Modulated structure and weak ferromagnetism in Nd3Ge5 studied by neutron diffraction and magnetic measurements

Journal of Magnetism and Magnetic Materials 82 (1989) 99-108 North-Holland, Amsterdam

99

M O D U L A T E D S T R U C T U R E AND WEAK F E R R O M A G N E T I S M IN Nd aGes S T U D I E D BY N E U T R O N DIFFRACTION AND M A G N E T I C M E A S U R E M E N T S P. S C H O B I N G E R - P A P A M A N T E L L O S Institut far Kristallographie und Petrographie, ETHZ, CH-8092 Ziirich, Switzerland

and

K.H.J. BUSCHOW Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands Received 2 May 1989; in revised form 11 June 1989

The crystal structure and the magnetic properties of the compound NdaGe 5 were investigated by neutron diffraction and magnetic measurements. The compound Nd3Ge 5 has an orthorhombic structure derived from the tetragonal a-ThSi 2 structure, the ordened Ge vacancies being described by a density and displacive modulation. Below Tc = 18 K NdaGe 5 orders ferromagnetic but only the Nd atoms at one of the two crystallographic Nd sites give rise to magnetic ordering. R3Ge 5 compounds isotypic with Tb3Ge 5 were found for R = La, Sm, Gd, Tb and Dy.

1. Introduction Although rare-earth disilicides and digermanides have been known for some considerable time [1-5], several aspects of their magnetic properties need more careful investigation. This is true especially with regard to the pronounced tendency in those systems, of compound formation at nonstoichiometric compositions, such as RGe2_ x and RSi2_ x [6-10]. In recent investigations of the TbGe2_ x [11,12] and TbSi2_ x systems [13], we have found that for the composition TbGe1.67 the missing Ge atoms are ordered and a superstructure with a six times larger cell than the basic tetragonal structure of the a-ThSi 2 type (space group 141/amd ), has been identified. The superstructure, on the other hand, can be considered as a stoichiometric compound with the chemical formula Tb3Ge5. The same kind of structure was found by other investigators for Y3Ge5 [14,15]. A new structure type also related to the a-ThSi2 type has been proposed for the T b G e 2 composition [13]. The stoichiometric compound T b S i 2 does not form in the Z b S i 2 _ x system. Instead two com-

pounds with the previously reported structure types: (a) hexagonal A1B2 and (b) orthorhombic a-GdSi 2 type occur; but both compounds form defect structures with quite narrow homogeneity ranges and are separated from each other by a two-phase region near the critical composition ZbSil.67.

The compound TbSil.67_~ of the hexagonal A1B2 type has a statistical vacancy distribution of the missing Si atoms. In the orthorhombic a-GdSi2 modification of the compound ZbSil.67+8 , the vacancies are preferentially distributed within the layers of the Si(2) site 4e at z = 0.965. However, the presence of a single modulation wave vector has not yet been certified as in the case of Tb3Ge 5 [k = ½ ½ 0]. Also it seems that in the latter compound the magnetic properties are strongly controlled by the relation of the vacancy distribution to the underlying structure. The presence of various incommensurate wave vectors within the (x, y, 0) orthorhombic planes has been reported for LaOe2 and CeGe 2 [16] of the a-GdSi 2 type. It seems of interest to extend this study to other members of the rare-earth family in view of the large variety of their magnetic properties [9,17]. In

0304-8853/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

100

P. Schobinger-Papamantellos, K.H.J. Buschow /Modulated structure in Nd 3Ge5

the present investigation we will report on the nuclear structure, the magnetic structure and magnetic properties of the Nd3Ge 5 compound and will give some preliminary results about the formation of the R3Ge 5 type within the rare earth family.

2. Crystal structure and magnetic properties of NdG%_ x compounds Earlier investigations of the NdGe2 system referred to the existence of a single stoichiometric compound NdGe2 which crystallizes with the tetragonal a-ThSi 2 type (I41/amd, a = 4.24 A, c = 13.98 ,~, z = 4, with Nd at 000 and Ge at 00z, z = 0.412 [18]), common in RGe 2 and RSi 2 compounds. The main characteristics of this structure types, displayed in fig. 1, are the double layers of rare-earth trigonal prisms, centered by Ge or Si atoms. The latter atoms are supposed to form a three-dimensional network reminiscent of the graphite structure. Based on the quite short interatomic G e - G e distances of 2.36-2.45/k the structure was considered to contain covalent Ge bonds [181. o

c

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N d G e 1.67 b

I "~z;"IE

I

1 41/amd

'.,Ect;yl-l-, ;:;t, ." •

Ge vacancies at 8e Ge at 8e

C) Nd at 4a

a"

/

Fig. 1. Schematic representation of the a-ThSi z type of structure.

