From Paracrystalline Ru(CO)4 1D Polymer to Nanosized RutheniumMetal: A Case of Study through Total Scattering AnalysisAntonio Cervellino,† Angelo Maspero,‡ Norberto Masciocchi,*,‡ and Antonietta Guagliardi‡,§

†SLS, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland‡Dipartimento di Scienza e Alta Tecnologia, Universita dell’Insubria, via Valleggio 11, 22100 Como, Italy§Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, via Amendola 122/O, 70126 Bari, Italy

*S Supporting Information

ABSTRACT: The highly defective 1D polymer [Ru(CO)4]nspecies, in which Ru atoms are arranged in parallel chains wellseparated by the ligand shell, is here investigated by totalscattering Debye function analysis on synchrotron powderdiffraction data. A new chain packing model based on thepresence of 2D paracrystalline effects in the ab plane issuccessfully proposed and well accounts for the unusualcombination of sharp and very broad diffraction peaks notcompatible with conventional size or strain models. Uponthermolysis, the collinear metal arrangement of the parallel[Ru(CO)4]n bundles in the polymer is not maintained in the Ruparticles, whose nanocrystals are not significantly elongated and show a faulted hcp structure with much smaller domains than inthe parent organometallic species. These results thus dismiss the appealing hypothesis that, by using a chain-like precursor, highlyanisotropically shaped Ru-metal nanorods can be formed upon controlled pyrolysis.

■ INTRODUCTIONThanks to their peculiar geometric and electronic features,nanosized one-dimensional molecular metal wires and one- andtwo-dimensional superclusters that are assembled through M−M bonds are very interesting low-dimensional molecularmaterials finding applications in molecular electronics,1 nano-lithography,2−4 and catalysis.5,6 Useful precursors to thesesystems are organometallic polymers containing M−M bondsin the backbone: these species are somewhat rare but inherentlyconstitute potential precursors to high aspect ratio nanomateri-als, thanks to the presence of metal atoms arranged in chainsand the relatively inert ligands wrapped around them.7 Asrecently demonstrated by Zacchini,8 these polymers couldultimately provide ultrathin nanowires, as long as the mainproblem of preventing the aggregation of the metal chains,when the ligands are removed, is solved. In this regard, manyhomoleptic carbonyl species have been synthesized through theyears, aiming, in some cases, at the formation of low-dimensional species [see, for example, the HRe(CO)4 polymer

9

or the [Pt3(CO)6]nm− stacks of D3h fragments10]. However, the

tendency of second and third row transition metals inmaximizing the metal-to-metal connectivity plays a major rolein driving high-nuclearity carbonyl clusters toward a closed,convex, often polyhedral shape, i.e., a zero-dimensional, trulymolecular, connectivity,11 making the isolation of 1D species arare event. In this regard, the long known12 [Ru(CO)4]npolymer seems to be a promising starting material, due to thepresence of a very anisotropic metal atom distribution, withincollinear chains of (covalently bonded) ruthenium atoms.

Accordingly, several papers have appeared in the recentscientific literature, addressing its chemical and functionalproperties, for the formation of ruthenium nanowires uponcontrolled thermal degradation,13 or in search for electrical andnanotechnological applications,11 along with specific catalyticactivity.14,15 Worthy of note, if compared to those derived fromruthenium salts or osmium precursors, Ru metal nanoparticles,pyrolytically generated at 200 °C from the polymeric[Ru(CO)4]n organometallic precursor,16,17 show enhancedcatalytic activity, e.g., in CO oxidation (with reducedconversion temperatures and increased conversion yields),their appealing properties being attributed to the presence ofelongated Ru nanorods (TEM evidence). After havingdetermined, nearly two decades ago,18 (by unconventionalstructural powder diffraction methods) the elusive crystal andmolecular structure of [Ru(CO)4]n, and prompted by therenewed interest in its potential nanotechnological properties,we decided to give a deeper insight into the still unsatisfactory(or incomplete) microstructural interpretation then proposed,by focusing on the undisclosed defectiveness of such ananosized 1D polymer, and on the way its reported thermaltransformation to ruthenium nanowires can be potentiallyaffected by the defects.Ruthenium tetracarbonyl is a cornerstone of modern

organometallic chemistry;19 in its stable and commercial

Received: April 3, 2012Revised: May 13, 2012Published: May 21, 2012

Article

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form, it normally appears as a cyclic trimer, the widely knownRu3(CO)12 molecule. In the trimer, ligand-unsupported Ru−Rubonds generate a triangular cluster of idealized D3h symmetry.

