Neutron powder diffraction study of the nuclear and magnetic structures of HoNi1.985Co0.015B2C and HoNiBC

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Physica C 271 (1996) 311-318

Neutron powder diffraction study of the nuclear and magnetic structures of HONil.985Coo.olsB2C and HoNiBC Q. Huang

a, J.W. Lynn a,b, A. Santoro * ,a, B.C. Chakoumakos c, R.J. Cava d, J.J. Krajewski d, W.F. Peck, Jr. d

a Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA b University of Maryland, College Park, MD 20742, USA c Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA d Lucent Bell Laboratories, Murray Hill, NJ 07974, USA Received 22 July 1996

Abstract

The nuclear and magnetic structures of HoNil.985Co0.015B2C and HoNiBC have been analyzed by neutron powder methods. The first of these materials crystallizes with the symmetry of space group I4/mmm, lattice parameters (at room temperature) a = 3.52011(9), c = 10.5286(3) /~, and is isostructural with the undoped compound HoNi2B2C. Cobalt and nickel atoms are disordered over the same crystallographic sites. Well below the Nell temperature ( ~ 8 K), the magnetic moments of the Ho atoms are ordered ferromagnetically in the a, b planes and antiferromagnetically along the c-axis, with a magnitude of the moment of 8.26(6)p, a. The Co doping, even at such low concentrations, destroys the superconducting behavior of the undoped phase. The compound HoNiBC is also tetragonal (space group P4/nmm), with lattice parameters (at room temperature) a = 3.5631(2), c = 7.5486(6) ,~, and is isostructural with LuNiBC. Below the Nell temperature (9.82(8) K), the Ho magnetic moments are ordered ferromagnetically in the bi-layers (HoC)(HoC) with spins lying in the a, b planes, and successive bi-layers are ordered antiferromagnetically along the c-axis. The magnitude of the moments is 6.74(9)/xa. No structural distortions associated with the magnetic ordering were observed in either compound.

1. Introduction

The discovery of superconductivity in quarternary metal boro-carbides has revitalized the interest in the crystallography and crystal chemistry of these materials [1-3]. Compounds of formula RNi2B2C (R = Lu . . . . . La, Y) have been characterized by X-ray and neutron diffraction methods [4,5]. They crystallize with the symmetry of space group I 4 / m m m and

* Corresponding author. Fax: + 1 301 921 9847.

have a structure that can be described as (HoC) rock-salt type monolayers interleaved, along the caxis of the unit cell, with (B) (Ni 2) (B) fluorite-type blocks in which square-planar arrays of Ni atoms are sandwiched between two boron layers. Superconductivity in these materials was found not only for the non-magnetic rare earth elements, but also for magnetic elements such as Ho, Er and Tm, with transition temperatures of 7.5, 10.5 and 11.0 K, respectively. Eisaki et al, [6] have shown that antiferromagnetism and superconductivity do co-exist in these phases, and experiments with an applied magnetic field revealed strong anomalies near the antiferro-

0921-4534/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 5 3 2 - 1

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Q. Huang et al. / Physica C 271 (1996) 311-318

magnetic ordering temperature. The most pronounced anomalies were observed in HoNi2B2C which has a transition to a superconducting state at about 8 K with re-entrant behavior. The magnetic structure of HoNi2B2C has been elucidated by neutron powder and single crystal methods [5,7] and the relationship between magnetism and superconductivity has been discussed in detail [6-8]. Doping HoNi2B2C with small quantities of Co suppresses superconductivity. It has been shown that the impurity Co atoms replace Ni in the structure. The nuclear and magnetic structures of the doped and undoped materials remain identical, however, demonstrating that the superconducting behavior is controlled by the magnetic ordering present at each temperature, and not vice-versa [9]. If the layer sequence of HoNi2B2C is modified by inserting a second (HoC) layer next to the original one, we obtain the structure of HoNiBCo Lattice parameter measurements show that this compound is isomorphous with LuNiBC [4] and has tetragonal structure (space group P 4 / n m m ) in which (HoC)(HoC) bi-layers having a rock-salt atomic arrangement, alternate along the c-axis with the (B)(Ni2)(B) fluorite blocks. No superconducting behavior has been found in this material, but a magnetic transition occurs at about l0 K. Since in the sequence of HoNiBC the adjacent (HoC) layers are separated by a short distance of about 2.4 A, it is reasonable to expect that this compound has a different magnetic ordering from that observed in HoNi2B2 C. To our knowledge, no studies of the nuclear and magnetic structures of HoNiBC have been carried out. We have therefore analyzed the compound with neutron powder diffraction in order to determine precisely its nuclear structure and to

