A study of the lattice spacings, magnetic susceptibilities and 151Eu Mössbauer spectra of some palladium-europium alloys

Jownal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

281

A STUDY OF THE LATTICE SPACINGS, MAGNETIC SUSCEPTIBILITIES AND 151Eu MOSSBAUER SPECTRA OF SOME PALLADIUM-EUROPIUM ALLOYS

I. R. HARRIS Department

of Physical

15, Birmingham

Metallurgy

and Science of Materials,

University

of Birdngham,

Edgbaston

(Gt. Britain)

AND

G. LONGWORTH Matevials (Received

Physics

Division,

A.E.R.E.,

Harwell

(Gt. Britain)

October Sth, 1970)

SUMMARY The lattice spacings, magnetic susceptibilities and 15rEu Miissbauer spectra of some palladium-europium alloys have been examined. The solid-solubility of europium in a-palladium is IO%* at about 600°C and this is succeeded by an intermediate phase at about 14 to 16% Eu, probably Pd5Eu. The lattice spacings, magnetic susceptibilities and Mijssbauer isomer shifts indicate that the europium atoms are in the 8-valent (7F,) state in the solid solution alloys, the Pd5Eu phase and in PdsEu. In the alloys corresponding to PdzEu, PdEu, PdEuz and PdEua, the magnetic susceptibilities and Mijssbauer isomer shifts indicate that the europium atoms are in the 2-valent (%7/z) state. PdaEu, PdEu, PdEuz and PdEus are magnetically ordered at 4.2”K with hyperfine fields at the europium nucleus of 17 f IO, 162 fi 5, 148 + IO and 244f 15 kOe, respectively.

INTRODUCTION

The lattice spacings of a series of LIZ-type, PdsR phases (R=rareearth metal), indicate that the europium atoms are in the 8-valent state in Pd&ur. This valency state has been confirmed by subsequent measurements of the 15iEu Mossbauer isomer shifts and the magnetic susceptibilitys. The Mossbauer work2 also indicates that the europium atoms are in the divalent state in the Cr+type, PdzEu phase. The present work is an extension of previous studies on palladium-rare earth systemsQ-5 and provides information on the valency state of europium in the solidsolution alloys and in the Pd-Eu intermediate phases. The use of the 151Eu Massbauer resonance provides a simple and unambiguous determination of the valence states of the europium atoms. These investigations are also intended to supplement previous work on band filling in palladium alloys using the Mijssbauer effect’+. * All percentage compositions in this paper refer to atomic %. J. Less-Common Metals,

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282 PREVIOUS WORK

The lattice spacings of some Lrz-type PdsR phases exhibit a smooth variation of u-spacing with atomic number (2) and the lattice spacings of P&Eu and P&Yb indicate that these metals are in the 3-valent state in these phasesi. This behaviour can be contrasted with the isomorphous SnaR and InaR phases where the lattice spacings indicate that the EU and Yb atoms are in their normal, divalent statelo. The trivalent state of Eu in PdaEu has been confirmed by some 15iEu isomer shift measurements2 and these measurements also showed that, in the Crg-type phase, PdaEu, the Eu atoms are in the normal divalent state. These measurements indicated a hyperfine field (Hhyp) of < 20 kOe for the latter phase and zero for PdaEu, which is in accordance with the 3-valent state of Eu in this phase. Lattice-spacing measurements on some or-palladium-ytterbium alloys5 clearly show that the ytterbium atoms are in the 3-valent state both in the solid solution range and in PdsYb. There does not appear to be any information on the lattice spacings of the corresponding (Ypalladium-europium alloys. MATERIALS AND EXPERIMENTAL

METHODS

The palladium used in the present investigation was supplied by Messrs. Johnson Matthey and Co. Ltd., and the spectrographic analysis is given in Table I. The europium was supplied by Rare Earth Products Ltd., and a typical analysis is given in Table I. TABLE

I

PURrTrESOF THE MATERIALS

USED

IN

THE

PRESENT

WORK,

AS QUOTED

BY

THE

SUPPLIERS

--

Palladium Europium:

