Hannes Krüger1, Václav Petříček2, Volker Kahlenberg1

1Institute of Mineralogy and Petrography, University of Innsbruck2Institute of Physics, Academy of Sciences of the Czech Republic, Prague

A phase transition in Ca2Fe2O5 with coexistence of a 3d and

a (3+1)-dimensional phase

Ca2Fe

2O

5

(Srebrodolskite)C

2F

x=0

Imma(00)s00above 710°C

Ca2FeAlO

5

(Brownmillerite)C

4AF

x=0.5

Ca2Al

2O

5

C2A

x=1

The Ca2(Fe

1-xAl

x)

2O

5 solid solution series

x=0.28

x=0.7Pnma 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)

ca

c

b

Basic building units of brownmillerite-type structures

1) Layers of corner-sharing FeO

6 octahedra

Ca2Fe

2O

5 at room

conditions. Pnma

a

cBasic building units of brownmillerite-type structures2) zweier single chains of distorted FeO

4 tetrahedra

Ca2Fe

2O

5 at room conditions. Pnma

R

L

R/Ldisorderedas in Imma

Pnma

b

a

L

R

c*

b*c*

b*

(1kl)

(0kl)

827(5)°CMain Reflections indexed witha=5.493(1), b=15.037(3),c=5.651(1)ÅSpace group: Imma

Satellite reflections indexed withq=(0,0,0.588(2))(3+1)-dimensional Superspace group: Imma(00)s00

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 on½ of the period in x

4

Fe2 Fe2'

O3' O3

Lambert et al. (2002)

x4 x

4

x1

x1

x1-x

4 sections of the 4-dimensional F

obs synthesis

Fe2, x2=0.25, x

3=0.569 O3, x

2=0.25, x

3=0.626

L L R L R L R R L R

„t-map“ (physical space section) at t=0, x2=0.25, calculated from

4-dimensional Fobs

synthesis

-5.9

-5.8

-5.7

-5.6

-5.5

-5.4

-5.3

-5.2

-5.1

600 620 640 660 680 700 720 2.8

2.9

3

3.1

3.2

3.3

3.4

3.5heating (y1)cooling (y2)

DTA measurement Ca2Fe

2O

5, 5K/min, 100 µl Pt-crucible, 95.39mg sample

Temperature [°C]

He

at F

low

[µV

]

(1kl)

c*

b*

Reconstructed reciprocal space, measured at 697(5)°CCa

2Fe

2O

5

Reflectionsh+k+l ≠ 2n

andSatellites

are present!

This is observed fora temperature range of (~20K)

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 ≠ 2nImma(00)s00 hklm: m ≠ 0

0

2

4

6

8

10

12

14

16

640 660 680 700 720 740 760 780 800 0

1

2

3

4

5

6

Temperature (C)

P-Reflections heatingP-Reflections cooling

Satellites heatingSatellites cooling

aver

age

I/(

I) o

f 113

P-r

efle

ctio

ns, (

hkl0

):h+

k+l≠

2naverage I/

(I) of 182 satellite reflections, (hklm,m

≠0):h+k+

l=2n

688°C 709°C

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 samenumber of dimensions

calculation of combined structure factor:

0

10

20

30

40

50

60

70

80

90

100

688 690 692 694 696 698 700 702 704 706 708

Vol

ume

frac

tion

[%]

Temperature [C]

Volume fractions of Pnma and Imma(00g)s00 structures

fraction Pnma (heating)fraction Imma (heating)fraction Pnma (cooling)

Conclusions

Pnma and Imma(00)s00 phases of Ca2Fe

2O

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 TEMSubstitution Fe/Al

Acknowledgements

Waltraud WertlEduardo Hernandez BocanegraKaren FrieseJuan Manuel Perez-MatoMichael Czank

Thank you for your attention

0.596

0.598

0.6

0.602

0.604

0.606

0.608

0.61

0.612

0.614

0.616

680 700 720 740 760 780 800 820 840

valu

e of

gam

ma

IPDS-II sample Temperature (C)

temperature dependence of q-vector

heatingcooling

Pnma

I2mb

b

a

L

R

R

R

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: 43R

w (all; main; sat): 0.055; 0.053; 0.059

Acta Crystallogr. B61, 656-662

300 400 500 600 700 800 900 1000-35

-30

-25

-20

He

at

Flo

w [

mW

]

T [°C]

DTA-Experiment Ca2Fe

2O

5

T1

T2

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 transitionImma or I2mb Berastegui et al. (1999)incommensurate? Redhammer et al. (2004)

Experimental setup (single crystal X-ray diffraction)

Diffractometer: Stoe IPDS-II, 2 circleHeating method: Heated N

2-stream

Temperature: 827(5)°CCrystal size: 0.18x0.18x0.06 [mm]Absorption Correction: integration, using 8 indexed facesRadiation: Mo K

max

(°): 29.31

Measured Reflections

Main: 2208Satellite: 3932R

int: 0.057