Ferromagnetism of the chimney-ladder compound Cr11Ge19

Journal of

ELSEVIER

Journal of Alloys and Compounds 262-263 (1997) 363-365

Ferromagnetism of the chimney-ladder compound CrllGel9 P. P6cheur a'*, G. Toussaint a, H. Kenzari a, B. M a l a m a n b, R. W e l t e r b "Laboratoire de PILvsique des mat~riaztt; U.R,4. no 155, Ecole des Mines de Nancy, Parc de Saumpt, 54042 Nancy Cedex, France bLaboratoire de Chbnie du solide mbz~ral, U.R.A. no 158, Universif~ H. Poincar~, boite postale 239, 54506 Vandoeuvre-les-Nancy Cede~, France

Abstract

Using a non-self-consistent tight-binding-LMTO method, we have obtained the densities of states for Cq;Ge~9 and MntiSilg. The calculations show a deep minimum in the density of states at an energy corresponding to a valence electron concentration of 14 per Cr (Mn) atom. For CrliGel,~, the Fermi level lies in a high density of states region, suggesting magnetism. Experimentally we have fimnd that this compound is indeed ferromagnetic below 100 K. © 1997 Elsevier Science S.A. Kowords:

lntermetallie compounds: Band structure; Magnetism

I. Introduction

Crl0Gel,~ and Mnl~Sipj have the same crys,allographic structure; their tetragonal unit cell includes 12|| atoms with a large c parameter (48 ,~ and 52/~). They belong to a large family of transition metal compounds, the so-called 'chimney-ladder' [I]. The stability of these compounds is controlled by the valence electron concentration (VEC), defined as the total number of valence electrons divided by the number of transition metal atoms. This VEC practically never exceeds the 'magic' number of 14. Moreover, a VEC of 14 appears to be a good criterion for the occurrence of a band gap in these materials (Ru,Si~, RuAl,, Mn~tSiv~) which suggests an energy stabilisation of the structure duc to the lowering of filled bonding states and the rise of empty antibonding states. Nothing much is known of the physical properties of these compounds, except that Mn~Sit,, is a natu-

*Corlesponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pl l S0925-8388(97)0041~|-6

rally p-doped semiconductor, which has been used in thermoelectric devices. We have performed band structure calculations and magnetic measurements on both CrliGel,~ and Mn,iSil,~. 2. Computational method and results

The calculations have been performed using the tight binding LMTO method and the LMTO parame~ ters tabulated by Andersen [2]. The recursion proce~ dure in teal space was used to obtain the densities of states [3]. This procedure avoids the diagonalization of large matrices. The atomic sphere radius for Mn (Cr) and Ge (Si) have: been chosen so as to obtain the local neutrality. The calculations are not selfocon° sistent, however, the method gives reasonable results for TiSi: and RuAI 2 where a comparison with other calculations is possible. There are no adjusted parameters. The calculations show a deep minimum in the density of states at an energy corresponding to a VEC of 14 (Figs. I and 2). This minimum, between bonding and antibonding states is related to the hybridization

3~

P I¥cheur t,t at,/,lotmml ,:(Alloys and Om~l~otmds 262-263 tl~71 363~365

and Si (powder, 99.9%). Pellets of a stoichiometric mixture were compacted using a stee! die and then introduced into silica tubes sealed under argon (200 mm Hg). Preliminary homogenization were conducted at 1273 K. The samples were then annealed for 1 week at 1373 K and 1173 K f o r Mn~nSi~,~ and CrnnGe,~, respectively The purity of the final product was checked )y a powder X-ray diffraction technique (Guinier, Cu K a). Refinement using silicon as an internal standard gives the following parameters (S.G. P - 42 n):

IR:L~ cJO St at e s

i~llll|

ceil)

!

70 60

// -14

-12

-10

-8

-6

-~

o

-2

2

4

6

Fig. 1. Density of slates Of Mntl Sit,~,Th~ Fermi level is at E = 0.

between the s and p orbitals of Si CGe) and the d orbitals of the transition element. The minimum appears within the partial density of d states. It is also found in all 'chimney-ladder' compounds for which we have ~ r b r m e d similar calculations and is indeed related to the stability of these peculiar structures. For Mn~tSit+, the Fermi level (VEC = 13.9) is in a low density of states region, slightly below the minimum. Although the method we use. which is in fact a reconstruction of the densities of states from their moments, cannot distinguish between a deep minimum (quasi gap) and a real band gap, the calculation strongly su~ests that this minimum corresponds to the ¢~rimental gap of Mn,,Si,0. For Cr~Ge~+. the

