PRECIPITATE PHASESThe Suzuki phase in NaCl : Cd2+ and NaCl : Mg2+

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PRECIPITATE PHASESThe Suzuki phase in NaCl :

Cd2+ and NaCl : Mg2+

A. Guerrero, E. Butler, P. Pratt

To cite this version:

A. Guerrero, E. Butler, P. Pratt. PRECIPITATE PHASESThe Suzuki phase in NaCl :

JOURNAL DL PHYSIQUE Colloyue C6, supplimc.r~t uu nu 7 , Tome 4 1, Juillet 1980, page C6-363

PRECIPI

TA

TE PHASES.

The

Suzuki phase in NaCl

:

c d 2 + and NaCl

:

A. L. Guerrero, E. P. Hutlcr a n d P . L. Pratt

Department of Metallurgy and Materials Science, Irnpcrial College of Science and Technology, London, SW7, U.K.

Rhumb. - Les precipites Suzuki ont Cte observes directement avec un microscope klectronique en transmission

a haute tension dans NaCI : C d 2 + et NaCI :Mg2' pour plusieurs concentrations d'impuretes et temperatures de recuit. De faibles intensltes de faisceau electronique et des temperatures de I'ordre de 10 K ont systtmatiquement Cte ut~lisees pour eviter I'irradiation des lames minces. Des mesures de conductivite ionique ont kt6 ftaites sur des echantillons issus des mCmes plaquettes que celles utilisees en M.E.T. L'analyse de la conductivite ion~que deve- loppee par Guerrero et al. [ I ] a ete a nouveau utilisk et reliee aux observations de M.E.T.

Abstract. - Suzuki precipitates in NaCI :Cd2' and NaC1: Mg2+ have been observed directly with a high voltage

transmission electron microscope for several impurity concentrations and annealing temperatures. Low beam curtcnts In the microscope and temperatures of about 1 0 K were used throughout the experiments to prevent radiation damage to the specimens. Ionic conductivity measurements were carried out on specimens belonging to the same platelets as those used in the T.E.M. experiments. The analysis of ionic conductivity developed by Guerrero er al. 111 was applied to these data and correlated with the T.E.M. observations.

1. Introduction.

-

In 1961 Suzuki [2] demonstrated the existence o f a new metastable phase in NaC1 : C d 2 + crystals, having t h e c o m p o s i t i o n

6 NaCI.CdCI2. Since then, a number of precipitates

in other alkali-halide/divalent cation systcms have been reportcd as having the same structure, a n d these are generally known a s Suzuki precipitates. T h e structure is well known ; it is a f.c.c. superlat- ticc of NaCl having a latticc parameter approxima- tely twice that of thc host lattice. Details of the ato- mic arrangement can be found in the literature [ 2 ] .

Several techniques have been used in recent years t o gain a better understanding of both the precipita- tion process a n d the characteristics a n d properties of these Suzuki precipitates. Although from ionic conductivity d a t a [ I , 3, 41 it is possible t o get infor- mation regarding the solubility of the aggregates a n d the kind o f species present in solid solution prior t o the precipitation, the use of electron microscopic techniques is nccessary if detailed infor- mation about the precipitates is t o be obtained. However, the high radiation damage rate of ionic crystals imposes a serious limit t o their direct obser- vation. Hobbs [ 5 ] was the first t o show that the spe- cimen lifetime can be extended by the use of speci- men temperatures of less than 30 K t o impede diffu- sion of defects. A combination of low temperature, extremely low beam current density, a n d high acce- lerating voltage, to minimise the ionisation cross- section, enables useful studies t o be made in the electron microscope [6].

In the present work, we examinc crystals of NaCl : C d 2 + having different Cd" concentrations,

using both tcchniques, viz. low temperature trans- mission ilectron microscopy a n d iollic co~lductivity. In addition, a few specimens of NaCl : Mg2+ were examined with t h e microscope t o investigate t h e dis- location structure associated with the Suzuki preci- pitates.

2. The specimens and their preparation. - T h e

crystals wcrc grown from the melt, using a modified Kyropoulos technique. Platelets were cleaved from the crystal transverse t o the growth axis and small blocks for microscopy, conductivity a n d analysis were cut from adjacent points in each platelet. The dopant content was dcterrnined by atomic absorp- tion, the errors being

-

2 3 O/o.

