Some physical properties of LaFe4P12 type compounds

J. Ph,w. C’hcm Solids Vol. 45. No. 8/9. pp. 877-886, Printed in the U.S.A.

1984

cQz2-3697/84 Pergamon

SOME PHYSICAL PROPERTIES TYPE COMPOUNDS

53.00 + .oo Press Ltd.

OF LaFe,P,,

F. GRANDJEANand A. Gw Institut de Physique BS, Universitk de LiZge, B-4000 Sart-Tilman, Belgium D. J. BR~UN~ and W. JEITSCHKO$ Abteilung Chemie der UniversitHt Dortmund, D-4600 Dortmund, West Germany (Received 21 November 1983; accepted 15 December 1983) Abstract-Several pnictides with the filled skutterudite (LaFe*p,* type) structure were investigated by X-ray diffraction, magnetic susceptibility and electrical conductivity measurements and M6ssbauer spectroscopy. The structure refinement of CeFe,P,, from single crystal X-ray data resulted in a conventional residual value of 0.022 for 11 variable parameters and 317 reflections. LaFe$,, and CeFe,P,* are paramagnetic at room temperature. EuFe,P,, is a ferromagnet with a Curie temperature of 99 K. CeFe,P,, and CeFe4As,t are ~~ndu~ng. IaFe,Pn,, (Pn = P, As or Sb) and CeFe,Sb,, show me&llic conductivity. The room temperature “Fe Miissbauer spectra of LaFe,P,,, CeFe,P,, and EuFe4P,, are symmetrical doublets. The isomer shifts are compatible with low spin or metallic Fe. At 1.5 K the s7Fe MBssbauer spectrum of EuFe,P,, under an external field shows that Fe does not carry any magnetic moment. 15’EuMijssbauer spectra of EuFe,P,, at 4.2 and 300K show unusual hyperfine parameters. 1. ~ODU~ON

type corn~un~ which were also characterized by “Fe and “‘Eu Massbauer spectroscopy. Fnrthermore we relate the results of a structure refinement of CeFe,P,* from single crystal X-ray data which serves as an introduction to the structural characteristics of these phases. It is also of interest in comparison with the structure of LaFe,P,,[8] where La is, as usually, trivalent while Ce is at least partially tetravalent in CeFe4P,,.

The skutterudite (CoAs,) type structure occurs for all nine combinations of the transition metals T = Co, Rh, or Ir with the pnicogens Pn = P, As, or Sb[l-31. These compounds are diamagnetic semiconductors or semimetals[d] while isotypic Nip, [2] with one addi-

tional electron per formula unit is believed to be a metallic conductor[7J Recently ternary phosphides [8], arsenides [9], and antimonides [lo] were reported which crystallize with a “filled-up” version of this structure. Their composition is represented by the formula LnT,Pn,, where T = Fe, Ru or 0s. In these compounds the large voids formed by the TFn, (N T4F+n,3 “frame work” are filled with lanthanoid (Ln) atoms (Fig. 1). In addition it can be expected that it should be possible to prepare such compounds where these voids are filled with other large electropositive metal atoms, e.g. earth elements or actinoids, of which some thorium containing compositions have already been rcportedjl21. Figure 2 shows the volume of the body centered cubic cell of the known Ln containing representatives with this structure. The varying deviations of most of the Ce, Sm, and Eu containing compounds from the linear volume decrease suggest mixed valencies for these compounds. The present paper reports magnetic and electrical conductivity measurements of some representative compositions of the ternary filled-up skutterudite

2. SAMPLEPREPARATION The samples

were prepared by reaction of the elemental components in evacuated sealed silica tubes essentially as described before[8-lo]. All metals were of stated purity 99.9% or greater. The lanthanoids were in the form of filings prepared under Ar and were not exposed to air prior to the reaction. Ultrapure red phosphorus (Hoechst-Knapsack AG) was used and the arsenic (99.5%) was purified by subIimation. Crystals of the phosphides were obtained from a tin flux with the atomic ratio Ln:T:P:Sn= 1:4:20:50. These samples were annealed at 1050 K for one week and then cooled slowly (2K/h). The tin-rich matrix was then dissolved in hydrochloric acid which does not attack the crystals of these phosphides. 3. CRYSTALs~ucruRE

OF ceFe&

Single crystals of CeFe,P,, were investigated in Buerger precession and Weissenberg cameras. The patterns were entirely compatible with the body centered cubic cell of space gr&p Im3 determined for LaFe,P,,[8]. It was confirmed during the structure

$ Present address: Vereinigte Aluminium Werke, D-5300 Bonn, West Germany. #Present address: Ano~ni~h-Chemi~h~ Institut der Universitit Miinster, D-4400 Miinster, West Germany. 877

878

F. GRANLVEAN et

al.

