George H. Kwei

Neutron Diffraction Evidence of Microscopic Charge Inhomogeneities in the CuO{sub 2} Plane of Superconducting La{sub 2-x}Sr{sub x}CuO{sub 4} (0 {<=} x {<=} 0.30 )

High-resolution atomic pair distribution functions have been obtained using neutron powder diffraction data from La2-xSrxCuO4 over the range of doping 0<or=x<or=0.30 at 10 K. Despite the average structure getting less orthorhombic, we see a broadening of the in-plane Cu-O bond distribution as a function of doping up to optimal doping. Thereafter the peak abruptly sharpens. The peak broadening can be well explained by a local microscopic coexistence of doped and undoped material. This suggests a crossover from a charge inhomogeneous state at and below optimal doping to a homogeneous charge state above optimal doping.

Charge-stripe ordering from local octahedral tilts: Underdoped and superconducting La{sub 2{minus}x}Sr{sub x}CuO{sub 4}thinsp(0{le}x{le}0.30)

The local structure of La2-xSrxCuO4, for 0<x<0.30, has been investigated using the atomic pair distribution function (PDF) analysis of neutron powder diffraction data. The local octahedral tilts are studied to look for evidence of [110] symmetry (i.e., LTT-symmetry) tilts locally, even though the average tilts have [010] symmetry (i.e., LTO-symmetry) in these compounds. We argue that this observation would suggest the presence of local charge-stripe order. We show that the tilts are locally LTO in the undoped phase, in agreement with the average crystal structure. At non-zero doping the PDF data are consistent with the presence of local tilt disorder in the form of a mixture of LTO and LTT local tilt directions and a distribution of local tilt magnitudes. We present topological tilt models which qualitatively explain the origin of tilt disorder in the presence of charge stripes and show that the PDF data are well explained by such a mixture of locally small and large amplitude tilts.

Crossed beam studies of the dynamics of electronic energy transfer: Quenching of Na(3p 2P3/2) atoms by N2, O2, CO, and NO molecules

The dynamics of quenching of electronically excited Na atoms by the diatomic molecules N2, O2, CO, and NO has been studied using crossed molecular beam techniques. Distributions in both laboratory scattering angle and recoil velocity provide information on the quenching process and the disposal of energy. For N2, O2, and CO, we observe a direct process which occurs in less than one rotational period of the collision system; all the molecules are scattered forward, along their original direction. For N2 and CO, an average of 30%–40% of the available energy is transferred to internal energy of the diatomic molecules. A comparison with data from infrared absorption studies of the Na*–CO system shows that very little of this internal energy is taken up by rotational modes of the CO molecule. For O2, a greater fraction of the electronic energy is partitioned into internal modes but the details of this partitioning are further complicated by the accessibility of the low lying O2 excited electronic states. For N...

Neutron powder diffraction refinement of boron carbides nature of intericosahedral chains

Abstract Our Rietveld structure refinements of neutron powder diffraction data of boron carbides with 10, 13, 16, and 20 at.% carbon indicate 15–25% vacancies at the central position of the intericosahedral chains. In addition, we find a comparable number of interstitials residing off the intericosahedral chains. However, X-ray diffraction studies on unirradiated boron carbide single crystals do not generally find these defects. This discrepancy may indicate that the observed defects are the remnants of defect cascades created by the energetic fission products of 10 B during the neutron diffraction measurements. The tendency to form vacancies in the center of the intericosahedral chain is consistent with the weak and soft binding of chain-center atoms that we previously inferred from other measurements.

Structure and valence from complementary anomalous X-ray and neutron powder diffraction

Abstract We introduce a new type of crystallography whose major aim is to study the crystal chemistry and the electronic structure of constituent atoms or ions in crystals, rather than the crystal structure itself. These results are obtained from recent experiments that take advantage of the complementary nature of anomalous X-ray and neutron diffraction data, and of the fact that anomalous scattering can provide substantial changes in the scattering factors for atoms or ions with similar atomic number Z or with the same Z but different valence. Examples presented include: the determination of constituent ordering among several sites in Fe/Co/V alloys and in certain high temperature superconductors; the study of ordering that arises via valence fluctuation in metallic cerium; the determination of anomalous X-ray scattering factor variation for copper with oxidation state; and finally, the determination of site-specific valence of the Cu ions in the chain and plane sites in the high temperature superconductor YBa 2 Cu 3 O 6+ x as a function of oxygen stoichiometry.

Local lattice disorder in the geometrically frustrated spin-glass pyrochlore Y 2 Mo 2 O 7

The geometrically frustrated spin-glass Y{sub 2}Mo{sub 2}O{sub 7} has been widely considered to be crystallographically ordered with a unique nearest-neighbor magnetic exchange interaction, J. To test this assertion, we present x-ray-absorption fine-structure results for the Mo and Y K edges as a function of temperature and compare them to results from a well-ordered pyrochlore, Tl{sub 2}Mn{sub 2}O{sub 7}. We find that the Mo-Mo pair distances are significantly disordered at approximately right angles to the Y-Mo pairs. These results strongly suggest that lattice disorder nucleates the spin-glass phase in this material. (c) 2000 The American Physical Society.