T h e crystal blocks for microscopy (typical size 5

x

5

x

25 mm) were divided in two groups accord- ing t o the thermal treatment given. T h c first g r o u p of specimens was annealed in purified N2 at 550 " C for about IS hours a n d thcn slowly cooled down t o room temperature a t a rate of 0.16 OC/min. T h e crystals of the second group were annealed a t 550 " C in the same way, and then slowly cooled to 250 "C (340 " C for NaCI : Mg2-r) a n d kept a t that temperature for 30 hours (6 hours for NaCl :Mg2+). They were then rapidly quenched into air a t room temperature by using a copper sheet.

A key t o the nomenclature of t h e specimens, their impurity concentration (in molar ppm) a n d their thermal treatment is given in table I.

3. Electron microscopic observations. - Suitable

samples for transmission electron microscopy were

C6-364 A . L. GUERRERO, E. P. BUTLER AND P. L. PRATT TABLE I Doping level NaCl : Cd 120 mppm 170 mppm 420 mppm 700 mppm N'ICI : Mg 1900 mppm

Electron microscopy I . Conductivity

slowly cooled to RT annealed at 250 "C and quenched

Al A2 A3

B I B2 B3

CI C2 C3

Dl D2 L) 3

slowly cooled to RT annealcd at 340 O C and quenched

MI M2

prepared by cleaving small squares from the original blocks down to a typical size of 3

x

3

x

0.3 mrn and using the chemical jet-polishing technique des- cribed by Hobbs 171. The samples were examined in an AEI EM7 electron microscope fitted with a cold stage with the specimen at a temperature of 10

+

2 K . The microscope was operated a t 1 000 k V and low levels of illumination. Micro- graphs were recorded on high sensitivity X-ray film. Typical electron micrographs for the NaCl : Cd2+ system are shown in figures 1 to 4. Specimens A l and BI, with low Cd2+ concentration, show small cube-shaped precipitates having characteristic strain field contrast, as seen in figure 1 . For A1 the den-

Fig. I. - Coherent Suzuki precipitates in specimen BI.

sity of the precipitates was lower and they were about 90 nm on side, compared with 120 nm for BI. As expected from the solubility diagram of the Suzuki phase [3] specimens A2 and B2 did not show any precipitates that could be identified as of Suzuki typc.

Figure 2 is a micrograph of specimen Dl. Cuboid Suzuki precipitates and two types of dislocations (matrix and interfacial) can be seen. The matrix dis- locations are long and have been pinned, in their apparent movement along [010], either by invisible small precipitates or by other dislocations out of contrast. Interfacial dislocations can most clearly be seen in precipitates A and B. There is a clear align- ment of the interfacial dislocations along [OIO]. These dislocation arrays and their alignment along [010] were also found in specimen CI, although the precipitates were of a smaller size (about 280 nm). Figure 2 shows two further significant features :

a ) The incipient transformation of the

1

100 1 faces of the Suzuki precipitate into apparently morc sta-

Fig. 2.

-

The microstructure of S P C C ~ I I I C I I 1)1 \I~o\ving matrix and interfacial dislocations associated with the Suzuki phase.

ble ( 110 ] or ( 11 1 ] planes

-

as seen in precipitates C and D -, and b) circular patches of debris (hav- ing an average radius

-

1.5 pm) which surround most of the precipitates.

THE SUZUKI PHASE IN NaCI : Cd2+ A N D NaCl : Mg2+ C6-365

which illustrates a typical Suzuki precipitate in spe- cimen MI tilted 23" ( + 10 To) along the [110] dia- gonal. A set of dislocations running along TO] on the (001) faces can be seen. By counting the kinks along the bottom I0101 edge, to avoid confusing top and bottom interface networks, the dislocation spac- ing in the interface, along [110], was measured as 75 nm ( _+ IS %). This is in excellent agreement

with the theoretical value calculated by Yacaman et al. [Y], for a set of perfect

<

110

>

interfacial dis- locations in this system. Hence, there seems to be no need to postulate, as these authors do, the exis- tcnce of partial dislocations at the interface.

