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Shear transformation in the layered compound KAlF4 :low temperature phase structure and transformation

mechanismJ.M. Launay, A. Bulou, A.W. Hewat

To cite this version:J.M. Launay, A. Bulou, A.W. Hewat. Shear transformation in the layered compound KAlF4 : lowtemperature phase structure and transformation mechanism. Journal de Physique, 1985, 46 (5),pp.771-782. �10.1051/jphys:01985004605077100�. �jpa-00210019�

771

Shear transformation in the layered compound KAIF4 :low temperature phase structure and transformation mechanism

J. M. Launay (*,~), A. Bulou (*), A. W. Hewat (+),A. Gibaud (*), J. Y. Laval (~) and J. Nouet (*)

(*) Laboratoire de Physique de l’Etat Condensé, E.R.A. n° 682, C.N.R.S., Faculté des Sciences,72017 Le Mans Cedex, France(~) Laboratoire d’Etude et de Synthèse des Microstructures, E.R.A. n° 912, C.N.R.S., ESPCI, 10, rue Vauquelin,75231 Paris Cedex 05, France(+) Institut Laue Langevin, 156 X Centre de Tri, 38042 Grenoble Cedex, France

(Reçu le ler octobre 1984, révisé le 23 novembre 1984, accepté le 8 janvier 1985)

Résumé. 2014 Le composé lamellaire KAIF4 subit une transition de phase structurale au voisinage de 250 K. Lesdeux phases ont été étudiées par diffraction de rayons X, d’électrons et par affinement du profil des raies de diffrac-tion de neutrons. La phase ambiante (groupe d’espace quadratique D54h 2014 P 4/mbm 2014 Z = 2; a = b = 5,045 A,c = 6,159 A) dérivée du type T1A1F4 a été confirmée. Nous avons déterminé la structure basse température (mono-clinique, P 21/m, Z = 4; am = 7,340 Å, bm = 7,237 Å, cm = 6,407 Å, 03B2 = 106,8° à 4 K) qui est très voisine de lastructure de KFeF4. Nous montrons par des arguments structuraux que cette transition de phase du premier ordreest principalement caractérisée par un glissement dans la direction [100] des feuillets successifs ce qui permetd’expliquer après transition la désorientation de 16° entre les microcristaux mâclés mise en évidence par diffractionde rayons X et par microscopie électronique.

Abstract. 2014 The layered compound KA1F4 undergoes a structural phase transition in the vicinity of 250 K.Both phases have been studied by X-ray and electron diffraction and by profile refinement of the neutron powderdiffraction patterns. The room temperature structure (tetragonal space group D54h 2014 P 4/mbm 2014 Z = 2 ;a = b = 5.045 Å, c = 6.159 A) derived from the T1A1F4 type is confirmed. We have determined the low tempera-ture structure (monoclinic, P 21/m, Z = 4; am = 7.340 A, bm = 7.237 A, cm = 6.407 Å, 03B2 = 106.8° at 4 K) closelyrelated to the KFeF4 structure. From structural argument it is shown that this first order transition is mainlycharacterized by a gliding of the successive sheets in the [100] direction. This can explain the 16° misorientationof twinned microcrystals resulting from the transition as shown by X-ray diffraction and by electron microscopy.

J. Physique 46 (1985) 771-782 MAl 1985, 1

Classification

Physics Abstracts64.70K - 61.60

1. Introduction.

The structures of the tetrafluoroaluminates AAlF4(A = K, Rb, Tl, NH4) are closely related to the idealtetragonal TlAlF4 structure described by Brosset

(P 4/mmm- Dih; Z = 1 ; a=3.616(3)Å, c=6.366 (3) A}[1, 2]. These compounds consist of infinite layers ofAIF6 octahedra sharing four Feq atoms in the (001)plane; unshared fluorine atoms are denoted FaX andlie along the c axis. Each octahedron is centred in asquare based parallelepiped of cations A (Fig. 1).The ideal structure (denoted phase I) is encounteredin TlAlF4 and RbAlF4 at high temperature [3, 4]. Onthe other hand, this ideal structure is not observed forKAIF4, even at high temperature. In this case, thestructure is derived from the ideal one by correlatedrotations of the AlF6 octahedra around the fourfoldaxis and the space group is P 4/mbm - Dlh, Z = 2

