RNi9In2 (R= Pr, Nd, Eu) compounds and their hydrides

I. Bigun1,2, a, M. Dzevenko1,2, L. Havela2, Ya. Kalychak1

1Department of Analytical Chemistry,Ivan Franko National University of Lviv, Kyryla and Mefodia Str. 6, 79005 Lviv, Ukraine

2Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic

ae-mail: biguninna@gmail.com

Keywords: indides; hydrogen absorption; crystal structure; magnetic properties

Abstract. RNi9In2 and their hydrides crystallize in the YNi9In2 structure type. Low pressure

hydrogenation leads to absorption of 1.5-3.4 H/f.u. RNi9In2 (R= Pr, Nd) as well as PrNi9In2H1.7 and

NdNi9In2H1.5 exhibit Curie-Weiss behaviour and lack of magnetic ordering down to T = 2 K.

EuNi9In2 reveals ferromagnetism below approximately 40 K, which may be due to a secondary

phase. Its hydride is Pauli paramagnet, indicating the Eu3+

state.

Introduction

The investigation of hydrogen absorption in numerous intermetallic compounds raised considerable

interest for both fundamental and practical reasons, especially related to compounds of the LaNi5

type [1]. Therefore hydrogen absorption properties of RNi5 (CaCu5 type structure) [2] binary

compounds are well known. In the R-Ni-In ternary systems, the series of compounds with

compositions RNi4In, RNi5In and RNi9In2 exist in the region close to RNi5 [3]. The interaction of

hydrogen with RNi4In (MgCu4Sn type) [4] and RNi5In (CeNi5Sn type) [5] compounds was studied

for all the rare earths. The existence of hydrides RNi5InHx (R = La, Ce, Nd) was reported in [6].

RNi4In do not interact with hydrogen. Here, we describe the synthesis, structure and basic magnetic

properties of RNi9In2 (R = Nd, Pr, Eu; YNi9In2 type) compounds and their hydrides.

Experimental

Polycrystalline samples of 1:9:2 stoichiometry were prepared by arc-melting of pure components

(the purity of ingredients is better than 99.9 wt.%) under high-purity argon atmosphere. The ingots

were remelted twice to ensure homogeneity. The weight were less than 1%. For the hydrogenation,

the alloys were subsequently crushed in sub-millimeter pieces and heated to T = 473 K in vacuum

(better than 1*10-6

mbar) to remove adsorbed impurities. After introducing of hydrogen gas,

samples started to absorb hydrogen readily at room temperature and hydrogen pressure 700 mbar.

The hydrogen absorption was monitored by the pressure change in the system. The stoichiometry

was determined by the decomposition in closed volume. The desorption curves of the hydrides

point to a single-stage process at T ≈ 770 K. The X-ray diffraction data for structure refinement

were collected on Bruker D8 advance diffractometer (CuKα-radiation, graphite monochromator,

20.00° - 100.00° 2Θ range, step size in 2Θ = 0.03°, scan time 23 s/step). The program Fullprof [7]

was used for the determination and refinement of the crystal structure. Magnetic measurements

were performed in the temperature range 2 - 300 K and magnetic field up to 6 T by means of

extraction magnetometer (Quantum Design PPMS) in the form of powder sample with randomly

oriented grains fixed by a glue.

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Results and discussion

RNi9In2 compounds belong to the family of

transition-metal rich compounds, which crystallize

with the YNi9In2 structure type (SG P4/mbm) [8].

The crystal structure of YNi9In2 was solved and

lattice parameters for other isostructural compounds

have been reported [8]. Therefore, we present the

results of a complete crystal structure investigation

for PrNi9In2, NdNi9In2, and EuNi9In2 by means of

X-ray powder diffraction. Crystal structure data and

the details of structure refinement are listed in

Tables 1 and 2. Fig. 1 shows the X-ray diffraction

patterns of PrNi9In2 compound.

Fig. 1. Observed (٠) calculated (-) and

difference X-ray diffraction patterns of

PrNi9In2.

The YNi9In2 structure type is an ordered superstructure to the CeMn6Ni5 type [9] with 3 sites

occupied exclusively by Ni and 1 site by In atoms. All atoms in the structure have high coordination

numbers: 22 for the rare-earth component, 15 for indium, and 12 for nickel. The structure represents

complex three-dimensional (Ni9In2)n-net with short interatomic In-In distances δ = 0.27 nm

(Fig.2a). On the other hand, it can be considered as the three-dimensional networks of NiNi8In4

centred icosahedra (Fig.2b).

a b

Fig.2. Three-dimensional Ni9In2-nets (a) and NiNi8In4-icosahedra networks (b) in PrNi9In2.

