Journal of Alloys and Compounds 589 (2014) 48–55

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Journal of Alloys and Compounds

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Pd5InSe and Pd8In2Se – New metal-rich homological selenides with 2Dpalladium–indium fragments: Synthesis, structure and bonding

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.11.172

⇑ Corresponding author. Fax: +7 (495) 9390998.E-mail address: alexei@inorg.chem.msu.ru (A.N. Kuznetsov).

Elena Yu. Zakharova a, Sergey M. Kazakov a, Anna A. Isaeva b, Artem M. Abakumov c,Gustaaf Van Tendeloo c, Alexey N. Kuznetsov a,⇑a Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, GSP-1, 119991 Moscow, Russian Federationb Department of Chemistry and Food Chemistry, TU Dresden, Helmholtzstraße 10, 01062 Dresden, Germanyc EMAT, Physics Department, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium

a r t i c l e i n f o

Article history:Received 16 October 2013Received in revised form 21 November 2013Accepted 24 November 2013Available online 1 December 2013

Keywords:IntermetallicsTransition metal alloys and compoundsCrystal structureElectronic band structureTransmission electron microscopy, TEMX-ray diffraction

a b s t r a c t

Two new metal-rich palladium–indium selenides, Pd5InSe and Pd8In2Se, were synthesized using ahigh-temperature ampoule technique. Their crystal structures were determined from Rietveld analysisof powder diffraction data, supported by energy-dispersive X-ray spectroscopy and selected areaelectron diffraction. Both compounds crystallize in tetragonal system with P4/mmm space group(Pd5InSe: a = 4.0290(3) Å, c = 6.9858(5) Å, Z = 1; Pd8In2Se: a = 4.0045(4) Å, c = 10.952(1) Å, Z = 1). The firstcompound belongs to the Pd5TlAs structure type, while the second one – to a new structure type. Mainstructural units in both selenides are indium-centered [Pd12In] cuboctahedra of the tetragonally distortedCu3Au type, single- and double-stacked along the c axis in Pd5InSe and Pd8In2Se, respectively, alternatingwith [Pd8Se] rectangular prisms. DFT electronic structure calculations predict both compounds to be 3Dmetallic conductors and Pauli-like paramagnets. According to the bonding analysis based on the electronlocalization function topology, both compounds feature multi-centered palladium–indium interactions intheir heterometallic fragments.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Mixed palladium and main-group metal compounds can featurealternating low-dimensional (2D) fragments with different types ofbonding interactions, e.g. a combination of heterometallic frag-ments with multi-centered delocalized bonds and fragments withbonds of covalent or ionic nature. To this day, only a few com-pounds of this type were reliably characterized, namely Pd5AlI2

[1], Pd7�dSnTe2 [2], Pd5TlAs [3], and Pd5HgSe [4]. The key structuralunits in all these compounds are [Pd12M] cuboctahedra, joined viacommon faces into the 2D layers in the ab plane. This cuboctahe-dra-based motif is very similar to the structure of Cu3Au, the struc-ture type that is rather common for d-block – p-block metal binaryintermetallics [5]. For the only halide in this series, Pd5AlI2, noisotypic compounds were reported. Pd7�dSnTe2 [2] belongs to theNi7�xMQ2 structure type and has a number of direct analoguesamong nickel – main-group metal chalcogenides (M = Sn, Sb, Ge,Si, Q = S, Se, Te) [6], however, it is the only non-nickel based com-pound belonging to this structure type that was found to this day.Pd5TlAs and Pd5HgSe belong to the same structure type, namedafter the former compound [3]. As reported in [3], a supposedly

large number of other ternary phases also crystallize in the Pd5TlAsstructure type, mainly palladium and platinum mixed pnictidesT5MPn (T = Pd, Pt, M = Al, Ga, In, Tl, Sn, Mg, Ag, Zn, Cd, Hg, Pn = P,As) [3,7], and also a couple of selenides Pd5MSe (M = Zn, Cd)[3,8]. Surprisingly, apart from recently characterized Pd5HgSe [4],no structure studies on any of them were reported ever since theinitial paper [3]. Yet the number of proposed compounds suggeststhat palladium, and even platinum, are as capable of forming thestructures based on the Cu3Au-type heterometallic fragments astheir lighter group 10 neighbor, nickel. Also interesting is the factthat in all reported palladium-based compounds the thickness ofa heterometallic block is one cuboctahedron only, while for thenickel-based compounds several cases of double-stacked cubocta-hedra have been reported [9].