Fig. 1. - Ideal structure of the tetrafluoroaluminates

AAIF 4 (A = Tl, Rb, K, NH4).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01985004605077100

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(phase II) [2, 5]. All these compounds undergo struc-tural phase transitions (SPT). In RbAlF4 and TlAlF4the transitions can be explained by octahedron rota-tions [3, 4]. In KAIF 4’ a discontinuous SPT is observedin the vicinity of 250 K [5-7] with a large heat of tran-sition (3.15 cal/g) as determined from DSC. The

attempts to describe the transition by octahedronrotations alone have always failed. Moreover the

crystal breaks up at the transition. The symmetrydetermination of the new phase (III) by X-ray diffrac-tion on crystal is thus very difficult. The determinationof the lattice symmetry of phase III has been performedon powder by long wavelength neutron diffractionand by electron diffraction which allows the selectionof small single domains. The structure parametershave been determined by neutron powder profilerefinement at 4 K. It must be pointed out that thethermal hysteresis is very large (- 100 K) so that thelow temperature structure is still present and can bestudied at room temperature.

2. Experimental.

Single crystals are prepared by the horizontalBridgman method from a non-stoichiometric mixture(0.51 KF + 0.49 AlF 3) chosen in reference to the

KF-AlF3 phase diagram [6]. The powder used for theneutron scattering experiments is obtained by pulve-rizing crystals in order to get high purity samples.However, this powder gives enlarged diffraction linesand it must be annealed at 350 OC before being studiedby the neutron powder profile refinement method.The neutron diffraction patterns were collected at4 K and at room temperature on the DIA neutronpowder diffractometer at the LL.L. (Grenoble-France).The angular range was 2 0 = 180 to 1600 in steps0.050 with an incident wavelength of 1.909 A. Therefinements were carried out with the Rietveld pro-gram [8] modified for thermal anisotropy [9]. Preli-

minary neutron diffraction powder patterns were

collected at 2.990 A in order to determine the latticesymmetry.

X-ray investigations have been performed in thetwo phases (II and III) both on powders and oncrystals which are always largely misorientated aftertransition.For electron diffraction experiments the samples

have been thinned by cleavage parallel to the basalplane and quenched to liquid nitrogen temperature.Electron diffraction diagrams have been recorded at100 keV on a Siemens Elmiskop IA and on a Jeol100 CX equipped with a cooling and temperatureregulating device.

3. Neutron diffraction study.

3.1 ROOM TEMPERATURE PHASE. - The room tem-perature structure (phase II) determined by Nouetet al. [5] is confirmed by neutron powder profile refine-ment. The results are given in table I. The large R factor

Table I. - Atomic coordinates j’or KAlF4 in the

space group P 4/mbm at room temperature obtainedfrom neutron powder profile refinement at a 1.909 Awavelength. R factors are defined by : RNUC =100 Z ) I (obs.) - SI (ca/.) /Z 1 (obs.) ; R pROF =100 E Y (obs.) - S Y (ca/.) liE Y (obs.) with 1 (obs.),1 (caL) the integrated intensity of reflections, Y (obs.),Y (caL) the intensity data point and S the scale factor.The Bij are defined by Bij = 8 a’ ui Uj > and aregiven in Å 2. Standard deviations are given in paren-theses.

is imputed to the presence in the diffraction powderpattern of several lines still larger than the instrumentalwidth which perturbs the classical Rietveld method.Analysis of both X-ray and neutron diffraction lineswidth is in progress.

3.2 Low TEMPERATURE PHASE. - In order to deter-mine the lattice type of the low temperature phase,we have recorded a neutron powder diffraction

pattern of KAlF 4 at 4 K with a A = 2.99 A wave-length so that the maximum splitting between thediffraction lines is obtained. The diffraction angle ismeasured with a great accuracy owing to the presenceof diffraction lines due to A/3. We have used a generalprogram for the search of the lattice parameters [10].The most reliable solution is obtained for an ortho-rhombic unit cell with parameters ao = 3.670 A,bo = 7.237 A, co = 12.267 A and from the conditionslimiting reflections, a B centred space group can be