:

7p.p.m.Si;~p.p.m.Fe;rp.p.m.Ca;<1p.p.m.~g;rp.p.m.Cu;<1p.p.m.Ag. < 1.0 wt.% other rare-earth metals, ~0.02 wt.% content not detected by spectrographic methods.

other base metals, tantalum --

The Pd-Eu alloys were prepared by arc melting in an argon atmosphere and, in general, small weight losses were obtained on melting. Selected alloys were chemically analysed and close agreement with the nominal composition was obtained (see Table II). The alloys were homogenised by heat treating for one week at an appropriate temperature. Some of the specimens were examined metallographically after grinding on successive grades of emery paper and then polishing with a preparation of diamond dust. The specimens were examined in the etched condition using concentrated nitric acid as the etchant. The powders for X-ray analysis were exposed to CuKa(Pd-rich alloys) and CoKa(Eu&h alloys) radiation in a Philips’ Debye-Scherrer camera ( I I .483 cm diam.) and the lattice spacings were derived from the diffraction pattern using an extrapolation technique for the elimination of systematic errorsii. The magnetic susceptibilities were measured using a modified Sucksmith ring balance or a Sartorious Vacuum Electra-microbalance, depending on the magnitude of the susceptibility, and it is estimated that the susceptibilities are accurate to within +0.5%. J.

Less-Common Metals, 23 (1971) 281-292

PALLADIUM-EUROPIUM TABLE

ALLOYS

283

II

THE LATTICE

SPACINGS

AND

CONSTITUTION

Nominal comQosittin (at.% solute)

Lattice s#w6+tg (-& o.oooa kX)

Pure Pd

3.8829 3.9001 3.9178 3.9368

2.01 3.98 5.97 7.96 (8.2 & 0.4)* 9.94 12.04

Pd-Eu

ALLOYS

Heat twatmant and constitution of alloys (S.S. = solid solution)

4.0869 4.0925 (PdsEu)

S.S. S.S. S.S. S.S. S.S. S.S. + S.S. + S.S. + S.S. + S.S. jPdsEu PdsEu PdsEu PdsEu PdsEu PdsEu PdaEu

7.7456 (PdaEu) 7.7458 4.4204

Pd,Eu PdEu2 + second phase***

;:;:;: 3.9687 (S.S.)

‘3.98

3.9674 (SS)

15.97

3.9663 (S.S.)

17.94 19.95 21.95 22.93 (22.6 3 0.4). 23.99 Ref. I: PdaEu 30.49

4.0689 (PdaEu) 4.0689 (PdsEu) 4.0698 (PdaEu) 4.0718 4.0771

31.92 74.9

OF SOYE

6oo’C for 2 h 6oo”C for 2 h 600% for 2 h 600°C for 2 h 600% for 2 h 6oo’C for 2 h 6oo’C for 2 h 24 h at 500°C 800% for 2 h 24 h at 500°C 24 h at 500°C 24 h at 5oo’C 800°C for 2 h 800°C for 2 h Soo’=C for 2 h 800°C for 2 h 8oo°C for 2 h

PdsEu PdsEu PdsEu*’ PdsEu + PdsEu PdsEu** + PdsEu + PdsEu + PdsEu

+ Pd2Eu

800°C for 2 h 500°C for 2 h

Analysed composition. ** Much stronger diffraction lines were obtained for PdaEu after the 500°C annealing treatment. l ** The diffraction pattern of this phase does not correspond with that of either Eu or EuaO2. l

The Mossbauer Spectra were obtained using a SmFs source incorporating i%m (30 mCi and 93 year half life). This source had a recoilless fraction, f zo.4 at 300°K and a linewidth of 2.2 mm set-1 against a thin, 3f europium absorber. Measurements were made using an electromagnetic drive system, designed by Dr. B. Window, to follow a linear velocity ramp, with a multichannel analyser operating in time modeiz. The source remained at room temperature while the absorber was either at room temperature or at 4.2”K (using a top-loading He dewar). The palladiumeuropium absorbers were prepared either by rolling the solid-solution alloys to produce foils (- 150 pm thick) or by crushing the intermediate phases and mounting in “Durofix” cement to form discs of thickness -2/p, where p is the electronic absorption coefficient. The spectra were fitted to Lorentzian line shapes and, in the case of the magnetically-split spectra, these were fitted to the full r&line hyperfine pattern, with the relative heights and spacings of the components fixed at the values determined by the known, nuclear level parameters. EXPERIMENTAL