Our systematic search for new ternary compounds based onheterometallic fragments of Cu3Au type in the Pd–In–Se system,that, considering the lack of published data, appears to have beencompletely overlooked in previous studies, has very recently re-vealed one new mixed selenide, Pd17In4Se4 [10], that features[Pd12In] cuboctahedral fragments and yet has an open-frameworkstructure. Here we report two new palladium–indium homologicalselenides, Pd5InSe and Pd8In2Se, that both have a 2D arrangementof cuboctahedral heterometallic fragments.

Fig. 1. Main SAED patterns for the Pd5InSe (top) and Pd8In2Se (bottom)compositions.

Table 2Refined coordinates and isotropic displacement parametersa for Pd5InSe and Pd8In2Se.

Atom Site x/a y/b z/c Biso (Å2)

Pd5InSePd(1) 1a 0 0 0 1.1(2)Pd(2) 4i 0 1/2 0.2807(2) 1.1(2)In 1c 0.5 0.5 0 0.8(2)Se 1b 0 0 0.5 1.2(2)

Pd8In2SePd(1) 2f 0 1/2 0 0.4(1)Pd(2) 4i 0 1/2 0.3580(7) 0.9(3)Pd(3) 2g 0 0 0.1810(2) 0.7(2)In 2h 1/2 1/2 0.1797(1) 0.2(1)Se 1b 0 0 1/2 1.0(2)

a The values for Pd5InSe and Pd8In2Se are taken from refinement 1 and refine-ment 2, respectively.

E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55 49

2. Experimental

2.1. Synthetic and analytical procedures

Palladium (powder, 99.8%), indium (shot, 99.999%), and elemental selenium(powder, 99.98%) were used for sample preparation. Mixtures of the elements(ca. 0.3–0.5 g in total per sample) were put into silica ampoules, sealed under vac-uum (ca. 20 mTorr), and annealed at various temperatures within the temperaturerange 550–750 �C for 10–24 days, and then allowed to cool down to room temper-ature (except for samples of Pd5InSe and Pd8In2Se compositions, where quenchingwas also tried after each stage of annealing). Mechanical homogenization bythorough grinding of the samples in an agate mortar, with subsequent pressing intopellets, was used between the annealing stages. The composition of the productswas studied by powder X-ray diffraction (XRD) using Nonius FR-551 Guinier cam-era (Cu Ka1 radiation). Flux and chemical transport (CTR) techniques were used inattempts to grow single crystals. In the former approach, indium, KBr, or KI wereused as fluxes. The samples were annealed for 7–12 days at 550–750 �C and slowly(�1–2 �C/h) cooled to 300 �C. However, this approach yielded only Pd3In crystals.For CTR, annealed sample of Pd8In2Se composition was placed into a silica ampoule(210 mm long, 16 mm in diameter) with a small amount of iodine, and put into afurnace with 550 �C/600 �C or 600 �C/650 �C gradient between zones. No mass-transport, apart from small amounts of PdI2, was observed after 7 days.

Selected area electron diffraction (SAED) was performed using a Philips CM20transmission electron microscope (200 kV, LaB6 cathode), the composition of themicrocrystallites was checked simultaneously by energy-dispersive spectroscopic

Table 1Data collection and Rietveld analysis parameters, and final residuals.

