The ternary system Ce–Si–Y

Journal of Alloys and Compounds 297 (2000) 129–136

L

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The ternary system Ce–Si–Y Phase relations, crystallographic and magnetic properties a,b ¨ H. Flandorfer a , J. Grobner , A. Kostikas c , C. Godart d,e , P. Rogl a , *, V. Psicharis c , A. Saccone f , R. Ferro f , G. Effenberg b a

¨ Physikalische Chemie der Universitat ¨ Wien, Wahringerstr ¨ Institut f ur . 42, A-1090 Wien, Austria b Material Science International, Nobelstr. 15, Postfach 800749, D-70507 Stuttgart, Germany c Institute of Material Science, NCSR, Demokritos, Aghia Paraskevi, Gr-15310 Athens, Greece d CNRS, UPR 209, Pl. A. Briand, 92195 Meudon, France e CNRS, LURE, University of Paris Sud, 91405 Orsay, France f Istituto di Chimica Generale, Universita` di Genova, Viale Benedetto XV. 3, I-16132 Genova, Italy Received 5 July 1999; accepted 19 August 1999

Abstract Phase relationships were reinvestigated for the isothermal section of the Ce–Y–Si system at 6008C using X-ray powder diffraction techniques and EMPA. From Rietveld refinements the alloy CeY 4 Si 4 was found to crystallize with the partially disordered Ce 2 Sc 3 Si 4 type. The existence of extended mutual solid solutions, as observed earlier, is generally confirmed, however, the extent of the homogeneity regions and the phase field distribution was changed. Magnetic Squid measurements of the binary phases with large ternary solubility show in most cases paramagnetic tripositive cerium down to liquid helium temperature.  2000 Elsevier Science S.A. All rights reserved. Keywords: Ce–Y–Si alloys; Phase diagram; Isothermal section; X-ray Rietveld powder refinement; Cerium valency

1. Introduction Phase relations in the Ce–Y–Si system were first derived by Bodak and co-workers [1,2] and were critically assessed by Rogl [3], and Bodak and Gladyshevskij [4]. Rather extended mutual solid solutions into the ternary were claimed for most of the binary rare earth silicides. This seemed to be of interest in respect to a possible concentration-dependent variation of the valence of the cerium atom. For a closer investigation of the magnetic properties, a series of ternary alloy samples were synthesized, which were also used for a detailed inspection of the phase equilibria. Magnetic data were hitherto only published for the binary phases CeSi 22x and Ce 5 Si 3 . Hill et al. [5] measured the resistivity, susceptibility and magnetization for polycrystalline samples of the alloys CeSi x with 1.6#x#2.0. They found, that samples with x#1.85 order ferromagneti*Corresponding author. Tel.: 143-1-427752456; fax: 42779524. E-mail address: [email protected] (P. Rogl)

143-1-

cally at temperatures below 15 K while those with x$1.90 remain paramagnetic. Hybridization was said to increase with Si content [6], accordingly ferromagnetic order occurs for x#1.83, whereas a Fermi liquid with no magnetic order is observed for x$1.83. CeSi 22x is an example for a Kondo lattice with Kondo temperatures of approximately 10 and 20 K for x50.3 and x50.14, respectively. Magnetic properties of Ce 5 Si 32x (0,x,1) have been evaluated recently [7,8] revealing a metamagnetic jump in the magnetization curve around 40 Oe at 1.3 K.

2. Experimental details All samples, each of about 2 g, were prepared by repeated arc-melting the high purity elements together in a Ti-gettered argon atmosphere. Weight losses due to the arc-melting process were checked to be less than 0.5 mass%. The following starting materials were used: Ce (ingot, m3N, Auer-Remy, Germany); Si (lumps, 99.99%, Johnson and Matthey, Germany) and Y (ingots, 99.9%, Strem Chemicals, USA). After melting the reguli were