Table 1. Crystallographic data of RNi9In2 (structure type YNi9In2, space group P4/mbm, Z = 2;

R = Pr, Nd, Eu) compounds and their hydrides

Compound a [nm] ∆a/a c [nm] ∆c/c ∆V/V0

[%]

RBragg

[%] Rf

[%]

Rp

[%] Rwp

[%]

PrNi9In2 0.8274(2) - 0.4860(1) - - 4.07 3.31 5.75 7.48

PrNi9In2H1.7 0.8293(2) 0.23 0.4882(1) 0.45 0.96 3.50 2.46 4.29 5.42

NdNi9In2 0.8267(2) - 0.4855(1) - - 4.74 3.78 4.99 6.51

NdNi9In2H1.5 0.8288(2) 0.25 0.4878(1) 0.47 0.96 5.46 3.26 4.96 6.42

EuNi9In2 0.8254(1) - 0.48535(7) - - 6.68 2.97 3.44 4.60

EuNi9In2H3.4 0.8288(2) 0.42 0.48764(9) 0.47 1.32 5.74 3.15 4.00 5.16

The hydrogenation process leads to a hydrogen absorption corresponding to approx. 1.5 H atoms

per formula unit for PrNi9In2, NdNi9In2, and 3.4 H atoms per f.u. for EuNi9In2. Hydrogen

absorption results in nearly isotropic small expansion of the unit cells with the volume increase by

about 0.96 % for PrNi9In2H1.7 and NdNi9In2H1.5 and 1.32 % for EuNi9In2H3.4 (Table 1). The atomic

coordinates of the initial compounds and their hydrides are presented in the Table 2.

46 Solid Compounds of Transition Elements II

Table 2. Atomic coordinates and displacement parameters of PrNi9In2 and PrNi9In2H1.7 phases

Atom PrNi9In2 PrNi9In2H1.7

coordinates Biso ×102 [nm

2] coordinates Biso ×10

2 [nm

2]

Pr 2(a) 0 0 0 0.9(6) 0.5(4)

Ni1 8(k) x 1/2+x z x = 0.178(2) 0.7(5) x = 0.177(2) 0.5(4)

z = 0.248(3) z = 0.249(3)

Ni2 8(j) x y 1/2 x = 0.064(2) 0.7(5) x = 0.066(2) 0.5(4)

y = 0.211(2) y = 0.209(2)

Ni3 2(c) 0 1/2

1/2 0.7(5) 0.5(4)

In 4(g) x 1/2+x 0 x = 0.623(1) 1.3(6) x = 0.6224(8) 0.5(4)

NdNi9In2 NdNi9In2H1.5

Nd 2(a) 0 0 0 0.9(4) 0.3(4)

Ni1 8(k) x 1/2+x z x = 0.179(2) 0.9(4) x = 0.180(2) 0.8(4)

z = 0.247(3) z = 0.251(3)

Ni2 8(j) x y 1/2 x = 0.063(2) 0.9(4) x = 0.066(2) 0.8(4)

y = 0.212(2) y = 0.208(2)

Ni3 2(c) 0 1/2

1/2 0.9(4) 0.8(4)

In 4(g) x 1/2+x 0 x = 0.6212(9) 1.0(4) x = 0.6219(8) 1.0(4)

EuNi9In2 EuNi9In2H3.4

Eu 2(a) 0 0 0 0.4(2) 1.2(2)

Ni1 8(k) x 1/2+x z x = 0.1781(5) 1.6(1) x = 0.1762(6) 0.8(2)

z = 0.251(1) z = 0.253(1)

Ni2 8(j) x y 1/2 x = 0.0613 (6) 1.6(1) x = 0.0638(8) 0.8(2)

y = 0.2058(7) y = 0.2092(8)

Ni3 2(c) 0 1/2

1/2 1.6(1) 0.8(2)

In 4(g) x 1/2+x 0 x = 0.6193(2) 0.6(1) x = 0.6198(3) 0.1(2)

