Hyperfine Interactions 42 (1988) 1063-1066 |063

ISlEu-MOSSBAUER INVESTIGATION IN SOME NEW TERNARY Eu2M3Si s SYSTEMS (M = Ni, Pd, Cu, Rh)

Sujata PATIL', R. NAGARAJAN + , L.C. GUPTA +, C. GODART", R. VIJAYARAGHAVAN + and B.D. PADALIA

�9 Indian Institute of Technology, Bombay 400 076, India + Tata Institute of Fundamental Research, Bombay 400 005, India ** CNRS, ER 209, Chimie Metallurgique des Terres F 92190, Meudon, France

The series Eu~M~Si. (M = Ni, Cu, Pd, Rh) have been synthesised forLt~e ~irst tlme.15~he Ni, Cu, Pd systems form in U.Co~Si. type structure. "Eu M6ssbauer spectroscopy shows Z t~at ) Eu is in valence fluetuatlng (VF) state in Nl system, while in Cu and Pd systems i t is In stable dlvalent state. These observations are further confirmed by magnetic susceptibi l i ty studies. The material Eu2Rh)SI 5 shows two lines in the M~ssbauer resonance, one of which shows VF behaviour.

I. Introduction

Many r a r e earth (RE) -based intermetal l ic compounds and alloys exhibit variety of interesting phenomena such as superconductivity, magnetism, coexistence of superconductivity and magnetic order, heavy fermlon, Kondo effect, and valence fluotuatlon (VF) behavlour. Reoentl~ discovered and extensively studied high-T superconducting oxides have rare earth elements as

�9 . C one of thelr constltuents. Permanet magnets with extra ordinary energy product (BH) contain Md or Sm and materials such as LaNI~ are excellent hydrogen storage materlals. I t has, therefore, been always rewarding to syntheslse new RE systems and study their properties, we have undertaken the study of the series, RpM~SI~ (R : rare earth, M = transit ion metal). In thls paper we report the --- EO Mossbauer studies of Eu2Ni)Sis, Eu2Cu)5i5, Eu2Pd)515 and Eu2Rh3Sk 5.

2. Experimental

The samples of the compounds Eu2M2Si ~ (M = Ni,Cu,Pd,Rh) were prepared by standard arc/induction melting procedure. Any loss in weight due to evaporation of Eu was compensated by addition of requisite amount of Eu and remeltlng. Due to reasons discussed below Ni-based compound was annealed for nearly one month at 900 .

X-ray powder d i f f ract ion pattern of Eu~Ni~Si., Eu2Pd~51., Eu2Cu~Si. showed that they form in U~CoxSi 5 type crystal str~ot~re~ Analysl~ o~ X-ray dif~raotlon pattern of Eu2Rh35i 5 ~as yet not been completed. Minority phases which could not be ident l f ied were also seen. I t may be noted that most of the 2-)-5 compou~ form in Sc~FeaSl ~ type-tetragonal structure w~th space 1~~ P4/mnc / I / . " Eu M6ssbaue~ s(udfes were performed against a 500 mcl. Smr 3 source uslng a conventional constant accelerator spectrometer In conjunction wlth a multisoaler analyser developed by one of us /2/. Spectra were taken over the temperature range 4.2K to )OOK using a gas flow type oryostat. DC magnetic susoeptlbilJty studies were carried out over the temperature range 1.6K to )OOK using a Faraday micro balance.

�9 J.C. Baltzer A.G., Scientific Publishing Company

1064 S. Patil et al., IZIEu-M.S. in some new ternary EuzM3Si 5 systems

3. Results

The lattice parameter of the orthoromblc unit cell were calculated using reflections (2 4 0), (3 2 I ) , ' (2 O 2) are as follows: Eu2Ni~Si 5 : a=9.467 A, b=11.96 A,c=5.695 A ,Eu2Pd.SI~: a=9.587 A,b=12.451 A,c=5.835 A, Eu~Cu.SI~ : a=9.564 A, b=12.271 A, o=5.~56)A.These values of the parameters so ob~al~ed)are oonsistant with the other refleetlons