0.94A-’ were measured with 8-28 scans, a scintillation counter and a pulse-height discriminator. The background was counted at both ends of each scan. Equivalent re&ctions were averaged and corrected for absorption assuming spherical crystal shape 01MoK,= 12Ocm-‘, pR = 0.48). The structure was refined by least-squares methods as described before for isotypic LaFe,P,,[8]. The conventions residual was R = 0.022 for 317 structure factors and 11 variable parameters. Positional parameters are listed in Table 1, interatomic distances in Table 2. A listing of the structure factors can be obtained from the authors. A MAGNEZWBUD Fig. 1. The LaFe,P,, type structure. Large spheres represent La atoms. The Fe atoms are in the centers of the P octahedra. One half of the structure is shown in the projection along an edge of the cubic cell.

The magnetic su~tibilities of LaFe,P,r, CeFe,P,, and EuFe,P,, were measured with a Faraday balance at varying field strengths (maximum field 18 kOe) between 20 and 300 K. The samples were sealed under argon in silica tubes. They were composed of several selected single crystals of between 0.1 and 0.25mm dia. and a total weight of about 1Omg for the La and Ce compounds and of 3 mg for the Eu compound. The measured susceptibility values were corrected for the diamagnetism by assuming a value of x = - 115 x 10-6cm3/mol for the correction. -This value is obtained by adding the ~ama~etic increments for Fe” [13] La”‘, and of - 3.4 x 10-6cm3/mol for the P atoms. The latter value corresponds to that of elementary silicon. This value was chosen because the bonding situation of the P atoms in these compounds is very similar to that of silicon in its elementary state and quite different from that of elementary phosphorus, where the P atoms possess a nonbonding electron pair. It should be noted, however, that the basic character of the reciprocal susceptibility curves is not much influenced by the particular value of the diamagnetic correction. Figure 3 displays typical inverse magnetic susceptibility values for LaFedP,r and CeFepP,,. Both

Table 1. Positional and thermal parameters of CeFe,Pu. Standard deviations in the least significxmt digits are given in parentheses. Thermal parameters are defined by T = exp [-2arz(U,,hz& + . . . + 2~,~~k~b~ + . . .)I; equivalent isotropic parameters B are in A2 La

ce

Pr

Nd

Pm

Sm

Eu

M

Fig. 2. Cell volume of compounds with filled skutterudite (LaFe,P,* type) structure. The broken lines indicate the corresponding volume decrease as observed for LnP compounds with NaCl type structurefl 11.

fm3

Fe

P 24(g)

2(a)

8(c)

x

0

l/4

0

Y

0

l/4

0.3522(l)

z

0 0.0053(l)

ull

The lattice constant a = 7.792 t_ 0.001 A was obtained from a least-squares refinement of Guinier powder data. X-ray intensity data were collected from a crystal of globular shape with graphite mon~~omati~ MO& radiation in a four-circle diffractometer. Al1 reflections within one octant up to (sin@)/1 =

Ce

1/4 0.0023(1~

refinement.

u22

"11

v33

51

"12 %3 '23

B

0 0 0 0.42(l)

"11 ull 0.0001(1) %2 u12 o.lsfzt

0.1501flf

O-0035(2) 0.0043(2) 0.0043(2) 0 0 0.0000(1) 0.32(Z)

819

Some physical properties of LaFe,P,, type compounds Table 2. Site symmetry, interatomic distances (A) and angles in CeFe,P,,. All distances shorter than 3.5 A are listed. Standard deviations are 0.002A and 0.1”. For comparison the corresponding values of LaFe4Pz2[8]are given in parentheses Ce

m3 12 P

Fe

2

P

m

8 Fe 6 P 2 Ce 2 Fe 1 P' 1 P' 1 Ce 4 P 1 P 4 P

2.983 3.374 2.244 3.374 2.244 2.304 2.338 2.983

(3.012) (3.391) (2.259) (3.391) (2.259) (2.288) (2.356) (3.012)