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Table II. - Atomic coordinates for KAlF4 in the

space group Bmmb at 4 K obtained from neutronpowder profile refinement at a 1.909 A wavelength.The R factors are defined in table L

proposed. This suggests a structure similar to KFeF4[11]. Refinements performed in the correspondingspace group Bmmb (D2h, no 63, Z = 4) give theresults of table II. The reliability is quite satisfactoryif compared to the corresponding one obtained atroom temperature for phase II where the structure isunambiguously known [5]. However some very weakintensity lines are observed in addition to the expectedlines. Such lines are also mentioned in KFeF4 [11].This is in particular the case of the intensity diffractedbetween 024 and 032 lines denoted by full arrowsin figure 2a. This figure represents the pattern preparedfor the refinement i.e. the experimental pattern inwhich the intensity has been set zero outside the regionwhere diffraction lines are expected (the diffractionlines are Gaussian peaks truncated to 1.5 times thefull width at half height on either side of the peakcentre). Clearly, some parts of the diffraction patternare not expected in the framework of this orthorhombiccell (dotted arrows) and particularly between the twolines mentioned above (dashed arrows). This intensityarises from at least two lines as evidenced on thediffraction powder pattern recorded at 2.99 A (Fig. 3).As can be seen in table III, any orthorhombic cellwith the lattice parameters of Bmmb space group,even though primitive, cannot account for the exis-tence of these two lines. The smallest cell which can

explain the presence of such diffraction lines is themonoclinic cell am = 2 ao, bm = bo, cm = ( - ao + co)/2(Table III, Fig. 4a). The monoclinic subgroups ofBmmb with this cell are either P 2,/m (C2 2h) or P 21/a(C’2h). Refinements have been performed in both

space groups and a better agreement between calcu-lated and observed powder patterns is obtained withP 21/m. The results of the refinement are summarizedin table IV and the observed and calculated patterns

Fig. 2. - Part of the neutron powder diffraction pattern ofKAlF 4 at 4 K with a 1.909 A wavelength : (a) experimentalpattern prepared for the refinement in the Bmmb spacegroup. The full arrows indicate the 024 and 032 orthorhombicdiffraction lines mentioned in the text. The dashed anddotted arrows indicate parts of the pattern where the inten-sity is not predicted in the Bmmb space group; (b) experi-mental pattern prepared for the refinement in the P 21/mspace group; (c) calculated pattern in the P 21/m spacegroup.

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Fig. 3. - Part of the neutron powder diffraction pattern ofKAIF 4 at 4 K with a 2.99 A wavelength. The full arrowsindicate the 024 and 032 orthorhombic lines as in figure 2a.The dashed arrows indicate the diffraction lines characte-ristic of the monoclinic symmetry.

Table III. - Indexation of the diffracted intensitylying between the 024 and 032 orthorhombic diffractionlines. The relationship between the orthorhombic andthe monoclinic cells is shown in figure 4.

are shown in figures 2b and 2c. The number of diffrac-tion lines used for the refinement is 414 instead of 128in the Bmmb space group. In order not to greatlyincrease the number of refined parameters the followingassumptions have been made about the mean-squaredisplacement matrix :

i) the directions of the semi-major axes are set

parallel to the axes of the pseudo-orthorhombic cell.We note by Bi° the corresponding components of themean-square displacement matrix expressed in this

pseudo-orthorhombic cell.

ii) the mean-square displacements for atoms havingthe similar surrounding have been set equal :

Fig. 4. - (a) Relationship between the orthorhombic Bcentred cell (parameters ao, bo, co) and the monoclinic pri-mitive cell (parameters am, bm, cm). (b) Relationship betweenthe room temperature cell (parameters at, ht, ct-tetragonalspace group P 4/mbm) and the monoclinic cell (parametersam, bm, cm-monoclinic space group P 21/m).

where the subscripts are defined in table IV.The mean-square displacements expressed in the

orthorhombic cell are given in table V. The absorptioncorrection on the Bij which is much smaller than thestandard deviation, has not been taken into account[12]. The results are discussed in chapter 6.

4. Electron diffraction and electron microscopy study.

As was shown in chapter 3, the low temperature phaseof KAlF4 is monoclinic but closely related to an

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Table IV. - Atomic coordinates for KAlF4 in the space group P 21 /m at 4 K obtained from neutron powder profilesrefinement at a 1.909 A wavelength. The R factors are defined in table I.

