119Sn Mössbauer spectroscopy studies of hydrides deriving from the stannide CeNiSn

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    82CuKeywords: Cerium; Hydride; Mssbauer spectroscopy; Magnetic transition

    1. Introduction

    The ternary stannide CeNiSn, crystallizing in the or-thorhombic -TiNiSi-type structure [1], is considered as aKondo lattice showing the opening of a pseudogap in theelectronic density of states at low temperatures [2]. Thiscompound, so-called Kondo semiconductor is classified as astrongly correlated f-electrons system. The occurrence of thepseudogap is generally argued to be formed by the cf hy-bridisation between 4f(Ce) and conduction electrons. A sys-tematic study has shown that the gap formation in CeNiSnis very sensitive to the degree of the cf hybridisation. Re-cently, it was shown that this ternary stannide undergoes un-der uniaxial pressure a transition from a pseudogapped stateto an antiferromagnetic ordered state (TN = 4 K) [3]. Alsosubstituting Cu, Pd and Pt for Ni in CeNiSn, the unit cellvolume increases and the nonmagnetic ground state is trans-formed into an antiferromagnetic state [47]. All these re-sults indicate that the physical properties of CeNiSn are verysensitive to the volume effects.

    Recently, the hydrogen sorption properties of CeNiSnwere investigated. Two hydrides (deuterides) were evi-

    * Corresponding author.E-mail address: chevalie@icmcb.u-bordeaux1.fr (B. Chevalier).

    denced: CeNiSn (H, D)1.0(1) and CeNiSnH1.8(1) [810].The first one crystallizes as CeNiSn in the orthorhombic-TiNiSi-type whereas the second belongs to the hexagonalZrBeSi-type. Hydrogen insertion leads to an increase of theunit cell volume, respectively, 2.6% and 8.0% per mole withincreasing H-content. Moreover, magnetic ordering is in-duced by the hydrogenation of CeNiSn. CeNiSnH1.0(1) is an-tiferromagnetic below TN = 4.5(2) K whereas CeNiSnH1.8(1)exhibits a ferromagnetic behaviour with TC = 7.0(2) K[8,10]. Investigation of these hydrides by electrical resis-tivity and specific heat measurements reveals a marked in-fluence of the Kondo characteristics [10]. It is worth tonote, the variation of volume promotes magnetic orderingfor the compounds deriving from CeNiSn. Hydrogenationdecreases the cf hybridisation between the 4f(Ce) electronsand the conduction electrons acts as application of a nega-tive pressure on intermetallics.

    In order to obtain more information on the evolution ofthe structural properties at hydrogenation, we have inves-tigated the CeNiSnHy system by 119Sn Mssbauer spec-troscopy. The local sensitivity of the diamagnetic 119SnMssbauer element allows measuring the hyperfine parame-ters, as influenced by structural and magnetic transitions. Inthis paper, we report on the results obtained on CeNiSnHywith y = 0, 1.0(1) and 1.8(1), Ce(Ni0.82Cu0.18)SnH1.7(1)Solid State Sciences 6

    119Sn Mssbauer spectroderiving from th

    B. Chevalier , A. WattiaInstitut de chimie de la matire condense de Bordeaux (ICMCB), CN

    33608 PessaReceived 18 November 2003; received in revise

    Available onl

    Abstract

    The systems CeNiSnHy (y = 0, 1.0 and 1.8), LaNiSnHy (y = 0 anby 119Sn Mssbauer spectroscopy. This study reveals: (i) a randoquadrupole splitting QS correlated with the increase of the H-contenthe Sn nuclei in the hydrides CeNiSnH1.0, CeNiSnH1.8 and Ce(Ni0. 2004 Elsevier SAS. All rights reserved.1293-2558/$ see front matter 2004 Elsevier SAS. All rights reserved.doi:10.1016/j.solidstatesciences.2004.03.0084) 573577www.elsevier.com/locate/ssscie

    opy studies of hydridestannide CeNiSnL. Fourns, M. PasturelPR 9048], Universit Bordeaux 1, avenue du Docteur A. Schweitzer,ex, Francem 16 February 2004; accepted 10 March 2004

    April 2004

    ) and Ce(Ni0.82Cu0.18)SnHy (y = 0 and 1.7) have been investigatedstribution of the H-atoms in the hydrides; (ii) an increase of thei) at 4.2 K the presence of a transferred magnetic hyperfine field at0.18)SnH1.7 in agreement with their magnetic properties.

  • 574 B. Chevalier et al. / Solid State Sciences 6 (2004) 573577[11] and LaNiSnH2.0(1). These results are compared withthose already published on CeNiSn [12,13].

