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Structural Heterogeneity inTwinned Yb2-x(Fe,Ga)17+2x Polytypes

Yaroslav Filinchuk

L’viv, 25-28 Sept 2002VIII Intl’ IMC Conference

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H. Birkedal 2,3 M. Hostettler 2

1 Laboratory of Crystallography, University of Geneva2 Institute of Crystallography, University of Lausanne3 Department of Chem. and Biochem., Univer. of California4 Department of Inorganic Chemistry, University of L'viv

Y. Filinchuk 1

R. Cerny 1

K. Yvon 1

O. Bodak 4

T. Yanson 4

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

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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 by Fe2 dumbbells, Fe atoms - by Ga atoms, respectively.

According to previous work Yb-Fe-Ga alloys crystallize with the hexagonal LuFe9.5 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

Yb12Fe64Ga24

alloy

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

H R

ordered ordered

disordereddisordered

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In this work we show thatIn this work we show that b both polytypes canoth polytypes can occuroccur within a within a coherently twinned crystal

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

(11 0 ) d irec t sp ace sec tio n

a bab plane ofplane of«large» hex cell«large» hex cell

6Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

Sa m p le p re p a ra tio nA rc -m e ltin g a t th e n o m in a lc o m p o s itio n Y b F e G a12 64 24

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)

Struc tureinve stig a tio ns

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[1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach,[1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach, Bulletin of the Czech and Slovak CrystallographicBulletin of the Czech and Slovak Crystallographic Association.Association. Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17).Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17).

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 computer program [1] that allows to refine several structures on a single data set.

[1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach,[1] H. Birkedal, M. Hostettler, W. Paciorek, D. Schwarzenbach, Bulletin of the Czech and Slovak CrystallographicBulletin of the Czech and Slovak Crystallographic Association.Association. Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17).Special issue B, ECM-18. 5 (1998), 200 (Abstract A4-P17).

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

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

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h 2 h l re c ip ro c a l sp a ce sec tio n o f th e “ c o m m o n c e ll”

R

R

R

H

H

H l h

C o m m o n

C o m m o n

C o m m o n

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

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H ( )1R ( )o b v 2

R ( )re v 32 H an d p o ly ty p es3 R

 

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

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

- R - d u m b b e l l ( a l i g n e d a l o n g c - a x i s )

R h o m b o h e d ra l - s tr. ty p e T h Z n 1 72

(o b v e rse )

o r

( re v e rse )

2 . R (o b v e rse ) : -h + k + l = 3 n

3 . R (rev e rse ) : -h + k - l = 3 n

1 . H : n o h k l ex tin c tio n s

H e x a g o n a l - s tr. ty p e T h N i 1 72H e x a g o n a l - s tr. ty p e T h N i 1 72

c

ba

C o n g lo m era tec ry s ta l

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

Data acquisition: ESRF (SNBL), 0.55091 Å; crystal selected from a bulk

the whole conglomerate domain type 1 domain type 2 domain type 3

crystal class hexagonal rhombohedral rhombohedral

space group P63/mmc R-3m R-3m

a (as = 8.615 Å) as as as as c (cs = 25.215 Å) cs cs/3 cs/2 cs/2 refined composition Yb3.46(2)Fe35.07(4) Yb5.006(2)Fe50.44(2)Ga1.878(13)

no. of parameters 41 18 20

volume fraction 0.320(4) 0.340(3) 0.340(3)

group of reflections

all com- mon

all own all own all own

R1(F2

o>3s(F2o)) 0.0644 0.0551 0.0635 0.0839 0.0597 0.0660 0.0591 0.0646

wR2(all) 0.1857 0.1553 0.2102 0.2700 0.1625 0.1679 0.1626 0.1680

Goof(all) 1.0084

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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

STRUCTURAL HETEROGENEITYIN A SINGLE CRYSTAL

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Two-phase region as a single crystal !!!

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

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”…)

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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

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A crystal from an Yb12Fe65Al23 alloy has the same structural features but shows an

extra scattering (H´), with diffuse streaks in the basal plane.

It suggests short-range order in the basal plane.

Diffuse streaks for the reflections from R (2 and 3) domains indicate frequent stacking faults along c.

Yb-Fe-Al system

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

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Yb-Fe-Al systemh 0 2 l re c ip ro ca l sp a ce sec tio no f th e “ co m m o n c e ll”

H ´

l h

C o m m o n

C o m m o n

C o m m o n

(3 0 0 )

(0 0 -1 2 )

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

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h + k /2 k /2 3 rec ip ro c a l sp ac e se c tio no f th e “co m m o n c e ll”

(-5 5 3 )(-4 4 3 )

(-4 3 3 )

H ´ H ´

H ´

k

h

Yb-Fe-Al system

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

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

Yb-Fe-Al system

R : 2 3

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p e ak s o f rh o m b o h e d ra l d o m a in s a n d a re

p e ak s c an b e in d e x ed in ce ll w ith a = 2 a ( ) = 2 a ( ) a n d c = c ( )

th e y h a v e n o d iffu se s tre ak s a lo n g c*

d iffu se a lo n g c* d ire c tio n

H ´: H ´ 4( ) 1 1

d iffu se s trea k s a lo n g a * an d b * b u t

frequent stacking faults along cfrequent stacking faults along c

local R-atom ordering in the basal planelocal R-atom ordering in the basal plane different fromdifferent fromthat observed in Ththat observed in Th22NiNi1717 and Th and Th22ZnZn1717 type structures type structures

20Y. Filinchuk L’viv, 25-28 Sept 2002 VIII Intl’ IMC Conference

Related topics:

1. What is a real TbCu7 structure? Isn’t it a highly disordered stacking of the layers that have a strong tendency to ordering in the basal plane?

2. D.S. Yufit et al. (2002). Acta Cryst., B58, 673-676 - two polytypes of organic substance in a coherently twinned crystal

3. H. Katzke. (2002). Z. Kristallogr., 217, 127-130 - variations of peak profiles for 2H&3R twinned polytypes of NbS2

4. D.L. Smith et al. (1981). Acta Cryst., B37, 1807-1812 - 2H&3R coherently twinned polytypes of CuNCS were refined

CrystSun1.unige.ch

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Another story – how to estimate the average domain size along c-direction?

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

Calculated X-ray diffraction pattern as a function of the stacking fault probability P

NbS2