Mayer and Esdhat [10] mention that all RGe2_xSi x compounds are defect structures. But it was only in a more recent study of the N d - G e phase diagram [19] that precise results were given of the non-stoichiometric compound existing around the composition 62 at% Ge. This concentration is displaced by 4.7% from the suggested ideal composition of NdGe 2, the latter compound not being found in the phase diagram. The authors report the existence of two modifications: a high temperature phase NdGel.60 (b) of the tetragonal a-ThSi 2 type and the orthorhombically distorted low temperature phase NdGel. 6 (a) of the a-GdSi 2 type. The phase transition between the two modifications occurs within a composition-dependent temperature interval of 680-615 ° C. The stability of the defect structure is restricted to a narrow region of about 1.5% Ge. Within the homogeneity region, for samples which had undergone the same thermochemical treatment, the authors also observed a contraction from 13.99 to 13.93 A of the c (orthorhombic) constant with increasing Ge content. The available crystallographic data of previous investigators and the more complete results of Eremenko et al. [19] are compared to our results in table 1. The spread in the lattice constant values reflects the existence of a homogeneity region. The largest cell volume was found for the hightemperature phase (b) of approximate composition NdGel.60 [19], but most probably its lattice constants refer to high temperature data (700 ° C). Comparing cell volumes of room temperature phases of both types, it appears that the defect compositions have smaller volumes than the previously assumed stoichiometric compositions "NdGe2" which were not confirmed in ref, [19]. This suggests a volume increase with Ge content and in this sense the compound NdGel.67 used in the present investigation prepared under the conditions described below, has a comparatively small cell volume which corresponds to the composition with the lowest Ge content found. Concerning the magnetic properties of NdGe 2, Matthias and Corenzwit, and Sekizawa [17,9] report that the tetragonal modification orders ferromagnetically below T~ = 3.6 K and Op = 7 K, while the ferromagnetic PrGe 2 orders at 19 K and Op = 22 K.

P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in Nd 3Ge5

101

Table 1 Formation and structural data of binary NdGe2_ x compounds Compound

Structure type space group

a (A,)

NdGe 2

a-ThSi 2 141/amd a-ThSi 2 141/amd a-ThSi 2 141/amd a-GdSi 2 Imma a-ThSi 2basic structure superstructure Fdd2

NdGe 2 NdGe].60 (high T) NdGe] 160 (low T) NdGel.67 N d 3Ges a) (NdGel.67)

c (,~)

c/a

4.224

13.904

3.29

248.07

[2,9,10]

4.24(1)

13.89

3.27

249.70

[3,4,18]

4.23(1)

14.15

3.34

253.0

[19]

14.03

3.32 3.36 3.35

246.9

[19]

244.5

this work

4.22(1)

b (,~)

4.17(1)

4.177 5.907

14.024 17.720

V (,~3)

14.024

Refs.

1467

this work

a) The superstructure of the NdGel.67 composition has a = vF2-at, b = 31/r2-bt, c = ct, V = 6Vt.

The saturation magnetization at 4.2 K in a field of 8000 Oe corresponds to 2.18/~s/Pr.

3. Experimental procedures and results

The samples of composition RGel.67 examined in this study were prepared by arc melting in an atmosphere of purified argon ga s . The purity of the starting materials was 99.9% for the rare-earth elements and 99.99% for germanium. The samples were investigated by standard X-ray diffraction using C u K a radiation. The samples proved to be single-phase and their structure to be isomorphic with the TbaGe s superstructure type [11] (space group Fdd 2, a -- 5.905, b = 3 x a = 17.712, c = 14.031 A, which is based on the a-Th.Si 2 tetragonal structure a = 4.1754, c = 14.031 A). These resuits differ from those of Eremenko et al. [19], who found the simple tetragonal a-ThSi2 type for NdaGe s. We will report in more detail on the crystallographic and magnetic properties of N d a G e s in the next section and return to the X-ray results obtained for several other RaGe s phases at the end of this paper. Besides the differences found in the lattice constants of Nd ages in comparison with the data of other investigations, we also observed a difference in the magnetic properties. The ordering temperature T~ = 18 K and the paramagnetic Curie tem-

perature Or, --- 15 K deduced from the temperature dependence of the reciprocal susceptibility and the magnetization, fig. 2, are much higher than the corresponding values of 3.6 and 7 K given in refs. [9,17]. The effective moment derived from the slope of the X-I(T) curve equals 3.66/~B/Nd. The 8