Refinement 1

Compound Pd5InSe PMass fraction 83.7% 1Data collection Bruker D8 advanceRadiation type, source X-ray, Cu Ka1

Data collection temperature (K) 295Range in 2h, step size (�) 10–95, 0.02Space group P4/mmm (no. 123)Unit cell content Pd5InSe, Z = 1 PFormula weight (g/mol) 725.878 1

Unit cell parametersa (Å) 4.0290(3) 4c (Å) 6.9858(5) 1V (Å3) 113.40(2) 1Rp, Rwp (%) 5.23, 7.46RBragg (%) 1.12 2GoF 1.90

(EDS) analysis on a built-in Inca + Energy (oxford instruments) analytical moduleby using the Pd Ka, In Ka, and Se La lines. Pure palladium metal, InP, and HgSe wereused as primary standards. Samples were prepared either by grinding polycrystal-line specimens in ethyl alcohol and depositing the suspension onto a carbon net, orby ion-milling of a prepressed pellet. The latter technique gave better access to thestacking direction (corresponding to the crystallographic c axis) in texturized sam-ples. EDS on separate microcrystals from bulk samples was carried out with a JeolJSM-5510 scanning microscope equipped with an Inca + Energy module. The datawere averaged from 10 to 15 spots on flat surfaces of each crystallite.

2.2. Crystal structure determination

X-ray powder diffraction patterns were recorded using a Huber G670 imagingplate detector (Cu Ka1-radiation, Ge(111)-monochromator, transmission geome-try) and Bruker D8 advance diffractometer (Cu Ka1-radiation, Ge(111) monochro-mator, reflection geometry, LynxEye strip detector). Crystal structures of Pd5InSeand Pd8In2Se were refined together using the Rietveld method as implemented inthe TOPAS package [11]. XRD patterns of two samples, with respective stoichiome-tries of Pd5InSe and Pd8In2Se, were used for the refinement. Rietveld refinementswere performed using the fundamental parameter approach for the peak shapedescription. The starting structural model for Pd5InSe was derived from the pub-lished data for the Pd5TlAs [3]. The initial model for Pd8In2Se was constructed usingthe unit cell established experimentally from powder XRD and SAED data, and theatomic coordinates calculated under assumption of Pd8In2Se having double-stackedcuboctahedral fragments along the c axis. A small amount of impurities Pd17Se15

and Pd3In (in case of Pd8In2Se) was found in the XRD data, and these phases wereincluded into the refinement. The preferred orientation was corrected using aspherical harmonics approach implemented in TOPAS.

3. Computational details

Band structure calculations were performed on a density-functional theory (DFT) level utilizing the all-electron full-potential linearized augmented plane wave method (FP-LAPW) as

Refinement 2

d8In2Se Pd5InSe Pd8In2Se3.7% 5.6% 74.6%

Huber G670X-ray, Cu Ka1

2958–95, 0.02P4/mmm (no. 123)

d8In2Se, Z = 1 Pd5InSe, Z = 1 Pd8In2Se, Z = 1159.956 725.878 1159.956

.0047(4) 4.0286(4) 4.0045(4)0.952(1) 6.9762(6) 10.952(1)75.65(4) 113.22(3) 175.63(4)

2.93, 4.29.81 1.59 1.34

1.23

Table 3Selected interatomic distances for the Pd5InSe and Pd8In2Se structures.

Atoms Pd–Pd Distance (Å) Atoms Pd–In Distance (Å) Atoms Pd–Se Distance (Å)

Pd5InSePd(1)–Pd(2) 2.811(1) Pd(1)–In 2.848(1) Pd(2)–Se 2.530(1)Pd(2)–Pd(2) 2.849(1) Pd(2)–In 2.811(1)

Pd8In2SePd(1)–Pd(1) 2.831(1) Pd(1)–In 2.808(1) Pd(2)–Se 2.535(1)Pd(1)–Pd(3) 2.817(2) Pd(2)–In 2.796(1)Pd(2)–Pd(2) 2.831(1) Pd(3)–In 2.831(1)Pd(2)–Pd(3) 2.787(1)

Fig. 2. Observed, calculated and difference Rietveld profiles for the Pd5InSe (top) and Pd8In2Se (bottom) samples.

