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Structural Heterogeneity in Twinned Yb 2-x (Fe,Ga) 17+2x Polytypes

Structural Heterogeneity in Twinned Yb 2-x (Fe,Ga) 17+2x Polytypes. Yaroslav Filinchuk. L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference. H. Birkedal 2,3 M. Hostettler 2. Y. Filinchuk 1 R. Cerny 1 K. Yvon 1. O. Bodak 4 T. Yanson 4.

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Structural Heterogeneity in Twinned Yb 2-x (Fe,Ga) 17+2x Polytypes

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  1. Structural Heterogeneity inTwinned Yb2-x(Fe,Ga)17+2x Polytypes Yaroslav Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  2. H. Birkedal 2,3M. Hostettler 2 Y. Filinchuk 1R. Cerny 1 K. Yvon 1 O. Bodak 4T. Yanson 4 1 Laboratoryof Crystallography, Universityof Geneva 2 Instituteof Crystallography, Universityof Lausanne 3 Department of Chem. and Biochem.,Univer. of California 4 Department of Inorganic Chemistry, University of L'viv Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  3. Yb12Fe64Ga24 alloy • Is of interest as a ferromagnetic material • Is a substitution derivative of R2Fe17 str. types (R = Rare earth) in which R atoms are partially substituted byFe2dumbbells, Fe atoms - by Ga atoms, respectively. • According to previous work Yb-Fe-Ga alloys crystallize with the hexagonalLuFe9.5 type (1, P63/mmc) and/or rhombohedral PrFe7 type (2, R-3m)structure. • 1 and 2 (AB and ABC stacking sequences)can be considered as polytypes with maximum degree of order(MDO polytypes). They have the cell parameter relationships a(1) = a(2) and 3c(1) = 2c(2). Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  4. ordered ordered H R disordered disordered Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  5. In this work we show that both polytypes canoccurwithin a coherently twinned crystal abplane of «large» hex cell Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  6. Crystals of the cast sample showed peculiar non-space group absences and cell metrics a(1) = a(2) = 8.615 and 3c(1) = 2c(2) = 25.215 A • Diffraction patterns: image plate detector synchrotron radiation (SNBL at ESRF) Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  7. Modelled by superposing three sorts of domains: one hexagonal (1)and two rhombohedral (2, 3 - twinned by reticular merohedry). Interference terms arising from possible stacking-faults were neglected. • Structure refinement was performed by a newly developed computerprogram [1] that allows to refine several structures on a single data set. [1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach, Bulletin of the Czech and Slovak CrystallographicAssociation. Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17). [1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach, Bulletin of the Czech and Slovak CrystallographicAssociation. Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17). Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  8. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  9. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  10. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  11. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  12. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  13. STRUCTURAL HETEROGENEITY IN A SINGLE CRYSTAL • 1 and 2(3) displayed different structure features: • 1 showed a higher rate of Yb substitution by Fe2 dumbbells than 2(3); • 2(3) showed partial substitution of Fe by Ga. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  14. Two-phase region as a single crystal !!! • Consequences • Twinned polytype (called “syntactic coalescence” by polytype scientists) is a link between two classical formalisms: • description of two-phase region composed of related structures by a simple mixture of two phases (often impossible to resolve them by X-rays…) • description of two-phase region composed of related structures by a single phase (solid solution) with variable stacking sequences (“just stacking”…) Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  15. Diffuse intensity • its amount in the streaks along c* enabled us to find out the average domain size along c axis • evaluated qualitatively at the level of a few tens of unit cell dimensions Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  16. Yb-Fe-Al system A crystal from an Yb12Fe65Al23 alloy has the same structuralfeatures but shows an extra scattering (H´), with diffuse streaks in the basal plane. It suggests short-range order in the basal plane. Diffuse streaks forthe reflections from R (2 and 3) domains indicate frequent stacking faults along c. Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  17. Yb-Fe-Al system Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  18. Yb-Fe-Al system Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  19. frequent stacking faults along c Yb-Fe-Al system local R-atom ordering in the basal planedifferent from that observed in Th2Ni17 and Th2Zn17 type structures Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  20. CrystSun1.unige.ch • Related topics: • What is a real TbCu7 structure? Isn’t it a highly disorderedstacking of the layers that have a strong tendency to orderingin the basal plane? • D.S. Yufit et al. (2002). Acta Cryst., B58, 673-676 - two polytypes of organic substance in a coherently twinned crystal • H. Katzke. (2002). Z. Kristallogr., 217, 127-130 - variations of peak profiles for 2H&3R twinned polytypes of NbS2 • D.L. Smith et al. (1981). Acta Cryst., B37, 1807-1812 - 2H&3R coherently twinned polytypes of CuNCS were refined Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

  21. Another story – how to estimate the average domain size along c-direction? Calculated X-ray diffraction pattern as a function of the stacking fault probability P NbS2 Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

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