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00569-1

H. Flandorfer et al. / Journal of Alloys and Compounds 297 (2000) 129 – 136

130

packed in molybdenum or tantalum foil, sealed in evacuated silica tubes and heat treated for at least 140 h at 6008C in a wire wound power-controlled tubular furnace calibrated against a Pt / PtRh thermocouple. After annealing the samples were quenched by casting the silica tubes into cold water. Precise lattice parameters and standard deviations were obtained by a least squares refinement of room temperature Guinier–Huber X-ray powder data (Cu Ka 1 ) employing an internal standard of 99.9999 mass% pure Ge (a Ge 5 0.5657906 nm). Rietveld refinements were performed on X-ray powder intensity data collected from flat specimens in a Siemens D5000 diffractometer (Cu Ka). Light optical microscopy (LOM) on selected alloys, which were polished and etched by standard methods, scanning electron microscopy (SEM) and microprobe analyses based on energy dispersive X-ray spectroscopy (Si(Li) detector) were used to examine phase equilibria and equilibrium compositions. For quantitative EMPA the samples were

analyzed employing an acceleration voltage of 20 kV for a counting time of 100 s. X-ray energy spectra and data were processed using the ZAF software package supplied by Link Systems Ltd., UK [9]. A cobalt standard was used for calibration in order to monitor beam current, gain and resolution of the spectrometer. Pure elements served as standards to carry out the deconvolution of overlapping peaks and background subtraction. Magnetic measurements were performed employing a Squid magnetometer from room temperature (RT) down to liquid helium temperature. The applied fields were 1.5, 3 and 5 T. The step was 5 K in the range from 5 to 120 K, and 10 K up to 300 K. The magnetic susceptibilities were calculated from the measured magnetization. X-ray absorption measurements were performed (RT and 10 K) at the French synchrotron radiation facility of LURE using the X-ray beam delivered by the DCI storage ring, working at 1.85 GeV–320 mA, on the EXAFS 2 station. Experiments were made on powdered samples in

Table 1 Binary phases of the Ce–Si–Y system Phase

Structure type

Pearson symbol

Space group

Lattice parameters [nm] a b

d-Ce g-Ce

W Cu

cI2 cF4

¯ Im3m ¯ Fm3m

b-Ce a-Ce Si b-Y a-Y CeSi

a-La Cu C diam. W Mg FeB

hP4 cF4 cF8 cI2 hP2 oP8

P6 3 /mmc ¯ Fm3m ¯ Fd3m ¯ Im3m P6 3 /mmc Pnma

0.412 0.5161 0.51556 0.51548 0.51494 0.51400 0.36810 0.485 0.54306 0.407 0.36482

CeSi 2

ThSi 2

tI12

I4 1 /amd

0.8240

1.1857

0.57318 0.3942

GdSi 22x

oI12

U 3 Si 2

Ce 5 Si 3

Cr 5 B 3

Ce 5 Si 4

Zr 5 Si 4

Y 5 Si 3

Mn 5 Si 3

Y 5 Si 4

Sm 5 Ge 4

YSi

CrB

YSi 22x

d-(Ce,Y) a

a

AlB 2 (def.) GdSi 22x ThSi 2 a-Sm

Metastable phase [13].

tP10 tI32 tP36 hP16 oP36 oC8 hP3 oI12 hP3 hR3

[12] [12] [19] [19] [19] [19] [12] [12] [12] [12] [12] [12] [20] [12] [21] [12] [22] [12] [23] [12] [24] [12] [25] [12] [16] [12] [25] [12] [26] [27] [27] [18] [19] [19]

1.3903 , |15608C 0.4123

1.3875 ,13908C

P4 /mbm 0.7817

0.4308

0.7868

1.373

0.793

1.504

0.8442

0.6353

,14008C

I4 /mcm

,13908C

P4 1 2 1 2

,18508C

P6 3 /mcm

,18408C

Pnma 0.739

1.452

0.764

0.4251 0.3849 0.4060 0.404 0.36588 0.36603

1.0526

0.3826 0.4147 1.339 1.342 2.6384 2.6395

,18358C

Cmcm P6 /mmm Imma I4 1 /amd ¯ R3m

798–7268C 726–618C At 300 K At 280 K At 200 K At 120 K 61 to 21778C ,-1778C ,14148C 1526–14788C ,14788C ,14708C

0.6017

Imma 0.4189

Ce 3 Si 2

Ref.