RESULTS

(a) X-ray measurements

The room temperature (23OC) lattice spacings and constitution of the Pd-Eu alloys are summarized in Table II. There is a linear variation of lattice spacing with o/OEu in the a-Pd solid-solution alloys up to the solubility limit of 10% Eu at about 6oo”C (see Fig. I). The diffraction patterns of the alloys in the range 14--16% Eu, 3.

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I. R. HARRIS, G. LONGWORTH

284

after annealing at 600°C for 2 h, indicate the presence of a second phase corresponding to about the 14-167~ Eu composition (probably PdsEu). There is a pronounced increase in the intensity of the lines due to this phase after annealing at 500°C for 24 h. The diffraction patterns of the alloys at 18% and 20% Eu indicate that these alloys consist of a mixture of Pd.=,Euand PdaEu after annealing at 800°C for 2 h.The lattice spacings of the alloys in the homogeneity range of P&Eu lie close to the linear behaviour of the solid-solution alloys and good agreement is obtained with the previous lattice spacing results on PdsEur.

10.0-

9.0-

0

-I 10 AlOMlC

20 % Eu

30

30

Atomic “10 Eu

Fig. r. The room-temperature lattice spacings of some a-Pd-Eu alloys. The half symbol represents the two-phase mixtures of either S.S. + PdsEu or PdsEu + PdsEu. Fig. 2. The room-temperature magnetic susceptibilities represent two-phase mixtures. a Ref. 3. The dotted and II.

of some oL-Pd-Eu alloys. The half symbols curve III represents the summation of I

The lattice spacing of the Cxg Laves phase, PdsEu, (7.7458+-0.0005 kX) is in good agreement with previous work on this phase2 (7.743f0.005 kX) and an identical lattice spacing is obtained in the two-phase alloy at 30.5% Eu (PdaEu+ PdzEu). This indicates that the Laves phase has a very limited range of composition on the palladium-rich side of the PdzEu composition. Diffraction patterns have also been obtained for alloys at the PdEu, PdEu2 and PdEus compositions and distinctly different patterns were obtained in all cases. The PdEua alloy exhibited strong lines characteristic of a f.c.c. structure with a-spacing = 4.4204 + 0.0002 kX and weak lines which did not correspond with either those of Eu or Eu2Oa. The diffraction pattern of PdEuz is extremely complicated, indicating a structure of low symmetry. (b) The magnetic susceptibility measwements The room temperature (23°C) magnetic susceptibilities of the cx-palladiumJ. Less-Conamon Metals, 23 (1971) 28r--292

PALLADIUM-EUROPIUM

ALLOYS

285

europium alloys and PdsEu are plotted against the composition in Fig. 2. There is a minimum between 5% and 6% Eu. For the alloys PdaEu and PdEuz, there is a linear variation of I/X~ with T in the range 100~-300~K but for PdEu, departures from linearity were observed below about 170°K. The values of @ and the effective moment of Eu (PM) (assumingpPd =o in these phases) are given in Table III. Good agreement is obtained between the value of ep and ,ueftfor Pd2Eu obtained in the present work and that reported previouslyg.

(c) The Miissbazler measureme& The isomer shifts (Au,), hyperfine fields (Haup), and line widths are collected

TABLE III Alloy

/Jeff*

e

Pd2Eu

7.8

2180

(OK)

2

80

**PdEu PdEua

;::

0

33

8.2

Ref. Present work Present work Present work

* In the %~,a state, europium has a theoretical effective moment of 7.94 ~ELB. ** Deviations from Curie-Weiss behaviour were observed below 17o’K.