50 E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55

implemented in the ELK code [12]. The Brillouin zone samplingwas performed using 244 (Pd3In TiAl3-type), 322 (Pd3Al ZrAl3-type), 112 (Pd5InSe), and 146 (Pd8In2Se) irreducible k-points. ThePBESol exchange–correlation functional [13] of the GGA-type wasused in the calculations. The muffin-tin sphere radii for the respec-tive atoms are (Bohr): 2.40 (Pd, In), 2.20 (Se). The maximummoduli for the reciprocal vectors kmax were chosen so that RMT-

kmax = 10.0. The convergence of the total energy with respect tothe k-point sets was checked. Atomic charges were analyzedaccording to Bader’s QTAIM approach [14]. The electron localiza-tion function (ELF) was calculated according to [15] using DGridpackage [16]. The calculations were performed using the IntelCore-i7-based laboratory cluster and the MSU Lomonosov super-computer [17]. Visualization of the ELF and its topological analysiswas performed using ParaView package [18].

4. Results and discussion

4.1. Synthesis and preliminary sample characterization

The details of sample preparation and the results of powderXRD of the products are given in Table S1 in Supplementary Mate-rials. Our search was based on the initial guess of obtaining thecompounds isotypic with Pd7�dSnTe2, therefore early samples wereprepared with the Pd:In:Se ratio of 7:1:2, respectively. Powder XRDof the samples after annealing has shown that no ternary com-pound with a target composition was formed. Diffraction patternswere dominated by a group of reflections belonging to a phase thathad similar crystallographic characteristics to Pd5TlAs with a sys-tematic peak shift. However, after subtracting the reflections ofthe Pd5TlAs-type phase and the ones arising from binary impuri-ties, another group of weaker reflections remained. Based on thesuggestion (not supported by structural studies) of the possibility

of the formation of Pt8In2Pn (Pn = P, As) [19,20] in the Pt–In–Pnsystems, the compounds that feature double-stacked platinum–in-dium cuboctahedral fragments, we assigned these reflections totheir hypothetical analogue, a ‘Pd8In2Se’ compound. A secondbatch of the samples was prepared with the Pd:In:Se ratios of5:1:1 and 8:2:1. Powder XRD patterns of these samples haveshown that neither samples with Pd5InSe composition nor withPd8In2Se one are phase-pure, and both contain the same twogroups of reflections assigned to ‘Pd5InSe’ and ‘Pd8In2Se’ phases.The effect of quenching rather than slow cooling on the samplecomposition was also investigated, but the amount of impuritiesin quenched samples only increases. In an attempt to get rid ofthe binary palladium selenide, samples with the composition be-tween 5:1:1 and 8:2:1 were also prepared, but a trace amount ofPd17Se15 was still present even after 23 days of annealing. Severalattempts were made to produce single crystals suitable for struc-tural studies (see Section 2.1), however, they were all unsuccessful.

In order to get more insight into the phase composition of thesamples, we have performed SAED in a combination with EDSanalysis. Elemental analysis of different microcrystallites in bothsamples confirmed the presence of two ternary phases, their com-position, averaged over a large amount of microcrystallites, wasfound to be Pd72(1)In13(1)Se15(2) and Pd74(2)In17(2)Se9(1), which is ingood agreement with the presence of Pd5InSe and Pd8In2Se, respec-tively. Electron diffraction patterns for both types of microcrystals(see Fig. 1) allowed us to establish crystallographic parameters oftwo phases. Simultaneous EDS analysis has shown one-to-one cor-respondence between SAED patterns and the above-mentionedcompositions.

According to the SAED data, both compounds crystallize intetragonal system and a primitive unit cell. Unit cell parameterswere estimated as a � 4.1 Å, c � 7.1 Å for Pd5InSe, and a � 4.0 Å,c � 11.1 Å for Pd8In2Se. Using the electron diffraction data, we haveindexed groups of reflections arising from both compounds in the

Fig. 3. Crystal structures of Pd5InSe and Pd8In2Se: the respective unit cells (a, b) and polyhedral representation of the structures (c, d).

