Magnetic behaviour of Ni3P Ni2P, NiP3 and the series Ln2Ni12P7 (Ln = Pr, Nd, Sm, GdLu)

J. Whys. Chem. Solids Vol. 54. No. I I. pp. 1527-1531, Printed in Great Britain.

@TX?-3697/93 S6.00 + 0.00 0 1993 Pergamon Press Ltd

1993

MAGNETIC BEHAVIOUR OF Ni, P, N&P, NiP, AND THE SERIES Ln,Ni,,P, (Ln = Pr, Nd, Sm, Gd-Lu) KAI ZEPPENFELD and WOLFGANG JEITSCHKO AnorganischChemisches Institut, Universitlt Miinster, Wilhelm-Klemm-Str. D-4400 Miinster, Germany (Received

8,

11 January 1993; accepted in revised form 15 July 1993)

Abstract-The magnetic properties of the title compounds were investigated with a SQUID magnetometer between 2 K and 300 K. N&P, Ni,P, NIP, and Lu,Ni,,P, are Pauli paramagnetic. Sm,Ni,,P, shows Van Vleck paramagnetism, while the magnetic susceptibilities of the other ternary phosphides obey the Curie-Weiss law resulting in magnetic moments corresponding to the free ion values of the lanthanoid atoms. Thus, the nickel atoms of these ternary phosphides do not carry magnetic moments. The ytterbium atoms in Yb,Ni,rP, show mixed or intermediate valence behaviour. No magnetic order down to 2 K was observed for the praseodymium, neodymium, holmium, erbium and thulium compound. Gd,Ni,*P, is antiferromagnetic with a Neel temperature of TN= 15 K, DysNi,rP, is ferromagnetic (Tc = 9 K), while Tb,Ni,,P, (TN = 12K) is metamagnetic. The existence of EuNi,P, is confirmed; Eu,Ni,,P, with Zr,Fe,,P,-type structure was not obtained. Keywords:

Magnetic properties, N&P, Ni,P, NIP,, lanthanoid nickel phosphides with Zr,Fe,,P,

type

structure.

1. INTRODUCTION Phosphides Ln,T,,P, with Zr,Fe,,P, type structure [l] are known for the series of the lanthanoids (Ln)

with iron, cobalt and nickel as transition metal component [2,3]. In the iron-containing phosphides with this structure the iron atoms do not carry magnetic moments [4, 51,while the series of the cobalt containing compounds shows ferromagnetism with Curie temperatures around 150 K for most compounds [4,6]. In the present paper we report the magnetic properties of the isotypic nickel-containing phosphides. The samples of these compounds occasionally contained binary nickel phosphides as minor impurities. For that reason we also investigated the magnetic properties of these binary phosphides.

2. EXPERIMENTAL Starting materials

DETAILS

for the preparation

of the binary

and ternary phosphides were filings of the rare earth elements (all of nominal purity >99.9%), nickel

powder (Merck: 99.9%) and red phosphorus in the form of small pieces (Hoechst-Knapsack: “ultrapure”). The filings of the light rare earth elements were stored under dried paraffin oil, which was washed away by dried n-hexane prior to the reactions. The remaining n-hexane was evaporated in a vacuum and the filings were not exposed to air before

they were sealed in evacuated silica tubes together with the other components. The stoichiometric mixtures were first annealed for about 3 days at 550°C. The reaction products were then ground to a fine powder in an agate mortar under argon, cold-pressed and annealed again for a week at 1000°C in evacuated, sealed silica tubes. The reaction products were crushed and treated in diluted (1:l) hydrochloric acid for about 2 days to remove possible impurities (the binary lanthanoid phosphides, LnP and elemental nickel dissolve, the binary and ternary nickel phosphides do not dissolve in hydrochloric acid). Of the known binary nickel phosphides [7,8] we have obtained N&P [9], N&P [lo] and NIP, [ 1 l] in pure form. N&P and N&P were obtained as described above from the elemental components and NIP, was prepared by the tin flux technique [ 12, 131.A mixture of the elements in the atomic ratio Ni : P: Sn = 1: 6 : 7 was sealed in evacuated silica tubes, slowly (IO’C h-‘) heated to 7OO”C,kept at this temperature for ten days and quenched in air. The tin-rich matrix was dissolved in diluted hydrochloric acid. Energy dispersive X-ray fluorescence analyses in a scanning electron microscope did not reveal impurities like silicon or tin. The samples were characterized by Guinier powder diagrams using Cu Kcc, radiation and a-quartz (a = 491.30 pm, c = 540.46 pm) as a standard. The hexagonal lattice constants of the Zr,Fe,,P, type

