R 552 I Philips Res. Repts 20, 337-348, 1965

THE LANTHANUM-ALUMINIUM SYSTEM

by K. H. J. BUSCHOW

AbstractThe lanthanum-aluminium system is found to contain six intermediatephases: LaaAI (hexagonal MgaCd type), LaAI (orthorhombic CeAI type),LaAh (cubic MgCU2 type), Lao.eaále.re (hexagonal AIB2 type) andLaAla (hexagonal MgaCd type); LaAl4 is orthorhombic at room tem-perature but is transformed into a tetragonal modification (BaAI4 type)on heating. By means of thermoanalytical data the phase diagram of thesystem is constructed.

1. IntroduetionInvestigations of the lanthanum-aluminium system by thermoanalytical and

X-ray-diffraction methods 1-5) have been reviewed recently by Gschneidner 6).From these investigations it is known that four intermediate phases exist in thesystem mentioned, viz. La3AI, LaAI, LaAh and LaAI4. Recent studies in thislaboratory on some of these compounds revealed, however, that additionalintermetallic compounds are present, while some of the already known com-pounds exhibit a crystal structure deviating from findings by other authors.Furthermore the behaviour of some lanthanum-aluminium alloys was found tobe in disagreement with the accepted phase diagram of this binary system (seee.g. ref. 6). For those reasons the lanthanum-aluminium system has been re-investigated in the whole concentration region by means of metallographicmethods, X-ray diffraction and thermoanalysis.

2. Experimental

The alloy buttons were prepared by melting the charge metals together in anargon-are furnace. The purity of the lanthanum and aluminium used was 99·9 %and 99·99%, respectively. Major impurity elements in lanthanum were vanadium(~ 0·08%) and silicon (~ 0·01 %). To obtain single-phase samples some alloybuttons were heated fortwo weeks below the periteetic temperature in a ThOzcrucible inside an evacuated silica tube. Both the annealed and the arc-meltedsamples were investigated by means ofX-ray diffraction by means of a PhilipsX-ray Powder Diffractometer type PW1050/30. The X-ray diagrams wereobtained from powdered samples and from polished and weakly etched sur-faces of the samples.

3. The observed phases

LanthanumElementary lanthanum was observed microscopically and by X-ray measure-

338 K. H. J. BUSCHOW

ments in samples with less than 25 at. % aluminiutiJ.. The lattice constants ofpure a-La were found to be a = 3·762 ± 0·002 A and c = 12·170 ± 0·010 A.The addition of a few per cent of aluminium to pure lanthanum gives rise toa small decrease in the lattice parameters,· indicating a weak solubility ofaluminium in lanthanum.

LaàAl

Both metallographic and X-ray-diffraction results made it evident that thecompound richest in lanthanum is LaaAI, which agrees with landelli's results 4).It is noteworthy, however, that Iandelli found that the crystal structure ofLaaAI belongs to the cubic AuCUa type, whereas the present X-ray data show(see table I) that LaaAI possesses a hexagonal structure of the MgaCd type

TABLE I

X-ray data for LaaAl obtained bymeans of a polished surface of an alloy button(CuKa-radiation)

hkl lOa sin2 eexpo

dCA)

101 34·92 4·12 4·12- 110 45·89 3·60 3·60

200 61·01 3·12 3·12111 65·90 3·00 3·01002 78·59 2·75 2·75201 80·57 2·71 2·71210 107·07 ·2·354 2·356112 124·25 2·185 2·186202 139·36 2·063 2·063301 156·84 1·945 1·944220 182·9 1·801 1·800103 - 191·76 1·760 1·760302 215·4 1·660 1·659311 217·5 1·651 1·649203 237·26 1·581 1·581222 262·2 1·504 1·506004 313-02 1·377 1·376303 } 374·0 1·259 1·258204 1·259214 } 419·0 1·190 1-189403 1-188

expo I calc.

THE LANTHANUM-ALUMINIUM SYSTEM 339,

with the lattice constants a :- 7·195 ± 0·005 A, c = 5·503± 0·005 A; cla =0·77. As was, found by Van Vucht 7) in the case of Ce3AI, both the cubic and

,.l'hexagonallattice types are stable, though at different temperatures. A similarsituatio'n might explain the discrepancies observed for La3Al: Attempts weretherefore made to obtain the cubic modification of La3Al by quenching fromhigher temperatures.. but without" sucèess. Also thermoresistometric measure-ments point to the absence of a transition temperature in solid LaaAI aboveroom temperature, so,that as far as thepresent experiments are concerned thehexagonal form of Lá3Al seems to be the only stable one. By means of X-raydiffraction it became evident that the compound La3A1does not melt congruentlybut decomposes peritectically. Furthermore no other compounds were foundto be 'present between La3Al and LaAI.

