ELECTRON MICROSCOPY STUDY OF THE NATURE OF THE LOW-TEMPERATURE PHASE TRANSITION IN α-MnSe

HAL Id: jpa-00222166

https://hal.archives-ouvertes.fr/jpa-00222166

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ELECTRON MICROSCOPY STUDY OF THE NATURE OF THE LOW-TEMPERATURE PHASE

TRANSITION IN α-MnSe

J. van Landuyt, S. Amelinckx, H. van der Heide, C. van Bruggen

To cite this version:

J. van Landuyt, S. Amelinckx, H. van der Heide, C. van Bruggen. ELECTRON MICROSCOPY STUDY OF THE NATURE OF THE LOW-TEMPERATURE PHASE TRANSITION IN α-MnSe.

Journal de Physique Colloques, 1982, 43 (C4), pp.C4-351-C4-355. �10.1051/jphyscol:1982450�. �jpa-

00222166�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppl6ment au n o 12, Tome 43, dgcembre 1982 page C4-351

ELECTRON MICROSCOPY STUDY OF T H E NATURE OF T H E LOW-TEMPERATURE PHASE TRANSlTION IN

J. Van Landuyt

,

,

U n i v e r s i t y o f Antwerp, RUCA, Groenenborger Zaan 171, B-2020 Antwerp, BeZgiwn

* ~ n i v e r s i t y o f Groningen, Nyenborgh 16, NL 9747 AG Groningen, The N e t h e r l a n d s

(Revised text accepted 7 September 1982)

Abstract : Electron microscopy and electron diffraction has bee11 used to study the low temperature phase transition in a-MnSe by in situ investigations down to liquid nitrogen temperatures.

Direct observation allowed to characterize the transformation from the cubic NaC1-structure to the hexagonal NiAs-structure as a shear transformation on the close packed planes.

Various stacking schemes are discussed with the purpose of proposing a detailed mechanism for the shear transition in agreement with other physical measurements such as X-ray and neutron diffraction and Mossbauer spectroscopy.

On the basis of diffraction contrast analysis of the images at the transforna- tion faults in the cubic phase, a model is proposed whereby the NiAs-structure is obtained by the propagation of a suitable sequence of shockley partials.

Intermediate faults leave anions in trigonal prismatic environment which could explain some features of the physical measurements.

Electron microscopy and diffraction were used in view of elucidating uncertain- ties concerning the nature of the low temperature transition in a-MnSe from thecubic NaCl to the hexagonal NiAs structure and in order to shed some light on the results of Mossbauerstudies as a function of temperature above and below the transition temperature.

Single crystal NnSe fragments were ground and trepanned into 3 mm discs of about 100 vm thickness. These were mounted in a rotating holder for further thinning by bombardment with 5kV argon ions. The edges of the thus perforated discs are thin enough for investigation in transmission electron microscopy.

They are mounted in the cooling holder of a lOOkV electron microscope in which tempe- ratures close to liquid nitrogen temperature can be reached by controlled gasflow of boiling liquid nitrogen.

The observations were performed in bright field imaging conditions in strongly diffracting situations whereby structural phases and defects can be revealed by the diffraction contrast they produce (ref.1). Electron diffraction patterns are used to monitor phase changes, specimen orientation and diffraction conditions for imaging.

During the cooling cycle at about 1 9 0 ~ the rather uniform appearance of the specimen is disturbed by a sudden dynamic behaviour. Plate shaped bands traverse the thin foil in different crystallographic directions (fig.1); the bands are limited by

(111) planes of the high temperature phase. Such a banded texture is shown in fig.1, revealing the presence of a faulted NiAs phase as dark bands. These bands usually contain interfaces imaged by fringe patterns. These fringe patterns have the typi- cal contrast characteristics of stacking faults. These faults are often limited by partial dislocations. The plates appear to grow in a lateral direction by the move- ment of dislocation lines along the projected habit plane. After removing thecooling source the bands of NiAs phase reduce gradually in width by the inverse process but not all traces of the transformation disappear; a number of fringe patterns due to stacking faults or very thin plates in NiAs structure remain in the specimen at roonl temperature.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982450

C4-352 JOURNAL DE PHYSIQUE

In specimens which after cooling are observed above the transition temperature i.e. in the cubic phase, (sodium chloride structure) fringe patterns also occur

(fig.2) suggesting that the reverse transformation also leaves stacking faults.