20

At variance, the [Ru(CO)4]n polymer contains chains of trans-D4h-Ru(CO)4 fragments (see Figure 1), staggered by 45° oneto each other, with a Ru−Ru distance of 2.860(1) Å, a valuevery close to that found in solid Ru3(CO)12, 2.844(2) Å.

21

Staggering has also been derived from independent solid-state 13C NMR on nanosized [Ru(bipy)(CO)2]n polymer (n ≈20)22 and DFT methods on the pristine [Ru(CO)4]4 species.

23

Thus, the title compound constitutes a unique case of a neutralhomoleptic carbonylic polymer where metal atoms are arrangedin parallel chains, well separated by the ligand shell; therefore, itshould be of no surprise that is has been considered the mostappropriate candidate for the formation, by simple COdesorption, of highly anisotropic Ru metallic nanoparticles.In this article, we report on a detailed structural and

microstructural analysis of the [Ru(CO)4]n chain polymer andon its in situ thermally induced transformation to the nanosizedruthenium metal particles. X-ray synchrotron data modeledthrough an original total scattering approach enabled us todisclose a new, previously unforeseen, 2D paracrystallinestructural model relying on correlated chain displacements(briefly, a paracrystal is a crystalline species that has a highly

distorted lattice with unit cells of highly variable shape andsize). Surprisingly, instead of the purported nanowires,isotropically shaped (faulted) Ru metal nanoparticles werefound after the transformation, whose origin requires asignificant breaking of the metal chains.

■ EXPERIMENTAL SECTIONMaterials. A powdered sample of [Ru(CO)4]n was prepared in 4 h

following standard literature methods,12,18 using a UV lamp (emissionpeak near 290 nm).

X-ray Total Scattering Methods. Synchrotron X-ray powderdiffraction data, with high counting statistics and high angularresolution (thus, without significant instrumental contribution topeak broadening), were collected at RT on freshly prepared[Ru(CO)4]n at the Material Science Beamline at SLS (λ = 0.620639Å).

In a parallel set of experiments, [Ru(CO)4]n was measured with λ=1.00411 Å, from RT up to 470 K (where transformation to metallicruthenium was complete), in 10 K steps. Upon cooling back to RT,the full diffraction pattern was measured again and used formicrostructural analysis of metallic Ru. Prior to data analysis, rawdata were corrected for air and (0.3 mm Ø) glass capillary scattering,absorption effects, and, thanks to the evaluation of minor instrumentalaberrations detected on powders of NIST standard Silicon 640c, alsofor a slight instrumental misalignment.

Total scattering data analysis was performed using a modifiedversion of the DEBUSSY suite24 in which coding of the complexalgebra briefly presented in the Supporting Information was included.The remaining computations (Le Bail and Rietveld-refinements) wereperformed using the fundamental parameter approach implemented inTOPAS-R.25

■ RESULTS AND DISCUSSIONFacing a Complex Microstructural Problem. As

repeatedly observed in our laboratory, as well as by differentgroups adopting slight modifications of the original procedure,the [Ru(CO)4]n powder diffraction trace always shows a ratherannoying feature (see Figure 2): a few sharp peaks coexist withvery broad ones (wider than 2°, Cu−Kα radiation). In theoriginal Rietveld-like26 structure analysis, with diffractedintensities computed by taking into account Bragg scatteringonly, the shape and the width of these peaks were modeled by aphenomenological description (allowing smearing of thereciprocal lattice nodes by an anisotropically broadeningmodel) and tentatively attributed to lattice strain in the abplane. However, unraveling the complex structural defective-ness behind such experimental observation may be the key inunderstanding the structural and microstructural transforma-

Figure 1. Left: an 8-monomer sequence of staggered D4h Ru(CO)4fragments. Right: the pseudohexagonal Ibam crystal packing18 of thepolymeric chains in ab (viewed down c). The arrows refer to thecorrelated shift directions of a paracrystalline model later discussed.

Figure 2. Indexed XRPD trace of [Ru(CO)4]n, showing very anisotropic peak widths; insert, plot of the isosurface obtained by spherical harmonicsrepresenting the average crystal shape, resulting in a concave surface with no obvious physical counterpart (even if slightly concave Pd nanocubeshave been reported27).