Table 4Main geometrical parameters for the structural fragments in Pd3In, Pd5MQ and Pd8In2Se.

Compound Pd5TlAs [3] Pd5HgSe [4] Pd5InSe Pd8In2Se Pd3In (TiAl3-type) [22] Pd3In (ZrAl3-type) [22]

Q atomic radius (Å) 1.39 1.40 1.40 1.40 – –M atomic radius (Å) 1.71 1.57 1.66 1.66 1.66 1.66c (Å) 7.042 7.0378 6.9858 10.952 7.4759 15.2031hc for [Pd12M] (Å) 4.255 4.041 3.912 3.921 3.738 3.801wc for [Pd12M] (Å) 4.005 4.013 4.029 4.005 4.100 4.062Degree of distortion of [Pd12M], hc/wc 1.062 1.007 0.971 0.980 0.912 0.936Volume of cuboctahedron (Å3) 68.25 65.08 63.50 62.89 62.84 62.72dPd-M (parallel to ab) (Å) 2.832 2.837 2.849 2.831 2.899 2.872dPd-M (diagonal) (Å) 2.911 2.848 2.808 2.796, 2.808 2.774 2.766, 2.797hp for [Pd8Q] (Å) 2.817 2.997 3.073 3.110 – –wp for [Pd8Q] (Å) 2.832 2.837 2.849 2.831 – –Degree of distortion of [Pd8Q], hp/wp 0.995 1.056 1.078 1.098 – –dPd(2)-Q 2.448 2.504 2.534 2.535 – –

E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55 51

powder patters in P4/mmm space group, with a = 4.029(2) Å,c = 6.991(6) Å for Pd5InSe, a = 4.0038(4) Å, c = 10.953(1) Å for Pd8-

In2Se (for comparison, Pd5TlAs crystallizes in P4/mmm space group,with a = 4.005 Å, c = 7.042 Å [3]). And the overlap of hk0 reflectionsin their SAED patterns gives an extra support to the idea of thesecompounds being two homologues that differ in stackingsequence.

We have been able to prepare the samples of Pd5InSe andPd8In2Se where the respective title compounds would be the dom-inant phases. However, despite all attempts to produce phase-puresamples of each compound, we were not able to separate two com-pounds from each other completely. The analysis of literature datarevealed that this situation mimics the behavior of two palladium–indium binary intermetallics, Pd3In of the TiAl3-type and Pd3In of

Fig. 4. Two polytypes of Pd3In: TiAl3-type (left) and ZrAl3-type (right).

52 E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55

the ZrAl3-type, to which our compounds are structurally related(vide infra). According to [21–23], these two polytypes are alwayssynthesized as a mixture by a high-temperature route, and cannotbe obtained phase-pure. Another problem with powder sampleswas the presence of binary impurities, Pd17Se15 (in the sampleswith 5:1:1 element ratio) or Pd3In (in the most metal-rich ones).It has to be noted that the impurities cannot be completely elimi-nated by extending the time of annealing, since Rietveld analysis ofintermediate samples after 10 and 20 days of annealing has shownthat Pd17Se15 content decreased by less than 1% during that time.

Fig. 5. Total (TDOS) and projected (PDOS) densities of states near the Fermi level for PPd8In2Se (bottom right).

For the final Rietveld refinements we used two samples of therespective Pd5InSe and Pd8In2Se compositions that showed mini-mal amount of impurities.

4.2. Crystal structure description

Crystallographic data, atomic coordinates and selected bonddistances for Pd5InSe and Pd8In2Se are given in Tables 1–3. FinalRietveld refinement plots are given in Fig. 2.