Table V. - Mean-square displacements (8 n2 ui uj »)expressed in the pseudo-orthorhombic coordinates anddeduced from the refinement in the P 21/m space group.

orthorhombic B-centred cell. According to the lowdistortion of the orthorhombic cell and to the weak-ness of the spots arising from the monoclinic symme-try, it is convenient to analyse the electron diffractiondiagrams in the framework of the orthorhombic cellwhenever possible. The relationships between theMiller indices in both cells are :

4.1 SCANNING ELECTRON MICROSCOPY (S.E.M.). -In order to examine the samples after the transition,S.E.M. experiments have been carried out. The study

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Fig. 5. - Photographs of KAlF 4 in phase III obtained by Scanning Electron Microscopy. The scale given on each photo-graph is expressed in um : (a) Crystal surface after the transition. The breaks are evidenced owing to the edge contrast (clea-vage plane normal to the electron beam). (b) Detail of the surface in the vicinity of a break. (c) Evidence of twinned domains(cleavage plane tilted by about 52° with respect to the electron beam). (d) Evidence of sheets and twinned domains observedon the sample edge when the crystal has partially transited.

has shown the existence of breaks (Figs. 5a, 5b) on thesurface of the sample about 5 pm long, generallyparallel to [110] and [110] axes of phase II. Figure 5cshows the crystal surface near a break, when thesample is rotated by 520 for visualization. On figure 5bthe measured angles of the surface related to the flatsurface prior to the transition reach the value of 250.On figure 5c the orientation contrast shows the pre-

sence of twinned domains which were orthogonalbefore the transition.When the sample is examined parallel to the sheets

in phase III, it is shown that micronic twin domainsare tilted from the well-stacked configuration they hadbefore the transition (see Fig. 5d). The measured angleof this tilt, whose [010] axis is perpendicular to thesample edge, is about 1 5°.

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4.2 TRANSMISSION ELECTRON MICROSCOPY (T.E.M.).- Phase II of KAIF4 has been studied by T.E.M.experiments and compared to RbAlF4. The results arereported in [7].The low temperature phase is studied at room tem-

perature on quenched sample by a selected area

electron diffraction technique. (001)*, (101)*, (103)*,(01T)* and (311)* planes are observed in the pseudo-orthorhombic reciprocal lattice.When the electron beam is set perpendicular to the

cleavage plane (tilt = 00), (101)* plane is commonlyobserved (see Fig. 6a). This frequent observation of the(101)* plane can be explained by the tilting of twindomains revealed on the sample edge b1 S.E.M.(see Fig. 5d). The (001)* plane and the (103)* planeare simultaneously observed (Fig. 6b). Note that theformer corresponds to a 16.80 tilting around the

[010]* direction of the (101)* plane which is themost frequently observed. Table VI shows the agree-

Table VI. - Experimental and calculated ( from neu-tron results) interplanar spacings ratio in the (101)*,(001)* and (103)* planes.

ment between experimental and calculated data forseveral interplanar spacings. From these data, theexperimental values of ao and bo pseudo-orthorhombicparameters are deduced : ao = 3.7 (2) A and bo = 7.3(4) A. Moreover the doubling of the co parameter inrelation to phase II may be inferred from these results.

Furthermore, the existence rule h + I = 2 n corres-

ponding to the pseudo-orthorhombic B centred groupis confirmed. The (hko : k = 2 n) existence rule can beverified in spite of the frequent occurrence of doublediffraction. In figure 6a, weak extra spots arrowed inthe (101)* plane correspond to the doubling of theao parameter and can be indexed in the monocliniccell (am, bm, cm, P) proposed from the neutron diffrac-tion analysis. From neutron diffraction results, it canbe shown that the calculated intensities of these spots,indexed 130 and 310 in the monoclinic cell are not

negligible.When the sample is tilted by 300 around [100]*, the

(011)* plane is observed (Fig. 6c). The existence rulesh + 1 = 2 n and hko : k = 2 n are again well confirmed,but extraspots are evidenced on the diagram (seearrows). These extraspots belong to the (112)* planefrom phase II tetragonal cell. Table VII shows a verygood agreement between measured and calculatedinterplanar spacings for phase II and phase III. Thishypothesis of the simultaneous presence of phases IIand III is compatible with the evolution of the diffrac-tion diagram during observation. The transformationinto phase II is probably induced by the heatingcaused by the electron beam. A more complicatedreciprocal plane is shown in figure 6d which corres-ponds to (311)*. The experimental and calculatedinterplanar spacings are shown in table VIII. Thisdiagram confirms the pseudo-orthorhombic B-centredlattice. Extra spots caused by the doubling of the aoparameter are not evidenced in this plane owing totheir very small intensities.