The crystal structure of RNi9In2 was analysed for

possible interstitial sites for the accommodation of

hydrogen atoms. Three types of interstitials can be found

in the structure: R2Ni4 octahedra, In2Ni3 trigonal

bipyramids and Ni4 tetrahedra, surrounding the Wyckoff

position 2b, 4h and 8j, correspondingly. The analysis of

the interatomic distances shows that the interstitials with

Wyckoff position 4h do not satisfy geometrical and size

criteria (δH-Ni3 = 0.1410 nm, δH-Ni2 = 0.1483 nm for

PrNi9In2H1.7). Therefore, the favourable positions for

hydrogen atoms are in Wyckoff position 2b and 8j, and

they allow accommodating up to 5 hydrogen atoms per

formula unit (Fig. 6).

Fig.3. Three-dimensional

arrangements of interstitials in the

PrNi9In2H1.7 structure.

Magnetic susceptibility does not reveal any sign of ordering above T = 2 K except for Eu. They

exhibit the Curie-Weiss paramagnetic behaviour (Fig.4.) with weakly negative θp values (θp = -10 K

for PrNi9In2 and its hydride and θp = -3 K for NdNi9In2; θp = -4 K for NdNi9In2H1.5). The effective

moments of PrNi9In2 (µeff = 4.03 µB/Pr) as well as PrNi9In2H1.7 (µeff = 3.73 µB/Pr) are slightly

different from the expected f2 value 3.58 µB. In the case of NdNi9In2 hydrogenation does not

influence the effective moment, which remains µeff = 3.78 µB/Nd, close to the f3 theoretical value of

3.62 µB. The magnetic behaviour of EuNi9In2 also follows the Curie-Weiss law but with positive

paramagnetic Curie temperature, θp = 22 K suggesting a ferromagnetic coupling. The value of

effective magnetic moment (µeff = 4.24 µB/Eu) is significantly lower than expected one for Eu2+

(7.94 µB). This fact may be indicative of Eu intermediate valency in this compound. As the sample

contained 10-20% of an impurity, we cannot be conclusive about the ferromagnetic feature,

appearing at about 40 K. Its low moment (approx. 1.2 µB /f.u.) suggests that it may be due to a

secondary phase. Hydrogenation of EuNi9In2 results in the Pauli paramagnetism. It can be

Solid State Phenomena Vol. 194 47

explained by the influence of hydrogen, due to which the valence state of Eu is changed to the

nonmagnetic Eu3+

(Fig.5.). Eu atoms in the f6 state have smaller volume, thus more space in the

structure is available for hydrogen absorption and this can be probably the reason of twice higher

hydrogen content in EuNi9In2H3.4 than in Pr and Nd analogs.

Fig.4. Temperature dependence of magnetic susceptibility of PrNi9In2, PrNi9In2H1.7 (a), NdNi9In2

and NdNi9In2H1.5 (b) in magnetic field 6 T. Inset: Magnetization curves of PrNi9In2, PrNi9In2H1.7

(a), NdNi9In2 and NdNi9In2H1.5 (b) measured at temperature T = 2 K.

Fig.5. Temperature dependence of magnetic susceptibility of EuNi9In2 (a) and EuNi9In2H3.4 (b) in

magnetic fields 3 and 6 T. Inset: Magnetization curves of EuNi9In2 (a) and EuNi9In2H3.4 (b)

measured at temperature T = 2 K. The data reveal a ferromagnetic impurity with high TC

(presumably elemental Ni) in the hydride.

Summary

RNi9In2 (R=Pr, Nd, Eu) compounds have been synthesized and their crystal structure have been

determined by X-ray powder diffraction. RNi9In2 structure forms Ni9In2 nets with R atoms located

between them. RNi9In2 compounds absorb about 1.5-3.4 H/f.u., which is considerably lower than

the theoretical value (5 H/f.u.). Hydrogenation does not change crystal structure of compounds but

slightly expands their cells. RNi9In2 (R=Pr, Nd, Eu) and their hydrides (R=Pr, Nd) exhibit Curie-

Weiss behavior in the temperature range 2-300 K. Magnetic ordering is expected at the temperature

below 2 K. Hydrogenation of EuNi9In2 changes its magnetic properties to Pauli paramagnetic.

Acknowledgments

This work was supported by the Grant Agency of the Czech Republic under the grant No.

P204/12/0285. I. Bigun is indebted to International Visegrad Fund for support.

48 Solid Compounds of Transition Elements II

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