Phase transition in Ca2Fe2O5 Pnma Imma00gs00

1
A phase transition in Ca2Fe2O5 with coexistence
of a 3d and a (31)-dimensional phase
Hannes Krüger1, Václav Petrícek2, 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
x0.28
Ca2Fe2O5 (Srebrodolskite) C2F x0 Imma(00g)s00 ab
ove ?710C
Ca2FeAlO5 (Brownmillerite) C4AF x0.5
Taylor (1997) Kahlenberg et al. (2000) Redhammer
et al. (2004) Fukuda Ando (2002) Berastegui et
al. (1999) Krüger Kahlenberg (2005)
Ca2Al2O5 C2A x1
x0.7
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 a5.493(1), b15.037(3), c5.651(1)Å Space
group Imma
b
c
(0kl)
b
Satellite reflections indexed with q(0,0,0.588(2)
) (31)-dimensional Superspace group
Imma(00?)s00
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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
(31)-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, x20.25, x30.569
O3, x20.25, x30.626
x4
x4
x1
x1
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t-map (physical space section) at t0,
x20.25, calculated from 4-dimensional Fobs
synthesis
L L R L R
L R R L R
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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 hkl ? 2n and Satellites are
present!
c
(1kl)
This is observed for a temperature range of
(20K)
b
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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 lt7K all attempts in
primitive superspace groups failed
Exclusive sets of Reflections Pnma hkl0
hkl ? 2n Imma(00?)s00 hklm m ? 0
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688C
709C
average I/?(I) of 182 satellite reflections,
(hklm,m?0)hkl2n
average I/??(I) of 113 P-reflections,
(hkl0)hkl?2n
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Refinement in Jana2000 with a two-phase
model Integration I-centred lattice, with
q(0,0,?) hkl0 hkl ? 2n multiphase
refinement (via twinning option) both phases
are required to have the same number of
dimensions calculation of combined structure
factor
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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 1400C) 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
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Pnma
L
R
R
R
b
I2mb
a
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 Rw (all
main sat) 0.055 0.053 0.059 Acta
Crystallogr. B61, 656-662
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DTA-Experiment Ca2Fe2O5
2 thermal effects T1443C 0.2 J/g Néel
temperature Takeda et al. (1968) T2688C
0.5 J/g structural phase transition Imma or
I2mb Berastegui et al. (1999) incommensurate?
Redhammer et al. (2004)
T2
T1
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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 Me
asured Reflections Main 2208 Satellite
3932 Rint 0.057