,16208C 0.4192

CeSi 22x

Comments c

0.3967

|62 at.% Si, low temp. |66 at.% Si, high temp. 54.17 at.% Ce 55.89 at.% Ce

H. Flandorfer et al. / Journal of Alloys and Compounds 297 (2000) 129 – 136

131

3. Results and discussion

3.1. Binary systems

Fig. 1. System Ce–Y–Si: isothermal section at 6008C.

the range [5660–5840 eV] around the L III edge of Ce at fixed temperatures (10 and 300 K). Experimental details and the L III edge deconvolution technique were already described in one of our previous papers [10] (see also Ref. [11]).

The binary systems were essentially based on the data reported in Ref. [12]. The Ce–Y system is used without the d -phase, which was found to be metastable [13]. For the Y–Si and the Ce–Si system, contradictory literature data concern the disilicide region (see, for example, [12]). Instead of following various formulas such as Y 3 Si 5 , Y 2 Si 3 , YSi 2 and Ce 2 Si 3 , CeSi 2 we considered all these phases as defect disilicides, YSi 22x and CeSi 22x , respectively, at slightly different Si contents. In the Ce–Si system the CeSi 2 phase is reported to be of the tetragonal ThSi 2 structure, whereas the GdSi 22x structure was said to appear with a Si content smaller than 63 at.% Si [14]. The Ce–Si disilicides have been recently reinvestigated at 6008C by the authors [15] confirming the phase CeSi 22x at about 66 at.% Si with the ThSi 2 type in equilibrium with Si. According to Ref. [16] in the Y–Si system the phase at the lower Si content of |62 at.% Si has a defect AlB 2 -type structure at low temperatures and a GdSi 22x -type structure at high temperatures, respectively. A detailed analysis of the yttrium disilicide region [17] at 6008C agrees on the existence of a defect AlB 2 -type phase at 61.5 at.% Si but

Fig. 2. LOM-micrographs of Ce–Y–Si alloys annealed and quenched from 6008C. (a) Ce 33 Y 22 Si 45 , magnification 5003; bright phase is (Ce,Y) 5 Si 4 , gray matrix is (Ce,Y)Si; (b) Ce 16.6 Y 16.6 Si 66.8 , magnification 6253; bright phase is (Ce,Y) Si 2 , gray matrix is Si; (c) Ce 20 Y 20 Si 60 , magnification 5003; bright phase is (Y 12x Ce x )Si 2 , gray matrix is (Y,Ce)Si.

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132

Table 2 Crystallographic data of ternary Ce–Y–Si alloys, annealed at 6008C Alloy nom. comp., at.%

Phase analysis

Structure type

Space group

Unit cell dimension (nm) a b

Ce 7 Y 23 Si 70

(Y,Ce)Si 2 Si (Y,Ce)Si 2 Si (Ce,Y)Si 22x Si (Ce,Y)Si 2 (Y,Ce)Si 2 (Ce,Y)Si 2 (Y,Ce)Si 2 (Ce,Y)Si 2 (Ce,Y)Si 22x (Y,Ce)Si 2 a(Y,Ce)Si 2 a(Y,Ce)Si 2 (Ce,Y)Si 2 (Ce,Y)Si (Y,Ce)Si 2 (Y,Ce)Si (Ce,Y)Si a(Y,Ce)Si 2 (Ce,Y)Si (Y,Ce)Si (Ce,Y)Si (Ce,Y)Si 22x (Ce,Y)Si (Y,Ce) 5 Si 4 (Ce,Y) 5 Si 4 (Ce,Y)Si (Y,Ce) 5 Si 4 (Y,Ce)Si (Ce,Y) 5 Si 4 (Ce,Y)Si (Y,Ce)Si (Y,Ce) 5 Si 4 (Ce,Y) 5 Si 4 (Ce,Y)Si (Y,Ce) 5 Si 4 YSi (Ce,Y) 5 Si 4 (Ce,Y) 3 Si 2 (Ce,Y) 3 Si 2 (Y,Ce) 5 Si 3 (Y,Ce) 5 Si 4 (Y,Ce) 5 Si 3 (Ce,Y) 5 Si 3 (Ce,Y) 3 Si 2 (Ce,Y) 5 Si 3 (Y,Ce) 5 Si 3 g(Ce,Y) b(Ce,Y) (Ce,Y) 5 Si 3 (Y,Ce) 5 Si 3 g(Ce,Y) (Y,Ce) 5 Si 3 b(Ce,Y) a(Y,Ce)