TABLE Alloy (% EM)

2.OI 3.98 5.97 7.96 9.94

12.04 13.98 15.97 17.94 19.95 21.95 23.99 PdsEu 30.90 31.92 50 67

74.9

IV Isomer shift 300°K (mmlsec) 4.40 4.28 4.29 4.23 4.12 4.05 2.79 2.56

Hyperfine q.a’K (hoe)

& + -f & & xt zt f

0.03 0.05 0.01 0.01 0.03 0.05 0.05

f

0.02

0.17 3.55 & 0.14 2.82 f 0.2 4.22 f 0.3 2.78 f 0.08 4.20 f 0.04 4.14 + 0.02 4.28 f 0.02 4.34 zt 0.02 4.49 f 0.03 -9.13 f 0.02 4.37 f 0.04 -9.05 f 0.02 -8.30

-8.20 f 0.05 -11.70 f 0.02 1.60 f 0.10 -7.4 f 0.10 -II.0 f 0.20 0.69 f 0.08 -7.6 f 0.05

field

Line width (mmlsec) 3o0°K 2.26 2.74 2.27 2.30

f f f &

2.92

f

4.2”K 0.04 0.05 0.03 0.04 0.07

0.12 2.54 f 0.02 2.51 f O.PI 2.52 f 0.16 2.80 & 0.30 2.21 + 0.34 2.48 f 0.16 2.23 & o.II 2.78 & 0.10 2.40 & 0.08 2.48 f 0.03 2.3 f 0.2 2.4 f 0.1 2.4 f 0.2 2.34 & 0.04 2.48 f 0.11 3.41 -& 0.12 2.2 + 0.4 2.2 -& 0.6 3.2 & 0.6 3.6 f 0.6 2.7 f 0.3 2.80 f 0.08 3.30 f

I7 f I62 f 148 f

IO 5 IO

244 f

I5

J. Less-Common

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2.6 f 0.3 2.62 + 0.06 3.4 5.0 f

0.2

23 (1971) 281-292

5r

I

I

I

I

I

I 20

I

1

25

30

I

4-

t 3-

*+t+ 2-

I

I

t

5

IO

15

>

-8 -

-9 3

-‘So

I

t

40

50

t

I

I

I

I

60

TO

80

90

100

AT. “10 EUROPIUM

Fig. 3. The variation of isomer shift (A au) with europium concentration (Csu) in the palladiumeuropium alloys at 300°K. The half symbols represent the two-phase mixtures. The zero on the isomer-shift axis corresponds to the source (SmFa)Eu.

I

2 % Absorptm

-

-20 III1

-15 IIl~IIIII

-10

-5 IllI

0 III1 Vehy

5 IIII

IO /III

hl

15 III1

IIll

II/I

WC-‘)

Fig. 4. The rsrEu Mijssbauer spectra for PdEu, PdEus and PdEua at 300’K. velocity scale corresponds to the source (SmFa)Eu. J. Less-Comma

Metals,

23 (1971) 281-292

The zero of the

PALLADIUM-EUROPIUM

ALLOYS

287

SO%Eu

67XEu

75%Eu

iwhh -30

-20

-10

0

VeLmty himWC-‘)

Fig. 5. The 151Eu MBssbauer spectra for PdEu, PdEua and PdEus at 4.z’K. The zero of the velocity scale corresponds to the source (SmFa)Eu.

in Table IV and the isomer shifts are plotted against composition (Cx,) in Fig. 3. In the range o-25% Eu, there is a dip in the variation of Axu with Gnu in the twophase alloys between IO and 22% Eu. There is a linear variation of Axu with CE~ in the a-Pd solid-solution alloys and the value of Axu becomes progressively less positive with increasing europium content. This trend is reversed in the ordered alloys where there is an increase in Ax= with Cxu. Negative isomer shifts characteristic of the divalent form of europium are observed in the alloys, PdzEu, PdEu, PdEuz and PdEua, and the shifts become progressively less negative with increasing europium content. Magnetic splitting was observed in PdzEu, PdEu, PdEuz and PdEua at 4.2”K, and the values of Hhyp are given in Table IV. The Miissbauer spectra obtained for PdEu, PdEu2 and PdEua at 300’ and 4.2”K are shown in Figs. 4 and 5. The spectra for PdEu2 and PdEua at 300°K also contain lines corresponding to europium atoms in an additional z + phase and also in a 3 + phase. DISCUSSION OF RESULTS