    2. Experimental

    The samples CeNiSn, Ce(Ni0.82Cu0.18)Sn and LaNiSnwere synthesised by arc melting of a stoichiometric mixtureof pure elements (purity > 99.9%) in a high purity argonatmosphere. Then, the samples were turned and remeltedseveral times to ensure homogeneity. The weight loss duringthe arc-melting process was found smaller than 0.5 wt%. Anannealing treatment was made at 1073 K for one month aftersealing the samples in evacuated quartz tubes.

    Hydrogen absorption experiments were performed usingthe apparatus described previously [14]. An ingot of the an-nealed sample was heated under vacuum for 12 h up to523 K and then exposed to 5 MPa of hydrogen gas pres-sure. The amount of absorbed hydrogen was determined vol-umetrically by monitoring the pressure changes in a cali-brated volume. This method enabled us to prepare as well thehydrides CeNiSnH1.8(1) [8,10], Ce(Ni0.82Cu0.18)SnH1.7(1)[11] and LaNiSnH2.0(1) [15]. On the contrary, the hydrideCeNiSnH1.0(1) was obtained by heating CeNiSnH1.8(1) at523 K under a low hydrogen pressure (5 103 MPa).

    The quality of the ternary stannides and their hydrideswas checked by X-ray powder diffraction using Philips1050-diffractometer (Cu-K radiation). The crystallographicdata deduced from this analysis, agree with those reportedpreviously. CeNiSnH1.8(1), Ce(Ni0.82Cu0.18)SnH1.7(1) andLaNiSnH2.0(1) crystallise in the hexagonal ZrBeSi-type [8,11,15] whereas CeNiSnH1.0(1) belongs to the orthorhombic-TiNiSi-type [810].

    119Sn Mssbauer measurements were performed between4.2 and 300 K using a CaSnO3 source and a conventionalconstant acceleration spectrometer. The isomer shifts (IS)were quoted relative to that of CaSnO3 measured at roomtemperature. As the samples contain 15 mg natural tin percm3, for such a concentration the line broadening due tothe thickness of the sample can be neglected [16]. Thespectra were fitted to the sum of Lorentzians by least squarerefinements. All calculations were carried out without anyconstraint on the amplitudes and the widths.

    3. Results and discussion

    The crystal structures of CeNiSn, CeNiSnH1.0(1) (-TiNiSi-type; space group Pna21) and CeNiSnH1.8(1) (ZrBe-Si-type; space group P63/mmc) present some similarities(Fig. 1). The cerium three-dimensional network can be de-scribed by an intergrowth of trigonal [Ce6] prisms surround-ing alternatively the Ni and Sn atoms. However, the [Ce6]prisms are distorted in CeNiSn and CeNiSnH1.0(1). BesidesNi and Sn atoms form a hexagonal network, which is re-spectively buckled in CeNiSn and CeNiSnH1.0(1) and regu-Fig. 1. Projection of the crystal structure of CeNiSn, CeNiSnH1.0(1) ontothe (100) plane and CeNiSnH1.8(1) onto the (001) plane. White, black andgrey circles represent, respectively, the Ce, Sn and Ni atoms.

    Table 1Crystallographic data relative to CeNiSn and its two hydrides (a, b and care unit cell parameters, Vm the unit cell volume by mol and d(SnCe),d(SnNi) and d(SnH) the interatomic distances around Sn-atom)

    CompoundCeNiSn CeNiSnH1.0(1) CeNiSnH1.8(1)

    Space group Pna21 Pna21 P 63/mmca (nm) 0.7542 0.72720 0.4392b (nm) 0.7617 0.84951 0.4392c (nm) 0.46009 0.44021 0.8543Vm (nm3 by mol) 0.06608 0.06799 0.07136

    0.3183 0.3227 0.3095 0.3208d(SnCe) (nm) 0.3254 0.3285 0.3358 0.3382 60.3315

    0.3327 0.3335 0.3458 0.34850.2615 0.2665 0.2581 0.2769

    d(SnNi) (nm) 0.2686 0.2828 0.2818 0.2882 30.25360.2870 0.3063 20.2678

    d(SnH) (nm) 0.3143 0.3199 60.2995Reference [1] [9] [8]

    lar in CeNiSnH1.8(1). Thus, in the two hydrides, the H atomsare located inside the [Ce3Ni] tetrahedral sites as presentedon Fig. 1. In CeNiSnH1.8(1) nearly all the tetrahedra that areidentical, are occupied. On the contrary, in CeNiSnH1.0(1)two types of chemically similar but geometrically different[Ce3Ni] tetrahedra are formed; only one that has a nearlyregular Ce3 side is occupied by a hydrogen atom [810].The other [Ce3Ni] tetrahedra presents in the structure ofthe CeNiSnH1.0(1) hydride is empty and is significantly de-formed [9].

