Probing for Structural Features of Boron-rich Solids with EELS

Probing for Structural Features of Boron-rich Solids with EELS

K. Hofmanna, R. Gruehnb†, and B. Alberta,*a Hamburg, Institut für Anorganische und Angewandte Chemie der Universitätb Gießen, Institut für Anorganische und Analytische Chemie der Justus-Liebig-Universität

Received May 11th, 2002; revised July 17th, 2002.

Dedicated to Professor Rudolf Hoppe on the Occasion of his 80th birthday

Abstract. Crystal structures of boron-rich solids are characterizedby boron atom arrangements that are quite diverse: chains, sheets,and a variety of polyhedra like octahedra, pentagonal bipyramids,cuboctahedra, and icosahedra are observed. Probing by electronenergy-loss spectroscopy (EELS), these different structural featuresare mirrored by a pronounced variation of the energy loss near-edge fine structure (ELNES) of the BK ionization edges. For identi-fication, characteristics of these fine structures can be used as so-called “coordination fingerprints”, which is shown for solids like

Bestimmung struktureller Charakteristika in borreichen Festkörpern durchElektronenenergieverlustspektroskopie

Inhaltsübersicht. Die Kristallstrukturen von borreichen Festkör-pern zeigen stark variierende Boratom-Anordnungen: Man findetKetten, Schichten und eine Vielzahl verschiedener Polyeder wie Ok-taeder, pentagonale Bipyramiden, Kuboktaeder und Ikosaeder.Mittels der Elektronenenergieverlustspektroskopie (EELS) könnendiese strukturellen Charakteristika abgebildet werden, denn dieNahkanten-Feinstruktur (ELNES) der BK-Ionisierungskanten va-riiert signifikant mit der Boratom-Anordnung. Für Verbindungen

Introduction

Parallel electron energy-loss spectroscopy in a transmissionelectron microscope provides analytical information on thinspecimens of bulk materials that is of high spatial resolution(nanometer scale) combined with especially high energy res-olution (0.5�1.5 eV, LaB6 filament). This method yields in-formation about the quality of a sample, the quantity ofcertain elements contained, the coordination sphere of spe-cial atoms, and their electronic state.

The electron beam interacts with the solid in such a waythat fast electrons transmitted show a decrease in kineticenergy. The spectrum can be divided into several sections [2]:

a) the zero loss peak, resulting from elastically and quasi-elastically scattered electrons;

* Prof. Dr. B. AlbertInstitut für Anorganische und Angewandte ChemieUniversität HamburgMartin-Luther-King-Platz 6D-20146 HamburgFAX Int� (0)40-42838-6348Email: [email protected]

Z. Anorg. Allg. Chem. 2002, 628, 2691�2696 2002 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 0044�2313/02/628/2691�2696 $ 20.00�.50/0 2691

MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20,Na2B29, UB12, ZrB12, LaB2C2, CeB2C2, and CaB2C2. In addition,theoretical calculations of ELNES based on the density functionaltheory (FLAPW method) are presented for an example of boron-rich solids.

Keywords: Boron; Electron energy-loss spectroscopy (EELS); DFT(density functional theory) calculations

wie MgB2, TaB2, ZrB2, CaB6, SrB6, BaB6, NaB5C, KB5C, Na3B20,Na2B29, UB12, ZrB12, LaB2C2, CeB2C2 und CaB2C2 wird gezeigt,daß die unterschiedlichen Feinstrukturen als "Fingerabdruck" derKoordination der Boratome und damit der Identifizierung von Ver-bindungen dienen können. Zusätzlich werden für ein Beispiel bor-reicher Festkörper theoretische Berechnungen der ELNES, die aufder Dichtefunktionaltheorie basieren (FLAPW-Methode), vorge-stellt.

b) the low loss region (0�50 eV), caused by collectiveexcitations in delocalized orbitals (plasmons);

c) the high loss region (> 50 eV), corresponding to elec-tronical excitations from localized atomic orbitals (innershell) of a specific atom to unoccupied states above theFermi level.

In this study, we are mainly interested in the high lossregion that is called ionization edge. Its position and shapecontain information about

• the character of the atom investigated, the electronicaltransitions, charge transfer, and the position of the Fermilevel (onset),

• the density of unoccupied states above the Fermi level(0�50 eV from onset, energy-loss near edge structure,ELNES),

• the neighboring environment of a specific atom(50�100 eV from onset, extended energy-loss fine struc-ture, EXELFS).