elucidate the ordering of the Ho spins at low temperature. The results of this experiment will be described in this paper, together with the detailed determination of the nuclear and magnetic structures of HoNiI.985C0.015B2C, which was not reported in Ref. [9].

2. Experimental 2.1. Preparation o f the samples

The samples of HoNiBC and cobalt-doped HoNi2B2C were prepared by arc-melting and annealing. The starting materials were coarse powders of Ho (99.9% purity), Ni (99.99%), Co (99.99%), i i B(98.5%) and C (99.99%). The powders were first pressed into pellets, and then arc-melted three times under argon on a standard water-cooled copper hearth, turning the button over between each melt. The arc-melted buttons were then wrapped in Ta foil, sealed in evacuated quartz tubes, and annealed at 1050°C for 5 days. 2.2. Data collection and refinement o f the nuclear and magnetic structures

The neutron intensity measurements were made using the HB-4 high-resolution powder diffractometer at the high-flux isotope reactor at Oak Ridge National Laboratory, adopting the experimental conditions listed in Table 1. Intensity data from the sample of HoNiLgssCo0.015BEC were collected at room temperature, 18 K and 2 K. The powder patterns obtained in these experiments did not differ in any significant way

Table 1 Collection of neutron powder intensity data Monochromaticbeam: Horizontaldivergences: Sample container: 20 angularrange: Scattering amplitudes(l 0- i2 cm): Absorptionmeasurement:

115 reflectionofGe monochromatorwith A= 1.4174(2) ,~, and 311 with A= 2.221(1) ,~ for HoNiBCat 2.1 K. 12', 20', 6' of arc for the in-pile, monochromaticbeam, and diffractedbeam collimators, respectively. AI can with 4 mm diameter. I 1°- 135°, steps: 0.05°. b(Ho) = 0.808, b(Ni) = 1.03, b(Co)= 0.253, b(~tB)= 0.666, b(C) = 0.665. /.tr = 0.42 for HoNiBCand 0.10 for HoNil.985Coo.olsBzC.

Q. Huang et al./Physica C 271 (1996) 311-318

313

phase transitions of the nuclear structure were observed down to 2 K. Powder patterns of HoNiBC were obtained at room temperature, 11 K and 2.1 K. The reflections of the nuclear structure were readily indexed using lattice parameters similar to those obtained for LuNiBC [4], indicating that the two materials are isostructural. The refinements at room temperature and at 11 K, and those of the nuclear structure at 2.1 K, were therefore carried out in space group P 4 / n m m and using as initial parameters those reported for LuNiBC. The powder pattern recorded at 2.1 K shows in addition to the nuclear reflections, strong magnetic peaks that could be indexed in terms of a magnetic unit cell a m, bin, era , related to the nuclear

from those observed for the undoped compound at the corresponding temperatures, and therefore refinements of the nuclear and magnetic structures were carried out using as initial parameters those obtained in the previous work described in Ref. [5] assuming that the doping Co atoms replace Ni on the (Ni 2) layers. The structural parameters and selected interatomic distances obtained in these calculations are listed in Table 2. In Fig. 1 are shown the plots of the observed and calculated intensities and, on the same scale, of the difference Robs) - l(calc) for the powder pattern at 2 K. The nuclear and ground state magnetic structures of HONil.9ssCo0msB2C are schematically represented in Figs. 2a and 3a, respectively. As was the case for the undoped material, no