  • B. Chevalier et al. / Solid State Sciences 6 (2004) 573577 575

    1Fig. 2. 119Sn Mssbauer absorption spectra of CeNiSnHy system measuredat 293 K ((a) y = 0, (b) y = 1.0(1) and (c) y = 1.8(1)). The circles representexperimental values, while the lines are fits to the data.

    The structural transition from the orthorhombic -TiNiSi-type to the hexagonal ZrBeSi-type observed at hydrogena-tion of CeNiSn induces a modification of the crystallo-graphic environment of the Sn atoms (Table 1). The inter-atomic distances d(SnCe) in the trigonal [Ce6] prism arefound different in CeNiSn and CeNiSnH1.0(1) but are allequal in CeNiSnH1.8(1). The increase of the H-content leadsto an increase in the number of H atoms (4 8) neighbour-ing the Sn atoms. These structural trends should influencethe hyperfine parameters of the 119Sn atoms in CeNiSnHy .

    The 119Sn Mssbauer spectra recorded at 293 K areshown in Fig. 2 for CeNiSn and its hydrides. The spectrumrelative to CeNiSn can be fitted as a symmetric quadrupolesplit doublet (Fig. 2a) QS = 0.484(6) mm s1 with a line-width at half-height of = 1.024(9) mm s1. The isomershift was determined equal to IS = 1.900(6) mm s1. Thequadrupole splitting originates from the noncubic localsymmetry of the Sn sites. The value of QS determined hereis similar to that previously reported for CeNiSn taken at1.5 K (QS = 0.44 mm s1) [12,13]. The value of the isomershift IS is typical for a metal type Sn. For instance, in theparent stannides CaPdSn and CaPtSn crystallising also inthe TiNiSi-type, IS is 1.964(5) and 1.857(4) mm s at 77 K,respectively, [17].

    At 293 K, the quadrupole doublet relative to the hy-drides CeNiSnH1.0(1) and CeNiSnH1.8(1) presents an asym-metry (Fig. 2). A similar behaviour was observed recentlyat 26 K on the deuteride NdNiSnD [18]. In this case, theoccurrence of an asymmetric quadrupole doublet was ex-plained on the basis of a partial substitution of Ni and Snatoms in the ternary stannide NdNiSn. This explanation can-not be considered for, in the case of CeNiSnHy , since thequadrupole doublet relative to CeNiSn was found symmet-ric. The asymmetry character could result from a randomdistribution of the H-atoms surrounding the Sn probe. ForCeNiSnH1.8(1), this explanation looks valid since the H-atomoccupies only 90% of 4f-site (1/3 2/3 z = 0.4365) in thehexagonal ZrBeSi-type structure [8]. Also, the present studysuggests some hydrogen distribution between the two typesof [Ce3Ni] tetrahedral present in the structure of the hy-dride CeNiSnH1.0(1). The resulting Mssbauer parametersare given in Table 2. At 293 K, the isomer shift IS increasesin the sequence CeNiSn CeNiSnH1.0(1) CeNiSnH1.8(1)indicating an increase of the s-electron density at the Sn nu-clei. Similarly, the quadrupole splitting QS increases withthe hydrogen content in agreement with the increasing num-ber of H-atoms surrounding the Sn nuclei as illustrated inTable 1. In the other words, insertion of hydrogen in theCeNiSn lattice modifies strongly the 119Sn Mssbauer sig-nals.

    Similar 119Sn Mssbauer parameters are measured re-spectively at 293 and 4.2 K for the two isomorphous ternarystannides CeNiSn and LaNiSn (Table 2). The same remarkcan be made for those of CeNiSnH1.8(1), LaNiSnH2.0(1) andCe(Ni0.82Cu0.18)SnH1.7(1) measured at 293 K even if thesehydrides crystallise in the hexagonal ZrBeSi-type.

    Fig. 3 displays some selected 119Sn Mssbauer spectrarecorded on the CeNiSnH1.8(1) hydride respectively in theparamagnetic state (Fig. 3a and 3b) and below the ferro-magnetic transition TC = 7.0(2) K (Fig. 3c). Above TC,all the spectra could be fairly fitted assuming an asymmet-ric quadrupole doublet (Table 2). As the temperature is de-creased from 293 to 9.9 K, a smooth increase of IS andQS was observed. In the paramagnetic state, the thermal be-haviour of QS is expected to correspond to noncubic inter-metallic systems. It is empirically described by the relationQS(T ) = QS(0 K)[1T 3/2], where QS(0 K) is the electricfield gradient at zero temperature [19] and the parameteris related to the Debye temperature. Using this relation, weobtain a value of = 0.37 105 K3/2, which is typical formetallic systems.