2,960

(2.968)

3.283 3.374

(3.284) (3.407)

P-Fe-P P-Fe-P

P-Fe-P P'-P-P"

Fe-P-Fe

Fe-P-P' Fe-P-P* Ce-P-P' ce-P-Pm Ce-P-Fe

4x180.0 6x 82.5 6x 97.5 lx 90.0 1x120.5 2x110.8 2x110.3 1x156.9 lx 66.9 2% 79.0

(190.0) ( 82.1) ( 97.9) ( 90.0) (120.1) (111.1) (140.2) (157.0) ( 67.01 ( 78.7)

Fig. 3. Inverse magnetic susceptibility of LaFe,P,,( x ) and CeFe,P,,(@) vs temperature measured at 5.8 kOe..

compounds exhibit a slightly temperature and field dependent paramagnetism. According to the relation & = 2.&J&,,,, . T)“& the room temperature susceptibilities of our samples correspond to effective magnetic moments per formula unit of 1.46 @Bfor LaFedP,2 and 1.07 pB for CeFe,P,,. In Fig. 4, the inverse magnetic susceptibility of EuFe4Plz is shown as a function of temperature. The plot is practically linear, and is typical of a ferromagnet. The Curie temperature is 99 f2K. The associated magnetic moment deduced from the Curie constant is p = 6.2 + 0.1 &R per formula EuFe,P,,. 5. ELECIRICALMEASUREMENW The electrical conductivities were determined using a two-probe technique between 4.2 K and room temperature. Small single-crystals were used for the LnFe,P,&ype phosphides and cold-pressed pellets for the arsenides and antimonides. The results are only qualitative and show the metallic or semiconducting behavior of the samples. Because the contacted areas of the single crystals and the porosities of the pellets were not measured, no absolute values of the conductivities were determined. The temperature dependence of a particular sample, however, was judged to be much more reliable. The potential differences were determined with a compensator. In

zou

-I

300

T(H)

Fig. 4. Inverse magnetic susceptibility of EuFedP,2vs temperature. ail cases, the current-voltage relations were found to be linear for both directions of the current. Therefore we judged the conductivities not to be substantially influenced by surface barriers. The results are summarized in Figs. 5-7. CeFedP,, and CeFe.,As,, are semiconductors. The linear portion of the plot at about 3OK for ceFe,P,* to an activation corresponds energy-from R = Ro exp(E,lkT)-of E, = 0.015 eV. The extrap-

F. GRANDJEAN et al.

880

I

I

t

6

16

30

I

loo

9

I

300

Fig. 7. Electrical resistivity of LaFe,As,,( x ), CeFe,Sb,, (0) and LaFe,Sb,?(+) as a function of temperature. Fig. 5. Logarithm of the electrical resistivity of CeFe,P,,( x ) and CeFe,As,2(@) as a function of inverse temperature.

I

e.o60i 40 -

I

1

1

I

I

4

10

30

100

300

T(K)

Fig.

6. Electrical EuFe,P&)

resistivity of LaFe,PdO) as a function of temperature.

olation to higher temperatures for CeFe4As12 results in an activation energy of 0.01 eV. All other com-

pounds show temperature dependencies of their electrical resistivities typical for metallic conductors. The superconducting transition of LaFe4Plz at about 4 K found by Meisner[l4] was not investigated. 6.

M&SBAURR

MRAS-

All Miissbauer spectra of the LnFe.,P,, compounds with Ln = La, Ce, Eu were recorded with a constant acceleration type spectrometer, equipped with a 57Co source diffused in a Bb matrix for s7Fe spectra and with a ‘%mF, source for 15’Eu spectra. All “Fe