Other tilts have been used for studying the reciprocallattice : the diagrams are often difficult to index

owing to the occurrence of simultaneous reflectionsfrom different reciprocal planes and several mis-orientated twin domains. However, the four planesshown are sufficient to corroborate the cell deducedfrom the neutron scattering study.

Table VII. - Experimental and calculated interplanar spacings (dhkl) : (a) in the (011)* plane (phase III : ortho-rhombic cell); (b) in the (112)* plane (phase II : tetragonal cell):

778

©O*

7

uu e cC0

2

S

o

00 ..d ’-’ JS

.

. =’ 0

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Table VIII. - Experimental and calculated interplanar spacings (dhki) and angle in the (31T)* plane (phase III :orthorhombic cell).

5. X-ray diffraction study.

As was confirmed by S.E.M., KAlF4 crystals undergoimportant breaks at the transition and the phase IIIdomains are mutually misorientated. Owing to thesmall domains size the X-ray diffraction diagramarises from several crystallites and the analysis isdifficult. Figure 7 represents a typical diffraction dia-

Fig. 7. - Transmission X-ray diffraction diagram of

KAlF 4-phase III at room temperature. The X-ray beam isnormal to the cleavage plane with a 1.54 A wavelength(monochromatized CuKal X-ray radiation) focalized on acylindrical film.

gram obtained on a fixed sample with copper Kamonochromatic wavelength on cylindrical film. Theimportant number of spots and their spreading comefrom a distribution of misorientated microcrystals.Programmes have been developed in order to getquantitative information from these diagrams. Thisanalysis showed that the misorientation angles aremainly centred at ± 15° in agreement with the resultsof chapter 4. More detailed results will be publishedlater.

6. Discussion.

6.1 LOW TEMPERATURE STRUCTURE OF KAlF4. -As shown in chapter 3, the structure of KAlF4 at lowtemperature is monoclinic but very close to an ortho-rhombic B face centred cell. The fl angle of the pseudo-orthorhombic cell is 90.180. So it is convenient to

represent the structure in this pseudo-orthorhombiccell by the different projections shown in figures 8a,b, c. These projections clearly evidence that the octa-hedron distortion is small, as is also shown in figure 9.So we can describe this structure according to anideal orthorhombic one represented in figure 10 andcorresponding to the Bmmb space group (parametersao, bo, co) (Note that due to the K+ ion positions it isnot possible to describe this ideal structure in the cellao, bo/2, co as did Hidaka et al. [13] in the study ofKFeF4). Phase III of KAIF4 is derived from this idealstructure by :

i) octahedron tilts around [100] (about 10 degrees)and K+ displacements along [001] both consistentwith the Bmmb space group;

ii) octahedron tilts around [001] (about 4 degrees)which are responsible for the doubling of ao parameterand the subsequent monoclinic symmetry.Note that the absolute values of the B°ij (table V)

probably are not quite exact due to large uncertaintyon the background for diffraction angles greaterthan 800. However their relative values are in agree-ment with those expected i.e. the mean-square displa-cements are less important along the AIF bond thanin the plane perpendicular to this bond. The relativevalue of the B°ij for axial fluorine and equatorial fluo-rine is also consistent with those expected (except forB 22 (Fax) which would be greater). Then we canpredict that the thermal vibration mainly arises fromlibration of quite rigid octahedra. A last point to note isthe important decreasing of potassium fluoride inter-atomic distance (dKF = 2.85 A in phase II ; dKF =2.63 A in phase III). This is related to the change ofcoordinence of potassium ions since the theoreticalvalue [14] is d N 2.82 Å in phase II where the coordi-nence is 8 and d = 2.62 A in phase III where the coor-dinence is about 6.

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6.2 MECHANISM OF THE TRANSITION. - Comparisonof the structures of phase II and phase III of KAIF4 (1 )shows that the AlF6 sheets have undergone a ( - ao/2)displacement along the [100] pseudo-orthorhombicaxis. Moreover we note that in phase II the successiveoctahedra along the [001] axis are tilted around thisaxis in the same sense. In phase III this tilt angle is

(1) The space group P 21/m (phase III) is a subgroup ofP 4/mbm (phase II) with a 450 rotation of the unit cellaround the tetragonal axis (Fig. 4b).