The linear variation of lattice spacing with oh Eu (see Fig. I) is characteristic of the other or-palladium-rare-earth solid solutions investigated in the authors’ laborato@-6. The appreciable solubility of europium in Lw-palladiumis surprising in view of the nominal size factor of europium with respect to palladium of 48%.The fact that the lattice spacing of the solid-solution alloys extrapolate close to that of J. Less-Common

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I. R. HARRIS,

G. LONGWORTH

the PdaEu phase indicates that the europium atoms are in the 3-valent state in the disordered alloys as well as in PdaEu. A similar behaviour was observed in the ol-Pd-Yb alloyss. The expansion of the ol-palladium lattice is thought to be due, in part at least, to the filling of the q&state+ and thus the effective size of the europium atoms is smaller than that indicated by an extrapolation of the lattice spacings to IOO~~ solute, i.e., < 3.344 kX. This might explain the appreciable solubility of europium in or-palladium. The X-ray data indicate the presence of a non-cubic phase at about 14% to 16% Eu and as the amount of this phase is significantly increased by a low-temperature anneal, it is probably formed by a peritectoid reaction. In this respect, the Lx-Pd-Eu system appears to be different from the corresponding a-Pd-Gd system6 in which alloys at about this composition consist of two f.c.c. phases, namely, the solidsolution alloy and PdaGd. The reason for the difference in behaviour of these two systems is not clear at this stage as both 3 valent Eu and Gd have closely similar atomic diameters of 3.590 and 3.582 kX, respectively, and the Mijssbauer isomer shift data indicate that the europium atoms are in the 3-valent state in the PdhEu phase. The X-ray data indicate that PQEu is succeeded by PdzEu which is a Laves phase with the CI~ (MgCua-type) structure. The lattice spacings indicate a very limited range for this phase on the palladium-rich side of the stoichiometric composition. This can be contrasted with the isomorphous PtzEu phase which exists until at least the PtaEu composition 14. The limited range of PdaEu is probably due to the presence of the stable f.c.c. PdaEu phase. The lattice spacing of PdsEu is compared with the lattice spacings of the isomorphous PdzBa, PdaCa, and Pd&r phases15 in Table V, and it can be seen that there is a regular decrease in the lattice spacing with decreasing atomic diameter of the B-component of the AaB phase (using the divalent atomic diameter of europium). TABLE

V

Diameter of B (kXJ

Ref.

(kX) PdzBa

7.937

4.476

16

PdzSr

7.810

4.294

16

PdzEu

7.746

4.076

Present work

Pd&a

7.649

3.940

16

AzB-type

phase

a-spacing

The crystal structures of the alloys PdEu and PdEuz have not been determined in the present work. The diffraction pattern of PdEua was characteristic of the f.c.c. structure with a lattice spacing of 4.4204+0.0002 kX. This value is less than the lattice spacing obtained by extrapolation from P&Eu to the 75% Eu composition (4.495 kX). Thus, a simple comparison of the lattice spacings of PdaEu and PdEua would indicate a trivalent state for the Eu atoms in PdEua. Such a comparison, however, does not make allowance for the expansion of the PdaEu lattice due to the filling of the q&state 13. This expansion means that the effective size of Eu in PdaEu is appreciably less than its size in PdEua. This is consistent with J. Less-Common