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tion from the carbonylic polymer to metallic ruthenium and,possibly, in correctly interpreting the physicochemical proper-ties of the metal nanowires.Using the newly collected diffraction data, the widths of the

peaks were derived by single-profile fitting, which showednarrow h00, 0k0, and 00l peaks (providing microstructuralinformation along the a, b, and c axes, respectively), and verybroad hk0 peaks (h,k ≠ 0), providing information in the off-axes directions. Back transforming this information into realspace, vectorially spread apparent crystal sizes (ACS,determined by estimating the 1/cosθ-dependent Lorentziansize broadening) were estimated. Instead of defining a convexshape, ACS nicely fall on a concave, 2D plot (Figure S1,Supporting Information). This would imply questionable star-shaped nanocrystals with highly protruding beams, 10 timeslarger in a and b than along the diagonals, not observed byelectron microscopy.13 However, it is worth reminding here thewell-known caveat that scanning and transmission electronmicroscopies may see different objects (e.g., crystal aggregates)than diffraction methods (coherent domains).If, instead, this broadening is attributed to strain (with tanθ

dependence), no obvious physical counterpart can be devised.A full pattern simulation carried out using the (8th order)symmetrized spherical harmonics model of the TOPAS-Rprogram,25 and accounting for in-plane and off-plane micro-structural features, provided apparent average size and shape ofcrystals of mmm symmetry (see the inset of Figure 2). Indeed,the concave surface shown in the inset speaks for an awkwardperturbation effect. The analysis of the vectorial dependence ofthe ACS eventually suggested the occurrence of a conditionaldisorder, later attributed to anisotropic paracrystallinity of thechains packing in ab, and is discussed in detail in a followingsection.Total Scattering Debye Function Approach. Recover-

ing the structural defectiveness of [Ru(CO)4]n was possiblethanks to the use of a new total scattering approach based onthe Debye function,28 which is able to provide an exactmodeling, in the reciprocal space, of the whole samplescattering. In fact, by this method, both the Bragg (if any)and diffuse scattering, from orientationally isotropic systems,regardless of the presence of periodicity, can be simultaneouslyaccounted for. Indeed, diffuse scattering, commonly observed indisordered and/or nanosized materials, cannot be treated bythe conventional Rietveld approach in which only Braggscattering (confined within the diffraction peaks) is modeled.According to Peter Debye, whose original paper was

published in 1915,29 the scattered intensity from an isotropicpowder is described by the following equation:

∑π

π=

> =

I q b bqd

qd( ) 2

sin(2 )

2j i

N

j iij

ij1

where q = 2 sinθ/λ, is the scattering vector amplitude, dij are theinteratomic distances, bj the atomic form factors, and N is thenumber of atoms in the particle. Computing such an equationis highly time-consuming for large or disordered nanoparticles,unless suitable algorithms for sampling distances and patternmodeling are used.30 Recent advancements made it possible toemploy this technique in the characterization of a variety ofnanosized materials (metals,31,32 oxides and chalchoge-nides,33−37 and bioceramics38). To our knowledge, however,no covalently bound system has ever been studied by thismethod, which we propose here, for the first time, in the realm

of simple organometallic polymers (of known averagestructure), disclosing otherwise inaccessible structural features.

Paracrystalline Model Implemented in the DebyeFunction Approach. Following the seminal work byHosemann and Welberry,39 the Debye equation was suitablymodified to add anisotropic paracrystalline features of theaverage crystal structure.18 Specifically, by defining dampedcorrelations (in real space) between chain axes locations in ab(not any longer exactly C-centered), a probabilistic descriptionof the interatomic vectors in that plane was considered. Forsimplicity, we report here the equation valid for a 2D net, with1D displacements:

σ= × −

−− | | | |

⎛⎝⎜⎜

⎞⎠⎟⎟d

d dP K

r s( ) exp

12

( )

2 (1 )m na a0

2

a2

a a

where d is the vector separating two chains (in ab), ra and sa arethe so-called longitudinal and transversal correlation coef-ficients, and m and n are the location (in lattice units) of thepertinent chain. σa is the statistical variance (i.e., the amplitudeof uncorrelated shifts) of the packing periodicity along the aaxis; σa also represents the natural limit of the dispersion oflarge interatomic distances, thus taking the role of the a* rule inthe traditional approach to paracrystals.39 Deviation of ra and safrom unity represents the loss of correlation for moieties lyingalong a and for the laterally displaced ones, respectively (forshifts along a, as in left portion of Figure 3).