Fig. 8. - Projection of the ionic positions represented inthe pseudo-orthorhombic cell : (a) (100) plane; (b) (010)plane; (c) (001) plane.

Fig. 9. - Bond lengths and angles in the AIF6 octahedra(phase III P 21/m).

Fig. 10. - Ideal structure from which the low temperaturephase of KAlF4 is derived by AIF 6 tilts and K displacementsas indicated by arrows.

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preserved, although it is smaller, and the sequence ofoctahedra tilted in the same sense is encountered alongthe [TO1] pseudo-orthorhombic axis (Fig. 11). Thismeans that the transition arises from a gliding of theAlF6 planes, each plane gliding in the same directionwith respect to the preceding one (2). As shown infigure 11, this corresponds to a 16.80 rotation of theline joining consecutive octahedra of adjacent sheets.This mechanism is in good agreement with the obser-

Fig. 11. - Mechanism of the transition evidenced by thesense of the AIF 6 tilts in adjacent sheets. (a, c), (as, co) (am,cm) respectively represent the lattice parameters in the idealtetragonal, the pseudo-orthorhombic and the monocliniccells. Curved arrows indicate the rotation angle around thec and co axes. Straight arrows indicate the sheets displace-ments at the transition.

vation of 15° twinned domains (Fig. 5d and Fig. 12).Let us compare the SPT in KAlF4 to the SPT in

RbAlF4 which has the same structure at room tempe-rature. In RbAlF4 two SPT are observed (Table IX).The Tci and T C2 transitions arise from soft modecondensation of respectively the M3 and X3 modesof the Brillouin zone (BZ) of the ideal (phase I) cell[15, 16]. (The correspondence between phase I BZand phase II BZ are given in figure 13b; in particular,the X point of phase I BZ corresponds to M’ point ofphase II BZ). In KAlF4, the M3 mode is condensed

(2) Note that the alternative mechanism in which theAIF 6 successive sheets along the [001] axis would glide inopposite senses would lead to the noncentrosymmetricmonoclinic space group Pm with a greater unit cell. Norefinement was performed with this space group since noneof the expected additional diffraction lines were observed.

Fig. 12. - Cell orientation and domain structure of KAlF 4in the low temperature phase. The arrows indicate thegliding sense of the AIF 6 octahedra sheets

Fig. 13. - (a) Part of the Brillouin zone (BZ) of the idealcell (phase I-P 4/mmm) of the tetrafluoroaluminates repre-sented in figure 1. (b) Relationship between phase I BZ

points and phase II BZ points (primed symbols).

at room temperature. Moreover, a recent lattice

dynamics study together with inelastic neutron scat-tering experiments [16] revealed that a phonon branchsetting between X and M points (phase I BZ) softenswhen approaching the SPT of KAlF4 which is thenprobably induced by a displacive mechanism. Thesephonons correspond to tilts of the AlF6 octahedratogether with K+ displacements along the [001]axis. However such a mechanism cannot explainalone the SPT of KAlF 4 and we must assume thatafter the soft mode condensation the shear transfor-mation occurs. This hypothesis is supported by thestudy of KAlF4 doped with rubidium [17] whichexhibits an intermediate phase between phases IIand III of pure KAlF4. The determination of thisintermediate structure is in progress to explain theSPT by gliding of AIF 6 sheets which looks like the

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Table IX. - Comparison of the successive phases of RbAlF4 and KAIF 4.

martensitic transformations observed in metallic

alloys.In conclusion the structure of the low temperature

phase of KAlF4 has been determined using neutronpowder profile refinement, electron diffraction and

X-ray diffraction. It has been possible to deduce, fromstructural arguments, that the transition arises fromgliding of AIF 6 sheets which explains the texture ofcrystals in the low temperature phase. KAlF 4 providesa very interesting case of a phase transition close toa martensitic transformation while the other tetra-

fluoroaluminates generally exhibit transitions by tiltsof the AlF6 octahedra.

Acknowledgments.

The authors are indebted to C. Ridou for preliminaryX-ray investigations of the transition. They are grate-ful to G. Niesseron for crystal growth and to G.Ripault and A. Dubon for technical assistance withX-ray and scanning electron microscopy experiments.

References

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[16] BULOU, A., ROUSSEAU, M., GIBAUD, A., HENNION, B.and NOUET, J. (to be submitted).

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