isomer shifts are referred to Fe metal and “‘Eu isomer shifts to EuF,. Figure 8 presents the “Fe Miissbauer spectra of LaFe,P,,(A), CeFe,P,,(B), EuFe.,P,,(C) at room temperature. All are symmetrical doublets with the hyperfine parameters (6 = quadrupole splitting, 6 = isomer shift, r = linewidth) given in Table 3. The values for LaFe4Pu are in agreement with those found by Shenoy et al. [15] at room temperature. At 77 K, EuFe,P,, presents a slightly broadened and asymmetric doublet which was nevertheless fitted as a symmetrical doublet with the hyperfine parameters also listed in Table 3. The increase of the linewidth from 0.24 mm/s at 300 K to 0.42 mm/s at 77 K reflects this broadening. At liquid nitrogen and liquid helium temperatures, the “Fe M&sbauer spectra of LaFe.,P,, and CeFe,P,, are unchanged; their hypetine parameters are also given in Table 3. The evolution of the isomer shifts is compatible with the second order Doppler shift. The evolution of the quadrupole splitting and the isomer shift of LaFe,P,r are slightly different from that observed by Shenoy et al.[15]. The “Fe and “‘Eu low temperature Mijssbauer results on EuFe4P,, were presented elsewhere[l6]. Thus, only the conclusions from these data are recalled here. 57Fe measurements in an external magnetic field show that Fe is nonmagnetic in this lattice, whatever its charge state. The ls’Eu isomer shift amounts to - 6.0 + 0.2 mm/s and is constant from 4 K up to 300 K. Below 100 K, “‘Eu typical Zeeman patterns are observed as shown in Fig. 9 at 77 K. By applying an external magnetic field, the “‘Eu hyperfine field is found antiparallel to the magnetic moment and amounts at 4.2 K to - 670 f 10 kOe and at 77 K to - 534 + 6 kOe. These unusual values of the hyperIme

Some physical properties of LaFe,P,2 type compounds

881

parameters are discussed in detail in [ 161 and in the following

+

3 i_ n

I -I

-1.0

0 vlmms-1

-05

0.5

Fig. 8. Room temperature s’Fe MBssbauer spectra of LaFe,P,,(A), CeFe,P,,(B) and EuFe,P,,(C). The solid lines are the results of least squares fits with symmetrical doublets.

Table 3. “Fe Mkbauer hyperfine parameters in LnFe,P,, compounds. The values of the quadrupole splittings 6, the isomer shifts 6 (relative to Fe) and the linewidths r (at half-height) are in mm/s. Accuracy on all values: f 0.01 mm/s

La

0.33

0.14

0.26

ce

0.37

0.15

0.28

BU

0.57

0.16

0.42

IF. La

0.36

0.17

0.30

ce

0.41

0.14

0.29

section.

7. DISCUSSION From a crystal chemical point of view the LaFe,P,, type compounds belong to the large number of polyanionic compounds which can be rationalized on the basis of classical two-electron bonds[7, 171. In applying this simple concept, two electrons are counted for all short Fe-P and P-P interactions while the interaction of the lanthanoid components with the neighboring P atoms may essentially be assumed as ionic. For CcFe,P,,, where the cell volume suggests (at least to a first approximation) the oxidation +4 for Ce, we may thus write number Ce4+[Fe4PJ-. The oxidation number of the iron atoms in the (three-dimensional infinite) polyanion [Fe,P, j”- can be obtained as usual by counting the electrons of the short Fe-P interactions as belonging to the P atoms. This results in the oxidation number + 2 for the Fe atoms. In agreement with the structural systematics of many related transition metal polyphosphides[l8) (where the metal atoms are also octahedrally coordinated by P atoms) one may thus assume a spin compensated (low spin) d6 system for the Fe atoms. These bonding characteristics were summarized for the polyanion [Fe,P,J4- N [FeP,]earlier[8] in a simple MO diagram. Although we will see that this diagram is too simplistic to account for all properties of the compounds, we use it as a starting point for the discussion. We will discuss the physical data and their possible interpretation one after the other. It will be apparent at the end of the paper that it is difficult to correlate all data with a simple band structure model. To facilitate a comparison of the various compounds their most important properties are summarized in Table 4. Electrical conductivity It was noted before{81 that the polyanion [Fe4P,J4- _ [FeP,-] is saturated electronically and, like the isoelectronic and isostructural compound COP,, it should be semiconducting. The conductivity data of CeFe,P,, (Fig. 5) indeed show an increase of the conductivity with temperature. This is also the case for the corresponding arsenide CeFe,As,r, while the antimonide CeFe.,Sb,r (Fig. 7) shows metallic (or semi-metallic) behavior. This agrees with the generally observed tendency for homologous compounds of a decreasing band gap with increasing atomic weight. A similar behavior is observed for the analogous series COP,, CoAs,, CoSb,, where it was proposed that the band gap between the bonding and antibonding Fe-P bands is getting closed in the arsenide and antimonide[6]. The only slight inconsistency with this is, that the band gap of 0.015 eV for CeFe,P,r seems already rather small. It was, however, computed from a part of the diagram (Fig. 5) at rather low temperature and it may be due to impurities while the intrinsic region at higher tem-

882

F.