The above equation can be extended to 2D displacements byadding new rb, sb, and σb terms (our case). This is schematicallyshown in the four boxes of Figure 3, where ra and sb are thelongitudinal correlation coefficients and sa and rb are thetransversal ones. Briefly, the longitudinal coefficients describeshifts along the same row of the initial perturbation (thehorizontal a row in the top left box and the vertical b row in thebottom right box, respectively) but do not apply to the parallelones, here defined as transversal; correlated shifts of these latterrows are indeed described by the transversal coefficients. Amore exhaustive description of the complex analytical

Figure 3. Schematic drawing of the geometrical relationships amongthe different correlation coefficients used in this work. Within eachframe, movements depicted by red arrows cause the shifts drawn ingreen. Longitudinal correlation coefficients ra and sb describe shiftsalong the same line of the initial perturbation, while transversal ones,represented by the rb and sa scalars, describe shifts of the parallel(transversal) rows.

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expressions developed for the 2D case can be found in theSupporting Information.Microstructure of [Ru(CO)4]n. Within this approach, we

were able to reproduce the experimentally observed data (peakheights, widths, and shapes), as shown in Figure 4. The best

matching relied on the following model choices: (i) we adoptedrigid [Ru(CO)4]n chains of idealized P42/mmc rod symmetry(Ru−C 1.95 Å, CO 1.15 Å, Ru−C−O 180°, and Ru−Ru2.86 Å); (ii) using the I-centered orthorhombic lattice and thepolymeric structure defined in part (i), we generated atomisticmodels of prismatic rod-shaped crystals with a fixed base (50 ×50 nm2 in ab, as derived from the h00 and 0k0 peak widths)and variable heights (up to 70 nm) along c, and calculated thesampled interatomic distances of each nanorod; (iii) weadjusted by a grid-search approach the correlation coefficientsto the values ra = 1.00; sa = 0.97; rb = 0.97; sb = 1.00; and σa = σb= 3.0 Å (see Figure S2, Supporting Information).As far as the domain size information is concerned, refined

size along c gave an average value of 44.7 nm and a sizedistribution 15.4 nm wide (in the log-normal assumptionincluded in our size-distribution model). Therefore, the averagedomain size in ab and along c are comparable, witnessing anearly isotropic domain shape of the nanoparticles. Addition-ally, thermal parameters corresponding to rms vibrationalamplitudes of 0.0065 nm (Ru) and 0.0173 nm (C and O) werealso refined. Concerning the paracrystalline refined parameters,the unity value for the longitudinal correlation coefficients, raand sb (see Figure 3), indicates that pushing, or pulling, aRu(CO)4 chain along a, or along b (but not diagonally!)induces an analogous shift of the neighboring chains on thesame rows. These shifts are equal in size and direction and arepossibly related to the short O···O interactions present along a(3.08 Å) or, even more evidently, between chains adjacentalong b (2.81 Å, in the repulsive regime). At variance, thetransversal correlation coefficients, rb and sa, are responsible forthe large broadening effect of the hk0 peaks. Although theyappear close enough to unity (as in a truly periodic crystal),their elevation to the |m|th or |n|th power induces a rapidlygrowing coherence loss at large ds. An easy-to-catch pictorial2D representation of the distorted centered-rectangular lattice of[Ru(CO)4]n is given in Figure 5, where each node is theprojection of one polymeric Run chain, running along c, i.e.,perpendicularly to the ab plane.This model, which could be derived by our thorough

modeling through the DFA approach, is further idealized inFigure 6, where each chain is schematically depicted by astacking of disks (the trans-D4h-Ru(CO)4 fragment). Why in

Figure 6 these stacks are not perfect cylinders (as in discotic orcolumnar liquid crystals) stems from an additional hypothesis,which we tested as described in the following.Inspired by the recent work of Macchi et al.41 in which the

flexibility of the Mn2(CO)10 molecule was studied underhydrostatic pressures, new simulations were performed. Verymuch as in the high pressure phase of the manganesepentacarbonyl dimer, a nonnegligible bending of the axialsequence was allowed (with staggered Ru(CO)4 fragments ca.0.22 Å off-axis or, equivalently, with Ru−Ru−Ru angles downto 170°) but gave no relevant differences in pattern matchingcompared to the case of collinear chains (apart from theobvious occurrence of tiny superstructure peaks). This meansthat, if present, a random chain twisting (Figure 6d), possiblyreleasing some intramolecular strain, cannot be (easily) detected bydif f raction methods.42