1527

KAI ZEPPENFELD and WOLFGANGJEITSCHKO

1528 Table 1. Lattice constants

of lanthanoid

with Zr,Fe,,P, Compound

nickel phosphides

type structure

0 (Pm)

c (Pm)

V (nm’)

Pr,Ni,,P,

911.7 (2)

Nd,Ni,,P, Sm,Ni,,P, Gd2Ni,,P, I%Ni,,P, DyzNiuP, Ho,Ni,,P, Er,Ni,,P, Tm2NiuP, Yb,Ni,,P, Lu,Ni,,P,

910.2 (2) 907.7 (2) 906.9 (2) 906.4 (2) 905.1 (2) 905.3 (3) 904.6 (1) 904.4 (1) 904.8 (4) 904.4 (2)

374.9 (1) 373.8 (1) 371.0(l) 370.0 (1) 368.8 (I) 367.8 (2) 367.6 (1) 366.5 (1) 365.8 (1) 365.1 (2) 364.6 (1)

0.2698 0.2682 0.2647 0.2635 0.2624 0.2609 0.2609 0.2597 0.2591 0.2588 0.2582

Standard deviations are given in parentheses in the place values of the last listed digits.

phosphides are listed in Table 1. They agree well with the literature values [3], although the presently reported values were obtained from the samples prepared by direct reaction of the elemental components, while the earlier reported lattice constants were from samples prepared by the tin flux technique. Small differences may be ascribed to homogeneity ranges. The europium compound cannot be confirmed, in

agreement with other investigations [14]. Instead the powder pattern of EuNi,P, [15] was found, which is similar to the pattern expected for a Zr,Fe,,P,-type compound. The magnetic susceptibilities were determined in a SQUID magnetometer with powdered samples of between 2 and 20 mg depending on the magnitude of the susceptibilities. The samples were placed in thinwalled silica tubes of about 1 mm diameter. Corrections for the magnetic susceptibilities of the containers were applied whenever the susceptibility of the sample was low. The samples were cooled in zero magnetic field to 2 K and heated to room temperature in a magnetic field of generally 0.1 T. To check for ferromagnetic impurities this procedure was repeated at least once with a stronger magnetic field (e.g. 1 T). No such impurities were observed.

rationalized as Pauli paramagnetism. The magnetic susceptibility of N&P was already determined earlier by Gambino et al. [ 171. Their value of 2.3 x low9 m3 mol-’ is somewhat higher, possibly because of a minor amount of a paramagnetic impurity. While metallic conductivity and Pauli paramagnetism is to be expected for the compounds N&P and N&P because of their high nickel content, the metallic behaviour of NIP, deserves some comment. NIP, crystallizes with the cubic skutterudite (CoAs,) structure. The isotypic compound COP, is known to be a diamagnetic semiconductor [18, 191. Thus, the metallic behaviour of NiP, can be ascribed to the additional electron of this compound as was discussed before [18-211. Of the ternary lanthanoid nickel phosphides with Zr,Fe,,P, type structure the lutetium compound is again showing weak, nearly temperature-independent paramagnetism (Fig. 2) and in view of the high metal content the compound can be expected to be a metallic conductor. The magnetic susceptibility of this compound at room temperature was measured with a value of xmol= 7.0 x 10e9 m3 mol-I. This value may be slightly too high because of an unknown amount of a paramagnetic impurity, to which we ascribe the increase of the susceptibility with decreasing temperature. However, the greater part of the paramagnetism of this compound most likely results from

the

conduction very

This netism). Lu,Ni,,P,-where

electrons (Pauli paramagweak paramagnetism of

the Lu3+ ions

do not

carry

250

so0

a

3. RESULTS AND DISCUSSION

The magnetic susceptibilities of the binary nickel phosphides N&P, N&P and NiP, are shown in Fig. 1. The susceptibilities are essentially constant between 100 and 300 K. The upturns at lower temperatures can be ascribed to paramagnetic impurities or to paramagnetic surface states. The absolute values of the susceptibilities at room temperature (x 10e9 in m3 mol-i) are 0.89 f 0.03 for Ni,P, 1.60 f 0.05 for N&P and 2.4 + 0.2 for NiP3. These values are of about the same magnitude as observed for other metallic conductors, e.g. 2.1 for chromium, 1.1 for molybdenum and 6.9 for palladium [16] and may be

50

loo

150

200

T [RI Fig. 1. Magnetic susceptibilities of the Pauli paramagnets Ni,P, NhP, and Nip,. The values for Ni,P and N&P were obtained from relatively large powder samples, while those for NiP, were measured for few very small selected single crystals resulting in large scatter. The increase of the susceptibilities at low temperatures is most likely due to paramagnetic impurities or surface states of the powder samples.