Tl0.5Pb0.5Sr2CuO5: a nonsuperconductor despite an “optimum” hole concentration in the Cu-O sheets

Abstract Doping of La 3+ for Ba 2+ in TlBa 2 CuO 5 leads to superconductivity in the material TlBa 1.2 La 0.8 CuO 5 (with T ≈52 K). An increase in the Cu-O in-plane bond distance [1.9240(1) A] from that in TlBa 2 CuO 5 (1.915 A) is consistent with a lowering of the average hole concentration, p , in the Cu-O planes from 1.0 to approximately +0.4. We have doped Pb 4+ for Tl 3+ in TlSr 2 CuO 5 to give an average hole concentration of +0.53, but find that Tl 0.5 Pb 0.5 Sr 2 CuO 5 is not superconducting down to 5 K. The structure is similar to TlBa 2 CuO 5 and TlSr 2 CuO 5 , belonging to the space group P4/mmm. Structural refinement using neutron powder diffraction data collected at 300 K show that the planar lattice constant [ a =3.7309(3) A] is considerably smaller than that for TlBa 1.2 La 0.8 CuO 5 . This leads to the substantially smaller Cu-O in-plane bond distance of 1.8654(15) A. The atoms in the Tl/Pb-O(3) planes are again disordered, and both the cation and the oxygen site occupancies are close to stoichiometry. This stoichiometry and the Tl/Pb oxidation states suggest that the hole concentration is close to the optimum value for these materials. Several reasons for the absence of superconductivity are discussed.

Ne intercalated C 60 : Diffusion kinetics

{ital In situ} neutron powder-diffraction has been used to probe the Ne content and diffusion kinetics in C{sub 60} by monitoring the C{sub 60} lattice parameter vs time following step changes in applied Ne pressure. In the face-centered-cubic phase where the C{sub 60} molecules are freely rotating, diffusion is slower for higher Ne pressure. In the primitive cubic phase where the C{sub 60} molecules are orientationally ordered, the diffusion rate is slower and is essentially independent of pressure. These observations can be understood in terms of three unequal competing effects: (1) Increasing the external Ne pressure increases the driving force for diffusion; (2) compression of the C{sub 60} lattice slows diffusion with increasing pressure; and, the major one, (3) C{sub 60} molecular dynamics enables and enhances diffusion by a thermally activated {open_quotes}paddle wheel{close_quotes} effect. The activation energy for such Ne intercalated C{sub 60} reorientational hopping or rotation ({approximately}1500 K or 13 kJ/mole) is less than that in pure C{sub 60} ({approximately}2600 K or 22 kJ/mole), suggesting the Ne atoms act as {open_quotes}roller bearings.{close_quotes} {copyright} {ital 1999} {ital The American Physical Society}

The diboride compounds of molybdenum: MoB2−x and Mo2B5−y

Abstract The phase relations of the molybdenum diboride compounds MoB 2− x and Mo 2 B 5− y and the structure of Mo 2 B 5− y have been re-examined with samples prepared between 1400 and 1800°C. Two types of six-membered boron ring, nearly planar and puckered, occur within the Mo 2 B 5− y structure. The sites at the centers of the puckered rings may be unoccupied (K′ layers) or occupied by boron atoms (K layers). Neutron diffraction refinements show that these sites are completely unoccupied (K′ layers) in Mo 2 B 5− y samples of several batch compositions. This result, combined with the insensitivity of the unit cell dimensions of Mo 2 B 5− y to batch composition, shows that this phase has a narrow single phase region, with y close to unity, between 1400 and 1800°C. MoB 2− x also has a narrow single-phase region centered at about 61 at.% boron over this temperature range.

Combined X-ray and neutron powder diffraction study of the structure of La1.8Sr0.2CaCu2O6: Cation ordering and oxygen stoichiometry

Abstract La 1.8 Sr 0.2 CaCu 2 O 6 exhibits superconductivity around T c =60 K after a HIP (Hot Isostatic Pressing) treatment at 1020°C for 50 h under a total pressure of 1000 atm and an oxygen partial pressure of 200 atm. A joint neutron/X-ray powder diffraction study of this superconductor was carried out to analyze both cation ordering and oxygen stoichiometry. This revealed: (1) a preferential occupation of Ca for the 2a site and of both La and Sr for the 4e site, (2) a Cu-O(1) bond length which is the shortest among known 326 cuprates, and (3) that the O(2) site contains ∼3.5% vacancies and the O(3) site is occupied at a level of ∼2.3%. A comparison of the structure of La 1.8 Sr 0.2 CaCu 2 O 6 with those of four nonsuperconducting or weakly superconducting doped 326 cuprates suggests structural differences that may be responsible for the observed differences in behavior.