The crystal structures of two selenides are depicted in Fig. 3.According to the Rietveld refinement data, Pd5InSe is fully isotypicwith Pd5TlAs [3]. The key feature of its structure are heterometalliclayers of indium-centered palladium cuboctahedra [Pd12In], joinedvia common faces along the (001) plane, alternating along the caxis with the layers of rectangular prisms [Pd8Se], joined via com-mon edges. Cuboctahedra and prisms are connected in the c-direc-tion via common Pd(2)–Pd(2) edges. As is very common in thecuboctahedra-based intermetallic motifs, polyhedra in Pd5InSeare tetragonally distorted, being slightly shortened along the c axis,while in Pd5TlAs they are slightly elongated along the same direc-tion. The degree of distortion was estimated using the height-to-width ratio for the cuboctahedra (hc/wc, for explanation of hc andwc see Fig. 3a), and the data are summarized in Table 4. As seenfrom the table, the least distorted [Pd12M] fragments are observedin Pd5HgSe [4]. Similar estimations of the height-to-width ratio(hp/wp, Fig. 3a) of palladium–arsenic and palladium–selenium frag-ments show that [Pd8Se] rectangular prisms in Pd5InSe, as well asin Pd5HgSe, are also elongated in the c-direction (albeit slightly lessso in the latter case) and thus can be described as the fragments ofthe PtHg2 structure type. Pd5TlAs, on the other hand, features

d3In TiAl3-type (top left) and ZrAl3-type (top right), Pd5InSe (bottom left) and for

Fig. 6. Band structures of Pd5InSe (top) and Pd8In2Se (bottom) from DFTcalculations.

E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55 53

almost perfectly cubic [Pd8As] fragments (hp/wp = 0.995, seeTable 4) that are more properly described as belonging to theCaF2 type [24].

Pd8In2Se shares all the main structural features with Pd5InSe,the main difference being that the cuboctahedral fragments in itsstructure are doubly stacked along the c axis. This fact has somebearing on the geometry of the respective fragments: while cuboc-tahedra in Pd8InSe have almost the same degree of distortion hc/wc

as in Pd5InSe (see Table 4), palladium–selenium fragments aremore affected and have hp/wp of �1.10, which is the largest valueamong all the compounds in question.

As we mentioned in the Introduction (Section 1), Pd5InSe islikely to be a member of a Pd5MQ series along many palladium/platinum and main-group metal pnictides, although formally upto now it is only a third structurally characterized compound ofthe Pd5TlAs type, which makes this type a relatively rare one. Pd8-

In2Se represents a new structure type and is the first palladium-based ternary compound with double-stacked layers of cuboctahe-dra. Yet, the key features of their metallic blocks can be traced backto the palladium–indium binary intermetallics.

Heterometallic layers in Pd5InSe and Pd8In2Se bear very strongresemblance to the fragments of tetragonal Pd3In intermetallics,existing in two polytypic modifications, TiAl3 and ZrAl3 [21,22],based on the Cu3Au motif. Both polytypes are built from [Pd12In]cuboctahedral fragments and differ primarily in their packing(see Fig. 4). In the TiAl3-type Pd3In, layers of cuboctahedra arestacked along the c axis in such way that each next level is shiftedby 1/2 (a + b). In the ZrAl3-type Pd3In, these layers are double-stacked, i.e. third and fourth layers are shifted with respect tothe first and second, while within each pair cuboctahedra are

perfectly aligned. Thus we can call Pd3In (TiAl3-type) a parentstructure for the Pd5InSe structure, and Pd3In (ZrAl3-type) a parentstructure for the Pd8In2Se one.

The distances between palladium atoms (see Table 3) in ternaryselenides are slightly longer than in palladium metal and are veryclose to those in their parent intermetallics of the TiAl3 and ZrAl3

type (2.774–2.899 Å and 2.774–2.899 Å, respectively [22]), andalso to those in Pd5HgSe (2.838–2.848 [4]). Palladium–indium dis-tances in selenides are also very similar to the ones found in Pd3In(2.774–2.899 Å and 2.764–2.872 Å, respectively), while palladium–selenium contacts are slightly longer than in Pd5HgSe (2.504 Å)and closely match the respective distances in Pd17Se15 (2.433–2.586 Å [25]).