We will focus on the BK ionization edges of different bo-ron-rich solids. In principle, these BK edges look similar forall boron-containing compounds and they show up in thespectra at characteristic energy values. However, their fine

K. Hofmann, R. Gruehn, B. Albert

structure varies with the different coordination spheres ofthe boron atoms and the different electronic statuses. TheELNES of several compounds will be discussed in the con-text of different structural features of the boron atom ar-rangement.

The BK edges in EEL spectra of several compounds havebeen described in the literature. We will not cite work thatwas aimed mainly at the detection and quantitative analysisof boron. However, a short overview of earlier work con-cerning the coordination fingerprint of different boron-con-taining compounds will be given here.EELS has been performed, e.g., to study borate minerals, emphas-izing the distinction of tetrahedrally and trigonally coordinated bo-ron atoms [3, 4]. The BK edges of hexagonal and cubic boron ni-tride (again tetrahedral vs. trigonal coordination) were compared[5, 6]. Icosahedral structures in different modifications of elementalboron have been investigated, sometimes doped by metal atoms likelithium or vanadium [7, 8] or by carbon or oxygen atoms in“B13C2” or “B6O” [9]. Metal borides like BeB6 and BeB2 have beendescribed tentatively [10]. TiB2 appears to be a single example of aboride that has been investigated not only experimentally by EELSand ELNES, but also theoretically by comparing several methodsof deriving BK edges on the basis of band structure calculations ormultiple scattering theory [11].

Except for a BK edge of β-rhombohedral boron (shownfor comparison), in this publication we will show a collec-tion of BK edges that others have not measured (or at leastpublished) before.

The combination of recording experimentally the finestructure of ionization edges and comparing them withtheoretically derived edges is highly promising. Hereby, fea-tures of the observed fine structure of the energy-loss spec-trum are correlated to the electronical situation of the com-pound. Of course, the electronical situation depends on thestructural arrangement in the solid. Especially for crystal-

Fig. 1 Experimental ELNES for MgB2, TaB2, and ZrB2.

Z. Anorg. Allg. Chem. 2002, 628, 2691�26962692

line solids, methods like the APW (augmented plane wave),ASW (augmented spherical wave), FLAPW (full potentiallinearized augmented plane wave), or LMTO (linear muf-fin-tin orbital) can be employed within the density func-tional theory to calculate the unoccupied DOS of theground state on the basis of a given structural model. Inthe present study, the FLAPW method was used [12]. Toderive a theoretical ELNES from these calculations, tran-sition probabilities from the ground to the excited stateshad to be taken into account [13]. We will compare ob-served and calculated spectra for an example of boron-rich compounds.

The compounds we are going to discuss here have beencharacterized structurally before [14�31]. However, theirELNES is shown for the first time, and we suggest thatits analysis may be employed to widen the scope of boronchemistry and its methodology. Details of the crystal andelectronical structure will be elucidated by an extremely ver-satile access in the transmission electron microscope.

Results and Discussion

Diborides MgB2, TaB2, and ZrB2

These metal diborides crystallize hexagonally in spacegroup P6/mmm [14, 15], exhibiting layers of condensed bo-ron atom hexagons and metal ions between these sheets.Portions of their EEL spectra showing the BK edges withan onset at about 185 eV are given in Fig. 1. The similaritybetween the three spectra is obvious (∆AC is 8.1 (Mg), 8.6(Zr), and 7.9 (Ta) eV). The fine structure is dominated bytwo features, the 1s � π* transition (typically seen for sp2

hybridization) and the 1s � σ* transition. However, similarto what has been shown for TiB2 [11], for the tantalum andthe zirconium compound the σ* band is broad and slightlystructured (four bands A,B,C,D with ∆AB � 6.1 (Zr) and6.0 (Ta), ∆BC � 2.5 (Zr), and 2.9 (Ta), and ∆CD � 3.2(Zr) and 3.8 (Ta) eV), but forMgB2 it is much narrower.Apparently, the divalent state of the cation (the lower num-ber of electrons available) is reflected here experimentally.