The M6ssbauer spectrum of Eu2Ni~Si ~ at 30OK exhibited a major l ine with isomer shi f t (IS) ~-2.8 mm/seo and tw6 mfnor l ines, one with IS~+0.4 mm/sec and another with IS ~-8.2 mm/sec. These minor l ines were taken as an indication of some amount of divalent and t r iva lent phase. One batch of samples were annealed at 900K for a period of one month. Though, no perceptible difference was seen in the x-ray di f f ract ion pattern of the annealed sample, there was a marked reduction in the minor l ines of the Mossbauer spectrum. The M6ssbauer spectra of this sample, at various temperatures are shown in Fig. I . The solid l ine is the Lorentzian f i t for the spectra taking into account the presence of the remaining divalent impurity. The IS of the main l ine is seen to be strongly temperature dependent. The IS at 3OOK is -2.8 mm/sec and at 4.8K is -1.1 mm/sec. The variation of IS with temperature is shown in Fig. 2.

The M6ssbauer spectra, at some selected temperatures, of Eu.Pd.Si~ and Eu~Cu.Si~ are shown in Fig. 3 and 4. The IS at 300K for the Pd- a~d JCu~based systems ) is same (= -9.2 mm/sec) and independent of temperature. The MBssbauer spectra for both the systems show magnetic hyperfine sp l i t t ing at 4.8K (Fig. 3 and 4).

4. Discussion

In Eu compounds Eu is usually,found either in divalent or in trlvalent state. The IS of known trivalent Eu compounds lie in the region of ~0 mm/s to 4.5 mm/sec whereas the IS of known divalent Eu compounds lie in the region of -8 mm/sec to ~ mm/sec /M. Considering that the usual observed line width is 2.5 mm/sec, "--Eu M6ssbauer resonance is a sensitive tool to study the valency of Eu. The observed IS of -2.8 mm/sec at 300K for Eu2Ni)Si ~ clearly shows that Eu in Eu.Ni.Si 5 is in intermediate valence state or in VF state. The strong temperatur~ ~ependence of IS, much greater than the possible second order Doppler shift (~0.I mm/sec), further confirms theVF character of the Eu ion. The VF phenomenon arises because of the proximity of Eu L+ excited state to the Eu ~+ ground state. If this energy separation, E is comparable to thermal energy, the ion fluctuates between the two states.e~f the f luctuation is faster than the probe time of Mossbauer resonance (10- sec for Eu) , then one observes a 'time averaged' single l ine with an intermediate IS given by the relat ion /4/

S = P2S2 + P3S3

where S~ and S. and P2 and P. are the IS and popuiation of Eu 2+ and Eu 3+ states. P~ and ~. are ~be usual thermodynamic partition function except that temperature i~ defined as T" = (T + T ~)~/z, where T ~ is the fluctuation temperature which reflects the fluctuationS~requency, to ~ke into account the low temperature behaviour of VF systems /5/. The observed IS values of Eu>Ni3Si 4 were analysed with the above considerations using S 2 and S 3 as -9.5 mm/~ec and +1.0 mm/sec respectively. This analysis yields the values E = 730K and T J = 235K. It is interesting to note that one obtains a good agreement with obse~ed IS over the entire temperature range with one set of E and T ~, whereas it has usually been expected that these would be a functione~f temperature /6/. Figure 2 shows the obsrved and the calculated IS using these values. The temperature dependence of the average valence as inferred from the IS values is also shown in this figure. We would like to emphasise here that constant values of Eex and Tsf,

S. Patil et aL, t~tEu-M.S, in some new ternary EuzM~Si s systems 1065

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Fig. 3. 151Eu Mdssbauer spectra of Eu2Pd3SL-9 at various temperatures.

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Fig. 4, 151Eu M~ssbaue~-spectra of Eu2Cu3S % at vaPious temps.

