Electron Crystallographic Study of Incommensurate Modulated Structures Fan, Hai-fu Institute of Physics, Chinese Academy of Sciences, Beijing, China

What’s a modulated structure ? Muti-dimensional direct methods of solving modulated structures Incommesurate modulation in Bi-based supercondutors from electron crystallography

What’s a Modulated Structure ? T = 0 (mod t) or MOD (T, t) = 0 Commensurate modulation Þ superstructures T ¹ 0 (mod t) or MOD (T, t) ¹ 0 Incommensurate modulation Þ incommensurate structures T

Schematic diffraction pattern of an incommensurate modulated structure b* q

Conclusion In reciprocal space: The diffraction pattern of an incommen-surate modulated crystal is the projection of a 4- or higher-dimensional weighted lattice In direct space: An incommensurate modulated structure is the “hypersection” of a 4- or higher-dimensional periodic structure cut with the 3-dimensional physical space

Representation of one-dimensionally modulated incommensurate structures Lattice vectors in real- and reciprocal- space

Structure-factor formula Modulated atoms situated at their average positions

Modified Sayre Equations in multi-dimensional space

incommensurate modulated structures Strategy of solving incommensurate modulated structures i) Derive phases of main reflections using ii) Derive phases of satellite reflections using iii) Calculate the multi-dimensional Fourier map iv) Cut the resulting Fourier map with the 3-D ‘hyperplane’ (3-D physical space) v) Parameters of the modulation functions are measured directly on the multi-dimensional Fourier map

Electron Crystallographic Study of Bi-based Superconductors using Multi-dimensional Direct Methods

Why Electrons ? 1. Electrons are better for studying minute and imperfect crystalline samples 2. Electron microscopes are the only instrument that can produce simultaneously EM’s and ED’s for the same crystalline sample at atomic resolution 3. Electrons are better for revealing light atoms in the presence of heavy atoms

Scattering of X-rays and Electrons by Different Elements Bi Sr Ca Cu X-rays Relative scattering power Sinq /l ~ 0.31 Electrons O O

Bi-based Superconductors Bi2Sr2Can-1CunO2n+4+x n = 1 n = 2 n = 3 Bi-2201 Bi-2212 Bi-2223 Bismuth bi-layer Bi-O Bi-O Bi-O Sr-O Cu-O Ca-O c Perovskite layer Sr-O Cu-O Ca-O Sr-O Cu-O Bi-O Bismuth bi-layer Bi-O Bi-O

Electron diffraction analysis of the Bi-2223 superconductor Space group: P [Bbmb] 1 -1 1 a = 5.49, b = 5.41, c = 37.1Å; q = 0.117b* *The average structure is known*

Bi-2223 [100] projected potential Space group: P [Bbmb] 1 -1 1 a = 5.49, b = 5.41, c = 37.1Å; q = 0.117b* RsymM = 0.12 (Nref. =42) RsymS = 0.13 (Nref. = 70) Rm = 0.16 Rs = 0.17

Bi-2223 4-dimensional metal atoms a3 a4 cut at a2 = 0 and projected down the a1 axis Space group: P [Bbmb] 1 -1 1 a = 5.49, b = 5.41, c = 37.1Å; q = 0.117b* a1 = a, a2 = b - 0.117d, a3 = c, a4 = d

Image Processing of Bi-2212 EM image from Dr. S. Horiuchi Space group: N [Bbmb] 1 -1 1 a = 5.42, b = 5.44, c = 30.5Å; q = 0.22b* + c* FT FT-1 Phase extension

Image Processing of Bi-2212 (continued) 8 4 1 Bi Sr Cu Ca Original image Enhanced image c Oxygen in Cu-O layer b

O atoms on the Cu-O layer Electron diffraction analysis of Bi-2201 Space group: P[B 2/b] -1]; a = 5.41, b = 5.43, c = 24.6Å, b = 90o; q = 0.217b* + 0.62c* O atoms on the Cu-O layer RT = 0.32 Rm = 0.29 RS1 = 0.29 RS2 = 0.36 RS3 = 0.52 Bi-O Sr-O Cu-O b c O (extra)

Experimental B and M Bi-2201 Influence of thermal motion (B) and Modulation (M) to the dynamical diffraction B set to zero B,M set to zero M set to zero

Bi-2201 The effect of sample thickness ~300Å ~200Å ~5Å ~100Å Oxygen in Cu-O layer Bi-O Sr-O Cu-O Extra oxygen