abstract The superconducting properties of YBa 2 Cu 3 O 7−x have been shown to depend markedly on the overall oxygen stoichiometry and on the ordering of oxygen vacancies on the available sites. YBa 2 Cu 3 O 7−x has been studied in situ by high-temperature neutron powder diffraction in controlled oxygen atmospheres in order to learn the structural properties of the system in thermodynamic equilibrium. Additionally, metastable, oxygen-deficient samples produced by quenching into liquid nitrogen from high temperature and various oxygen partial pressures have been studied at room temperature or low temperature in order to correlate the structural and superconducting properties. Such experiments have shown that a systematic relationship exists between the superconducting transition temperature, T c , the number of oxygen vacancies, and the degree of oxygen vacancy ordering. This brief paper reviews our work in these areas.

The magnetic structure of Bi2Fe4O9– analysis of neutron diffraction measurements

The compound Bi2Fe4O9 belongs to the space group Pbam ( D92h), with two formula units per unit cell. Neutron diffraction measurements showed that it is paramagnetic at room temperature and undergoes a transition to an antiferromagnetic state at TN = (264 ± 3) K in agreement with previous susceptibility and Mossbauer measurements. Analysis of the 80 K neutron diffraction pattern yielded a magnetic structure with the following features: (a) The basic translations ao, bo, co of the chemical lattice change into antitranslations in the magnetic lattice. (b) The spins are perpendicular to co. (c) The magnetic structure belongs to the PC2/m space group and is a basis vector to an irreducible space under the Pbam irreducible representations, in accord with Landaus theory of second-order phase transition. The position parameters of the Fe3+ ions in the unit cell were refined. The magnetic moment of the compound was found to be (4.95 ± 0.08) μB, compared with the value of 5 μB for the Fe3+ free ion. The temperature dependence of the { 131 } magnetic reflection peak intensity was measured and found to be in agreement with the sublattice magnetization predicted by the molecular field approximation.

Interstitial site occupation in ZrNiH

In situ X-ray diffraction measurements during stepwise hydrogenation of the intermetallic compound ZrNi confirmed the existence of a stable compound ZrNiH. Neutron diffraction measurements on ZrNiD allowed a determination of the site occupied by deuterium. This site is tetrahedrally coordinated by four zirconium atoms as predicted earlier from a model based solely on geometric considerations. Occupation of this site causes the lattice to distort from an orthorhombic to a triclinic structure. The trihydride is formed when two completely different types of sites are filled, to the exclusion of the sites occupied originally, causing reversion of the lattice to an orthorhombic structure. Other confirmed predictions of the geometric model are discussed.

Scaling of transition temperature and CuO2 plane buckling in a high-temperature superconductor

A characteristic feature of the high-temperature superconductors is the existence of a chemical composition that gives a maximum transition temperature, Tc, separating the so-called under-doped and over-doped regimes, . This behaviour is thought to be universal for high-temperature superconductors. In practice, there are only a few high- Tc compounds for which the composition can be varied continuously throughout the entire doping range. Here we report a study of correlations between structure and Tc in a compound with the ‘123’ structure in which both the under-doped and over-doped regimes can be accessed. We observe a clear scaling between Tc and the buckling of the copper oxide planes; both go through a maximum at the same oxygen composition (and hence doping level), so implying a common origin. Previous work has shown that, for a fixed chemical composition, increased CuO2 plane buckling lowers the transition temperature. Thus the observation of a maximum in the buckling at the maximum T c indicates that, as the composition is changed to increase T c, there is a structural response that competes with superconductivity.

Structural study of Sr2CuO3+δ by neutron powder diffraction

Abstract Average crystal structures of superconducting Sr 2 CuO3+ δ synthesized under high pressure and nonsuperconducting Sr 2 CuO3+ δ synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen-deficient La 2 CuO 4 -type tetragonal T structure with oxygen vacancies located in the CuO 2 planes, not in the Sr 2 O 2 layers. This result raises important questions regarding the nature of superconductivity in Sr 2 CuO3+ δ reported to be a 70 K superconductor.

Magnetic ordering in the superconducting weak ferromagnets RuSr 2 GdCu 2 O 8 and RuSr 2 EuCu 2 O 8

Neutron powder-diffraction measurements for RuSr{sub 2}GdCu{sub 2}O{sub 8} and RuSr{sub 2}EuCu{sub 2}O{sub 8} show that both compounds, below their magnetic ordering temperatures of 133 and 120 K, respectively, have the same G-type antiferromagnetic structure in which Ru moments are antiparallel in all three crystallographic directions. The ferromagnetism in these compounds, which is clearly indicated by hysteresis loops, can be explained if the Ru moments are canted to give a net moment perpendicular to the c axis. In such a model, one would expect an induced ferromagnetic ordering of the magnetic Gd ions, which have large ({approx}7{mu}{sub B}) moments, as a result of the net field at the Gd site, while Eu, being nonmagnetic, would exhibit no such response. The dramatically larger hysteresis loops for RuSr{sub 2}GdCu{sub 2}O{sub 8} compared to those for RuSr{sub 2}EuCu{sub 2}O{sub 8} are consistent with this hypothesis.

The site preference of Cu substituted Zn and Ni in YBa2Cu3O7−γ

Abstract Neutron diffraction measurements were performed at room temperature on samples of YBa2(Cu1−xMx)3O7−γ (M = Zn, Ni) with x = 0.05. The commonly used full pattern analysis is found to be insensitive to whether M substitutes for Cu in the chains or in the planes. The few neutron reflections which have the higher sensitivity to the M position have been studied. It is found that both Zn and Ni substitutes for Cu in the planes. A discussion, which also includes Fe and Co substitutions, on the relation of this result to the structural transport, and magnetic properties of these mixed phases is presented.

Neutron diffraction study of Ce2O3

Abstract A powder sample of Ce 2 O 3 was investigated by neutron diffraction and magnetic susceptibility measurements. The position parameters were determined and found to be similar to those in the isostructural La 2 O 3 . The effective magnetic moment was found to be 2.17 μ B /molec. No evidence was found for the existence of a magnetic or structural phase transition in connection with a specific heat anomaly previously found at 8.5 K.

Neutron diffraction study of dicalcium ferrite

Abstract Neutron diffraction was carried out on Ca 2 Fe 2 O 5 at room temperature. The results are consistent with a magnetic cell equal to the chemical cell and the magnetic space group P c m‘n’.