GdSi 22x C diamond GdSi 22x C diamond GdSi 22x C diamond ThSi 2 GdSi 22x ThSi 2 GdSi 22x GdSi 22x GdSi 22x AlB 2 AlB 2 AlB 2 GdSi 22x FeB AlB 2 CrB FeB AlB 2 FeB CrB FeB GdSi 22x FeB Sm 5 Ge 4 Zr 5 Si 4 FeB Sm 5 Ge 4 CrB Zr 5 Si 4 FeB CrB Sm 5 Ge 4 Zr 5 Si 4 FeB Sm 5 Ge 4

Imma ¯ Fd3m Imma ¯ Fd3m Imma ¯ Fd3m I4 /mmm Imma I4 /mmm Imma Imma Imma P6 /mmm P6 /mmm P6 /mmm Imma Pnma P6 /mmn Cmcm Pnma P6 /mmn Pnma Cmcm Pnma Imma Pnma Pnma P4 1 2 1 2 Pnma Pnma Cmcm P4 1 2 1 2 Pnma Cmcm Pnma P4 1 2 1 2 Pnma Pnma

Zr 5 Si 4 U 3 Si 2 U 3 Si 2 Mn 5 Si 3 Sm 5 Ge 4 Mn 5 Si 3 Cr 5 B 3 U 3 Si 2 Cr 5 B 3 Mn 5 Si 3 Cu a-La Cr 5 B 3 Mn 5 Si 3 Cu Mn 5 Si 3 a-La Mg

P4 1 2 1 2 P4 /mbm P4 /mbm P6 3 /mcm Pnma P6 3 /mcm I4 /mcm P4 /mbm I4 /mcm P6 3 /mcm ¯ Fm3m P6 3 /mcm I4 /mcm P6 3 /mcm ¯ Fm3m P6 3 /mcm P6 3 /mmc P6 3 /mmc

0.4089 0.4013 1.3430 0.5432 0.4124 0.4076 1.3514 0.5434 0.4134 0.4092 1.3573 0.5431 0.4169 1.3789 0.4127 0.4087 1.3535 0.4149 1.3695 0.4074 0.3987 1.3405 0.4162 0.4111 1.3834 0.4099 0.4086 1.36453 0.3914 0.4237 0.3914 0.4233 0.3937 0.4256 0.4119 1.3542 0.8047 0.3887 0.5797 0.39137 0.42280 Traces Traces 0.3934 0.4255 0.8009 0.3871 0.5766 Traces 0.8177 0.3927 0.5888 Traces 0.8166 0.3933 0.5838 0.7674 1.4498 0.8498 Traces 0.8035 0.3878 0.5782 0.7479 1.4795 0.7790 Traces 0.78063 1.44818 0.82003 0.39491 0.59836 Traces 0.7453 1.4720 0.7766 0.7838 1.5098 Traces 0.7435 1.4664 0.7722 Traces 0.7870 1.4647 0.7709 0.4398 0.7793 0.4225 0.8473 0.6439 Traces 0.8533 0.6441 0.7759 1.3766 0.7654 0.4394 0.7836 1.3719 0.8491 0.6442 Traces Traces Powders oxidize easily, only few very diffuse lines on the X-ray film

Ce 14 Y 17 Si 69 Ce 16.6 Y 16.6 Si 66.8 Ce 30 Y 5 Si 65 Ce 17 Y 18 Si 65 Ce 25 Y 10 Si 65 Ce 5 Y 31 Si 64 Ce 32 Y 5 Si 63 Ce 18.7 Y 18.7 Si 62.5 Ce 20 Y 18 Si 62 Ce 25 Y 15 Si 60

Ce 20 Y 20 Si 60

Ce 20 Y 25 Si 55

Ce 35 Y 15 Si 50 Ce 28 Y 25 Si 47

Ce 20 Y 33 Si 47

Ce 33 Y 22 Si 45

Ce 16.1 Y 39.4 Si 44.4 Ce 47 Y 9 Si 44 Ce 8 Y 48 Si 44 Ce 42 Y 16 Si 42 Ce 25 Y 35 Si 40