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PALLADIUM-EUROPIUMALLOYS

289

the Miissbauer isomer shift data which indicates that an appreciable number of europium atoms are in the divalent state in PdEue. The room-temperature magnetic susceptibilities (23°C) of the ol-Pd-Eu solidsolution alloys exhibit a pronounced minimum between 5% and 6% Eu. There are two main contributions to the magnetic susceptibility of the palladium alloys: (I) the decrease in the matrix susceptibility due to the decrease in the number of vacant 4.&states and (2)the increase in susceptibility due to the increasing amount of 3-valent europium which has a small, localised moment. The first contribution can be estimated assuming that it is identical with the or-Pd-Y alloys4 as both Y and Eu are in the 3-valent state in these alloys and yttrium has no localised moment. The second contribution can be estimated assuming a linear increase in xB between o and the value for PdaEu. The addition of these two variations (I and II in Fig. 2) should give a composite curve which agrees with the observed experimental variation. Figure 2 shows that the experimental and calculated curves (dotted line) are in reasonable agreement, particularly at concentrations up to 4% Eu. The magnetic susceptibilities of the two-phase alloys between the limit of solid solution and the Pd-rich limit of the PdaEu alloys vary linearly with composition and indicate that there is no divalent character of the Eu atoms in PdsEu. The variation of susceptibility with temperature for the PdaEu phase has been studied recentlya; the results were consistent with the Eu atoms being in the 3-valent state and good agreement with the observed behaviour was obtained using the VAN-VLECK analysis16 of J-states which lie close to the ground state in Euaf. The magnetic susceptibilities of the remaining Pd-Eu alloys exhibit CurieWeiss behaviour and the effective moment of the Eu atoms indicate a divalent state for Eu in all the alloys, which is consistent with Mijssbauer data. The Pd-Eu alloy exhibits a surprisingly low value of 8p ( N 0°K) in view of the departures from the CurieWeiss behaviour observed below 170°K. The Mijssbauer effect measurements have been carried out using the 21.6 keV transition in 151Eu. This is particularly useful in phase determination work since the isomer shift is sensitive to small changes in the s-electron density at the nucleus. Thus, there is a large decrease in the isomer shift corresponding to approximately 5 source-absorber line widths between europium in the trivalent and divalent states, due to the increased shielding of the s-electrons from the nuclear charge by the additional 4f-electron. In the divalent state, the 4f-shell is half filled with a sS7/~ ground state and magnetic hyperfine fields at the nucleus of 200-300 kOe have been observed in z+ europium salts and alloysl7~r* in the magnetically-ordered state. The trivalent state is based on a 7Fo ground state and 3f europium alloys usually have no hyperfine fields. There is an approximately linear variation of Axu with CxU in the a-Pd-Eu solid-solution alloys and Ax,, becomes less positive with increasing europium content. The value of Axu at infinite dilution in or-Pd is 4.5fo.05 mm see-i which is the largest shift observed to date for trivalent Eu. This variation could indicate a gradual change in the effective valency of the europium atoms from 3+ to 2+ (4fS + 4f7) but if this was the case there would be a corresponding increase in the lattice spacings. The lattice spacings in the solid-solution range, however, extrapolate close to the spacing of P&Eu and the latter is known to be characteristic of g-valent Eur. This suggests that the decrease in shift in the solid-solution region is not due to an increased J.