III

III

-4

-6

GRANDJEANet

-2

al.

I

0 vkmh)

II

11

2

4

Fig. 9. lslEu Miissbauer spectrum of EuFe,P,, at 77 K. The solid line is the result of a least squares fit as described

in [16].

Table 4. Summary of the results compound

LaFe4P,2 ,,eff=l.4i$1

supracond. T

c

=44K* zero

7.7920

CeFe4P,*

7.8055

EuFe4P,*

paramagn.

semi-

"eff'l.07uS

conducting

ferromagn.

metallic

Tc=99K veff=6.2u*

CeFe4As,

2

nwment

I b ’

8.3252

LaFe4Ay2

magnetic on Fe

UIlUSUal 151 Eu parameter

9

metallic

s.2g5gi

semiconducting

LaFe4Sbt2

9.1395

’

CeFe4Sb12

9.1350

I

metallic metallic

! I *

ref.

or

semimetallic

1141

peratures may not have been reached in our sample. The conductivity data of LaFe,P,,, LaFe,As,,, LaFe,Sb,, and EuFe.,Pi2 (Figs. 6 and 7) indicate metallic conductivity. This is on the one hand satisfactory because La and Eu (in contrast to Ce) cannot provide four valence electrons and therefore these compounds must be electron deficient. On the other hand these are the first results which are somewhat at variance with the previously proposed[8] MO diagram. The consistency of the semiconducting behavior of CeFe4P12 and CeFe4As12 and the metallic behavior of the corresponding La compounds suggests that a rigid band structure could account for the physical data of these compounds. To be consistent

with the metallic conductivity, the Fermi level has to cut through a band which extends throughout the structure. Thus the MO diagram as drawn in [8] could only be essentially correct if the 3d levels of the Fe atoms were not localized but had some band character. Since the Fe-Fe distances of 3.9 A are too great for such a direct interaction, this could be achieved via the La atoms. (The La-Fe distances are only about 13% longer than the La-P distances!) Another band which extends throughout the structure is the Fe-P bonding band. In order for it to account for the metallic conductivity we would need to raise it above the non-bonding Fe 3d levels. Actually we will learn from the discussion of the magnetic and Miissbauer data that a combination of these two rationalizations

Some physical properties of L.aFe,P,, type compounds seems even more attractive: The metallic conductivity could be achieved via an interaction of the La atoms with the Fe-P bonding band, while the non~n~ng Fe 3d (+a) states remain localized, filled and below the Fermi level. Interatomic distances Some indications about the occupation of bonding and anti~nding levels cau be obtained from a comparison of the in~ratomic distances of CeFebP,, with those of LaFe,P,, (Table 2). The lattice constant of the Ce compound is 0.5% smaller than that of the La compound. As could be expected, the Ce-P and &-Fe distances are shorter (by 1.0 and 0.5%) than the corresponding distances in the La compound. The Fe-P distances, however, are also 0.7% shorter in the Ce compound, although this would not need to be, because the free positional parameters of the P atoms could adjust to keep the F*P distances constant. This may be taken as an indication that the Fe-P bonding band in LaFe4P,, is not completely filled and that the metallic condu~ti~ty of this compound (and LaFe,Aslz) is accomplished through this band. In this structure the Fe-P distances cannot vary without also affecting the P-P distances: If the lattice parameter is held constant, a shortening of the Fe-P distances requires a stretching of at least some P-P distances and vice versa. Of the two inde~ndent P-P distances of the two structure refinements, one is shorter and the other is longer, both by 0.7%. Since the lattice constant of CeFedPIZ is smaller, the dilatation of the one P-P distance in CeFe4Pls is very remarkable. It may be a reaction to the complete filling of Fe-P bonding levels, but it could also be due to partial filling of antibonding P-P levels. (An incomplete filling of bonding P-P levels can be ruled out in view of the relatively high electronegativity of phosphorus.) Regardless of whether the P-P antibonding leveh are partially occupied or not, this should not have a great influence on the temperature dependence of the electrical conductivity because the P-P interactions do not extend throughout the structure. Magnetic susceptibility and Mihbauer measurements According to the simple band structure of Ref. [S] CeFe,P,, should be diama~eti~ because Ce4+ does not carry a magnetic moment and because the polyanion [r;e4P,,14- should be saturated and diamagnetic like the isoelectronic compound CoP,[6]. Instead paramagnetism is observed with a magnetic moment corresponding to k = 1.07pB at room temperature. For LaFe4P,, pa~ma~etism was originally expected because it lacks one electron per formula unit. If this electron was missing from the nonbonding localized Fe 3d states (as the MO diagram in Ref. [8] suggests), one quarter of the Fe atoms would have a (low spin) d5 configuration. This should have resulted in a magnetic moment of p = 1.731s, per formula unit as compared to the experimental value of * = 1.46~~ The 57Fe isomer shifts in Ce-, La- and EuFe4P,2 are