4. Ionic conductivity measurements.

-

Ionic con- ductivity measurements were performed on speci- mens A3-D3. A Wayne Kerr Bridge at v = 20 kHz was used for temperatures above 400 "C. At lower temperatures, the meas~lrements were taken with a Keithley 616 Digital Electrometer. Experimental

Fig. 3. - Suzuki prec~pitate in specimen Dl showing interfacial

transformation to the ] 1 1 1 1 h a b ~ t planes. Dark field image details and conductivity plots will be published in wing g = [020j. the near future.

The analysis proposed by Guerrero et a/. [ I ] was carried out for this experimentai data, and 6 vs. xdp which divalent impurities and cation vacancies alter- _{plots were obtained (%, is the dipole concentration}

nate. _{and x:, 8 is the total impurity present in aggregates}

The origin o f the debris circles surrounding the _{higher than dipoles).}

_values

_{of the parameters used} precipitates has not yet been determined. One possi- _{are : A , = 9.0 ( Q - I cm- 1 K), H, = 0.72} bility is that they are dislocation loops related to the _{(eV) and EII; = 0.38 (eV) (notation as in 111).} precipitation process, and they mark peripheral _{A typical 6 plot for temperatures slightly above} regions around a precipitate from which Cdzf

-

_{the precipitation is shown in figure 5a. The error}

vacancy dipoles were removed to form the ~ r e c i ~ i - _{bars have been calculated assuming 3 % error both}

tale. _{in a and in the total impurity concentration. Since}

studies on ~ a C l : Mg2+ using gold decoration _{the presence o f trimers give a positive slope} techniques frequerltly show a set o f parallel cleavage _{to the 6 vs. qp plotS, no aggregation species higher} steps coming out from the precipitate

[PI.

These are _{than dimers seem to be present.}

usually attributed to the interaction of the clcavage crack with screw dislocations surrounding the preci-

pitate, arising from the misfit between the precipi-

-

10

-

12-

tate and the matrix. That a network of dislocations does exist at the interface, is confirmed by figure 4,

8

-

10 - r 8- d 1 L

-P

₉

-

( 0 ) 6- 1 1 50 0 1 1 1 20 60 100 1CO 1bptrnppm)-

-

Fig. 5.

-

Plots of 6 against dipole concentration, xdp.

C6-366 A. L. GUERRERO, E. P. BUTLER AND P . L . PRATT The precipitation of the Suzuki phase at 250 "C

in the more heavily doped specimens, C3 and D3, can be seen in figure 5b. This result is in agreement with the solvus o f Chapman and Ldey [3] and with our T.E.M. observations of specimens A2-D2 annealed at 250 "C. More ionic conductivity experi- ments in this batch of crystals are being carried out to get sufficient experimental data to find the bind- ing energy of the most stable dirners.

Acknowledgments.

-

We wish to thank Professor L. W. Hobbs for his invaluable help with specimen preparation and the design and installation of the low temperature stage, the S.R.C. for their financial support, the Central Research Fund of the Univer- sity of London for the provision of a Keithley 616 Digital Electrometer, the Materials Development 1)ivision A.E.K.E. (Harwell) for the use of thinning facilities, and Conicit (Venezuela) for a Research Scholarship to support Mr. A.L. Guerrero.

References [ I ] GUEWRERO, A. L., JAIN, S. C and PRATT, P L., Phys. Status

Solidi (a) 49 (1978) 353.

(21 SUZUKI, K., J. Phys. Soc. Japan 16 (1961) 67.

(31 CHAPMAN, J A. and LILLEY, E., J. Physique Colloq. 34 (1973) C9-455.

141 FIGUEKOA, D. and LAREDO, E., Solid State Commun. 1 1 (1972) 1209.

[5] HOBBS, L. W., J. Physique Colloq. 34 (1973) C9-227 ; in Surface

and deject properties of solids, Vol. 4 (The Chemical Society, London) 1975, p. 152.

[6] YACAVAN, M . J., HOBBS, L. W. and GORINGE, M. J., Phys.

Stalus Solitli (a) 39 (1 976) K 85. [7] H o e s , L. W., J. Phys. E 3 (1970) 85.

[8] KHAN, A. R., Ph. D. Thesis, Univ. of London (1967). [9] YACAMAN, M. J., G o ~ e , A. and H I R ~ , J. P., Philos. Mag. A