Structural properties of rare-earth molybdenum chalcogenides REMo6X8

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Structural properties of rare-earth molybdenum chalcogenides REMohXs 0. PERA*, C. GEANTET, I-I. SCHMITT, F. LE BERRE and C. HAMARD Laboratoire de Chimie du Solide et Inorganique MolCculaire (LCSIM), UMR 65 11 CNRS-UniversitC de Rennes-I, 35042 Rennes Cedex, France

(received June 12, 1999; accepted June 19, 1999.)

Dedicated to Dr Marcel SERGENT on the occasion of his retirement

ABSTRACT.- Singlecrystalsof rare-earth-based ChevrelphasesREMo6X,(X = S, Se) have beengrown. In this work, we summarizeall our structuraldata obtained up to now for almost all lanthauidesin the caseof the sulfidesseriesREMo,S,, and for light rare-earthsin the caseof the selenides seriesREMo,Se,.A full comparison is madebetweenthesetwo examples,discussingthe effects of the size and of the oxydation stateof the RE ion.

INTRODUCTION The ternary molybenumchalcogenides MMo& (X = chalcogen),discoveredin Rennesin 1971 by Chevrel, Sergent and Prigent [l], were soon found to be remarkablematerialsfor basicresearchandappliedphysics.They presenteda fairly high superconductingtemperature(up to 15 K) and an extremely large critical field (about 50 Tesla at liquid helium) [2,3]. Other crystallochemicaland physical propertiesof thesecompounds,known commonly asChevrel phases,can be found in the thoroughreview publishedby Chevrel andSergentin 1982[4]. Becauseof their particular crystal structure, the three-dimensionalsublattice basedon pseudo-molecular clustersMo,X, interactsvery weakly with the cation network, even if this later is composedof highly-magneticions (M = rare-earths RE). As a result, it was possiblefor both antagoniststatesto coexist in the same Solid State Sciences, 1293-2558/99/7-8/O

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material. Many works were performed in the early ’80s on these so-called magnetic superconductors, alI of them performed on polycrystalline samples (see, for instance, compilations in ref. 5-7 and the review article of Pefia and Sergent published in 1989 [S]). Obviously, the best way to study the true coexistence of competing phenomena is by performing careful, and if possible simultaneous, experiments in a single crystal specimen. In addition, well crystallized samples would allow to easily identify any anisotropic behaviour iu these materials. Materials elaboration becomes then, of primary importance in order to separate out extrinsec effects due to secondary phases, from intrinsic parameters. In this context, a program on crystal growth of ternary rare-earth molybdenum chalcogenides was started at the University of Rennes under the impulse of Marcel Sergent, and a large number of works were devoted to this problem [g-12]. Part of the results were published iu the late ’80s mainly concerning the sulfides series [8,13,14], while the selenides series is still under investigation [15,16]. The present report compares single crystal results obtained in the sulfide and selenide series of the rare-earth-based Chevrel phases, giving a special importance to crystallochemical aspects (e.g., electronic and steric effects on the crystal structure).

EXPERIMENTAL

Single crystals were obtained by high-temperaturemelting. Due to the noncongruent behaviour of most of these compounds, the starting charges, of approximatecompositionsREJ,Mo,,S,, and RE,,Mo,,Se,,, were mainly composed of mixtures of an already smteredstoichiometric phase (RE,Mo,X,) and the corresponding binary rare-earth chalcogenide. A few exceptions should be mentioned: in the caseof divalent cations(Yb” and Eu’+), congruentmelting was observed and the use of a stoichiometric charge was favorable to obtain monophasedcrystals. Same approachwas adoptedfor the selenidecompound PrMo,Se, [ 171.More detailson growth mechanisms can be found in references13 and 15. It is to be noted that, for intrinsic reasonsdue to their specific microstructure, no single crystals of heavy rare-earthsspecimens(i.e., Gd + Yb) have ever beenobtainedin the selenidesseriesREMo,Se, [ 18,191. Use of an aluminacrucible was absolutelynecessaryin the caseof sulfides in order to avoid a direct contactof thecrucible with the melt. In the caseof selenides, reactionof seleniumvaporswith the crucible’swalls becameimportantonly above 1600 T, leading to the formation of a Mo,Se, coating which slowed down a further reactionwith the melt. Thus,no intermediatecruciblewasneededduring the high-temperaturemelting of selenium-based charges. The final productobtainedafter melting consistsmainly of a mixture of crystals of the ternary phase, imbeddedin a ram-earthchalcogenidecrust. This latter is dissolvedin a solutionof ethyl alcoholand hydrocloric acid. The REMo,X, single crystals are collected and some of them used, after crushing, to perform X-ray analysis. Additional analysisby EDXBEM were used to check their expected stoichiometry.Further testswere usuallyperformedin order to verify their quality ; in this way, magneticsusceptibilitymeasurements give the exact overall rare-earth content, while a single-stepsuperconductingtransitionis usually a good proof of single-phasespecimens. TOME