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4f-character but rather to a decrease in s-character. Any increase in d-character with increasing Eu additions would not be reflected in an isomer shift change since the shielding of the d-electrons is negligible ls. According to the simple rigid band picture, there should be a decrease in the number of vacant 4d states with increasing Eu content, which is consistent with the good agreement between the observed magnetic susceptibilities and the calculated variation based on a comparison with the ol-Pd-Y alloys. If the alloys between the limit of solid solubility (s.s.) and the Pd@Eu phase were simply a mixture of S.S. + PdsEu, then the values of Anu would lie on a line between the values of Anu at the limits of the two-phase region. This is not the case however (see Fig. 3) and there is a pronounced dip in the values of An” in the twophase alloys indicating the presence of an additional trivalent phase. This observation is consistent with the X-ray diffraction results which indicate the presence of a noncubic intermediate phase at about the 14-167~ Eu composition, probably based on PdbEu. This phase is probably formed by a peritectoid reaction, as a single-line spectrum was only obtained after annealing the 14% Eu alloy at 500°C for 24 h. The linear variation of magnetic susceptibility with CnUacross the two-phase region between the limit of S.S. and Pd@Eu confirms that the Eu atoms are in the trivalent state in the PdsEu phase, as any divalent character would result in a pronounced increase in the susceptibility. The variation of AE~ with CnU for alloys in the Pd@Eu range indicates an increase in s-density with increasing europium content and this behaviour would be consistent with a full 4d-band in these alloys and a gradual filling of the s-band by the higher valency Eu atoms. The isomer shift clearly indicates the divalent character of europium in the alloys based on Pd@Eu, PdEu, PdEu@ and PdEus, and the value of Axu becomes progressively less negative between Pd@Eu and europium. This implies an increase in s-electron density. If one uses the fact that one Gs-electron in Eu@+ produces a change in shift of N 15 mmjsec r@then the change in shift between Pd@Euandeuropium corresponds to an increase of ~0.1 Gs-electron. It is known that palladium has 0.36 valence s electron per atom@@, while europium is thought to have ~0.5 6selectron per atoml7. The values of hyperfine field determined in the present work are shown in Table IV. There is a large increase in field between Pd@Eu and europium, which is consistent with the observation of WICKMAN et a,?.2 that large variations in field occur in alloys in which d ligands are involved. The narrow line width of the PdEu spectrum at 4.2”K implies that all the europium sites have the same field. The field in PdEu@ is less well defined, and the spectrum at 300°K contains an additional divalent and a trivalent line. These could be due to partial oxidation of the sample. It is not clear how to interpret the spectra for PdEu@. There is clearly more than one value of hyperfine field at 4.2”K and the 300°K spectrum contains a pronounced trivalent line. The mean field of 244& 15 kOe is close to that of europium (258 kOe)@r but there is no X-ray evidence of free europium in this alloy. The trivalent phase is either due to oxidation of the sample or, possibly, to an additional phase being present. The additional divalent line at NII mm/set in PdEu@ and PdEua is probably due to EuO and this could be, at least, partially responsible for the increased width of the spectrum for PdEu@ at 4.2”K. J. Less-Common

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CONCLUSJONS

(I) The solid-solubility of europium in a-palladium is 10% at 600°C despite a nominal size factor difference of 48%. (2) There is an intermediate phase at about 16% Eu which is probably formed by a peritectoid reaction. The isomer shift of the intermediate phase at PdsEu corresponds to the Eu being in the trivalent state. (3) The lattice spacings of the solid-solution alloys vary linearly with composition and extrapolate close to the spacing of PdaEu. This indicates that the europium atoms are in the 3-valent state in both the solid solutions and in PQEu. (4) Unlike PtsEu, the isomorphous PdeEu phase has a very narrow range of composition on the Pd-rich side of the stoichiometric composition. (5) The alloys corresponding to PdEua have a f.c.c. structure. (6) The magnetic susceptibilities of the Pd-Eu solid-solution alloys are consistent with the simple rigid band picture. (7) The variation of isomer shift (Au,) with Eu concentration for z-Io~/~ Eu implies a decrease in s electron density at the Eu nucleus. (8) The variation of AEu with CEu for alloys in the PQEu range indicates an increasing s-density with increasing CE u, which is consistent with there being a full qd-shell in this phase. (9) The values of AE~ and susceptibility indicate a divalent state for Eu in the alloys PdaEu, PdEu, PdEus and PdEu3 with an increase in s electron density corresponding to "0.1 6s electron between PdnEu and europium. (IO) The alloys PdsEu, PdEu, PdEus and PdEua are all magnetically ordered at 4.2”K with values of hyperfine field of 17 + IO, 162 f5,148 f IO, and 244+ 15 kOe, respectively. ACKNOWLEDGEMENTS

The authors thanks are due to the S.R.C. for the provision of a grant for the study of the structure and constitution of alloys. Thanks are also due to B. HAWTHORNE, to

Dr.

W.

T. B. MERRYFIELD E.

GARDNER

for

and W. DALZELL

valuable

discussions

for their over

the

technical magnetic

assistance

and

susceptibility

results.

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