Table 2 Structural parameters and selected interatomic distances (~,) of HONi 1985Cooo~B2C at 296 K (first line), 18 K (second line), and 2 K (third line). Nuclear structure: Space group 1 4 / m m m (No. 139). a N = 3.52011(9) A; c N = 10.5286(3) ,~; a N = 3.511(3(1) A; c N = 10.5319(4) A; a n = 3.5108(1)/~; c N = 10.5319(4) ~, at 296, 18 and 2 K, respectively. Magnetic structure at 2 K: Shubnikov space group Pm'mm' a~.I = b M = a n , c i = cN, p.(Ho) = 8.26(6) ~B Atom

Site

x

y

z

B(/~2)

Ho

2a

0

0

0

0.25(6) 0.00(9) 0.01(6)

Ni/Co a

4d

0

l/2

1/4

0.28(3)

0.26(4) 0.58(4) B

4e

0

0

0.3589(3) 0.3587(4) 0.3600(3)

0.59(4) 0.66(6) 0.67(5)

C

2b

l/2

l/2

0

0.58(7) 0.7(1) 0.58(9)

Rp(%) = 6.31 9.41 9.26

Rwp(%) = 8.03 11.51 11.24

X 2 = 1.912 1.274 2.101

Selected interatomic distances Ho-B

×8

Ho-C

X4

a N i / C o = Nil.985Coo.ol s.

2.899(2) 2.894(2) 2.887(2) 2.48909(6) 2.48265(8) 2.48250(8)

Ni-B

×4

Yi-Ni

×4

B-C

x I

2.100(2) 2.096(2) 2.104(2) 2.489O9(6) 2.48265(8) 2.48250(8) 1.485(3) 1.488(4) 1.474(3)

314

Q. Huang et a l . / P h y s i c a C 271 (1996) 3 1 1 - 3 1 8 r

400

i

[

HONil,985Coo.o15B2Cat 2

I

I

I

K

300 +

200

+

+ +

~100

'1'11'1 't ' II' 'q ll' ~'1' I'1'11'1 't~11'1"1

II'l"l'll

'1 ' I 1 ' 1 "

I

It'

-100 I 20

I 40

I 60

I 80

I 100

I 120

20(deg)

Fig. 1. Plot of observed and calculated intensity profiles for the powder pattern of HoNi~.985Coo.olsB2C at 2 K. The plot of the difference l(obs) - l(cal) is shown in the lower part of figure. The short vertical marks indicate the angular positions of the magnetic (top) and nuclear (botom) reflections. The angular intervals excluded from the refinements are those affected by AI and/or impurity lines.

with no restrictions on h and k, and therefore, there is no o v e r l a p p i n g o f the nuclear and m a g n e t i c peaks. The m a g n e t i c structure consists o f four H o atoms

cell a 0, b 0, c o by the transformation matrix (1, 0, 0 / 0 , 1, 0 / 0 , 0, 2). M a g n e t i c reflections are o b s e r v e d w h e n l = 2 n + 1 and are absent for l = 2 n,

~Ho/Co

.....................

J

0

o

" - ' F : I ........... ...........? 1

/]

i

,

/

/1

i

,c

t

::::::::::::::::::::::::'E .... (

Ni

.45............X.....

~ .~r

~

ii---f---

::©::i? .2..I Ni2% layer

Ni2B 2

,oyer

......... [ O" ....... [ ........................

o

j--u ," a

(a)

(b)

Fig. 2. Schematic representation of the crystal structures of (a) HoNi2B2C and (b) HoNiBC. The distance between successive (HoC) layers in (a) is c / 2 . In (b) the distance between contiguous layers in the bi-layers (HoC) (HoC) is about 2.5 A..

