1. Hannes Krüger 1, Václav Petříček 2, Volker Kahlenberg 1 1 Institute of Mineralogy and Petrography, University of Innsbruck 2 Institute of Physics, Academy of Sciences of the Czech Republic, Prague A phase transition in Ca 2 Fe 2 O 5 with coexistence of a 3d and a (3+1)-dimensional phase

2. Ca 2 Fe 2 O 5 (Srebrodolskite) C 2 F x=0 Imma(00  )s00 above  710°C Ca 2 FeAlO 5 (Brownmillerite) C 4 AF x=0.5 Ca 2 Al 2 O 5 C 2 A x=1 The Ca 2 (Fe 1-x Al x ) 2 O 5 solid solution series x=0.28 x=0.7 Pnma I2mb ambient conditions elevated pressure Taylor (1997) Kahlenberg et al. (2000) Redhammer et al. (2004) Fukuda & Ando (2002) Berastegui et al. (1999) Krüger & Kahlenberg (2005)

3. c a c b Basic building units of brownmillerite- type structures 1) Layers of corner-sharing FeO 6 octahedra Ca 2 Fe 2 O 5 at room conditions. Pnma

4. a c Basic building units of brownmillerite-type structures 2) zweier single chains of distorted FeO 4 tetrahedra Ca 2 Fe 2 O 5 at room conditions. Pnma R L R/L disordered as in Imma

5. Pnma b a L R

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

7. The superspace mirrorplane Imma(00  )s00 glide vector ½ along x 4 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 x 4 Fe2 Fe2' O3' O3 Lambert et al. (2002)

8. x4x4 x4x4 x1x1 x1x1 x 1 -x 4 sections of the 4-dimensional F obs synthesis Fe2, x 2 =0.25, x 3 =0.569 O3, x 2 =0.25, x 3 =0.626

9. L L R L R L R R L R „t-map“ (physical space section) at t=0, x 2 =0.25, calculated from 4-dimensional F obs synthesis

10. DTA measurement Ca 2 Fe 2 O 5, 5K/min, 100 µl Pt-crucible, 95.39mg sample Temperature [°C] Heat Flow [µV]

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

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 Pnmahkl0: h+k+l ≠ 2n Imma(00  )s00hklm: m ≠ 0

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

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:

16. Conclusions Pnma and Imma(00  )s00 phases of Ca 2 Fe 2 O 5 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

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

19. Pnma I2mb b a L R R R

20. 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 R w (all; main; sat):0.055; 0.053; 0.059 Acta Crystallogr. B61, 656-662

21. DTA-Experiment Ca 2 Fe 2 O 5 T1T1 T2T2 2 thermal effects: T 1 =443°C 0.2 J/g Néel temperature Takeda et al. (1968) T 2 =688°C 0.5 J/g structural phase transition Imma or I2mb Berastegui et al. (1999) incommensurate? Redhammer et al. (2004)

22. Experimental setup (single crystal X-ray diffraction) Diffractometer: Stoe IPDS-II, 2 circle Heating method:Heated N 2 -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 R int :0.057