Magnetic bchaviour of some binary and ternary nickel phospbides

1529

W%2pt 50

loo

190

200

2x)

t’~2%2p7 300

so

too

l!So

200

250

so0

50

loo

150

200

250

so0

t 40

6

3

SO

&E? ‘:0

20

Lu2Ni,2P7

l

$10

g

\x

T [Kl -)

I

50

loo

150

200

250

300

T I’0 Fig. 2. Magnetic susceptibilities x or reciprocal magnetic susceptibilities of the compounds Ln,Ni,,P,. The insets show the behaviour at low temperatures. Most curves were obtained with magnetic field strengths of 0.1 T. The susceptibility of the Pauli paramagnet Lu,Ni,,P, was measured at 1.0T. The magnetic moments p,, as obtained from the slopes of the linear portions of some plots above 100 K are given where appropriate.

J

KAI

1530

ZEPPENFELD and

WOLFGANGJEITXHKO

Table 2. Magnetic properties of the phosphides Ln,Ni,,P, Compound Pr,Ni,,P, Nd,NiuP7 Sm,Ni,,P, GdrNiuP, Tb,Ni,rP, Dy,Ni,2Pr Ho, Ni,* P, Er,Ni,*P, Tm,Ni,,P, Yb2NiuP7 Lu,Ni,*P,

Il..p (/+I) 3.1(l) 3.2 (1) 1.55 (5) 7.89 (5) 9.2 (1) 10.64 (5) 10.44 (5) 9.58 (5) 7.54 (5) 3.9 (2) 0

fieR(Ln’+) (pa) 3.58 3.62 1.66 7.94 9.72 10.64 10.61 9.58 7.56 4.54 0

with Zr,Fe,rP,

type structure

Type of magnetism

@ (K) -10(l) -17(l) -5(l) -5(2) *o(l) -1 (1) *o(l) It-O(l) -88 (10) I-

Curie-Weiss, antiferro? Curie-Weiss, antiferro? Van Vleck paramagnetic antiferromagnetic, T, = 15 (1) K metamagnetic, T, = 12 (1) K ferromagnetic, Tc = 9 (1) K Curie-Weiss Curie-Weiss with magneting ordering temperatures, T, or T, <2 K intermediate valence paramagnetism Pauli paramagnetic

The experimentally determined effective magnetic moments per lanthanoid atom peXpare compared with the effective moments calculated for the free Ln3+ ions according to pti = g [J (J + I)]“* us. The value of pdl.for Sm’+ contains the temperature independent contribution calculated according to Van Vleck [22] for 300K with a screenine constant of D = 34. The Weiss constants 0 are also listed. Estimated errors in the place values of the last listed digits are given in parentheses.

moment because of their filled f shell-also shows that the nickel-phosphorus poly-“anion” is essentially nonmagnetic and this can also be assumed to be the case for the other phosphides of this isotypic series. All these, with the exceptions of the samarium and the ytterbium compounds, show Curie-Weiss behaviour. The magnetic moments pexpcalculated from the straight portions above 100 K of the l/x vs temperature plots correspond to the effective magnetic moments precalculated for the free Ln3+ ions from the relation pert= g [J (J + l)]‘/*~~ (Table 2). The paramagnetic Curie temperatures (Weiss constants) 0 are practically zero for the dysprosium, erbium and thulium compounds, while they are negative for the others. For the dysprosium compound ferromagnetic order is observed with a Curie temperature T, = 9( 1) K and the susceptibility decreases around this temperature with increasing field stgrengths as is normal for ferromagnets. No magnetic order was observed down to 2 K for the praseodymium, neodymium, erbium and thulium compounds, although antiferromagnetic order can be assumed to for the occur at still lower temperatures praseodymium and neodymium compounds, because of their negative Weiss constants. Gd2Ni12P, behaves like a normal antiferromagnet with a NCel temperature of TN = 15( 1) K, while the low temperature behaviour of the magnetic susceptibility of Ho~N~,~P, is somewhat unusual. The susceptibility of this compound is independent of the magnetic field strength over the whole temperature range, as is normal for an antiferromagnet. However, no minimum is observed in the l/x vs T plot. Instead the susceptibility is independent of the temperature below about 8 K, possibly because of crystal field effects. The terbium compound is antiferromagnetic with a Ntel temperature of TN = 12( 1) K. The discontinuity in the l/x vs T plot just above this temperature magnetic