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Data for structural refinement were collected on an automatic X-Ray EnrafNonius CAD-4 four-circle diffractometer. The structural-determination-package (SDP) adapted to a PDP minicomputer, or the MolEN program for Microvax, were used for structural calculations. The crystal structures were solved in the space group R-3 (n” 148) assuming that they were isotypical to Chevrel phases containing large cations, such as PbMo,S, [4] : the 6 molybdenum MO and 6 Se, atoms were placed in a 6fposition, 2 Se, atoms were placed in a 2c position, and the rare-earth RE was fixed in the la site. RESULTS Structural

AND

DISCUSSION

description

The Chevrel phase crystal structure [l] is composed of Mo6Xs blocks, in which an octahedral cluster of 6 molybdenum atoms is situated insrde an almost regular cubic cage of 8 chalcogen atoms. The Mo,X, unit is rotated by about 25 ’ with respect to the three-fold axis inside a rhombohedral cationic lattice. Two particular distances define the size of the MO, cluster : d(Mo-Mo), is associated to metal-metal bonds within a same plane perpendicular to the three-fold axis, while d(Mo,-Mo,) corresponds to metal-metal bonds between MO atoms situated in different planes inside each cluster (fig. 1). Separation between Mo,X, units can be described through several intercluster distances ; however, the most important ones are connected to the strongest (MO-MO) and (MO-X) bandings between clusters.

Fig. 1 - crystal structure SOLID

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The rare-earth atom located at the origin of the rhombohedral unit cell is surrounded by eight chalcogen atoms belonging to eight different units : it shares two short bandings {REX,) with the two axial chalcogen atoms X,, and six long bondings (RE-X,) with the X, atoms occupying the 6f positions (fig. 1). All these parameters are represented in figures 2 through 5 as a function of the rare-earth ionic radius {20]. A detailed list of these and other interatomic distances can be obtained from the authors on request. Electronic

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The physical properties of these ternary compounds greatly depend on the third component, the M cation, which has a stabilizing effect on the crystal structure Its influence on the physical properties is due essentially to a transfer of valence electrons from the ionic M component toward the electron-deficient metallic Mo,X, cluster. Also important in these materials are covalent bandings which influence the charge transfer mechanism. Thus, if a ionic assumption allows to consider sulfur as strictly divalent, this is not the case of selenium (Se*“.75) or tellurium (Te-1.33), according to ref. 21. Thus the conduction band fills mom rapidly for selenium and tellurium than for sulfur compounds and the “degree of insertion” of metallic cations will differ from one series to another. As a consequence, the influence of both size and oxydation state of each one of the constituents should be considered when comparing the different compounds. Atomic size will be important through chemical pressure effects inside the unit cell, while the electronic charge will change the total number of electrons in the metallic unit MO, and, by this way, modify its size. The conjunction of these two parameters play an enormous role on the physical properties of a special group of compounds M”Mo,X, (M = Ca, Sr, Ba, Eu), characterized by a structural transition of electronic origin [22], but this problem stays beyond the scope of this work. Obviously, another way to modify the total number of electrons is by changing the cation concentration. This was done on the L%Mo,Se, system, for which a thorough structural study was performed on three single crystals exhibiting critical temperatures from 9.40 K up to Il.00 K [ 151. All parameters were found to vary quite linearly with the lanthanum concen~~on, as expected for an increasing ratio of large-sized atoms occupy~g the origin site of the structure. Such a linear dependence will allow us to safely compare single crystals of the selenides series RE,Mo,Se, for which the rare-earth content x(RE) could differ from sample to sample, as discussed below. Influence

of the ionic radius

Experimental growth conditions described above, yielded single crystals of the sulfides series in which the elements are in an exact stoichiometric ratio 1:6:8. However, all trials performed in the case of selenides, including those which followed the same procedure as for sulfides, resulted in sub-stoichiometric crystals, with a rare-earth content usually comprised between 0.82 and 0.94. Intrinsic causes to this fact must be related again to the formal covalent charge of Se, as compared to S (SeV’.7sand Sa ; see above). For this reason, all comparisons described below ‘TOME