Ce 25 Y 36.5 Si 38.5

Ce 57.5 Y 5 Si 37.5 Ce 19 Y 45 Si 36

Ce 50 Y 20 Si 30

Ce 20 Y 60 Si 20

0.8439 0.3680 0.3688

c

0.6365 1.1780 0.5853

EMPA data in at.% Ce Y

Si

7.7

27.6

64.7

27.5

8.4

64.1

26.6

10.6

62.8

24.3

16.4

59.3

15.5

33.9

50.7

29.6 26.3 30.7

20.6 30.0 23.6

49.8 43.7 45.7

20.0

34.4

45.5

35.9 43.6

20.9 15.1

43.2 41.3

20.1 18.8 21.8 54.1

42.0 35.7 39.6 35.8

37.9 45.5 38.6 10.1

11.0

51.3

37.7

54.1 22.7

8.6 40.2

37.3 37.1

58.3

5.8

35.9

H. Flandorfer et al. / Journal of Alloys and Compounds 297 (2000) 129 – 136

133

also claims the formation of a GdSi 22x -type phase at a rather stoichiometric composition of 66 at.% Si. The ThSi 2 -type phase was shown to form at 6008C stabilized by small amounts of Ni (1.5 at.% Ni (YSi 1.94 Ni 0.05 ) [17]. A binary high temperature phase with the ThSi 2 -type was only reported by Perri et al. [18]. A summary of the crystallographic data of the unary and binary phases in the Ce–Y–Si system is given in Table 1.

3.2. Ternary system The isothermal section of Ce–Y–Si at 6008C is shown in Fig. 1. The results of the crystallographic investigation of ternary alloys are presented in Table 2, including the EMPA data of selected alloys. According to these information and data of LOM the existence of extended ternary solubilities is generally confirmed, however, homogeneity ranges and tie line directions appear greatly changed with respect to earlier data in the literature [4]. Fig. 2 shows the micrographic images of three selected alloys. The dependency of the lattice parameters versus the Y-content of different compositions on the Si-rich boundary of of the (Ce,Y)Si 2 solution crystallizing in the ThSi 2 or GdSi 22x structure is given in Fig. 3. In contrast to the data [4] there is a continuous (second-order type) transformation from the tetragonal ThSi 2 -type to the orthorhombic GdSi 22x -type around 0.12 mol fraction Y.

3.3. Rietveld refinement of ( Ce x Y12 x )5 Si4 (x50.2; Sm5 Ge4 -type) A full profile full matrix Rietveld refinement was performed on powder X-ray diffraction data of the alloy CeY 4 Si 4 . All results are summarized in Table 3. The low residual values, R I 50.066 and R F 50.028, respectively confirm isotypism with the Sm 5 Ge 4 -parent type. As the scattering factors of Ce and Y are well distinguished it was possible to determine their distribution over the rare earth atomic positions (4c, 8d(1) and 8d(2)). Cerium is only found in the 8d(2) sites which it occupies up to 50%, the rest is filled with yttrium. Based on this scheme of occupation, the structure can be considered as a partially disordered Ce 2 Sc 3 Si 4 -type in which cerium preferentially occupies one of the 8d positions.

Fig. 3. Lattice parameters versus Y-content of (Ce,Y)Si 2 compounds.

ple holder was neglected since a measurement of the sample holder showed a 10 22 –10 24 times lower signal compared with the samples. For a correction for possible small amounts of ferromagnetic impurities introduced during the preparation of the samples, the magnetization was measured at three different saturating magnetic fields (15, 30 and 50 kOe) and extrapolated to 1 /H → 0. This extrapolation proved to be negligibly small. Only the samples, Ce 47 Y 9 Si 44 , Ce 31 Y 7 Si 62 and Ce 32 Y 5 Si 63 , show field dependence at temperatures below 20 K (see Figs. 7–9).

3.4. Magnetic properties The reciprocal susceptibilities per mol of Ce of the ternary alloys investigated in the temperature range from 5 to 300 K are shown in Figs. 4–9. The paramagnetic data ( meff and up ) derived from a least squares fit to the Curie–Weiss law are presented in Table 4 together with the temperature range used for the evaluation. A correction of the magnetisation data according to the diamagnetic sam-

Fig. 4. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 35 Y 15 Si 50 and Ce 16.1 Y 39.4 Si 44.4 .