Therefore, further details of the packing disorder wereinvestigated by molecular mechanics, aiming at estimating therelative orientations of the [Ru(CO)4]n Swiss-cross columns(see the Table of Contents or Abstract graphic). By using asimple Lennard-Jones potential for the O···O contacts andperturbing (in the xy plane) only one column within a clusterof six neighboring chains fixed to their average lattice positions(see Figure 1), we found that the columns can move moreeasily along the b direction than along a (see Figure S3,Supporting Information). If the column rotations about the Ruhinges (Rz) are also relaxed, for small translations, a deepminimum is observed for the 22.5° offset (with respect to the aaxes), in agreement with the actual (refined) carbonyl

Figure 4. Experimental powder diffraction trace for [Ru(CO)4]n (red);DFA simulated trace for the paracrystalline model with the correlationterms quoted in the text (green). Horizontal axis, 2θ (deg). Verticalscale, intensity (a.u.). The lower curve (blue trace) represents thedifference plot. Conventional profile agreement factor40 Rp = Σi|yic −yio|/Σiyio = 0.066, for 570 observed intensity values (yio).

Figure 5. Graphically exaggerated paracrystalline centered rectangularlattice (dashed red lines), whose displacements from the averageperiodicity are not random (isotropic strain) nor 100% correlated(ideal crystal).

Figure 6. (a) Drawing of the packing of flat Ru(CO)4 moieties(idealized by coins) within the (para)crystals. For the perfect (b) andimperfect (c) stacking of linear Ru chains, see text. The actual shapeand interlocking of the Ru(CO)4 crosses makes the whole averagecrystal orthorhombic (a/b = 2.0) and not truly hexagonal (a/b = √3).

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orientations, the optimal angle differing by at most 4° aftershifting by 0.7 Å (Figure 7).

Thermal Evolution of [Ru(CO)4]n. Once the paracrystal-line features providing the peculiar diffraction pattern wereclarified, the thermal evolution of [Ru(CO)4]n was investigated.Using the conventional Rietveld-like approach implemented inTOPAS-R, lattice parameter changes of [Ru(CO)4]n, in the30−190 °C range, were calculated and linear thermal expansioncoefficients (∂lnx/∂T) derived therefrom 56, 104, and 20M K−1, for the a, b, and c axes, respectively (see Figure 8). Thecombined volumetric value, ∂lnV/∂T, is nearly 181 M K−1 andcan be mainly attributed to the large thermal expansion sufferedby the crystals in the ab plane, likely due to the swingingmotion of the carbonyl groups hinged about the centralruthenium atoms (if not to chain belly-dancing, see Figure 5c).Interestingly, the conventionally accepted linear thermalexpansion coefficient for ruthenium metal43 is nearly one-third of that of [Ru(CO)4]n along the c axis, witnessing therather stiff nature of this Ru−Ru link in this molecular,nonmetallic, compound.Near 200 °C, [Ru(CO)4]n transforms into the hcp Ru metal.

Previous determinations12,13 of the stability of [Ru(CO)4]n

indicated 126 °C (under vacuum) or 170 °C as itsdecomposition temperature. Likely, the significantly highervalue observed in this case is due to the sealed environment andby pressure effects exerted by the freed CO.Conventional Rietveld-like analysis of the hcp metallic Ru