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concern single crystals of approximate composition RE,,,Mo,Se,, while those for sulfides are better described by the formula RE,$Jo,S,. An exception is the data plotted for “CeO,,,Mo,Se,” : since we found no crystals with such stoichiometry, we present a hypothetical data which is the average of experimental results obtained on two single crystals of formulae Ce,,,Mo,Se, and Ce,,,,Mo,Se,, respectively. Assuming the linear dependence of parameters, as discussed above, a good comparison can be made with respect to the other crystals of the selenides series. Figure 2 shows the rhombohedral lattice parameters for single crystals of REMo,S, and REMo,Se,. It is to be noted that the opposite evolution of a, and a,,, implies an elongation of the unit cell with the increasing size of the inserted ion. The increase of the overall volume is mainly connected to the accompanying increase of the (MO-MO) intercluster distance and, in a less extent, to the &lo-X,) one (fig. 3). Figure 4 shows the two (MO-MO)‘“‘” distances which define the MO, cluster, one of which stays quite constant (d(Mo-Mo)J since it concerns the same MO, triangle perpendicular to the three-fold axis [4], the other one decreasing with the ionic radius. This last effect must be related to the external pressure exerted by the rareearth atoms, and it is only observed for selenides. As a result, the Mo,Se, cluster contracts and becomes more regular when larger cations are introduced at the origin site. This variation seems negligible in the case of sulfides, perhaps due to the smaller S, pseudo-cubic cage compared to Se,. An important parameter of the structure is the size of the origin site. Two main distances, (RE-X,) and (RE-X,), define the available volume in which the lanthanide atom is inserted. Figure 5 shows its linear variation, perfectly correlated

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Fig. 2 - Rhombohedral lattice parameters. Sulfides, RI$,Mo,S, ; REl = La, Ce, Sm, Gd, Dy, Ho, Er, Tm, Lu. Selenides, RE-o,,Mo,Sea ; RE = La, Ce, Pr, Nd, Sm. Fig. 3 - Intercluster distances (Mo-Mo)“~~ and (Mo-X,)‘~” SOLID

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Fig. 4 - MO-MO distances which define the MO, cluster : (MO-MO), and (MO,-MO,) Fig. 5 - Distances which define the origin site : @E-X,) and @E-X,)

with the lanthanidcs contraction. It is to be noted that this increase is accompanied with a slight modification of the site’s geometry, since (RE-X,) changes mom rapidly than the &E-X,) distance, implying a slight elongation of the site along the ternary axis. At this point, and for completeness, we should recall our results on the divalent cases (Yb ‘MO&, Eu”+Mo,S, and Et?‘Mo,Se,). These examples were not fully treated here, since the overall evolution of thetr interatomic distances does not always follow the behaviours shown in figures 2-5. Full reports concerning these phases can be found in ref. 23, 24 and 25, respectively. As mentioned before, the departure from linearity is due to charge transfer mechanisms since we are now dealing with a divalent cation instead of trivalent ones. Their most important influence on the crystal structure is the increase of the size of the MO, cluster, mainly through the d(Mo,-Mo,) distance (see fig. 1). Also, the origin site contracts slightly since d(RE-X,) decreases while d(RE-Xi) increases, resulting in a larger a, value (- 0.5 ’ above a RE%Mo,X, compound involving an equivalent rare-earth ionic radius). We should also mention the interesting result obtained in some crystals of Eu’+Mo,Se,, for which a slight displacement of the Eu ion (of the order of 0.86 8, along the three-fold axis) occurred when small amounts of vacancies were present in the Se sublattice. Such shift concerned only a limited quantity of ions (- 8 96). exactly equals to the amount of point defects on the axial Se, site (selenium vacancies or presence of oxygen atoms). Such phenomenon was found to be correlated with different features in the electrical transport properties [25].

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CONCLUSION This full report on the structural data obtained in single crystals of rare-eaxthbased Chevrel phases shows the importance of both steric and electronic effects, the former due to the internal chemical pressure exerted by the inserted metallic ion, the latter due to the charge transfer toward the MO, cluster. These data allow to unambiguously characterize the structure of these materials since, for the first time, crystallographic results were deduced from single-crystal data analysis. The work outlined here is being continued on the telluride series, beiig conscious of the difficulty to insert a trivalent ram-earth ion in the Mo,Te, matrix [4]. However, our present approach consists in a progressive substitution of selenium by tellurium, inserting decreasing quantities of RE atoms [26]. Hem again, electronic arguments will be essential to understand the restricted domain of solubihty of the cation in a telhnium-based matrix.

ACKNOWLEDGMENTS Authors sincerely acknowledge the collaboration of Profs. R. Horyn and A. Wojakowski from the Institute of Low Temperamm and Structure Research (Wroclaw, Poland) during all stages of this work (1982- 1999).

REFERENCES

PI t:; t-i; [Sl 161 t71

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author (e-mail : [email protected]

; far: +33-2-99635704)

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