LaAI'This intermetallic compound crystallizes with an orthorhombic lattice and

belongs to the CeAI type of structure 7). The lattice constants are found to bea = 5:809± 0·005 A, b = 7·734 ± 0·006 A, c = 9·531± 0·006 A. The unitcell contains 8 formula units. The 'X-ray data indicate that LaAI is formedperitectically .

LaAlzIt has been established by X-ray measurements that the compound LaAlz

belongs to the group of cubic Laves-phase ([CIS], MgCU2type) compounds;its lattice constant is ei = 8·153 ± 0·005 A which agrees with the results ofWernick and Geller 8) who found a = 8·145± 0·005 A. Gschneidner 6), whenconsidering the spread in the values found in literature for this lattice constant,'suggested that a region of homogeneity may exist for LaAlz. The lattice con-stants obtained from X-ray diagrams of samples richer"and poorer in aluminiumthan would correspond to the compound LaAlz do, however, not point to aregion of homogeneity of any significanee for this compound.

The compounds LaAla and LaAla;In addition to the compounds thus far reported in literature two more inter-

mediate phases were found to exist in the system lanthanum-aluminium, viz.'LaAla and LaAla;. Powder diagrams of LaAla showed that the arc-melted alloybutton was not a single phase; after annealing, however, the powder diagramwas single-phase and could be indexed hexagonally with the lattice constants'a = 6·662± 0~003A, c =4·609 ± 0,003 A; cja = 0·69. The La.Als structureproved, to be isotypic with SmAla and GdAla 9) and thus belongs to the Mg3Cdtype of structure. Spacings and intensity data for Le.Als are given in table Il.When samples varying in composition between LaAls and LaAl4 were

quenched from different temperatures it could be shown that above about

340 K. H. J. BUSCHOW

,TABLE II

X-ray data for LaAIa (powder method; CuKa-radiation)

hkl103sin2 e d(A)

Iexp• *)expo expo calc.

100 18·0 5·74 5·76 15101 46·0 3·59 3·60 120110 53·9 3·32 3·33 66200 71·7 2·88 2·88 39201 99·6 ' 2·44 2·44 160002 112·1 2·30 2·30 27210 125·1 2·178 2·177 21102 129·6 2·140 2·141 2211 152·9 1·970 1·970 28300 160·5 1·922 1·921 10112 165·2 1·895 1·894 30202 182·9 1·801 1·799 16301 188·5 1·774 1·773 2220 214·4 1·664 1·664 24310 232·4 1·598 1·598 2212 236·5 1·584 1·583 16311 259·9 1·511 1·510 35103 269·1 1·485 1·484 ~ 26302 272·2 1·476 1·475400 285·4 1·442 1·441 5401 313·5 1·376 1·375 18203 3~2·6 1·356 1·356 ~ 54222 325·7 1·350 1·349312 343·7 1·314 1·313 3213 366·7 1·272 1·271 18

213~ 375·6 1'·257 1·258 ~ 19410 1·255

*) Iexp• in arbitrary units.

1165 oe the phase LaAI3 becomes unstable and decomposes into LaAI4 andLaAl!;. The compound LaAle is poorer in aluminium than LaAI3; a single-phase sample LaAIx was obtained by quenching an alloy with x ~ 2·4 from1200°C. The powder diagram of LaAl; was indexed hexagonally: a = 4·478± 0·004 A, c = 4·347 ± 0·004 A; cla = 0·97. Spacings,and intensities of this

THE LANTHANUM-ALUMINIUM SYSTEM 341

phase are given in table Ill. The powder diagram of LaAl» was found to bevirtually identical with that of ThAh 11) which means that LaAlz seems tobelong to the [C32] AIB2 type of structure, The pycnometric density amountsto 4-1 ± 0-1 gfcm3_

LaAl4Until now this compound was considered to be the only intermediate phase

TABLE IIIX-ray data of Lao-ssAh-12 quenched from 1200 °C (powder method, CuKa-radiation)

hkl103 sin2 () dCA) Iexp. *)

expo expo calc.