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134

Table 3 Crystallographic data a of CeY 4 Si 4 , quenched at 6008C Atom parameters

Atom

Site

x

y

z

B in 10 22 nm 2

Occupation

Y1 Y2 (Ce,Y) Si 1 Si 2 Si 3

4c 8d 8d 4c 4c 8d

0.346(1) 0.3211(6) 0.0237(5) 0.005(3) 0.227(2) 0.125(2)

1/4 0.6211(3) 0.0975(3) 1/4 1/4 0.5384(9)

0.4890(9) 0.3195(7) 0.3234(6) 0.616(2) 0.108(2) 0.038(2)

0.9(1) 1.1(1) 1.7(1) 0.5(2) 0.5(2) 0.5(2)

1 1 0.5 Ce10.5 Y 1 1 1

Interatomic distances

Central atom: Y 1

(nm)

Ligand atom

Distances

Ligand atom

Distances

Ligand atom

Distances

1 Si 1 1 Si 2 1 Si 2 2 Si 3 1 Si 1 2 Y2 2 Y2 2 Y3 2 Y3 Central atom: Si 3

0.2734 0.2938 0.3092 0.3151 0.3290 0.3432 0.3462 0.3538 0.3565

Ligand atom

Distances

0.2791 0.2801 0.2904 0.2929 0.2965 0.3127 0.3432 0.3462 0.3727 0.3801 0.3803 0.3885 0.3912

1 1 1 1 1 1 1 1 1

0.2255 0.2801 0.2904 0.2929 0.3083 0.3144 0.3151 0.3230 0.3748

1 Si 1 1 Si 3 1 Si 3 1 Si 3 1 Si 2 1 Si 1 1 Y1 1 Y1 1 Y3 1 Y2 1 Y3 2 Y2 1 Y3 Central atom: Si 2

0.3083 0.3144 0.3186 0.3201 0.3204 0.3230 0.3538 0.3565 0.3729 0.3748 0.3803 0.3899 0.3902 0.3912 0.3991

Ligand atom

Distances

1 Si 3 1 Si 3 1 Si 2 1 Si 2 1 Si 1 1 Si 3 1 Y1 1 Y1 1 Y2 1 Si 3 1 Y2 2 Y3 1 Y2 1 Y2 1 Y3 Central atom: Si 1

1 Si 1 1 Y1 2 Y2 1 Y1 2Y 3 2 Y3

0.2714 0.2938 0.2965 0.3092 0.3186 0.3201

Ligand atom

Distances

1 1 2 2 2 1

0.2714 0.2734 0.2791 0.3127 0.3204 0.3289

Si 3 Y2 Y2 Y2 Y3 Y3 Y1 Y3 Y3

Central atom: Y 2

Central Atom: (Ce,Y)

Si 2 Y1 Y2 Y2 Y3 Y1

a

Method: Full profile refinement of room temperature X-ray powder diffraction data. Number of reflections used in refinement: 977, 258#2u #1048. Flat specimen for Siemens D5000 automatic diffractometer. Lattice parameters: a50.7463(1) nm, b51.4742(2) nm, c50.7778(1) nm, V50.8557 nm 3 . Structure type: Partially disordered Ce 2 Sc 3 Si 4 -type (Sm 5 Ge 4 -type). 16 ¯ Z54. Space group: Pnma 2D 2h , No. 62, origin at 1, Residual values: R I 50.066, R F 50.028, R P 50.090, R wP 50.122.

Fig. 5. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 25 Y 10 Si 65, Ce 30 Y 5 Si 65 and Ce 17 Y 18 Si 65 .

Fig. 6. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 19 Y 45 Si 36 , Ce 57.5 Y 5 Si 37.5 and Ce 20 Y 18 Si 62 .

H. Flandorfer et al. / Journal of Alloys and Compounds 297 (2000) 129 – 136

Fig. 7. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 47 Y 9 Si 44 . The inset shows the low temperature dependence of the susceptibility measured at 15, 30 and 50 kOe.

Fig. 8. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 31 Y 7 Si 62 . The inset shows the low temperature dependence of the susceptibility measured at 15, 30 and 50 kOe.

