Lattice distortion in CeCu6

"~12

Journal of Magnetism and Magnetic Materials 63 & 64 (1987) 312-314 North-Holland Amsterdam

LATTICE DISTORTION IN CeCu6 E G R A T Z +, E B A U E R +, H and B B A R B A R A °

NOWOTNY*, H

M U E L L E R +, S Z E M I R L I °

+Instttute of Experimental Phystcs, T U Vwnna, Austrta *Institute of Theorettcal Physws, T U Vtenna, Austria °Lab Louts Ndel, CNRS, Grenoble, France

The temperature mduced transmon from orthorhomblc to the monochmc symmetry m CeCu6 has been studied The lattice parameters below and above this Iransmon temperature (~220 K) were obtained using a low temperature X-ray diffraction experiment The influence of this transmon could also be observed on the temperature dependence of the thermal expansion, the electrical reslstlwty and the elastic constant cs~

1. Introduction Within the C e - C u phase diagram a total of five compounds have been identified CeCu2 and CeCu6 form congruently, CeCu, CeCu4 and CeCu5 form peritectlcally Except CeCu5 which crystalhzes In a hexagonal structure, all the others crystallize In different orthorhombic crystal classes [1] In many of these C e - C u compounds unusual physical properties are observable CeCu6 has been classified as a K o n d o lattice system with a nonmagnetic ground state Besides the anomalous high electronic specific heat coefficient 3' (~ 1500 mJ/mol K 2) [2] many of the other physical properties of CeCu6 are strikingly similar to those of CeAI~ which IS considered as a typical K o n d o lattice system Recently, Onuki et al [3] pubhshed measurements of the lattice constants of the pseudobmary (CexLal-x)Cu6 series at room temperature They also reported on a change of the symmetry from the orthorhombic D ~6 to the monochnIc C 25hspace group when 60% of Ce is substituted by La Furthermore, they supposed the existence of a similar structural change in CeCu6 by cooling down the sample Measurements of the elastic constants and neutron diffraction experiments also reveal hints for such a transition [4,5] The main purpose of this paper is to present low temperature X-ray diffraction measurements, which have been performed to study the structural

transition in CeCu6 All these measurements were performed using the Cu-K~ radiation The temperature dependence of the diffraction profiles were measured on a Siemens dlffractometer with a He-flow-cryostat, mounted on the dlffractometer in place of the standard specimen holder For testing the cahbratlon of the dlffractometer w~th the mounted cryostat at any temperatures we mixed tungsten powder to the CeCu6 powder This enables us to measure simultaneously the well known temperature dependence of the tungsten lattice constants which then was used as a reference The CeCu6 sample for the X-ray diffraction experiment was prepared by high frequency melting, crushed, sieved (< 50 Ixm) and homogenized to release the stress in the grams of the powder

2. Results and discussion T o see, whether this structural transition can be observed In other physical properties of CeCu6, we have started with experiments such as electrical resistivity (p), thermal expansion ( A l / I ) , elastic constants (c55) and susceptlbdlty (X) As can be seen from fig 1, where these results are collected (except x ( T ) ) , there are anomalies around 220 K in the electrical resistivity, the elastic constants and in the thermal expansion It was interesting to see that in the temperature dependence of the susceptibility no hints for the

0304-8853/87/$03 50 © Elsevier Science Publishers B V (North-Holland Physics Publishing Division)

E Gratz et al / Lamce &storaon

313

m CeCu6

Table 1 Miller lnd]ces

I orthorhombtc20]~'x'O"

rnonocL,ntc 735

~

C5~ M "~...,..._~.. T j . %.% . ~ . . . ' ~ . . -

/~= •

,"

20-position

[hkl]orthorhornb, c

[hkl]

35 5 37 7

[020]o [302]0

387

[ 3 1 1 ] o ~ ~ [213]o~[132] [022]0

[200]~. [023]., 15],, [[l113],~ [132],, m [220]=

t9-

"

--I

38 8 39 5

720 80

•

" 120 '

.....