883

compatible with only one type of Fe either low spin F$ + or low spin Fe3 + or metallic Fe. In any case, whatever its formal charge state, Fe carries no magnetic moment in La- [ 151or EuFe4PLz as shown by the Miissbauer measurements in an external field and does not see any magnetic field in CeFe4P,, at 4 K. Thus Fe must have a very similar electronic configuration in the three compounds. This suggests that the nonbon~ng Fe 3d ( - t&) states are @led in all of these compounds and well below the Fermi energy. Thus they also cannot be responsible for the magnetic properties and we have to look for other rationalizations. In view of the metallic conductivity of LaFe,P,, the relatively weak (considering the large formula unit) par~agnetism of 1.46 fig at room temperature could be rationalized as a kind of strong Pauli paramagnetism. The paramaguetism of CeFe,P,, may be associated with a mixed or intermediate valence of the Ce atoms. As can be seen from the smooth slope of the volume plot (Fig. 2) the Ce atoms in the series LnOs,Sb,, have oxidation number + 3. In the other series more or less pronounced dips in the volume curves occur for the Ce compounds, indicating varying amounts of Ce4+. The deepest dip occurs for the Ce compound in the series LnFe,P,,, but this does not prove that Ce is fully +4 in CeFe,P,,. This rationalization of the parama~etism of CeFe,P,, can only be consistent with both the semiconductivity of CeFe4P,z and the metallic conductivity of LaFe,P,, if the valence levels of the lanthanoids (4fSd6s) participate in the formation of a band which is partially filled in the La compound and completely so in CeFe4P,,. The properties of EuFe,P,, are extremely surprising. It is a ferromagnetic compound where only Eu can carry magnetic moments, with a Curie temperature of 99 K, an effective magnetic moment of 6.2 PJformula unit and Eu-Eu distances amounting to about 6.8 A. The ‘$‘Eu MGssbauer parameters are also unusual: a temperature independent isomer shift of - 6 mm/s and a saturated giant hyperfine field of - 670 kOe. In order to get a consistent picture of this compound, the first question to answer is: what is the Eu valency? A pure + 3 valency may be excluded on the basis of the magnetic movements, the IsiEu isomer shift and hyperfme field values and the volume curve. Then, we shall first assume that Eu is + 2 in EuFe,P,, and see how this is compatible with all experimental results. The observed effective moment of 6.2 &formula unit is lower than the theoretical effective magnetic moment of Eu’+ :7.94pB[19], but it is not unusual to observe such a difference. in Eu2+ Au,Si,[20], for instance, a magnetic moment of 6.7 &formula unit is observed with a Curie temperature of 15.5 K and an “‘Eu isomer shift of - 11.8 mm/s. Then, the temperature independent isomer shift observed in “‘Eu amounting to -6.0 mm/s retative to EuF, is a particularly large value for a Eu*+