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-

Fig. 2. Temperature dependence of the reciprocalsusceptibility (X-]) for the compound NdsGe5. The inset displays the temperature dependence of the magnetization and its field dependence at 4.2 K.

P, Schobinger.Papamantellos, K.H,J. Buschow /Modulated structure in Nd 3Ge5

102

Neutron

Wavelength

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2-Theta (Degrees) Fig. 3. Neutron diffraction pattern of Nd3Ge5 in the paramaguetic state at 293 K. The points represent the observed intensities corrected for absorption and the background. The trace represents the profile calculated by least squares fitting. The difference diagram Iobs-- I=lc is given at the top of the figure.

field dependence of the magnetization at 4.2 K shows (inset of fig. 2) that saturation is not reached in the ultimate field strength considered by us. The magnetization value of 1200 k A / m corresponds to the ordered moment value of 1.4/~ B/Nd. More details of both the chemical and the magnetic structure were derived from neutron diffraction results on a polycrystaUine sample at 21 and 293 K (h = 1.7065 A) in the paramagnetic state and at 4.2 and 1.5 K in the magnetically ordered state. The data were collected with the D M C (Double Axis Counting System) at the Saphir Reactor, Wiirenlingen. The step increment of the diffraction angle 20 was 0.10 °. The data were corrected for absorption and evaluated by the line profile analysis method [20]. The nuclear scattering lengths and magnetic scattering factors used are from refs. [21] and [22], respectively. Neutron patterns are displayed in figs. 3 and 4.

3.1. Modulated crystal structure: N d 3Ge5 The neutron diffraction pattern of N d 3 G e s in the paramagnetic state at 293 K, fig. 3, displays the same features as that of Tb3Ge 5 [11]. One can distinguish between a set of strong or main reflections (upper part of fig. 4) pertaining to the tetragonal b o d y centered lattice of the a-ThSi2 type of structure and a large number of weak reflections that can be indexed as satellite reflections of the basic lattice with the wave vector k = [{ { 0]. The same diffraction image m a y be produced by the commensurate orthorhombic cell (indexing for the space group F d d 2 is given in the lower part of fig. 4) as for Tb3Ges, w i t h the lattice constants as given in tables 1 and 2. The refined structural parameters from the r o o m temperature nuclear intensities are summarized in table 2. The resulting R-factors, R n = 8.3% for the integrated nuclear

P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in Nd zGe5

103

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2-Theta (Degrees) Fig. 4. Neutron diffraction pattern of Nd3Ge 5 in the magnetically ordered state at 1.5 K. The points represent the observed intensities corrected for absorption and the background. The trace represents the profile calculated by the least-squares fitting. The difference diagram lobs - 1~c is given at the top of the figure. (A list of observed and calculated integrated neutron intensities is available from the authors upon request.)

intensities, Rwp = 12.9% for the weighted profile and Rex p = 4.7% for the expected value, reflect a satisfactory agreement between the observed and calculated data.

What characterizes the modulated structure type found for R3Ge 5 (R = Nd, Tb, Yb) is the occurrence of ordered Ge vacancies within the four Ge double layers of the a-ThSi 2 structure type at z

Table 2 The refined parameters from the 293 K neutron data in the paramagnetic state and the 1.5 K data in the magnetically ordered state for Nd-Ge. The ideal parameters of the a-ThSi-type basic structure are given in the middle part of the table for the enlarged cell Fdd2 atom

Site 293 K

Nd(1) Nd(2) Ge(1) Ge(2) Ge(3) B (,~,)

8a 16b 8a 16b 16b

a, b, c (/k) R-factors

Basic structure

1.5 K x

x

y

z

x

y

z

0 0.7607(19) 0 0.7960(14) 0.7389(17) 0.66(5)

0 0.0808(6) 0 0.0703(4) 0.0864(4)

0 * 0.2563(11) 0.4413(14) 0.6591(15) 0.8385(14)

0 0.75 0 0.75 0.75

0 0.083 0 0.083 0.083

0 0 0 . 2 5 0.7414(26) 0.413 0 0.663 0.7892(20) 0.837 0.7247(23) 0.55(7)

5.9076(3) 17.720(1) Rn=8.3% R,vp=12.9%

* Fixed parameter.

