-.Solld State Communzcatzons, Vol.65,No.3, pp.183-184, 1988. "-~_~e Printed in Great Britain.

0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.

FIRST ORDER PHASE TRANSITIONS IN RARE-EARTH HEXABORIDES*

Naushad All Department of Physics

Southern Illinois University Carbondale, Illinois 62901 U. S. A.

and

MoJtaba Kahrizi and M. O. Steinitz Department of Physics

St. Francis Xavier University Antlgonish, Nova Scotia Canada B2G IC0

(Received September 2a, 1987 by M.F. Collins)

Thermal expansion measurements made using capacitance dilatometry to determine the first or second order nature of the N~el transitions and incommensurate-commensurate transitions in PrB6, NdB 6 and GdB6, are reported.

The compounds PrB6, NdB 6 and GdB 6 order antiferro-magnetically at low temperatures. PrB 6 is known to show two low temperature phase transitions I , one to an incommensurate antiferromagnetic state at about 7 K and a second to a commensurate state at about 4.5 K. NdB 6 has only one transition 2, to a commensurate state, at about 7.5 K. GdB 6 has a similar transition3 at about 15 K, although in the absence of magnetic structure determination in this highly neutron-absorbant material, one can only make the assumption that the 15 K transition is to a commensurate state, while speculating on the nature of a sample dependent transition found around 10 K by Nozaki et a13. Specific heat results on GdB 6 4 NdB6 5 and PrB6 1,6 have been interpreted in terms of second order transitions to date.

Resistlvlty measurements on PrB 6 at the 4.5 K transition showed hysteresis, but thermal expansion measurements I did not clearly indicate that this transition is of first order. The 7 K transition showed no hysteresis.

We have undertaken a dilatometric investigation on highly perfect single crystals

of PrB 6 and NdB 6 and two polycrystalline samples of GdB 6 in order to clarify the first or second order nature of these transitions. A capacitance dilatometer7 was used to monitor length changes as the temperature was allowed to drift up from 4.2 K at a rate between I and 5 mK/sec.

Figure 1 presents results on PrB 6 and clearly shows a discontinuity in length at the lower transition, which occurs around 5 K on warming. There is also a small step at the 7

* Work supported in part bY the Natural Sciences and Engineering Research Council of Canada (M. S.) and a mini-sabbatical grant and a Special Research Fellowship to N. A. from ORDA Southern Illinois University.

>

n -

I I I

I 1 I I 4 5 6 7 8 9

Temperature ( Kelvins )

~2 < m

~r

R 5x 10- m

_~. o

o

Figure I. Relative expansivity (a) and change in thermal expansion coefficient (a') of

single-crystal PrB 6 relative to the brass dilatometer frame on warming from 4.2 K to 9 K.

K transition, but this is so small that the transition is presumably only weakly first order, Wlth the major feature being the change in thermal expansion coefficient at this transition. Thus we see that the 4.5 K transition is clearly first order and that the 7 KNeel transition is weakly first order.

Figure 2 shows that the Ngel transition in Nd86 is of first order, while Figure 3 indicates that the N~el transition in GdB 6 is also of first order, although presumably somewhat smeared out due to local strains as this was a polycrystalline sample.

Of the two samples of GdB6, prepared by arc melting in argon atmosphere, one was annealed for 8 hours in vacuum at 1073 K and

183

1 8 4

IT- ~ 5x10-" >

• 2__ W

I I

(]i

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FIRST ORDER PHASE TRANSITIONS IN RARE-EARTH HEXABORIDES

\

I I I I 6 ? 8 9 T e m p e r a t u r e ( K e l v i n s )

~u

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n o

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Figure 2. Relative expansivity (a) and change in thermal expansion coefficient (a') of single-crystal NdB 6 relative to the brass dilatometer frame on warming from 6 K to 10 K.

d

~ T 01~I x 10 -5

i1,

r Y

I I I I

I I I I 13 14 15 16 17 18

T e m p e r a t u r e ( K e l v i n s )

Vol. 65, No. 3

one was not annealed. Figure 4 shows that both samples had a first order transition at about 15 K, which we speculate may be the N@el transition to a commensurate phase. The annealed sample, however, shows an additional feature, a broad peak in the sample length centered at approximately 5 K. We suggest that this may be related to hysteretic, sample dependent effects seen in resistivity studies of this material.

Under the assumption that the length changes at the first order transitions are isotropic, one can calculate the volume changes by multiplying the length changes by three. This assumption is somewhat perilous as one is assuming that the high temperature cubic structure is relatively undistorted in the lower symmetry antiferromagnetic phase, and that one can ignore domain effects. One would clearly like to do the measurements in a single domain sample, measuring length changes parallel and perpendicular to the sublattice magnetization. In the absence of such data, the calculated volume changes are: 43 +/- 2 ppm at 5 K and 11 +/- 2 ppm at 7 K in PrB6, and 65 +/- 2 and 14 +/- 2 ppm at the transitions in

NdB 6 and GdB 6 respectively. We note that the transition to the

commensurate state in PrB 6 and NdB 6 is a first order transition and that this transition coincides with the N~el transition in NdB 6.

We thank S. B. Woods for the loan of the single crystal samples.

"7

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I I I I I I I I I I I I I I

I I I I l I I I I I I I I I 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Temperature (Kelvins)

Figure 3. Relative expansivity of annealed sample of GdB 6 relative to the brass dilatometer frame on warming from 10 K to 20 K.

Figure 4. Relative expansivity (a,b) and change in thermal expansion coefficient (a',b') of two polycrystalline samples of GdB 6 relative to the brass dilatometer frame on warmlng from 4.2 K to 20 K (a) annealed, (b) as arc melted.

References

1. C. M. McCarthy, C. W. Tompson, R. J. Graves, H. W. White, Z. Fisk, and H. R. Ott, Solid State Comm. 36, 861 (1980).

2. C. M. McCarthy and C. W. Tompson, J. Phys. Chem. Solids 41, 1319 (1980).

3. H. Nozakl, T. Tanaka and Y. Ishizawa, J. Phys. C:Solid St. Phys. 13, 2751 (1980).

4. E. F. Westrum Jr., J. T. S. Andrews and G. A. Clay, Thermal Anomalies and Thermodynamic Properties of Gadolinium Hexaboride, U. S. Dept. Com., Clearinghouse Sci. Tech. Inform. AD 627224,9 (1965).

5. E. F. Westrum Jr., H. L. Clever, J. T. S. Andrews and G. Feick, "Thermodynamics of La and Nd hexaborides", Proc. Conf. Rare Earth Res., 4th, Phoenix, Ariz., 1964 p. 597 (1965).

6. K. N. Lee, R. Bachmann, T. H. Geballe and J. P. Malta, Phys. Rev. B~, 4580 (1970).

7. M. O. Steinitz, J. Genossar, W. Schnepf and D. A. Tindall, Rev. Sci. Instrum. 57 297 (1986).