Takashi Onozuka

Displacement Waves in La2CuO4-δ and La1.85Sr0.15CuO4-δ

Structural investigation of orthorhombic La2CuO4-δ and La1.85Sr0.15CuO4-δ was carried out by means of X-ray and neutron diffraction on the basis of the space group Cmmm. The periodic expansion/contraction type distortion of CuO6 octahedra was found in both orthorhombic compounds. The distortion is nearly one-dimensional in La2CuO4-δ but is two-dimensional in La1.85Sr0.15CuO4-δ. The existence of a charge-density wave, CDW, with a wave vector qCDW=(0 0 c*) is highly possible in the structures.

Crystal Structure of Tetragonal Form of La2NiO4+x

The crystal structure of the title oxide was studied by means of the X-ray and neutron single crystal diffraction measurements. At room temperature, the tetragonal crystal structure is P4 2 /ncm-type (No. 138), which is one of the subgroup of the space group I4/mmm. The lattice parameters of a sample annealed and slowly cooled in oxygen atmosphere from 673 K are a = b =5.4640(1) A and c =12.6719(2) A, while the oxygen content, x =0.10(4), was determined from obtained neutron data. The title oxide undergoes a tetragonal (P4 2 /ncm)/tetragonal (I4/mmm) phase transition at about 560 K. The transition temperature is almost identical both in the annealed and as-grown crystals.

High-Resolution Electron Microscopy and Electron Diffraction Study of an Incommensurate Bi1.8Sr1.5Ca1.5Cu2.2Ox Superconductor

Weak contrast modulations with a long period of about 15 nm along [010] are observed in high-resolution lattice images of a Bi1.8Sr1.5Ca1.5Cu2.2Ox superconductor. Contrast modulations with short periods, 4.5bo/2 (1.2 nm) and 5bo/2 (1.3 nm), along [010] are reproduced with a multislice computer simulation. Those contrast modulations as well as the intensities of incommensurate superlattice reflections are interpreted by a long period superlattice structure model consisting of alternative alignments of domains with the periods of 4.5bo and 5bo along [010].

A low-temperature electron microscopy and electron diffraction study of La1.84Sr0.16CuO4

A high-Tc superconducting compound, La1.84Sr0.16CuO4, has been investigated by electron microscopy and electron diffraction in the range from 10 K to ambient temperature. The tetragonal K2NiF4-type structure undergoes an orthorhombic distortion below about 130 K. In the low-temperature phase, extra diffraction spots and twin lamellae are observed reversibly on cooling and heating in situ. Based on the observed results, a plausible structure model with orthorhombic distortion is proposed.

Changes in modulation period of Bi2Sr2 (Ca1−xNdx) Cu2Oa + δ and their relations to oxygen content

Abstract Structural changes of long-period modulated structure (LPMS) of the Bi 2 Sr 2 (Ca 1− x Nd x ) Cu 2 O a + δ ceramics with x are investigated by means of electron diffraction. Increasing x, the modulation mode of the mixing of domains of two modulation periods with b=4.5b o and b=5b o changes to that with b=4.5b o and b=4b o through the modulation mode with only b=4.5b o at x=0.4−0.5. The wavenumber of the superlattice reflection shows a stepwise increase rather than a linear increase with x. Excess oxygen in a LPMS model with the site of excess oxygen is shown to be consistent with excess oxygen analyzed chemically by iodometry and its change with x.

Symmetrical staircase in the profile of lattice-modulation period versus Pr-concentration in Bi2Sr2(Ca1-xPrx)Cu2O8+δ

Variation of the lattice-modulation period p a of the “incommensurate” phase Bi 2 Br 2 (Ca 1- x Pr x )Cu 2 O 8+δ (0≦ x ≦0.72) with Pr-concentration x was examined by electron diffraction. The plot of p a versus x exhibits roughly a linear decrease but it has a staircase structure. The period of each stair, p a , agrees with a value expected for a commensurate structure. We observed nine stairs which are symmetrically arranged; the central stair corresponds to the primary structure with a simple modulation, and a stair on the left (or right) corresponds to a structure which is derived from the primary structure by introducing an advanced (or delayed) discommensuration. The observed structures cover a full range of commensurate structures predicted theoretically.

Incommensurate-Commensurate Transition in 2H-Tase2

Incommensurate-commensurate transition which occurs in 2H-TaSe2 is investigated in detail by means of high resolution transmission electron microscopy. The double honeycomb discommensuration model so far accepted for the incommensurate phase of 2H-TaSe2 is not found to be appropriate. There is a change in the modulation mode in the incommensurate phase and the commensuration process is found to occur in the lower temperature range. The commensurate process takes place as the motion of the jellyfish patterns with three legs. The change in the modulation mode also explains the change in the incommensurability in two steps.