14.024(1) Rexp=4.7%

5.8990(4) Rn=7.1%

y

z

u(u)

0 0.0804(7) 0 0.0695(6) 0.0858(6)

0 0.2545(13) 0.43382(20) 0.6533(20) 0.8312(21)

0.34(12) 2.70(9) -

17.696(1) Rm=10.5%

14.008(1) R,~a=12.3 % Rexp=3.5~

104

P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in Nd 3Ge5

around ~, ], ~ and ~, and of atomic diplacements that may be decribed by harmonic functions (V~ sin(2~rk • r + q~v)associated with the same wave vector k = [] ½ 0] as for the density modulation function extensively described in ref. [11]. The total displacement contains transversal and longitudinal contributions, with respect to the direction of the wave vector (b axis) which are out of phase by ~r/2. The atoms located in planes perpendicular to the wave vector and spaced ( l / k ) apart have the same displacements. As shown in table 2 the largest displacements relative to the ideal positions of the basic structure are found for the Ge,(2) atom, fig. 5. Its total displacement of 0.357 A is somewhat smaller than for the TbaGe5 compound (0.452 , ~ ) a n d this explains the lower intensities of the satellite reflections observed for the Nd compound. Interatomic

distances and coordination polyhedra are given in table 3. There is an analogy to the findings for the Tb compound, the structure does not contain any G e - G e bonds smaller than the covalent bond (2.4 .&). The two Nd sites have the same coordination numbers CN = 18. The Ge polyhedra around Nd(1) and Nd(2) have the same number of first neighbours (10) instead of 12 for the basic structure with interatomic distances ranging from 3.02(1)-3.45(1) ,~. Apparently the polyhedron around Nd(1) has a higher symmetry due to the presence of the twofold axis while Nd(2) has no symmetry at all. On the other hand, the Nd polyhedra are 8 coordinated with N d - N d distances varying between 4.013-4.254 ,~ and have almost the same symmetry for the two sites. The coordination numbers of the Ge atoms vary between 10-12. Only the Ge(3) atoms are

/•',

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~"

~ Gel

G, i~ac

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®

®

'%,,,,~.~

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P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in N d jGe 5

Table 3 Interatomic distances for Nd3Ge5 up to 4.2 A (coordination polyhedra: (a) NdaNdsGel0; (b) Nd2NdsGelo; (c) GelNd6 Ger; (d) GeZNd6Ge4; (e) C_re3Nd6Ger) Atoms coord, no.

Distance (,~)

Atoms coord, no.

Distance (,~)

2Ge(1) 3.066(5) Ge(1) 2Ge(2) 2.512(24) Nd(1) CN = 18 2Ge(2) 3.096(15) CN=12 2Ge(3) 2.533(17) 2Ge(3) 2Ge(3) 2Ge(2) 2Nd(2) 2Nd(2)

3.139(15) 3.154(10) 3.441(9) 4.013(14) 4.119(14)

2Nd(2) 4.166(11) Ge(2)

2Nd(2) 4.254(11) CN =10 Nd(2) CN = 18

Ge(2) Ge(2) Ge(3) Ge(2) Ge(3)

3.023(16) 3.045(20) 3.053(17) 3.071(17) 3.083(21)

Ge(1) Ge(3) Ge(1) Ge(3) Ge(2) Nd(1)

3.134(12) 3.179(15) 3.283(21) 3.293(17) Ge(3) 3.449(16) CN = 12 4.013(14)

Nd(2) 4.024(15)

2Nd(2) 4.114(20) Yd(1) 4.119(14) Yd(1) 4.166(11)

Yd(2) 4.206(15) Yd(1) 4.254(11)

2Ge(2) 2Nd(1) 2Nd(2) 2Nd(2)

4.188(8) 3.066(5) 3.134(12) 3.283(21)

Ge(3) Ge(3) Ge(2) Ge(1) Nd(2) Nd(2) Nd(2) Nd(1) Nd(1) Nd(2)