001 I 31·54 4·33 4·34 20

100 39·68 3·87 3·88 69

JOl 71·24 2·91 2·89 132

110 118·67 2·23 2·24 50

002 125·60 2·17 2·17 21

111 150·04 1·99 1·99 12

200 158·11 1·94 1·94 11102 165·18 1·895 1·895 14

201 189·43 1·770 1·770 33

112 242·98 1·562 1·558 25

210 276·28 1·469 1·466 9

202 283·43 1·447 1-444 9

211 307·53 1·389 1·389 39

103 321·31 1·359 1·357 10

300 354·65 1·293 1·293 9

I301 386·71 1·239 1·239 1

212 401·51 1·216 1·215 11203 439·81 1·161 1·162 5

220 473·31 1·120 1·120 4

320 480·1 1·112 1·111 8

311 543·7 1·044 1·044 14

213 557·6 1·031 1·031 12

222 598·3 0·996 0·995 6. 114 619·10 0·979 0·979 5

314 637·5 0·965 0·964 6

*) Iexp. in arbitrary units.

L__

'342 K. H. J. BUSCHOW

between LaAh and aluminium. Its structure was investigated by Nowotny 3)and more recently by Iandelli 4). These authors find that LaAl4 is tetragonal(a = 4·405' A, c = 10·140 A 6)), and is isötypic with BaAI4. In the presentinvestigation the X-ray reflexions of a powdered sample of LaAl4 did at firstsight agree with their findings; it appeared, however, that if an indexing wastried according to the unit-cell dimensions given by Iandelli and Nowotny acertain number of (chiefly minor) reflexions remained unindexed. Tests provedthat these extra reflexions are not due to the presence of a second phase in thesample. All observed reflexions together could be indexed orthorhombically<with a -;- 4·431 ± 0·005 Á, b = 13·142 ± 0·0010 A and c = 10·132 ,± 0·007 A.Spacings and intensity data are given in table IV; as may be seen, the sole

TABLE IV

X-ray data of LaAl4 (powder method, CuKa-radiation)

hkl 103 sin2 8 dCA)[expo *)expo

expo calc.

002 23·14 5·06 5·07 63101 36·09 4·05 4'05j031 ~ 36·67 4·02

. 4·02 115022 4·02130 61·09 3·12 3·12 57042 78·12 2·76 2·76 14103 82·2 2·69 2-69j033 82·77 2·68 2·67 282132 84·22 2·65 2·65141 91·03 2·56 2·56 ~ 52004 ·92·54 2·53 2·53123 ' 96·11 2·48 2·49 . 12

. 150 116·31 2·26 2·26 5200 120·82 2·22 2,21, '.3060 -123,80 2·19 2·19 3114 126·22 2'16B" 2·169 7150 ~ 144·0 2·030 2·030 ~ 9202 2·029062 146·6 . 2·102 ·2·010I044~ 147·7 2·004 ' 2·005 13·015 2·003134 153·8 1·964 '1·965 10

222 ~ 157·6 1·940 1·940 ~ 18231 . 1·939 .

THE LANTHANUM-ALUMINIUM SYSTEM 343

TABLE IV (continued)

hkl, 103 sin2 8expo I dCA)

. [expo*)expo I calc.

161 159·8 1·926 1·926 16

232 ~ 174·9 1·842'.8421105 1·842 39

240 175·7 1·838 1·837242 198·9 1·727 1·727 5233 203·6 1·707

'·706 ~163 205·8 1·698 1·69756

006 ~ 1·688144

208·4 1·6871·686

204 213·2 1·668 1·667 15064 216·1 1·657 1·656 10026 221·6 1·636 1·635 4252 230·5 1·604 1·606 3260 244·3 1·558 1·557 19136 269·4 1·484 1·484 32321 290·9 1·428 1·426 7235 296·3 1·415 1·415 ~ 31165 298·2 1·410 1·410330 302·7 1·408 1·399 8190 308·2 1·387 1·386 3

204 ~ 314·0 1·374 1.375~ 5037 r·374332 325·7 1·350 1·348 23206 329·0 1·342

'.3421192 ~ 331·6 1·3381·337 22

-- 066 1·337263~ 336·9 1·326 1·326 8

, *) [expo in arbitrary units.

criterion for the non-extinction of reflexions is. (001): 1= 2n, (hOI): h + I =2n and (OkI): k + 1= 2n. It is very likely, then, that the LaAl4 structurebelongs to one of the space groups Pnnm (No. 58) or Pnn2 (No. 34).Although thermoanalytically a rather sharp phase transition is observed in

the solid LaAl4 compound it has not been possible to obtain a diffractionpattern different from the one discussed above by quenching samples of LaA14from different temperatures above the phase-transition point. The diffraction

344 K. H. J. BUSCHOW

pattern taken from powdered LaAl4 at 1000 oe (in a hydrogen atmosphere)made it clear, however, that here the LaAl4 lattice has acquired tetragonalsymmetry with lattice constants a = 4·48 A and c = 10·42A. The ortho-rhombic symmetry reappeared on cooling.