1066 S. Patil et al., 151Eu-M.S. in some new ternary Euz~I3Si ~ systems

have been obtained in the other two known Eu-Ni based VF systems v i~ . , EuNiSi~ /7 / , EuNi2P 2 /8 / . Since Eu L+ " ~ ~+ " " c zon carried a magnetic moment and Lu zon is a Pauli paramagnet, VF behaviouc is ref lected in magnetic properties also. In VF system, the observed suscept ib i l i t y would also be given by a re la t ion s imi lar to one encountered in IS, v i z . , X = P~X~ + P~X~- However, i t would be impossible to

. . . Z L ~ �9 �9 ~ ~ L + . Eu3+ d is t ingu ish between a pnyslcaz mzx~ure or Lu ano and VF. Having establ ished that Eu is in VF state in Eu2NI3Si s through M~ssbauer spectroscopy, one can get a supporting evidence from magnetic s u s c e p t i b i l i t y . Our magnetic su~cept ib i l i ty results give Peff as 5.52 ~n against 7.94p~ expected for free

L+ Eu ion. We were able to get a consistent~f i t to the observed X (T) using the E and T ~ derived from the Mossbauer data. Detai ls of the suscept ib i l i t y ex _ST . . . . . measurements WlZl oe gzven elsewhere /9 / .

The temperature independent IS (= -9.2 mm/sec) for EupPdiSi s and Eu2Cu.Si 5 ind ica te that Eu is in stable d iva len t state in these two ~at6r iA ls . From ~he onset of broadening observed in Mossbauer spectra, the magnetic ordering temperature (T c) has been estimated as 20 K for both the mater ia ls. From the (X)- behavior of Eu)Pd~Sis ,which is l i nea r down to 18 k , e f f ec t i ve magnetic moment per Eu ion is-deduced to be (8.04 PB), suggesting stable d iva len t state of Eu. The Curie-Weiss temperature O~ = -8.OK is i nd i ca t ing that the i n t e r i o n i c i n te rac t ion is rather weak. The deviat ion from the Curie-Weiss behaviour of

X - I (T ) occurs below T = 18K. The MSssbauer spectra also exh ib i t broadening around that temperature. At 4.8K, MOssbauer spectra of Eu~Pd3Si. and Eu.Cu.Si. show hyperf ine s p l i t t i n g and hypecfine f i e l d at Eu calculated ~rom lineZ ~posztzons>. wi th respect to EuS is 375 kOe in Eu2Pd3Si 5 and 409 kOe in Eu2Cu3Si 5.

5. Conclusion

The new series Eu2M3Si S where (M = Ni, Cu, Pd) form in U~Co.S1.L �9 ) type orthorombic structure. Our R6ssbauer and magnetic suscept ib i l i t y measurements conclusively establish that Eu in Eu2Ni~Si 5 is in valence f luctuat ing state with E =730 K, T _=235 K andp --=5.5UB." Also, these measurments show that i t is in s~Xble diva~Tent state i~r~u2Pd3Si5 and in Eu2Cu3Si 5. and exh ib i t magnetic ordering .

Acknowledgement

We would l i ke to express our thanks to Dr. D. Rambabu fo r developing the graphics package and help in many aspects of programming and to Mr. S.K. Paghdar fo r assistance in data co l l ec t i on and dedicated equipement maintainance.

References

/ I / G.K. Shenoy, B.D. Dunlap and F.Y. Fradin, North - Holland Publ lcat ions, 'Ternary Superconductors'.

/ 2/ R. Nagarajan and K.3. Memon, TIFR Tech.Report.TFR/RN4 (1977). / 3/ E.R. Bauminger, G.M. Kalvius and I . Nowik, i b id , chap 10. / 4/ E.R. Bauminger, I . Felner, D. F ro ind l i ch , D. Levron, I . Nowik,

S.Ofer and R. Yanovsky, 3 .Phys. (Par is) , Colioq.,35:(1974)C6-61. / 5/ B.C. Sales and D. Wohl~eben, Phys.Rev.Lett.35(1975)1240. / 6/ 3. Rohler,D. Wohlleben,G. Kaindl and H. Balster,Phy.Rev.Lett .49(1982)65. / 7/ R.Nagarajan, Sujata P a t i l , L.C.Gupta and R.Vijayraghavan, 3.Mag.Mag.

Mater,54-57(1986)349. / 8/ R. Nagarajan, G.K. Shenoy, L.C. Gupta, and E.V. Sampathkumaran,

Phys.Rev.B32(1985)2846. / 9/ to be published