2.554(27) 2.647(15) 3.466(11) 3.512(24) 3.023(16) 3.045(20) 3.070(17) 3.096(15) 3.441(9) 3.449(16)

Ge(1) 2.533(17) Ge(2) 2.554(27)

Ge(2) 2.647(15) 2Ge(3) Ge(3) Nd(2) Nd(2) 2Nd(1) Nd(2) Nd(2)

4.043(24) 4.164(12) 3.053(17) 3.083(21) 3.139(15) 3.179(15) 3.293(16)

three-coordinated and therefore the supposed three-dimensional graphite-like Ge network is not realized. The Ge zigzag chains that extend to the tetragonal a or b directions are interrupted by the ordered vacancies and the Ge atoms adjacent to the vacancy move towards the hole created. This shift contains either Ax, Az or Ay, Az components (referring to tetragonal coordinates) depending on the direction of the chain (along a or b). Owing to the existence of a two-component wave vector [~ ½ 0] the description of this structure as a modulated one needs harmonic functions with transversal and longitudinal contributions. The superstructure has an orthorhombic symmetry because only one wave vector arm has been observed. The symmetry of the modulated structure may also be described by a four-dimensional super-

105

space group which results in a parameter reduction.

3.2. The magnetic structure The low-temperature neutron data 4.2 and 1.5 K in the magnetically ordered state shown in fig. 4, contain additional weak reflections at reciprocal lattice positions associated with a wave vector k = 0 of the orthorhombic (Fdd2) structure and which would not be allowed by the tetragonal symmetry of the basic structure. Given the fact that the positional parameters of the Nd atom do not essentially deviate from the tetragonal symmetry, the break in symmetry associated with the appearance of new reflections has to be related to different moment values on the two Nd sites. As an example we note that the structure factor of the first observed magnetic reflection (022): (8/~a - 8 . 4 9 # 2 ) + i0.48/~ 2 would not be observed unless/~t ~/~2- F r o m the observed (004) magnetic intensity one can assume that the moments are confined to the (xyO) plane. This restricts the choice of the possible magnetic space groups associated with k = 0 to one: the space group Fd'd~ (Sh2~6) which allows an Fy ferromagnetic mode and two antiferromagnetic modes (CxAz) for Nd(2) at 16(b) and only two modes CxFy for Nd(1) at 8(a) [11]. Including all free parameters allowed by the magnetic space group, the refinement resulted in rather small/~x and/~z moment values with larger errors (less than twice the standard deviation) leaving thus an Fy uniaxial ferromagnetic arrangement as a unique choice. As expected the two Nd positions have unequal moment values. Nd(1) has an ordered moment value almost equal to zero (0.34(12)# B at 1.5 K), while Nd(2) has a moment value of 2.70(9)#B/Nd at the same temperature (table 2). At 4.2 K the refined Nd(2) moment value is, as expected, slightly lowered to 2.34(15)/~ B while Nd(1) again has no ordered moment. The R-factors of the refinement are R , = 7.1%, Rm = 10.5%, R ~ = 12.3% and Rexp = 3.5%. Their comparatively low values are indicative of the correctness of the model proposed which is shown in fig. 5.

106

P. Schobinger-Papamantellos, K.H.J. Buschow /Modulated structure in Nd3Ge 5

c

a

c

b

!

:

~

b

a

b

Fig. 6. (a) Stereoscopic view of the ferromagnetic structure of Nd3Ge s. Large circles represent N d atoms. Small circles with vertical bars [a], a black dot [b], or open small circles [c] correspond to the Ge x, Ge 2, Ge 3 atoms, respectively. The ordered Ge vacancy is represented by an asterisk. (b) Stereoscopic view of the ideal a-ThSi 2 structure in the enlarged orthorhornbic cell. Large circles, N d atoms; small open circles, Ge atoms; small full circles, the ordered Ge vacancy.