increases with decreasing field strengths; it is probably due to a very small amount of an unknown ferromagnetic impurity. Below the N&e1temperature, however, the susceptibility becomes field dependent in a way typical for a metamagnet. This is demonstrated with the magnetization curve recorded at 5 K (Fig. 3). The magnetization increases with increasing field strength, as is typical for an antiferromagnet up to about two T. Above that field strength the magnetization shows a hysteresis as is observed for metamagnets. If that hysteresis were due to the ferromagnetic impurity mentioned above, that hysteresis would need to pertain down to zero field strength. The magnetic moment calculated from the saturation magnetization (actually the complete saturation was not reached; instead we used the value obtained at 5.5 T) is P_~(~,,,) = 6.53 ~(a. This compares well with the theoretical moment of P,~~,,,,= 9.0 pe calculated from the relation p+,,) = g J, considering that the experimental moment was measured for a powdered sample with random orientation. The metamagnetism of Tb,Ni,,P, (where the antiferromagnetic order is destroyed by

Bo-

Tb2Ni12P7

Fig. 3. Magnetization per formula unit M,, vs magnetic field strengths B,,, of the meta magnet Tb,Ni,,P, measured at 5K.

Magnetic behaviour of some binary and ternary nickel phosphides

moderate magnetic fields) fits well considering that the magnetic order of the gadolinium compound is antiferromagnetic (this order could be destroyed only with extremely high external fields) and the dysprosium compound is ferromagnetic (where the magnetic moments are already aligned parallel at zero magnetic fields). The magnetic behaviour of Sm,Ni,,P, and Yb,Ni,,P, is different from that of the compounds just discussed. The magnetism of Sm,Ni,rP, can be ascribed to the Van Vleck paramagnetism of the Sm3+ ions. The theoretical effective moment p,s = 1.66 pe, calculated from Van Vleck’s formula [22] for 300 K assuming a screening constant of cr = 34, is close to the experimentally determined value p,,r = 1.55(5& calculated from the susceptibility at room temperature according to peXp= 2.83 (TX/2)“*pc, The l/x vs T plot of the ytterbium compound is typical for the mixed or intermediate valence of the ytterbium ions found in such compounds, e.g. YbNi,P, [23]. This is seen immediately from the behaviour at low temperature. However, also at temperatures above 100 K the l/x vs T plot is slightly curved (only poorly visible at the scale used in Fig. 2). Nevertheless, we have calculated the magnetic moment from this plot according to the Curie-Weiss law and obtained a value of pexp= 3.9(2)pB per ytterbium atom, which is in between the theoretical values for Yb3+ and Yb*+ of per = 4.54 pB and per = 0.0 pa, respectively. The mixed or intermediate valence of the ytterbium atoms in this compound is remarkable in view of the fact that the cell volume of this compound (V = 0.2588 nm3) is only slightly larger than the volume of V = 0.2586 nm3 interpolated from Tm,Ni,,P, and Lu2Ni,*P7. Chemical bonding in the Zr2Fe,rP, type phosphides with the many transition metal-transition metal interactions is certainly different from that in the phosphides with ThCr,Si, type structure. Nevertheless, the magnetic behaviour of the transition metal atoms in the series Ln,T,,P, and LnT,P, (T = Fe, Co, Ni) is similar. In both series, Ln,Co,,P, [4,6] and LnCo,P, (with the exception of EuCo,P,, where europium is divalent) [24-271, the cobalt atoms carry magnetic moments, while the iron and nickel atoms of the series Ln,Fe,,P, [4, 51, LnFe,P, [S, 241, LnNi,P, [23,24,28] and the presently reported compounds Ln,Ni,,P, are nonmagnetic.

1531

Acknowledgements-We thank Dr G. Hiifer (Heraeus Quarmchmelze) and the Hoechst AG, Werk Knapsack for generous gifts of silica tubes and ultrapure red phosphorus. This work was also supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

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