Aluminium

Elementary aluminium was observed in all X-ray diagrams of samples withmore than 80 at. % aluminium. The X-ray data indicated that little or nolanthanum is soluble in aluminium.

4. The equilibrium diagram '

In fig. 1 the phase diagram of the lanthanum-aluminium system has been

1

10 4 '70--, at. .,. aluminium __...La

80 100Al,

Fig. 1. Phase diagram of the system lanthanum-aluminium.

THE LANTHANUM-ALUMINIUM SYSTEM 345

drawn with the aid of results obtained from thermoanalytical measurements.Additional information was obtained from microscope studies and from X-rayexaminations. As far as the pure element lanthanum is concerned no clearindication could be found regarding the a -?- (3 transition temperature; the(3 -?- Y transformation was observed at 855 oe. As the error in the temper-ature data is about 5° it was not possible to obtain significant data to showwhether the transition temperature is lowered by aluminium. The phases La3Aland LaAI melt incongruently (see fig. I), a fact which was already suggested bythe X-ray examinations and which is further confirmed by microscopie obser-vations (figs 2-4). From quenching experiments combined with X-ray analysisit has become evident that the compound La.Al; is only stable between 1090 oe

Fig.2. Arc-melted alloy with 25 at. % aluminium; etched with diI.HCI; = 75 >toPrimary crystals of LaAI are surrounded by La3Al. Dark spots: the La-La3AI eutectic.

Fig. 3. Arc-melted alloy with 33 at. % aluminium; etched with dil. HCI; - 75 >toPrimary crystals of LaAI are surrounded by La3Al. Dark regions: the La-La3AI eutectic.

346 K. H. J. BUSCHOW

Fig. 4. Arc-melted alloy with 50 at. % aluminium; etched with dil. Hel; --- = 30 ~ (po-larization microscope). Primary crystals of La Als are surrounded by LaAI which in turn issurrounded by La3Al. Dark region between the crystals: the La-La3Al eutectic.

and 1240 °C; on cooling it decomposes at the former temperature into La.Al,and LaAla. The phase LaAl» on the other hand is only stable up to 1165 °C;it decomposes at this temperature into La Al; and La Ala. As already mentionedin the preceding section a sharp thermal arrest (at about 915° C) is observedon cooling or heating an LaAl4 sample. The X-ray measurements stronglysuggest that the thermal arrest corresponds to an a ---;.. f3 transition witha-LaA14 orthorhombic and f3-LaAI4 tetragonal.

5. Discussion

As observed earlier for cerium 7), lanthanum forms a stable compoundR3AI *) but the compounds R2AI and R3Al2 do not occur. As these compoundscertainly are observed for the heavier rare-earth metals it must be concludedthat these intermediate phases will become unstable if the radius of the rare-earth metal exceeds certain limiting values. Regarding the crystal structure ofLa3AI the disagreement between landelli's 4) results and the present findingsis not understood. From the existence of both a- and f3-CeAh it might beconcluded, however, that the difference in stability of the cubic and the hexag-onal modification of La3A1 will not be large, in which case one form may bestabilized by minor impurities.

Very recently MeM asters and Gschneidner 12) studied the crystal structuresof rare-earth intermetallic compounds in connection with the electron-atomratio eja and the radius ratio rR/rM (M = non-rare-earth metal). As far as thecompounds RAb are concerned, their work shows that due to a relatively highrR/rM-value compared to the other rare-earth metals La.Als may be considered

*) R = rare earth.

THE LANTHANUM-ALUMINIUM SYSTEM 347.

a borderline case and may-crystallize as a cubic [CI5] MgCJ.lz-type compoundas well as a hexagonal [C32] AIBz-type· compound, Both. types of crystalstructures have been observed in the present investigation.Tt is interesting tonote, however, that the [C32] AIBz-type compound does not occur at theproper stoichiometrie composition; instead a more aluminium-rich compoundLaAl; is found (see sec. 3). The deviation from stoichiometrie composition canbe realized by either a lanthanum deficiency or an occupation of some lantha-num sites by aluminium atoms;

Keeping in mind the value x = 2·4 arrived at in sec. 3 we may represent thecompound in question by Lao.s3Mz in the case of a lanthanum deficiency andby Lao.ssAlz.12 in the case of a partial replacement of lanthanum atoms by