The nature of such faults (intrinsic or extrinsic) can in principle be deduced from the knowledge of the active diffraction vector: in a two beam situation and from the nature of the outer fringes in the dark field image of a single fault in sufficiently thick foils (ref.2). In fig. 3 we have reproduced a bright and dark field image of the same fault in a cubic matrix. The active diffraction vector, which is of the type 220, was indicated on the image as well. Assuming the faults to be single and using the method derived in ref.2,the nature of the fault turned out to be extrinsic.

These observations clearly suggest a shear type transformation between the cubic MnSe with NaC1-structure to the hexagonal NiAs-structure. The transformation appears to be rather sluggish in that not all the material is transformed at once and also because after reheating residual defects (stacking faults) are left behind in the

heavily faults NiAs-phase is observed as dark bands in crystallographic directions.

?ig.2 : Tne reverse transformatron leaves cubic phase with several some- times overlapping faults.

By cooling the specimen around the critical temperature and applying controlled heating by transmitting current through the thermocouple used for temperature control, temperature cycling is possible whereby successive stages of the nucleation and growth of the low temperature phase can be observed as illustrated in fig. 4.

Individual stacking faults are observed to grow or shrink by the passage of partial dislocations.

The electron diffraction patterns confirmed the X-ray results; at temperatures above the transition temperature a FCC pattern is produced, whereas beLow the transi- tion temperature they reveal a hexagonal lattice.

The orientation relationsqip between the two lattices is such that [OOO1lhex is parallel with [l 1 1

lcub.

This means that the close packed planes are parallel in the two structures-Some disorder is evident as well in the cubic as in the hexagonal phase. In the diffrac- tion pattern this is revealed by streaking along the direction perpendicular to the

Fig. 3 : B r i g h t f i e l d ( a ) and dark f i e l d images a s used f o r c h a r a c t e r d e t e r m i n a t i o n o f t h e s t a c k i n g f a u l t s .

Fig.

-

and c o o l i n g a g a i n from (b) t o ( c ) , (d) one o b s e r v e s s h r i n k a g e and growth o f t h e f a u l t s .

c l o s e packed p l a n e s ; i n t h e images s t a c k i n g f a u l t s can be r e v e a l e d d i r e c t l y . The o r i e n t a t i o n r e l a t i o n s h i p between t h e two phases s t r o n g l y s u g g e s t s t h a t t h e t r a n s f o r - mation should proceed by s h e a r i n g along c l o s e packed l a y e r s . Such l a y e r s c o n t a i n only a s i n g l e chemical s p e c i e s , a s w e l l i n t h e sodiumchloride s t r u c t u r e (sch. 1 ) a s i n t h e n i c k e l a r s e n i d e s t r u c t u r e (sch. 2 ) . The s u c c e s s i o n o f l a y e r s i s t h e same b u t t h e i r s t a c k i n g mode i s d i f f e r e n t i n t h e two phases. The t r a n s f o r m a t i o n can t h u s b e performed by t h e propagation of s e p a r a t e Shockley p a r t i a l s a l o n g t h e s e p l a n e s changing t h e s t a c k i n g form c u b i c t o hexagonal and vice-versa f o r t h e r e v e r s e t r a n s - formations.

S i n c e t h e s t a c k i n g f a u l t energy i s e f f e c t i v e l y n e g a t i v e i f t h e c r y s t a l i s kept a t a temperature f o r which t h e m a t r i x i s u n s t a b l e , s t a c k i n g f a u l t s w i l l have a ten- dency t o become i n f i n i t e l y wide by t h e s e p a r a t i o n o f s i n g l e p a r t i a l s .

We s h a l l now i n q u i r e about t h e r e l a t i o n between t h e s h e a r v e c t o r and t h e s t r u c - t u r e of t h e s h e a r plane. On t h e one hand one might imagine t h a t t h e g l i d e o p a t h i s mainly determined by t h e b i g anions ( t h e i o n i c r a d i u s of selenium i s 1.98 A ) . The s m a l l e r c a t i o n s ( i o n i c r a d i u s of Mn i s 0.82

5 )

I n t h e l a t t e r c a s e t h e i r displacement would be d e s c r i b e d by a v e c t o r , d i f f e r e n t from t h e Burgersvector of t h e p a r t i a l d i s l o c a t i o n s and t h u s a l s o d i f f e r e n t from t h e displacement v e c t o r o f t h e s t a c k i n g f a u l t . Synchro-shear motion can a l s o be des- c r i b e d a s r e s u l t i n g from t h e p r o p a g a t i o n of a d i p o l e o f p a r t i a l d i s l o c a t i o n s l o c a l i - zed on e i t h e r s i d e of t h e l a y e r o f moving i o n s ( r e f . 4 ) .