Q. Huang et al./Physica C 271 (1996) 311-318 I

which are located in the magnetic unit cell at the positions:

Z

No

•I .........................

./"

........ 1 .......................... I I I I

I

(1) ~1, ~ l, z;

I

1

I I

L I I

.......

....... I ......................... I I I I I I L I I I I

7t

....... I ......................... I I I I I J I I

1

I

(a)

(b)

Fig. 3. Ground state magnetic structures of (a) HoNiL985Coo.ol sB2C and (b) HoNiBC.

I

1

(2) ~3 , ~3 , 1

(3) Z , ~ , 7 + z ;

I I I I I

I I I I I I

315

3

3

1

(4) a, ~, ~

Z

The extinction condition on the magnetic reflections hkl (l = 2n) establishes that atoms (1) and (3) and atoms (2) and (4) are coupled antiferromagnetically. In addition, since the z-coordinate of the Ho atoms is about (1/12)c, and since reflections with h + k = 2n and l = 12n + 3 or l = 12n + 9 are unobservable, the atoms (1) and (2) must be coupled ferromagnetically. The magnetic structure of HoNiBC is therefore determined without ambiguity. The best agreement between observed and calculated magnetic intensities was obtained by ordering all the moments in a plane perpendicular to the c-axis. The results after the final refinement are given in Table 3 together with the relevant bond distances. Schematic representations of the nuclear and magnetic structures of HoNiBC are shown in Figs. 2b and 3b, respectively, and the plots of the observed

I

t

z;

I

I

I

+

HoNiBC at 2.1 K (~ =2.221,~) 100

5C

I

~

I"

I "

I

I

20

40

I1~ '' ' l l

*t'

I

60 20(deg)

I'L ' t ' 1 " 1 " 1

II

"L'

I

I'

I'l

L

I

I

80

100

120

Fig. 4. Plot of observed and calculated intensity profiles for the powder pattern of HoNiBC at 2.1 K. The difference plot, and angular positions of magnetic and nuclear reflections are ordered as in Fig. 1. The excluded regions are those affected by extra intensifies.

Q. Huang et al./Physica C 271 (1996) 311-318

316

Table 3 Structural parameters and selected interatomic distances (,A) of HoNiBC at room temperature (first line), 11 K (second line), and at 2.1 K (third line). Nuclear structure: Space group P 4 / n m m (No. 129). a N = 3.5631(2) .~; c N = 7.5486(6) ,~,; a N = 3.5563(2) ,~, c N = 7.5494(5) ,~,; a~ = 3.5560(2) ,~; c N = 7.5487(5) ,A at 296, 11 and 2.1 K, respectively. Magnetic structure at 2.1 K: Shubnikov space group Pro'ran'. a M = b M = aN, c M = 2c N, /,t(Ho) = 6.74(9) tza. The temperature factors at 2.1 K were fixed at the values obtained in the experiment at 11 K Atom

Site

x

y

z

B (,~2)

Ho

2c

1/4

1/4

0.1668(4) 0.1672(4) 0.1656(7)

0.29(5) 0.06(5)

Ni

2b

3/4

1/4

1/2

0.65(9)

0.31(4) B

2c

3/4

3/4

0.3503(5) 0.3509(5) 0.3520(9)

0.87(6) 0.82(7)

C

2c

3/4

3/4

0.1529(5) 0.1541(6) 0.1534(9)

0.96(6) 0.60(6)

Rp(%) = 6.65, 7.52 11.66,

Rwp(%) = 7.93 9.04 13.04

X 2 = 1.031 1.255 1.327

Selected inleratomic distances Ho-B

×4

Ho-C

×4

Ho-C

× 1

2.875(2) 2.872(2) 2.882(4) 2.5217(3) 2.5166(3) 2.5162(4) 2.413(4) 2.425(5) 2.408(7)

and calculated intensities and of the difference l ( o b s ) - l(calc) are shown in Fig. 4 for the powder pattern measured at 2.1 K.