I .aluminium atoms. From the X-ray data given for LaAlz in sec. 3 we calculatea theoretical density of 3·74 gfcm3 and 3·96 gfcm3 for Lao.s3Alz and Lao.ssAlz'lZ,respectively. A comparison of these values with the experimental density4·1 ± 0·1 gfcm3 suggests that the presence of substitutional aluminium is morelikely than a mere lanthanum deficiency. Also from an energetic point of viewthe former situation seems to be the more probable one: the metallic valencesoflanthanum and aluminium are equal whereas the metallic radius of aluminiumis smaller than that of lanthanum; a replacement of lanthanum atoms byaluminium atoms would thus require less change in free energy than the creationof lanthanum vacancies. As no indication was found experimentally for anappreciable homogeneity range nor for a superlattice the substitution of alumi-nium atoms occurs at random. This fact gives rise to an additional entropywhich especially at higher temperatures will contribute much to the stabilityof the compound Lao.ssAlz.lZ as compared to LaAlz. This would account forthe fact that Lao.ssAlz'lZ is only stable at high temperatures.

A similar deviation from stoichiometrie composition seems to have beenobserved for some rare-earth-silicion compounds also belonging to the [C32]AIBz type of structure 13). In this case, however, the aluminium content is toolow rather than too high and because of the rather small metallic radius ofsilicon the only explanation seems to be a silicon deficiency. If use is made ofthe composition R3Sis determined by Lundin 13) for the [C32] AIBz-typecompounds the electron-to-atom ratio is nearly 3, as found for the lanthanum-aluminium compound discussed above.

As mentioned previously 9) the relative stability of the compounds RAlaand RAl4 is mainly governed by a size effect. The existence of a compoundRAla both for lanthanum and the heavier rare-earth elements from samariumon suggests that a compound RAla should also exist for cerium, praseodymiumand neodymium. Recent investigations indeed pointed to the existence of thecompounds CeAla, PrAl3 and NdAla. The corresponding binary systems arereinvestigated at present.

The relation ofthe crystal structure of ,B-LaAI4 (Be.Ala-type, tetragonal) with

K. H. i.BUSCHOW

~-LaAI4 (orthorhombic) is obvious: one of the two (identical) a-axes in the,B-LaAI4unit cell becomes about three times as large in the case of the a-LaA14unit cell whereas the remaining two axes are left intact.

Closer examinations of the compounds CeAI4, PrAl4 and NdAl4 showedthat the structures stable at roomtemperature are isotypic with (orthorhombic)a-LaAI4. In the case of SmAl4 11) this form was obtained once but it could notbe reproduced, however, so that it may have been due to some unknown con-tamination, The results mentioned above will be treated in detail in a separatepaper. A structure determination for these orthorhombic compounds is inprogress.

AcknowledgementThe author wishes to express his gratitude to Dr J. H. N. van Vucht for his

useful advice and for his interest in the investigation, and to Messrs Th. P. M.Meeuwsen and E. van den Brand for their help in the experimental work.

Eindhoven, December 1964

REFERENCES

1) G. Canneri, Met. Ital. 24, 99, 1932.2) R. Vogel and T. Heuman, Z. MetalIk. 35,29-42, 1943.3) H. N owotny. Z, MetalIk. 34, 22-24, 1942.4) A. Iandelli, The physical chemistry of metallic solutions and intermetallic compounds,

Her Majesty's Stationary Office, London, 1959.ó) F. Gaume-Mahn and M. Cohen, J. recherches centre nat. recherche sci. labs. Bellevue

(Paris) 38, 64, 1957.6) K. A. Gschneidner Jr, Rare-earth alloys, D. van Nostrand Company, Princeton, 1961.7) J. H. N. van Vucht, Z. MetalIk. 48, 253-258, 1947.8) J. H. Wernick and S. Geller, Trans. AIME 218, 866-868, 1960.9) J. H. N. van Vu ch t and.K. H. J. Buschow, Philips Res. Repts 19, 319-322, 1964.

10) J. H. N. van Vucht, Philips Res. Repts 16, 1-40, 1961.11) 1<. H. J. Buschow and J. H. N. van Vucht, Philips Res. Repts 20, 15-23, 1965.12) O. D. McMasters and K. A. Gschneidner Jr., Nuclear Metallurgy Series X, 93-157,

1964.13) C. E. Lundin in E. V. Kleber (ed.), Rare earth research, MacMillan Co., New York,

1961,p. 306.