] ....

o

I ,~o

200 '

2a,

28O

Fag 1 Temperature dependence of the electrical res]stw]ty p, the thermal expansion/Xl/l and the elastic constant css m the vlmmty of the structural transmon temperature

transition could be g a m e d although this m e a s u r e m e n t has been done very carefully T h e p-, (Al/l)- and x - m e a s u r e m e n t s have been performed on a polycrystallme sample which was prepared by htgh frequency melting and subsequently annealed at 700°C during 72 h T h e elastic constant data have been measured on a " p u r e " single crystal and on a single crystal in which 5% of c o p p e r has been substituted by alumlnlum A comparison of the css results yields that in the single crystal with 5% alumInium content the change of the symmetry is shifted towards lower temperatures (from 217 K for CeCu6 down to 145 K) We therefore conclude, that the existence of formgn phases or other impurities causes a lowering of the transition temperature Thts 'alloying' could possibly explain, why in the literature differences In the transmon t e m p e r a t u r e of about 50 K can be found (see e g refs [4, 5]) In table 1 the Mtller indices in the orthorhomblc and the m o n o c h m c description of those reflexes which have been used for the determmatlon of the lattice parameters are given T h e line profile of the reflexes with the Miller lndmes [020]o, [302]o and [022]0 will not be spht by the change of the lattice symmetry, whereas the [311]o and the [213]o line profile becomes split after the symmetry IS lowered T h e X - r a y line profiles of the [3111o and [213]o

reflexes which appear close together m the orthorhomblc structure under investigation are given at 290 K m fig 2a T h e diffraction pattern in the same 20 angle region obtained at 4 2 K is depicted in fig 2c A meaningful Interpretation of

tO

x103counts Q~B • 0 00

T= 2 9 0 K 8 6

% f 37~of 273Jo

•%

I[

i

""

2O

380 .".°

8

T= 2 0 0 K

%,°°

6

ee • ~0

o~, eI

~e e

o•

".

,I

, I ,I

,"-.r 2O

380 • I

8

T= ~ 2 K

6

,e

o 13s~

[132J~

. . . . .

_.." 380

II 38~

38 8

39 2

" 20

Fig 2 The X-ray hnc profiles of the orthorhomblc [311]o and [213]o reflexes and thmr variation due to the structural transmon into the monochnlc symmetry below 220 K

314

E

Gratz et al / L a t t t c e dlstorUon m CeCu6

B

I

916 ~ ~ - ~ - _ ~

908]

9oo I

monochn/c

~

~ ~-

orthorhombtc

'

latttce parameters

volume/umt cell

I017 10.15 ~20

1013 I0 II

'" 810 808 806

the various K,~l-llnes From this analys~s ao, bo, co of the orthorhomblc structure m the temperature range from 290 to 220 K and am, bm, Cm and the angle J~mOf the monochnlc structure below 220 K have been deduced The temperature dependence of the latt=ce parameters and the calculated volume of the unit cell is gwen m fig 3 As it can be seen from this figure, the angle/3m ~S90 ° as long as we have the orthorhomblc symmetry, whereas below the transmon temperature /3., increases continuously up to 91 5 ° at hqmd He temperature* The variation of the volume of the umt cell w~th the temperature shows a distract change of the slope at the structural transmon The value of the gradient ts smaller below 220 K than above The same tendency can also be seen m the thermal expansion m fig 1

5O9

507 50

ioo

15o

2oo

25o

T[K2

Fig 3 Temperature dependence of the lathce parameters and the volume of the umt cell below and above the structural transmon temperature

these data can only be made assuming the existence of the monocllmc symmetry at that temperature, I e assuming that the [3111o and the [213]0 reflexes both are split into two new reflexes due to this m o n o c h m c symmetry (see also table 1) Fig 2b demonstrates the situation at 200 K (about 20 K below the begin of the crystal d~stortlon) The precise 2 0 values of the different X-ray line profiles have been obtained fitting Lorentz functions to the experimental data and taking their centre of gravitation as the 2 0 angles The vertical hnes m fig 2 indicate the posmon of

Part of the work was supported by the " H o c h schuljubllaumsstlftung der Stadt Wlen"

References [1] K A Gschneldner, Jr and M E Verkade m Selected Cerium Phase Diagrams, IS-R1C7 (Rare Earth Information Centre, Iowa State U m v , September 1974} [2] G R Stewart, Z Fisk and M S Wtre, Phys Rev B 30 (1984) 482 [3] Y Onukl, Y Shlmlzu, M Nlshtkawa, Y Machn and q Komatsubara, J Phys Sac Japan 54 (1985) 1964 [4] T SuzuM, ql Goto, A TamaM, T Fupmura, Y OnuM and T Komatsubara J Phys So{- Japan 54 (1985) 2"~67 [~] Y Noda, K Yamada, ! Hirosawa Y Endoh Y OnuM and q Komatsubara, J Phys So{. Japan ~4 (198% 4486 [6] H Asano. M Ummo, Y 6nukl, T Komatsubara. F Izuml and N Watanabe, J Phys So{. Japan 55 (1986) 454 * These results are Jn agreement to these data of the lattice parameters obtained for CeCu. at 65 K [6]