    The comparison of the 119Sn Mssbauer spectra ofLaNiSnH2.0(1) and CeNiSnH1.8(1) recorded at 4.2 K showsclearly in the hydride the presence of a transferred mag-netic hyperfine field (Bhf) at the later Sn nuclei. The originof Bhf is due to the ferromagnetic arrangement of the Ceatoms surrounding the Sn one. At 4.2 K, Bhf = 2.5(2) T forCeNiSnH1.8(1). The 119Sn Mssbauer spectra of the ferro-

  • 576 B. Chevalier et al. / Solid State Sciences 6 (2004) 573577

    nd thFig. 3. Thermal evolution of 119Sn Mssbauer spectrum of CeNiSnH1.8(1)at T = 41 K (a), 10 K (b) and 4.2 K (c). The spectrum (d) is relative toLaNiSnH2.0(1) measured at 4.2 K.

    magnetic hydride (TC = 4.6(2) K) Ce(Ni0.82Cu0.18)SnH1.7(1)exhibits a broadening resulting from the presence of thetransferred magnetic hyperfine field (Fig. 4). Thus, theBhf value of 1.5(2) T is smaller than that determined for

    Fig. 4. Comparison at 4.2 K of the 119Sn Mssbauer spectra of Ce(Ni0.82-Cu0.18)SnH1.7(1) (a) and CeNiSnH1.8(1) (b).

    CeNiSnH1.8(1). Such a low value can be explained by therelatively low value of TC of the Ce(Ni0.82Cu0.18)SnH1.7(1)hydride.

    Fig. 5 shows the 119Sn Mssbauer spectra of the antifer-romagnet CeNiSnH1.0(1) hydride above (a) and close to (b)the Nel temperature TN = 4.5(2) K. Here also, the spectracan be described by an asymmetric quadrupole splitting (Ta-ble 2). However, at 4.2 K we find an anomalous increase ofthe full width i at the half maximum of the lines relative tothe determined value for CeNiSn (Fig. 5c). Between 10and 4.2 K the average i value for CeNiSnH1.0(1) increasesup to 11.4%. An explanation of such an anomaly in termsof a transferred magnetic field that would appear if the an-Table 2Results from fits to the 119Sn Mssbauer data in various ternary stannides aelectric quadrupole splitting and the linewidth

    Compound T (K) IS (mm s1)CeNiSn 293 1.900(7)

    4.2 1.925(4)CeNiSnH1.0(1) 293 1.917(6)

    40.5 2.000(5)10 1.998(5)4.2 2.006(7)

    CeNiSnH1.8(1) 293 1.983(4)41 2.047(3)10 2.046(2)

    LaNiSn 293 1.888(7)4.2 1.940(6)

    LaNiSnH2.0(1) 293 1.999(4)4.2 2.070(4)

    Ce(Ni0.82Cu0.18)SnH1.7(1) 293 1.972(4)10 2.041(6)eir hydrides deriving from CeNiSn. IS is the chemical isomer shift, QS the

    QS (mm s1) 1 (mm s1) 2 (mm s1)0.484(7) 1.024(9)0.541(4) 1.230(5)0.968(6) 1.073(8) 1.125(9)1.003(5) 1.201(7) 1.249(7)1.009(5) 1.190(7) 1.250(7)1.005(7) 1.324(9) 1.394(9)

    1.464(4) 1.081(7) 1.086(7)1.487(3) 1.116(6) 1.201(6)1.494(2) 1.129(4) 1.209(4)

    0.449(7) 0.987(8)0.474(6) 1.216(7)

    1.410(4) 1.067(7) 1.091(7)1.436(4) 1.153(6) 1.249(6)1.404(4) 1.142(7) 1.171(8)1.430(6) 1.234(11) 1.310(11)

  • B. Chevalier et al. / Solid State Sciences 6 (2004) 573577 577Fig. 5. Thermal evolution of 119Sn Mssbauer spectrum of CeNiSnH1.0(1)at T = 10 K (a) and 4.2 K (b). The spectrum (c) is relative to CeNiSnmeasured at 4.2 K.

    tiferromagnetic arrangement of Ce moments, leading to cre-ate...

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