3. Discussion As we mentioned earlier, the superconducting compound HoNi2B2C and the nonsuperconducting doped phase HoNi 1.985Co0.015B2C have identical nuclear and magnetic structures (Figs. 2a and 3a). Since these have been described in detail elsewhere [5], no further discussion of their features will be given here. The compounds HoNi2BzC (or HoNil.gssCoo.015 B2C) and HoNiBC are the first and second members of the homologous series Ho, Ni2B2C n. The rela-

Ni-B

X4

Ni-Ni

X4

B-C

× 1

2.110(2) 2.104(2) 2.100(4) 2.5195(2) 2.5146(2) 2.5145(2) 1.490(6) 1.486(6) 1.500(9)

tionship between the two structures is schematically illustrated in Figs. 2a and b. In HoNi2B2C the mirror planes perpendicular to the four-fold axis of space group I 4 / m m m are coincident with the (HoC) layers, so that the Ho and C atoms are constrained to be co-planar, and the n-glides are coincident with the planes of the nickel atoms. In HoNiBC, on the other hand, n-glides, rather than mirrors, are perpendicular to the c-axis. One of these is coincident with the (Ni 2) layers and is therefore coincident with the glide plane present in the unit cell of HoNizB2C, while the other is located half way between the adjacent (HoC) layers. As a consequence of this configuration, the Ho and C atoms are no longer constrained to be co-planar and the Ho atoms are in fact shifted along the c-axis away from the plane of the C carbon atoms towards the adjacent (B) layer.

Q. Huang et al./Physica C 271 (1996) 311-318

317 HoNiBC

Also, the presence of two contiguous (HoC) layers in HoNiBC causes the shortest separation between (Ni 2) planes to be about 7.8 ,~, i.e. much longer than the distance of 5.3 ,~ found in HONiEB2C. One of the major differences between the structures illustrated in Fig. 2 involves the coordination of Ho. In going from HoNi2B2C to HoNiBC, four of the boron atoms originally present in the coordination sphere of Ho (Fig. 2a) are replaced by one carbon atom. Consequently, each Ho atom in HoNiBC is surrounded by four boron and five carbon atoms forming a capped nine-fold coordination polyhedron. It has been shown [4] that in the compounds of general formula RNiEB2C the a-parameter increases with increasing ionic radius of the lanthanide element (from Lu to La), while the c-parameter decreases. A comparison of the lattice constants of LuNiBC and HoNiBC (Table 4) shows that the compounds of formula RNiBC behave in the same way, i.e. the structure of HoNiBC is compressed along the c-axis and expanded along the a- and b-axes, compared to that of LuNiBC. As shown in Table 4, the increase in the a-parameter in going from LuNiBC to HoNiBC is accompanied by corresponding increases of the R-C, R-B and Ni-Ni bond distances, while the Ni-B separation is not significantly different in the two cases. The shortening of the c-parameter, on the other hand, is reflected in a decrease of the R-C bond and in an increase of the

250

•' . . . . .

I . . . . . . .

I . . . . . . .

I . . . . . . .

I . . . . . . .

[ . . . . . . .

200

te

150

O •

100

O -

.

50

0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Temperature (K)

Fig. 5. Peak intensity of the 100 magnetic Bragg peak as function of temperature for HoNiBC.

Ni-B-Ni angle, while the B-C distance remains practically unchanged. A similar behavior of the atomic distances and angles is also observed for the Lu and Ho compounds of formula RNi2B2C. We now turn to the magnetic ordering behavior in HoNiBC. Fig. 5 shows the peak intensity of the {100} magnetic Bragg peak, measured on the HB-3

Table 4 Comparison of the bond distances (.~) and angles (°) of the Lu and the Ho compounds of the series LnnNi2B2C n at room temperature LuNiBC Ref. [4]