135

Fig. 9. Temperature variation of the reciprocal magnetic susceptibility per mol Ce for Ce 32 Y 5 Si 63 . The inset shows the low temperature dependence of the susceptibility measured at 15, 30 and 50 kOe.

The obtained reciprocal molar susceptibilities for most samples reveal a linear temperature dependence following the Curie Weiss law x 5 C /(T 2 up ). The least square fitting results in an effective paramagnetic moment for the cerium atom which is within an experimental error of about 5%. It corresponds in practically all cases to the ideal groundstate of Ce 31 : 2 F 5 / 2 . Deviations from the linear dependency of 1 /xmol versus T probably are due to crystal field effects. Particularly the disilicide samples (Ce 25 Y 10 Si 65 , Ce 30 Y 5 Si 65 and Ce 17 Y 18 Si 65 ) show already below 150 K significant nonlinear behaviour with small temperature dependence. The susceptibility of these samples at 5 K is about 5–10 times lower than that of the other phases in this system. Fig. 5 gives the susceptibilities per 1 mol Ce of the samples Ce 25 Y 10 Si 65 , Ce 30 Y 5 Si 65 (ThSi 2 type) and Ce 17 Y 18 Si 65 (GdSi 22x -type). xCe below 150 K decreases with continuous replacement of Ce by Y. Ce 25 Y 10 Si 65 shows the lowest xCe and the highest peak in

Table 4 Magnetic data of ternary Ce–Y–Si compounds in the temperature range 5–300 K Alloy nominal composition

Phase

Structure type

Temperature region for linear fit [K]

meff of Ce [ mB ]

up [K]

Ce 16.1 Y 39.4 Si 44.5 Ce 17 Y 18 Si 65 Ce 19 Y 45 Si 36 Ce 20 Y 18 Si 62 Ce 25 Y 10 Si 65 Ce 30 Y 5 Si 65 Ce 31 Y 7 Si 62 Ce 32 Y 5 Si 63 Ce 35 Y 15 Si 50 Ce 47 Y 9 Si 44 Ce 57.5 Y 5 Si 37.5

(Ce 0.29 Y 0.71 ) 5 Si 4 (Ce 0.49 Y 0.51 )Si 1.85 (Ce 0.3 Y 0.7 ) 5 Si 3 (Ce 0.53 Y 0.47 )Si 1.63 (Ce 0.72 Y 0.28 )Si 1.85 (Ce 0.86 Y 0.14 )Si 1.85 (Ce 0.81 Y 0.19 )Si 1.63 (Ce 0.86 Y 0.14 )Si 1.70 (Ce 0.7 Y 0.3 )Si (Ce 0.84 Y 0.16 ) 5 Si 4 (Ce 0.92 Y 0.08 ) 5 Si 3

Sm 5 Ge 4 GdSi 22x Mn 5 Si 3 AlB 2 ThSi 2 ThSi 2 GdSi 22x GdSi 22x FeB Zr 5 Si 4 Cr 5 B 3

5–300 150–300 30–300 50–300 250–300 230–300 150–300 100–300 5–300 150–300 150–300

2.59 2.53 2.57 2.44 2.77 2.61 2.49 2.32 2.48 2.43 2.57

223 2202 234 290 2275 2187 237 219 212 230 255

136

H. Flandorfer et al. / Journal of Alloys and Compounds 297 (2000) 129 – 136

1 /xmol below 150 K which corresponds to the solubility limit in the ThSi 2 -type (see Fig. 3). In the Y-richer sample Ce 17 Y 18 Si 65 the crystal structure is transformed to the orthorhombic GdSi 22x structure, where xCe increases again with the Y content.

3.5. X-ray absorption spectroscopy XAS measurements on selected alloys were performed at 10 K and room temperature. Data in all cases are consistent with a stable trivalent cerium ground state 2 F 3 / 2 . No significant difference was observed between the spectra at 10 K and room temperature.

Acknowledgements This research has been sponsored by the Austrian FWF under grant P8218 as part of a EU-HCM project ERB4050PL93-0370. H.F. and J.G. thank V. Vlessides for expert technical assistance with the SQUID measurements. H.F. and P.R. are grateful to the Austrian–Italian Scientific–Technological Exchange Program for fellowships in Genova and Wien, respectively (project N13). This exchange program has provided the starting activity in the early stage of this research co-operation.

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