recovered after cooling to RT (see Figure S4, SupportingInformation) highlighted the presence of significant residuals,which are compatible with a (still unknown) cubic phase of Ru(a0 = 3.86 Å). Such a finding suggests the presence of stackingfaults in the ideal hcp sequence,44 very much alike the well-known paradigmatic case of hcp cobalt.45 Observation of thesystematic broadening of the h − k ≠ 3N peaks46 and patternsimulations (using a specifically developed macro, similar tothat proposed by Whitfield et al.47) confirmed this structuralhypothesis, and a (small) growth faulting probability (β) of0.023(1) was found, well in line with similar results on faultedhcp metals and alloys. Furthermore, since 00l peaks are notaffected by the presence of faults, a Le Bail, structureless, fit(removing ideal hcp periodicity, see Figure 9) was carried outaiming at extracting the average domain size along thecrystallographic c-axis, that is, the elongation direction of theexpected nanowires. An average domain size of only 5.6 nm wasfound (nearly one-half that derived in ab), witnessing that thepolymeric chains, arranged as parallel bundles in the starting[Ru(CO)4]n nanoparticles, are broken during the thermaltreatment, and metal atoms heavily rearrange in significantlysmaller domains. Accordingly, the metallic Ru nanoparticles areneither a single ordered phase nor specifically elongated aboutone axis, and, therefore, the appealing hypothesis that, underpyrolytic conditions, “the presence of metal−metal bondsallows for retention of the rod-like arrangement of the metalatom chain”13 must be dismissed. Collapsing of the naked Ruchains (after CO removal) by simple translation in ab could inprinciple afford a bcc phase, whose existence and magneticproperties have been predicted.48 Worthy of note, alternativeways not using [Ru(CO)4]n for preparing high-aspect ratio andcatalytically active ruthenium nanocrystals have been veryrecently proposed.49

■ CONCLUSIONS

More than 25 years have now gone since [Ru(CO)4]n wasoriginally formulated as a rare homoleptic metal carbonylpolymer: in its first report (1986),12 only its synthesis andstoichiometry were correctly addressed; later (1993),18 whenpowder diffraction method began to be used as a quantitativestructural tool,50 a convincing structural model was proposed,

Figure 7. Plot of the angular variation (deg) from the 22.5° referencevalue obtained by minimizing the potential energy of a cluster of seven[Ru(CO)4]n chains as described above for Figure S3 (bottom,Supporting Information), showing that, even for a (rather large) 0.05nm displacement in xy of the Ru(CO)4 Swiss-cross, the refined angle isless than 4° off its nominal value. These results thus manifest the nearconstancy of the Ru(CO)4 orientation, regardless of its actual locationor displacement in the xy plane.

Figure 8. Plot of the relative lattice parameter dependence vs temperature, as obtained by a parametric whole-pattern refinement. Horizontal axis, T,°C; vertical axis, (xT − x0)/x0.

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and a tentative (semiquantitative) microstructural analysisreported; finally (this work), soon after the development oftotal scattering techniques (based on the Debye equationspecifically developed for the analysis of nanostructuredmaterials), physically sound quantitative estimates of un-expected disordering effects and chain length estimates becameavailable, adding a new dimension (and stereochemicalcomprehension) to the nanoscaled problem.The thermal desorption of the ligand shell from the

polymeric chains of [Ru(CO)4]n afforded faulted hcp Runanoparticles, of nearly isotropic shape, and not elongatedmetal nanowires, as originally reported. Accordingly, themorphology and the anisotropic structure of the parentorganometallic polymer are not maintained in the final product.Therefore, reasons different from the high aspect ratio of themetal nanoparticles must be invoked for the observed enhancedcatalytic properties.Finally, on the more methodological side, this work required

the development of new analytical expressions and their codinginto new computational tools; valid for polymeric [Ru(CO)4]n,these methods can well be adopted, or adapted, for otherfaulted chain-like structures, crystallizing in parallel bundles,where some kind of correlated disorder is present, as intechnologically relevant liquid crystalline copolymers.51

■ ASSOCIATED CONTENT

*S Supporting InformationDetails on the complex algebra developed for the adoption ofthe DFA protocol on anisotropic paracrystalline species.Ancillary plots including geometric and energetic analyses forRu(CO)4 and metallic Ru (Rietveld-type). This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: +39-031-2386627. Fax: +39-031-2386630. E-mail:norberto.masciocchi@uninsubria.it.

NotesThe authors declare no competing financial interests.

■ ACKNOWLEDGMENTS

Partial funding by Fondazione CARIPLO, Project Nos. 2009-2446 and 2011-0289. The X-ray powder diffraction data wererecorded at the MS4-Powder beamline of the SLS synchrotron,Villigen, Switzerland. We thank one anonymous referee forvaluable suggestions.

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Figure 9. Rietveld refinement plot in the structureless (Le Bail) mode and a conditioned broadening of the h − k ≠ 3N peaks. Horizontal axis, 2θ(deg). Vertical axis, intensity (counts).

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Crystal Growth & Design Article

dx.doi.org/10.1021/cg3004504 | Cryst. Growth Des. 2012, 12, 3631−36373637