($) c (~,) a

B-C Ho-C Ho-C Ho-B Ho-B

Ni-Ni Ni-B B-Ni-B

B-Ni-B B-(Ni) plane Ho-(B) plane Ho-(C) plane

X1 XI X4 X8 X4 X4 X4

3.4985(3) 7.7556(7) 1.52 2.438 2.475

HoNiBC Present work 3.5631(2) 7.5486(6) 1.490(6) 2.413(4) 2.5217(3)

LuNi2B2C Ref. [4] 3.4639(1) 10.6313(4) 1.47

HoNi2B2C a Ref. [5] 3.5182(1) 10.5253(3) 1.484(2)

2.449 2.855

2.48 18( 1) 2.8974(8)

2.867 2.474 2.11 108.05

2.875(2) 2.5195(2) 2.110(2) 107.00

2.449 2.10 108.75

2.4877( 1) 2.0995(9) 107.32

112.4

115.2

110.9

113.8

1.172 1.449 0.075

a Normalized to ORNL by multiplying by 1//0.99966.

1.130 1.385 0.105

1.192 1.466 0

1.150 1.484 0

318

Q. Huang et al./Physica C 271 (1996) 311-318

spectrometer with neutrons of a wavelength 2.35 .~. The intensity varies smoothly with temperature, and there are no indications either in these data or the structural refinement data of any structural distortions associated with magnetic ordering. There is also no evidence of any magnetic or structural transitions below the ordering temperature. The solid curve is a fit to the square of a mean-field Brillouin function, and we obtained a reasonable representation of the data with a fitted Ntel temperature TN = 9.82 + 0.08 K. The full refinements yield an ordered sublattice holmium moment of 6.7(1)~B. In conclusion, we have shown that HoNiBC is isostructural with LuNiBC and that below the Ntel temperature of 9.82(8) K the Ho moments are ordered ferromagnetically in the bi-layers (HoC)(HoC) and antiferromagnetically along the c-axis.

References [1] R. Nagarajan, C. Mazumdar, Z. Hassain, S.K. Dhar, K.V. Gopalakrislman, L.C. Gupta, C. Gotard, B.D. Padalia and R. Vijayaraghavan, Phys. Rev. Lett. 72 (1994) 274.

[2] R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, Jr., R.B. vail Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter and S. Uchida, Nature 367 (1994) 146. [3] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck, Jr., T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki and S. Uchida, Nature 367 (1994) 252. [4] T. Siegrist, H.W. Zandbergen, R.J. Cava, JJ. Krajewski and W.F. Peck Jr., Phys. Rev. B 51 (1995) 3701. [5] Q. Huang, A. Santoro, T.E. Grigereit, J.W. Lynn, R.J. Cava, J.J. Krajewski and W.F. Peck, Jr., Phys. Rev. B 51 (1995) 3701. [6] H. Eisaki, H. Takagi, R.J. Cava, K. Mizuhashi, J.O. Lee, B. Batlogg, J.J. Krajewski, W.F. Peck, Jr. and S. Uchida, Phys. Rev. B 50 (1994) 647. [7] A.I. Goldman, C. Stassis, P.C. Canfield, J. Zarestki, P. Dervenagas, B.K. Cho and D.C. Hohnston, Phys. Rev. B 50 (1994) 9668. [8] T.E. Grigereit, J.W. Lynn, Q. Huang, A. Santoro, R.J. Cava, J.J. Krajewski and W.F. Peck, Jr., Phys. Rev. Lett. 73 (1994) 2756. [9] J.W. Lynn, Q. Huang, A. Santoro, R.J. Cava, J.J. Krajewshi and W.F. Peck, Jr., Phys. Rev. B. 53 (1996) 802. [10] D.E. Cox, A.I. Goldman, M.A. Subramaniam, J. Gapalakrishnan and A.W. Sleight, Phys. Rev. B 40 (1989) 6998. [11] J.M. Hastings, L.M. Corliss, M. Blume and M. Pasternak, Phys. Rev. B 1 (1970) 3209.