Crystal structure and magnetic properties of HoPt2Si2

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

Solid State Communications, Vol. 52, No. 4, pp. 475-478, 1984. Printed in Great Britain.

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF HoPt2Si2 J. Leciejewicz Institute of Nuclear Chemistry and Technology, 03-195 Warszawa, Poland A. Szytu~a Institute of Physics, Jagellonian University, 30-059 Krak6w, Poland M. ~laski Institute of Physics, Technical University, 30-084 Krak6w, Poland and A. Zygmunt Institute of Low Temperatures and Structural Research, 50-950 Wroc~aw, Poland

(Received 2 May 1984 by E.F. Bertaut) The crystal structure of HoPt2Si2 was determined using powder neutron diffraction data. It is tetragonal, CaBe2Ge2 type (space group P4/n ram). Neutron diffraction and magnetometric measurements indicate that HoPt2Si2 remains paramagnetic at the temperature of 2.0 K. 1. INTRODUCTION INTERMETALLIC COMPOUNDS with the general formula RET2X2, where RE is rare earth or actinide atom, T = 3d, 4d or 5d transition metal atoms, X = Si, Ge, exhibit in majority the body-centered tetragonal ThCr2Si2 type of crystal structure. Recently Lalr2Si2 [1] and UPt2Si2 [2] were found to crystallize in the primitive tetragonal CaBe2Ge2 type. Investigating holmium intermetallics of ThCr2Si2 type we came across HoPt2Si2, which unlike the other Ho T2Si2 compounds, turned out to exhibit the CaBe2Ge2 type structure. We report below the results of our neutron-diffraction and magnetometric study performed on a polycrystalline sample of this compound. 2. THE CRYSTAL STRUCTURE

Table 1. Crystallographicparameters for HoPt2Si2 and LaIr2Si2 [1]

a (10 -1 nm) c (10 -1 nm)

c/a V (10 -x rim)3 zi z2 z3 B R (%)

HoPt2Si2

Lalr2Si2

4.117(2) 9.739(6) 2.36(5) 165.07(28) 0.7482(17) 0.3795(10) 0.1492(24) 1.7(2) 8.15

4.191(2) 9.944(4) 2.37 174.66 0.7447(1) 0.3745(1) 0.1262(7) 4.9

Neutron diffraction patterns of HoPt2Si2 taken at 2.0, 4.2 K and 300 K contain apart from fairly strong reflections obeying the h + k + l = 2n extinction rule, small peaks of rather small intensity with indices h + k + l -- 2n + 1 confirming thus the presence of a primitive tetragonal unit-cell (see Fig. 1). Neutron intensities were computed for the following crystal structure models. (I) ThCr2Si2 type, space group I4/m mm with atomic positions.

The sample of HoPtzSiz was synthesized from highpurity elements (Ho-4N, Pt-3N, Si-5N) by arc melting in argon atmosphere. The ingot was then homogenized and annealed at 900°C during 100 hr in a quartz capsule sealed under argon. X-ray pattern was obtained at the room temperature using FeKa radiation. It contains reflections with indices characteristic for a primitive tetragonal unit cell with lattice parameters, as listed in Table 1. Ho in 2(a) Neutron diffraction measurements were carried out Pt in 4(d) at 2.0, 4.2 K and room temperature. Powder diffractometer DN-500 at reactor EWA in ~wierk and neutrons with wavelength of 1.326 x 10 -1 nm Si in 4(c) were used in experiment. 475

O, O, O,

i ~ i , '~, 1. ~,

½,o,¼, o,½,¼; 0, 0, z,

½,½,½+z,

0, 0,~; 1

1

476

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF HoPt2Si2

t

.,03T 16 tt~ I.---

HoPt2Si 2

Z ::~ >,0:::

Vol. 52, No. 4

I-<[ >I-Z Z Z O IZ IZ

300 K

A I

I

1 10°

I

002

003021110111103I I 004

I

I I

100 101

I 15 °

I

I

I

20*

25*

I

i

30*

20(

II

II

202 211 114 105

I

I

35*

40 °

~'~'!

i

I !~o,~i~'~ I

006 20z, 213 i

45*

I i %~°

220 222 116 I

50*

55*

2

Fig. 1. Neutron diffraction patterns of HoPt2Si2 taken at 2.0, 4.2 and 300 K. Shaded peaks are reflections with

h+k+l=2n+

1.

(2) ThCr2Si2 type but with Pt and Si atoms distributed at random in 4(d) and 4(c) sites. A distribution coefficient was introduced in refinement procedure. (3) CaBe2Ge2 type, space group P4/n mm with atoms in the following sites. Ho in 2(c)

1 1

3 3

Pt

in 2(c)

~1, Vi4 , Z 2 ,

3 23[-, Z 2 , . 2[,

in 2(a)

~i , ~3, 0 ,

~3, , ~1,

in 2(c)

1 ;l~ , Z 3 , ;i,

3 ;~, 3 Z3 - ; 2~,

in 2(b)

1

Si

3

1

3

1

(ol

(b)

,

O;

1

The refinement of neutron intensities was performed using Rietveld line profile analysis method. Nuclear scattering lengths were taken after [3]. An overall temperature factor B was allowed for in refinement procedure. For the first two models, only reflections with h + k + l = 2n indices were taken for computations, while in the third case all reflections observed were used. The refined values of free atomic parameters