F. GRANDJEAN et al.

884

metallic compound[21] and approaches what seems to be the highest value observed to date which is near - 5.0 mm/s on Eu’+ diluted in S~,,,Y,,~sS[221. This high isomer shift value measured in EuFe.,P,, corresponds to a high s electron charge density at the nucleus just as the high hypet%ne field corresponds to a high unpaired spin density at the nucleus. Indeed, the saturation hyperflne field, H = - 670 kOe measured on ‘s’Eu in EuFe4P,, seems to be the highest presently observed for a Eu2+ metallic compound. The different contributions to the hyperhne field are discussed in detail in [16]. It can be concluded that the total hyperflne field is the sum of three negative terms: H”f, the core polarization field of ionic Eu2+ having the generally accepted value of - 340 kOe, Hsp(s, d), the hyperfine fields produced by the self ion polarization of the s and d conduction bands arising through contact and core polarization mechanisms respectively, H”Q d), the hype&e fields produced at a Eu nucleus by the polarization of the s and d bands due to the neighboring atoms. The sum HsP(s, d) + H”f’(s, d) amounts to - 330 kOe. When Hv and H”P are due to the polarization of the s band, they both are positive. Consequently, this negative value can only be explained by the polarization of the Eu 5d band which must play an important role in the magnetic properties of EuFeqPIZ. Since the Eu-Eu distances are rather large (6.8 A) for a direct magnetic Eu-Eu interaction, especially in view of the high Curie temperature of 99 K, this may perhaps be accomplished by an interaction of the Eu 5d6s orbitals with the Fe-P bonds. Such a weak P bonding interaction of the Ln atoms with the Fe-P bonds could also explain the electrical conductivity data and seems therefore an attractive rationalization. Similar magnetic orderingalthough at much lower temperature-was observed by Meisner[ 141 for PrFe4P12 and NdFe4P12. To our knowledge only EuPt2, where Pt is diamagnetic, has a similar Curie temperature, 105 K[23] with a magnetic moment of 7.8 &Eu, but with shorter Eu-Eu distances. In conclusion, even if the “‘Eu isomer shift, the hyperhne field and the magnetic moment are unusual

I

I

I

,

;_Ec

i::

: -0-t

P8

v

for a Eu2+ compound, they are compatible with the + 2 valency for Eu. Furthermore, taking into account the role of the Sd electrons emphasized by the high hyperfme field value on ‘*‘Eu, we may explain the measured magnetic moment which may result from the difference of the 4fusual contribution and of an orbital Sd contribution. Another explanation of the measured magnetic moment would be that Eu in EuFe4P12 is in a strong crystalhne field giving rise to a state different from 8S7,2(for instance a state 6J) with a lower magnetic moment. Another attractive hypothesis for explaining the behavior of EuFe,P,,, is an intermediate or mixed valence state for Eu. Indeed, the cell volume suggests a mixed valence and the magnetic moment is lower than the theoretical Eu2+ value. If some Eu3+ is present with a theoretical null magnetic moment, a simple calculation shows that the experimental magnetic moment of 6.2 pB corresponds to a percentage of 78% of Eu2+, i.e. an intermediate valency of + 2.22. Because of crystal field effects[24], the Eu3+ magnetic moment may amount to 3.5~~ at room temperature. The same simple calculation on the basis of the experimental magnetic moment of EuFe4P12 gives a percentage of 60% of Eu2+, i.e. a valency of +2.4. The plot of the magnetic susceptibility corrected for the ferromagnetic contribution, i.e. x( T - 99)/T as a function of temperature (Fig. 10) is intermediate between the theoretical Eu2+ and Eu3+ susceptibility curves but the experimental curve is parallel to the Eu2+ curve which means that the reIative quantities of Eu2+ and Eu3+ are temperature independent. The same plots are presented by Nowik and Felner for EuMn.&[25] and EuP&B4[26] but the experimental curves for both compounds deviate more and more from the Eu2+ curve as the temperature decreases indicating an increasing amount of Eu3+. For the past few years mixed or intermediate valency of Eu has also been recognized in various other compounds, for instance, EuFe,Al, [27], EuCu,Si,[28], EuNi2P2[29, 301 and EuPd,Si,[31]. All these compounds have been studied by 15’Eu Mossbauer spectroscopy and eventually by another tech-

0

I (00

, 150

Eu Ft. P,2

-o 1 200

E:

I 250

TIK)

Fig. 10. Magnetic susceptibility x e(T - 99)/T for EuFe4P12vs temperature. For comparison the theoretical Ed+ and Eu’+ susceptibility curves are also shown.