A temperature-dependent measurement of the magnetic intensities is not yet available due to the low magnetic intensities observed. In good agreement with the magnetic measurements ( Tc = 18 K) we found that in a neutron diagram at 21 K the magnetic reflections have disappeared, and we observed only a very slight change of the magnetic moment value between 1.5 and 4.2 K. The average ordered moment value of 1.56/~B,/Nd derived from neutron data at 4.2 K confirms the moment value of 1.4/xB/Nd derived from magnetization measurements at the same temperature, fig. 2. Apparently the ordering temperature of the Nd3Ge 5 sample is much higher than that found for the Ge-richer sample NdGe2_ x [9,17] of the tetragonal modification (see fig. 6). A further investigation with respect to the influence of stoichiometry on the chemical structure and the magnetic properties as well as the change in magnetic properties between a the high-temperature (tetragonal) and the low-temperature (orthorhombic) phase is imminent. As can be visualized from the arrangements of the N d - G e and N d - N d polyhedra shown in fig. 7 the N d - N d polyhedra (d) and (e) which concern the exchange interaction are rather similar while the N d - G e polyhedra are quite different. The polyhedron around Nd(1) has a higher symmetry due to the presence of the twofold axis than that

of the Nd(2) atom which has no symmetry at all. The different ordered moment values of the two N d sites are most probably related to differences found in the surrounding Ge polyhedra.

4. Concluding remarks We have shown that a compound of the composition Nd3Ge 5 exists and that its crystal structure comprises ordered Ge vacancies, the corresponding defect structure being describable by a density and displacive modulation with wave vector [½ ½ 0] of the tetragonal a-ThSi 2 type structure. Such a structure type was reported by us previously for Tb3Ge 5. The observation of the same structure type for N d a G e 5 and Y3Ge5 [15] indicates that it is fairly general and does not depend to a large extent on the ratio of the radii of the constituent rare-earth and germanium atoms. In fact, we observed the same type of superstructure lines in quite a number of R3Ge 5 compounds. These compounds can therefore be regarded as isotypic with Tb3Ge5 and Nd3Ge 5. The lattice constants of the latter compounds are given in table 4. We noted that there is a minor difference between the modulated structures of Tb3Ge 5 and N d 3Ge5 which concerns the total displacement of

P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in Nd 3Ges

107

0 e~

Z I

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108

P. Schobinger-Papamantellos, K.H.J. Buschow / Modulated structure in N d 3Ge5

Table 4 Lattice constants of the R 3Ges-type compounds

References

Compound

a (.A)

b (,~)

c (.A)

La3Ge5 Nd3Ge5 Sm3Ge5 Gd3Ge5 Tb3Ge 5 DY3Ge5

6.053 5.905 5.905 5.785 5.758 5.733

18.159 17.712 17.714 17.393 17.289 17.186

14.347 14.031 13.826 13.798 13.738 13.693

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the Ge(2) a t o m s f r o m its ideal p o s i t i o n in the b a s i c t e t r a g o n a l a - T h S i 2 structure. T h i s in t u r n resulted in a s o m e w h a t lower relative i n t e n s i t y o f the s u p e r s t r u c t u r e lines in N d 3 G e 5 t h a n in T b 3 G e 5. By e x t r a p o l a t i o n o n e m i g h t expect this t r e n d to c o n t i n u e w h e n going f r o m T b t o w a r d s the b e g i n n i n g of the l a n t h a n i d e series, the u p s h o t b e i n g t h a t eventually the m o d u l a t e d T b 3 G e 5 t y p e m a y b e less easily d i s t i n g u i s h e d f r o m the p a r e n t a - T h S i 2 t y p e w h e n using X - r a y diffraction. Since we d i d o b serve the s u p e r s t r u c t u r e lines in L a 3 G e s it is unlikely t h a t they h a v e b e c o m e t o o w e a k in Ce3Ge 5 a n d PraGe 5. I n fact, this m e a n s that o u r X - r a y d a t a l e a d to the c o n c l u s i o n t h a t the m o d u l a t e d T b 3 G e 5 structure t y p e d o e s n o t occur for R = Ce a n d Pr. T h e r e a s o n for this is n o t clear. A m o r e d e t a i l e d i n v e s t i g a t i o n o f the C e - G e a n d P r - G e systems in the c o n c e n t r a t i o n r a n g e a r o u n d R 3 G e 5 is c u r r e n t l y b e i n g u n d e r t a k e n .

Acknowledgements T h e a u t h o r s wish to t h a n k Dr. P. F i s h e r a n d Dr. A. F u r r e r for fruitful discussion, a n d the L a b o r a t o r i u m fiir N e u t r o n e n s t r e u u n g o f the E T H Ziirich for t e c h n i c a l s u p p o r t .