These two p o s s i b i l i t i e s a r e i l l u s t r a t e d f o r t h e case of a s i n g l e f a u l t i n an FCC m a t r i x by t h e s t a c k i n g schemes 3 and 4 where t h e p o s i t i o n of t h e a c t i v e s h e a r p l a n e h a s been i n d i c a t e d by means o f an arrow. The l a t i n l e t t e r s r e p r e s e n t selenium where- a s t h e greek l e t t e r s r e p r e s e n t manganese i o n s .

JOURNAL DE PHYSIQUE

On the other hand one could assume that the cations are sufficiently large to determine the glide-path. The result of a single shear movement is then as illus- trated in the stacking scheme 5. According to the first assumption (scheme 4) a single glide motion i.e. a single stacking fault gives rise to the formation of a lamella of NiAs structure with a thickness of one and a half unit cel.1. Whether or not synchro-shear takes place the resulting stacking fault is of the intrinsic type and all manganese ions remain in an octahedral environment.

According to the second assumption (scheme 5) there will be no synchro shear and one layer of manganese ions in a trigonal prismatic environment will now be formed in the wake of the first transformation partial (byb). The resulting stacking fault will now be extrinsic.

From a determination of the sign of the displacement vector of a single fault in an FCC matrix it is possible to conclude whether the fault is extrinsic or in- trinsic and thus it can be concluded whether the first or second sequence of events takes place.

Since observations have shown that the remnant faults in the cubic phase are extrinsic one might conclude that the second of the assumptions is realized in MnSe.

It should be stressed however that this result is obtained under the assumption that the fringe pattern under consideration is due to a single fault.

However this conclusion is n0.t: consistent with the Mossbauer measurements; the presence of a number of such faults would lead to a certain fraction of manganese ions situated in the centre of a trigonal prism of anions as represented by the stacking symbol b y b or b a b (depending on whether the manganese layer accompanies the upper or the lower crystal block). The implication would be that in MnSe no synchro shear seems to take place on transforming from cubic into hexagonal. It should be noted that in the perfect NiAs structure as well as in the perfect sodium- chloride structure : all cations occupy octahedral interstices in the anion sub- lattice. Only in faults left after the transformation or at the onset of the trans- formation does one find trigonal prismatic lamella. However the amount of cations in trigonal prismatic environment as found from Mossbauer studies (ref.5) would re- quire a very high density of faults, which actually were present in the hexagonal phase. A quantitative estimate is difficult, however and some doubts still remain with the authors whether the ions at the faults can explain the Mossbauer signal.

A certain reserve has to be made concerning a possible fault overlap for the examined cases. High resolution observations with the faults parallel to the electron beam should compliment the character determination and allow a further detailed model to be proposed.

REFERENCES :

1. S. Amelinckx, J. Van Landuyt, in : "Diffraction and Imaging ~echniques in Mate- rials Science", eds. S. Amelinckx et al., North Holland, Amsterdam, 1978, p.107- 151.

2. A. Art, R. Gevers, S. Amelinckx, Phys.stat.so1.

2,

3. M.L. Kronberg, Acta Met. - 9, (1961), 970.

4. S. Amelinckx, in : "Dislocations in Solids", vo1.2, Ed. Nabarro, North Holland, Amsterdam, 1979, p.401.

5. J.M. Sanchez, Univers. de Strassbourg : private communication.

STACKING SCMEMES :

1 . a y b a c

la

lay ...

a y b y a y b

...

a B c B a B c

...

b a c a b a c

...

a y b a c B a y b ~ ~ f a Y b C r c B a

. - .

synchro shear a y b a c B a y b y a y b a c B a i n t r i n s i c f a u l t

I N i A s

a y b a c B a Y b y b a c 6 u

N i A s

b y b t r i g o n a l p r i s m a t i c env.

e x t r i n s i c f a u l t

t r a n s f o r m a t i o n mechanism

J

I

l a t i n l e t t e r s : Se greek l e t t e r s : Mn