885

Some physical properties of LaFe,P,, type compounds

nique. They all show an isomer shift which is strongly temperature dependent and which indicates + 3 character at low temperature and + 2 at high temperature. EuFe4P,, has a constant isomer shift from 4K up to 300K amounting to - 6mm/s. Nevertheless, one could think that the valence fluctuation is so fast that even at 4 K, an average isomer shift corresponding to an average valency of + 2.5 is observed. In these conditions, one would have 50% of Eu2+ and 50% of Eu3+ ions from 4 to 300 K. The magnetic behavior of these mixed valent compounds is quite different. EuPd,B,[26] and EuCu,Si,[28] are not magnetically ordered above 4 K, and EuNi,P,[29,30] not above 77 K. In EuFe,Al,[27] the Fe sublattice orders below 140K and a magnetic hyperfine field is induced on the Eu ions as in pure Eu3+ cases. EuMn4Als[25] has a N&e1 temperature of 20 K due to ordering of Eu mixed valent ions, influencing the ordering of Mn. As noted by Felner and Nowik, this magnetic ordering in a mixed valent system is quite a unique phenomenon. As a mixed valent system, EuFe4P,, would be another unique case for its magnetic behavior, since it is a ferromagnet with T, = 99 K due to ordering of Eu mixed valent ions, without any magnetic influence on the Fe atoms. Since we have concluded that below 99 K, there would be about 50% Eu2+ and 50% Eu3 + and since every Eu atom has 8 Eu neighbors at a distance of 6.8 A, the magnetic exchange would take place between Eu atoms where on average every Eu atom has four Eu*+ and four Eu3+ neighbors at 6.8 A. Finally, if there is a mixing of Eu2+ and Eu3 + ions, the different contributions to the hypertine field on “‘Eu must be reexamined and must include the orbital contribution of the Eu3+ ions due to the exchange field produced by the Eu2+ ions, the only magnetic ions in this case. As shown by Armon et al. [32], that contribution is negative and may be very large (about -1000 kOe) in compounds where the exchange field is produced by transition metals. In these conditions, in EuFe,P,, the hypefine field of -670 kOe would result from the sum of 4 terms: the core polarization field amounting to about half the generally accepted value: - 170 kOe; the self ion and neighboring atoms polarization fields, which could be positive as usually observed; the orbital field which normally is negative and large. It seems possible that the last three terms amount to - 500 kOe. From the present experimental data, we are thus unable to chose between both explanations: either an intermediate valency of Eu, or a pure valency of + 2. Further evidence, for instance X-ray absorption spectra, would be needed to make a clear decision. 8. CONCLUSION

It is difficult to rationalize all physical data with a simple energy level diagram. Certainly the diagram of Ref. [8] is incomplete in that the 4f5d6s levels of the lanthanoid components need to be added. They

probably participate in the formation of the wnduction band. Indeed, the Miissbauer data have shown that the non-bonding Fe 3d statesare very likely filled in all compounds and thus well below the Fermi energy. The unusually long P-P bonds (already present in the binary semiconducting diamagnetic CoP,[2,6] and further stretched in the ternary compounds) can be rationalized by partial filling of antibonding P-P states, however, it is difficult to state where these electrons come from. In the absence of complementary data on these compounds we cannot improve much further on the diagram previously proposed[8]. In any case such a one-dimensional diagram can only give a very crude approximation of the bonding situation because the threedimensionality of the structure cannot be adequately represented. Acknowledgements-We want to thank U. Krieger arid Prof. C. Heiden for electrical conductivity measurements and H. Stockinger, Dr. G. Meyer and Prof. R. Hoppe (all

at the Universitit Giessen) for support of the magnetic measurements. Dr. J. A. Hodges and P. Imbert from C. E. N. Saclay are thanked for their support in the M&sbauer measurements under a magnetic field. We also would like to thank Dr. G. Heymer of Hoechst-Knapsack for a gift of ultrapure phosphorus. This work was supported by the F.N.R.S. of Belgium, by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. REFERENCES

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