As seen from Table 4, cuboctahedra in both intermetallics aretetragonally distorted in the same manner as in ternary com-pounds, i.e. slightly ‘squashed’ along the c axis, with the distor-tion being more pronounced for the intermetallics (hc/wc isfurther from 1). While showing similar degree of distortion ofcuboctahedra between each other, both Pd5InSe and Pd8In2Sehave less distorted [Pd12In] fragments as compared to their par-ent intermetallics. These fragments in the ZrAl3-type Pd3In areless distorted than in the TiAl3-type one. Another interestingobservation is the slight breaking of the inversion symmetry forthe cuboctahedra in Pd8In2Se: both In and Pd3 atoms are shiftedfrom the equatorial plane towards Pd2 atoms, which leads to thePd3–Pd2 and Pd2–In distances being ca. 0.04 Å shorter than therespective distances from those atoms to Pd1 (see Table 3). Thevalues of the shift along the c axis are almost identical for Pd3and In, as evidenced by their respective z coordinates beingwithin 0.0013 Å (see Table 2).

4.3. Electronic structure and bonding

Electronic structure of both Pd5InSe and Pd8In2Se, as well astheir parent intermetallics, was evaluated based on the DFT calcu-lations. Total and projected densities of states (DOS) near the Fermilevel for all compounds are shown in Fig. 5. Typically for the Cu3-

Au-type based chalcogenides (Pd7�dSnTe2 [2] and Ni7�xMQ2 [6]),main contributions to the DOS near the Fermi level arise fromthe d-states, 4d-states of palladium in this case, which are almostfilled and thus close to the d10-state. The contributions from sele-nium 4p-states and indium 5s-states reside in a significantly lowerenergy range. Rather low yet non-zero DOS at the Fermi levelindicates 3D metallic conductivity and a likely temperature-inde-pendent paramagnetism, as was observed for other Cu3Au-basedchalcogenides. This picture qualitatively matches the one obtainedfor binary intermetallics (Fig. 5), confirming that it is heterometal-lic fragments that largely define the peculiarities of electronicstructures of both selenides.

Band structures of Pd5InSe and Pd8In2Se (Fig. 6) are also consis-tent with 3D metallic behavior. A slight spatial anisotropy in theband dispersion is observed, manifesting in an unequal numberof bands crossing the Fermi level along different directions in thek-space, particularly for Pd8In2Se, which might be taken as anindication of a minor anisotropy of electric conductivity, but notto the extent that it would significantly affect an overall metallicproperties.

Bonding in both binary intermetallics and ternary selenides wasanalyzed by studying the topology of the electron localizationfunction (ELF). At high and medium values of the localizationparameter (g), the only features appearing in the ELF are atomicshells. As the ELF decreases below g = 0.3, localization domains inheterometallic part of the structure start to appear. The ELF iso-surfaces at g = 0.25 (see Fig. 7) show similar bonding patterns forall four compounds. In each case, there are 8 localization domainsplaced symmetrically around each indium atom in the centers of

Fig. 7. ELF localization domains (g = 0.25) for binary intermetallics (a – Pd3In, TiAl3-type; b – Pd3In, ZrAl3-type) and ternary palladium–indium selenides (c – Pd5InSe;d – Pd8In2Se) . Atomic shells were removed for clarity.

Fig. 8. The ELF cross-section passing through four localization domains aroundindium atom in Pd5InSe.

54 E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55

octants (Figs. 7 and 8). As it has been described before for Pd17In4-

Se4 [10] and Ni6SnS2 [6] (belonging to the Ni7�xMQ2 type andfeaturing cuboctahedral [Ni12Sn] fragments), these domains

Table 5Calculated QTAIM atomic charges in Pd3In (both types), Pd5InSe, and Pd8In2Se.