Ho

0:.. l

W

' .:0

,o - Q1 ~L"

O. (12

_.._ ,.:0 "

Ho

--,~

Fig. 2. Crystal structures of ThCr2Si2 type (a) and CaBe2G% type (b). The distribution o f Pt and Si atoms in tetrahedral (t) and pyramidal (p) sites is indicated. The origin of the unit-cell of CaBe2Ge2 has been shifted by ( ] , ] , ¼). z, unit-cell constants and the corresponding minimum values of reliability factor R are collected in Table 1. For comparison we include the parameters published recently for Lalr2Si2 single crystal sample [1]. Unit-cells of ThCr2Si2 type and CaB%Ge2 type

Vol. 52, No. 4

477

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF HoPt~Si2

L2o

2 I

I

lo

I

20

I

30

I

j

~

HoPt2Si2

/~'a=0Kv 0

/

o-~9

40 H[kOel

H =10 kDe

i

I

I

i

I

I

I

I

i

I

I

i

--

10

20

30

/~

50

60

70

80

90

100

110

T[K]

-

Fig. 3. Reciprocal magnetic susceptibility against temperature curve for HoPt2Si2. The inset represents magnetization curves in magnetic field up to 5 kOe measured at 4.2 K.

Table 2. Comparison o f calculated and observed intensities for HoPt2Si2 for three models of its crystal structure hkI

002 100 10 1 003 1 02 1 10 111 103 1 12 004 1 13 1 04 200 201 005 202 1 14 210 2 11 1 05 203 2 12 00 6

(II)

(III)

7727

2565

1 086

179

228

3 283

15890 58 262 6566

28060 49 028 2044

32 758

38 572

4520 14836

1 436 17271

477 8 102

75 14647

974

463

12145 10603

20535 3 160

1 15 213 204

hkl

Io~

Ic~a c

(I)

Table 2. Continued

1900 0 1 448 2440 4233 4653 11330 28317 48 583 1 894 549 939 38 604 34 2 000 995 15666 0 176 14075 2 420 2 200 411 98 19380 2738

3300 0 2 340 2856 4527 5 344 10384 28964 48 872 4766 1 013 1 010 40 800 138 2 078 1 076 18094 0 523 13 540 2 280 1 680 497 91 17710 3 120

lcalc (I)

106 2 14 220 22 1 1 16 205 222 007 R (%)

lobs (II)

15908

17934

16788

12055

2 378

723

42.9

20.1

(III) 1 565 800 16786 15 10200 3 670 468 132

1 252 560 17343 19 10870 3 860 528 153

8.15

The intensities were calculated for the following parameters: Model (I): z = 0.377(3). Model (II): z = 0.3778(10); e = 0.44(2) (e - mixing paranleter). Model (III): z parameters are listed in Table 1. compounds are displayed in Fig. 2. Their main feature is a completely ordered distribution of atoms, but with different coordination polyhedra. While in ThCr2Siz type each T atom is situated in the center of a tetrahedron composed of Si atoms with T - S i distance usually amounting close to the sum of the respective covalent radii, in the case of HoPt2Si2 (CaB%Si2 type), platinum atoms are placed in the center of a tetrahedron with P t - S i approach of 3.905 x 10 -1 nm and in the center o f a square pyramid with one P t - S i bond o f 3.174 x 10 -1 nm and four P t - S i bonds of 3.08 x I0 -1 nm in length.

478

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF HoPt2Si2

Considering the structure in terms of a sequence of monoatomic layers stacked along the tetragonal c axis, ThCr2Si= type structure can be visualized as R E - T S i - T - R e sequence, while in CaB%Ge2 type the R E - S I - T - S I - R E is observed. 3. MAGNETIC PROPERTIES Magnetometric measurements were performed in the temperature range from 4.2 to 300 K using a vibrating sample magnetometer. In addition we have obtained the magnetisation curves at 4.2 K in magnetic fields up to 5 kOe. The magnetic susceptibility of HoPt2Si2 obeys the Curie-Weiss law (see Fig. 3) with the paramagnetic Curie temperature of 0 K and effective magnetic moment amounting to 10 Bohr magnetons. Also the magnetization curve at 4.2 K (see inset in Fig. 3) exhibits typical paramagnetic character. This is confirmed by neutron diffraction patterns obtained at 4.2 and 2.0 K, where only peaks characteristic for a CaBe~Si2 type of

Vol. 52, No. 4

crystal structure are present, indicating complete absence of arty long-range magnetic ordering at hohnium sites even at 2.0 K. Many Ho T2Si2 compounds are antiferromagnetic at low temperatures (see for example [4, 5 ]) however, all of them show ThCr2Si2 type structure. The absence of long range magnetic order in HoPt2Si2 may be thus bound to the mixed-site occupancy characteristic for tile CaBe2Ge2 type structure.

REFERENCES 1. 2. 3. 4. 5.

W.F. Braun, N. Engel & E. Parth~, Phys. Rev. B28, 1389 (1983). J. Leciejewicz & A. Zygmunt (private communication). G.E. Bacon, Acta Cryst. A28,357 (1972). A. Szytut'a, J. Leciejewicz & H. Bificzycka, Phys. Status Solidi (a) 58, 67 (1980). M. Slaski, J. Leciejewicz & A. Szytut'a, J. Magn. Magn. Mat. 39,268 (1983).