Fermi level ( V ~ C ~ 12,g) is at a lower energy, in a region of high demity of st~es. The Stoner criterion i+ just s~flisfied (using t~ v~flue of 026 eV for the Stoner ¢ongtanl), which suggests itinerant nmgnefism (with ~1h ~ moment),

Cr~tGev~" a = 5.803 (1) ~,; c = 52.31 (1) .~, Mnll Sin,~" a = 5.508 (4)/~: c = 47.85 (6) A,

which are in good accordance with the previous values of Kolenda et al. [4] and Schwomma et al. [5]. The magnetic measurements were carried out between 4.2 and 300 K using a M A N I C S magnetosusceptometer in a field up to 1.7 T. 3.1. MnzzSizo

At the studied temperature, the MnltSi,,~ compound exhibits Pauli paramagnetic behaviour tFig. 3). 0,06

" ,

i+,.,i+,.++

(¢m.lmol) 0,04

=

Cr~Oe~. and Mn+~Si~+ were prepared from commercially available high purity elements: Mn, Cr

..........

,

0,0~

~

0,0|

J, S~mple F+pamt|o~ a.d magael|e measamm+.|s

==

0,01

IL-umm_

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0,00

0

SO

m""-~llla(O(D~l

I O0

150

200

I 2GO

ii °

300

Fig, o~, M~gneti¢ m~eplibilily of MntmSi1.. ++++ 8+

4s ~ , w - , , w ~ +0 m

¸ 1 Cr°'(;e'd

4o

I[l~fl}rV~

~+ #®°i =o

as

j

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+8

+6

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6

t~

$

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|:it+ °, ~ |)cnsi~++~.)~stat~s of Ct~(}e~+. The K'rmi l~vd is ~t E"~ 0, The Sto~r criterion is sati:siied,

~16 60

07 `+'+ ¢

i$

19 t~9 t$o 1to 21o 240 27o ~oo

Fig. 4, M+gneticsusceptibility of ChiGem+ above +toohqmv I b

P. P~cheuret al. ~Journal of Alloys and Compounds 262-263 (1997) 363-365

I CrllGel9 [ . I"

point, the temperature dependence of the inverse susceptibility obeys a Curie-Weiss law: The corresponding data is given below:

• N ' .n., I ' - ,urn..-m -,num--U.m .l

J

4

/

M

365

/~ff = 6.6 p J m o l ; / . % f f / C r = 1.99/~./mol; Op

U'"

= ]30 (5) K

f

(p,e/mol)

.a

3

i 4. C o n c l u s i o n s i

2.g

i

0

,

I

5000

,

I

10000

,

I

*

15000

20000

tl (G)

The expectations of the band-structure calculation have been confirmed experimentally: MnliSil9 is paramagnetic, while CrllGel,~ is ferromagnetic below 100 K with a moment of 0.5 P,a per atom of Cr. CrinGer,~ appears as an example of an electronic phase [1], whose structure is stabilized by a hybridization giving a low density of states above the Fermi level, but with a high density of states at the Fermi level itself, which leads to magnetism.

Fig. 5. Magnetizationof CrllGea, vs. F! at 5 K. References

The magnetic susceptibility values are about 4 . 1 0 " emu g-~ and 10 -~ emu g-t at 285 K and 5 K, respectively. 3.2. O'nGe w The CrDIGeI,~ compound is characterized by a ferromagnetic transition a! t}7 (3) K (Fig. 4). At 5 K, the maximum magnetization value is 5.4 # n / r e e l (Fig. 5), yielding 0.5 p J C r . The cocrcitive field is found to be veiny small, approximately I()!l G. Above ~hc Curie

[1] W. Jeitschko, E. Parth~, Acta Crystallogr. 22 (1967) 417. [2] O.K.Andersen, O. Jepsen, D. GI6tzel, in F. Bassani, F. Fumk M.P. Test (Eds.), Highlights of Condensed Matter Theory, North.Holland, New York, 1985,p. 59. [3] R. Haydock, in: H, Ehrcnreich, F, Scitz, D, Tumbu!l (Eds0), Solid State Physics no 35, AcademicPress, New York, 19~, p, 216. [4] M, K~dcnda, J, St~k, A, S~ymh~, J, M~gn, M~tgn, Maler, 20 (19~0) ~)9,

[5] O, Schwonml~l,A, Preisin~er, H, Nowotny,A, Wittm~|l~n~Mh, Chenl, =)5(1¢)64) 1527