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Hannes Krüger 1 , V á clav Pet říč ek 2 , Volker Kahlenberg 1

A phase transition in Ca 2 Fe 2 O 5 with coexistence of a 3d and a (3+1)-dimensional phase. Hannes Krüger 1 , V á clav Pet říč ek 2 , Volker Kahlenberg 1. 1 Institute of Mineralogy and Petrography, University of Innsbruck

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Hannes Krüger 1 , V á clav Pet říč ek 2 , Volker Kahlenberg 1

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  1. A phase transition in Ca2Fe2O5 with coexistence of a 3d and a (3+1)-dimensional phase Hannes Krüger1, Václav Petříček2, Volker Kahlenberg1 1Institute of Mineralogy and Petrography, University of Innsbruck 2Institute of Physics, Academy of Sciences of the Czech Republic, Prague

  2. The Ca2(Fe1-xAlx)2O5 solid solution series ambient conditions elevated pressure x=0.28 Ca2Fe2O5 (Srebrodolskite) C2F x=0 Imma(00g)s00 above 710°C Ca2FeAlO5 (Brownmillerite) C4AF x=0.5 Taylor (1997) Kahlenberg et al. (2000) Redhammer et al. (2004) Fukuda & Ando (2002) Berastegui et al. (1999) Krüger & Kahlenberg (2005) Ca2Al2O5 C2A x=1 x=0.7 Pnma I2mb

  3. Basic building units of brownmillerite-type structures 1) Layers of corner-sharing FeO6 octahedra Ca2Fe2O5 at room conditions. Pnma a c c b

  4. a R c Basic building units of brownmillerite-type structures 2) zweier single chains of distorted FeO4 tetrahedra Ca2Fe2O5 at room conditions. Pnma L R/L disordered as in Imma

  5. Pnma L R b a

  6. c* (1kl) 827(5)°C Main Reflections indexed with a=5.493(1), b=15.037(3), c=5.651(1)Å Space group: Imma b* c* (0kl) b* Satellite reflections indexed with q=(0,0,0.588(2)) (3+1)-dimensional Superspace group: Imma(00)s00

  7. Fe2 Fe2' O3 O3' The superspace mirrorplane Imma(00)s00 glide vector ½ along x4 Positions of O3-O3' and Fe2-Fe2' should not be occupied in any physical space section of the (3+1)-dimensional model Fe2 and O3 have to be defined on ½ of the period in x4 Lambert et al. (2002)

  8. x1-x4 sections of the 4-dimensional Fobs synthesis Fe2, x2=0.25, x3=0.569 O3, x2=0.25, x3=0.626 x4 x4 x1 x1

  9. „t-map“ (physical space section) at t=0, x2=0.25, calculated from 4-dimensional Fobs synthesis L L R L R L R R L R

  10. DTA measurement Ca2Fe2O5, 5K/min, 100 µl Pt-crucible, 95.39mg sample Heat Flow [µV] Temperature [°C]

  11. Reconstructed reciprocal space, measured at 697(5)°C Ca2Fe2O5 Reflections h+k+l ≠ 2n and Satellites are present! c* (1kl) This is observed for a temperature range of (~20K) b*

  12. Facts • PnmaPnma + Imma(00)s00Imma(00)s00 • only one thermal effect in DTA • can be indexed with P cell (continuous with t) + 1 q-vector • temperature gradient in sample <7K • all attempts in primitive superspace groups failed Exclusive sets of Reflections Pnma hkl0: h+k+l ≠ 2n Imma(00)s00 hklm: m ≠ 0

  13. 688°C 709°C average I/(I) of 182 satellite reflections, (hklm,m≠0):h+k+l=2n average I/(I) of 113 P-reflections, (hkl0):h+k+l≠2n

  14. Refinement in Jana2000 with a „two-phase model“ Integration: I-centred lattice, with q=(0,0,) + hkl0: h+k+l ≠ 2n multiphase refinement (via twinning option) both phases are required to have the same number of dimensions calculation of combined structure factor:

  15. Conclusions Pnma and Imma(00)s00 phases of Ca2Fe2O5 are coexisting in a range of 20K This can be handled as a special case of multiphase refinement in Jana2000 To do: experiments at higher temperatures (up to 1400°C) Satellite darkfield HT TEM Substitution Fe/Al

  16. Acknowledgements Waltraud Wertl Eduardo Hernandez Bocanegra Karen Friese Juan Manuel Perez-Mato Michael Czank Thank you for your attention

  17. Pnma L R R R b I2mb a

  18. Refinement 2 Crenel-functions, occupation of Fe2, O3 (2 parameters) Harmonic modulation functions (positional modulation): Fe1 in x (1 par.) Ca in x (2 par.) O1 in x, y (3 par.) O2 in x (2 par.) Equivalent reflections used: 934 (360 main, 574 satellites) Parameters refined: 43 Rw (all; main; sat): 0.055; 0.053; 0.059 Acta Crystallogr.B61, 656-662

  19. DTA-Experiment Ca2Fe2O5 2 thermal effects: T1=443°C 0.2 J/g Néel temperature Takeda et al. (1968) T2=688°C 0.5 J/g structural phase transition Imma or I2mb Berastegui et al. (1999) incommensurate? Redhammer et al. (2004) T2 T1

  20. Experimental setup (single crystal X-ray diffraction) Diffractometer: Stoe IPDS-II, 2 circle Heating method: Heated N2-stream Temperature: 827(5)°C Crystal size: 0.18x0.18x0.06 [mm] Absorption Correction: integration, using 8 indexed faces Radiation: Mo K max(°): 29.31 Measured Reflections Main: 2208 Satellite: 3932 Rint: 0.057

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