Pd3In (TiAl3-type) Pd3In (ZrAl3-type)

Atom Charge Atom Charge

Pd(1) �0.27 Pd(1) �0.28Pd(2) �0.23 Pd(2) �0.27In +0.79 Pd(3) �0.24

In +0.80

correspond to the four-centered 3Pd + In interactions. In bothintermetallics, these interactions form an unbroken network ofmulti-centered metal–metal bonds, while in the selenides theyare confined within the metallic fragments. The feature thatslightly differentiates the topology of these domains in Pd8In2Sefrom other compounds is the size mismatch between the domainsnear the border between the cuboctahedra and the ones closer totheir outer boundaries. As seen from Fig. 7, the former domainsare somewhat larger (and, more importantly, start to appear athigher g values) than the latter, while in other cases all 8 domainsaround each indium atom are fairly uniform. We attribute thesedifferences to the breaking of the symmetry of the cuboctahedrain Pd8InSe (vide supra).

Atomic charges calculated according to Bader’s QTAIM ap-proach [14] indicate that a significant charge transfer from indiumto palladium occurs in binary intermetallics (see Table 5), as wellas in ternary compounds. In both cases of Pd5InSe and Pd8In2Se,‘inner’ palladium atoms carry much stronger negative charge than‘outer’ ones that have selenium as their neighbor, this is particu-larly apparent for Pd8In2Se, where the charge difference betweenPd1 and Pd2 is significantly more pronounced and might accountfor the observed slight shift of Pd3 and In atoms towards Pd2.Almost no charge transfer between palladium and selenium isevident from the QTAIM charges, indicating non-polar nature ofbonding between these elements.

Pd5InSe Pd8In2Se

Atom Charge Atom Charge

Pd(1) �0.27 Pd(1) �0.31Pd(2) �0.12 Pd(2) �0.10In +0.82 Pd(3) �0.26Se �0.06 In +0.81

Se �0.04

E.Yu. Zakharova et al. / Journal of Alloys and Compounds 589 (2014) 48–55 55

5. Conclusions

Two new metal-rich palladium–indium selenides, Pd5InSe andPd8In2Se, were synthesized by a high-temperature route and theirstructures were determined from powder XRD data, supported byEDS and SAED data. Both compounds are built from cuboctahedral[Pd12In] fragments of tetragonally distorted Cu3Au-type structure,single-stacked in Pd5InSe and double-stacked in Pd8In2Se, alternat-ing along the c axis with [Pd8Se] rectangular prisms with PtHg2

structure. DFT calculations predict both compounds to be 3Dmetallic conductors and Pauli-like paramagnets. According to thebonding analysis based on the ELF topology and Bader’s QTAIMapproach, both compounds feature multi-centered palladium–in-dium interactions in their heterometallic fragments and significantcharge transfer from indium to palladium atoms. These com-pounds can be described as members of a homological series ofintergrowth selenides with single or doubled metallic fragments.Heterometallic blocks on Pd5InSe and Pd8In2Se can also be tracedback to their parent intermetallics, Pd3In polytypes. Interestingly,so far for nickel and palladium only the structures with single- ordouble-stacked cuboctahedra have been reported, while it wouldnot be completely beyond reasonable, considering the nature ofthe compounds in question, that the structures with alternatingsingle and doubled cuboctahedra, or even the structures with triplestacked metallic fragments might exist. Whether such compoundsmay be proven to exist remains to be seen for the time being.

Acknowledgments

This work was partially supported by Russian Foundation forBasic Research (Grant No. 12-03-00833a) and the Presidential Pro-gramme of Russian Academy of Sciences (Grant 8P23). The use ofthe resources of Supercomputing Center of Lomonosov MoscowUniversity is kindly acknowledged. Mr. L. Rossou (EMAT, Antwerp)is gratefully acknowledged for sample preparation using the ion-milling technique. A.I. is indebted to Belgian Science Policy (BEL-SPO) for the financial support of her staying at the University ofAntwerp.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jallcom.2013.11.172.

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