Hexaborides CaB6, SrB6, and BaB6

It is well known that hexaborides consist of three-dimen-sionally interconnected boron atom octahedra, each octa-hedron possibly stabilized by two extra electrons providedby the metal atom [16�19]. For divalent cations, semicond-ucting behavior is expected owing to a complete transferof two electrons into the framework [20]. The electronicalsituation for hexaborides of metals of higher valency is dif-ferent, and these compounds exhibit electrical conductivity.Even if there is a lot of interest in compounds like Ca1-

xLaxB6, which appear to exhibit high-temperature ferro-magnetism [21], little is known about slight differences be-tween the three alkaline earth hexaborides, CaB6, SrB6, andBaB6. Measurements of the electrical conductivity haveproven to be contradictory [16, 18, 19]. To demonstrate the


Fig. 2 Experimental ELNES for CaB6, SrB6, and BaB6.

variations of the electronical situation of seemingly similarhexaborides, it is highly recommendable to analyze the BK

edges in the EEL spectra of these compounds, as shown inFig. 2. At first sight, we admire the rather characteristicshape of the edges, which definitely look different from thediboride samples and very similar to each other. Obviously,it is possible to distinguish between different polyhedral bo-ron atom arrangements of boron-rich compounds just bylooking at the EEL spectra. This is one main result in itself.In addition, at second sight, we discover slight differencesin several spectral features, for example at one of the bandsat about 199 eV. It is broader (splitting of about 2 eV) forthe barium compound than for the other two. Apparently,these characteristics hint at a slight variation in the elec-tronic structure of the hexaborides of Ca and Sr on theone hand, and Ba on the other, similarly to what we earlierconcluded from the structural analysis, theoretical calcu-lations and electrical conductivity measurements for BaB6.The exact nature of these differences will have to be investi-gated further. We expect them to be caused by geometricalprerequisites, owing to the large Ba atom “blowing up” thehexaboride structure and hereby removing the electronicaldifferentiation between the interoctahedral 2e-2c bonds andthe intraoctahedral multicenter bonds [19].

NaB5C, KB5C

The carbaborides NaB5C and KB5C were found to be thefirst ternary compounds in the systems Na/B/C and K/B/C [22, 23]. They crystallize in space group Pm3̄m like thehexaborides with an octahedral framework consisting offive boron and one carbon atom per octahedron. The car-bon atoms are statistically disordered.

Z. Anorg. Allg. Chem. 2002, 628, 2691�2696 2693

Fig. 3 Experimental ELNES for NaB5C and KB5C.

Electronically, we expect the framework to be very muchthe same as the one found in alkaline earth hexaborides.Each of the electron-deficient octahedra is stabilized by anextra-electron provided by the alkali metal atom and by anextra-electron provided by a carbon atom. This situation issimilar to a boron atom octahedron in � for example �CaB6, where both of the electrons are provided by the metalatom, and it is similar to what we know from molecularunits in closo-hydroborates following the Wade andMingos rules.

The EEL spectra (Fig. 3) confirm our expectations byexhibiting a convincing similarity of the BK edges of alkalimetal carbaborides and alkaline earth metal hexaborides.Although the B and D peaks of the hexaboride spectrashow up only in form of shoulders, the positions of the Aand C peaks in Fig. 3 are exactly the same as in Fig. 2 (190eV and 200 eV, without parallel referencing, see experimen-tal part). The ∆AC is 9.2 (Na) and 9.8 (K) eV, similar towhat is found for the hexaborides (9.2 to 10.1 eV) and dif-ferent from the value found for a diboride such as TaB2

(7.9 eV).

Na3B20

What is EELS and ELNES telling us about compoundsthat contain boron atoms in different polyhedra? The BK

edge should exhibit overlaying features of both buildingunits. For Na3B20, the pentagonal bipyramid was disco-vered to be one of the building units, the other one beingthe well-known octahedron [24, 25]. An EEL spectrum ofthis compound can be compared with that of the hexabor-ides. There is no way of comparing it with a non-molecularcompound consisting of a pentagonal bipyramidal frame-work, since such a compound does not exist. In addition,not much is known about the electronical situation ofNa3B20. The ELNES shown in Fig. 4 exhibits a certainsimilarity to that of CaB6 shown in Fig. 2 (∆AC for ex-ample is 9.5 for CaB6 and 10.1 eV for Na3B20). However,


Fig. 4 Experimental ELNES for Na3B20.

the shape of the edge is less pronounced, blurred by over-laying features of the two different polyhedra. The bandsare broader, a shoulder at about 208 eV in the CaB6 spec-trum does not appear in the Na3B20 spectrum. Apparently,the method of analyzing the fine structures of elementaledges should not be taken further. The differentiation be-tween overlaying B6 and B7 features is something that can-not be expected to be resolved unambigiously by EELS.

Na2B29, ZrB12, and UB12

In this chapter, we intend to compare two structure typesthat are different with respect to both their building unitsand their symmetry. All of the frameworks of Na2B29,ZrB12, and UB12 contain B12 polyhedra, but it is icosahedra[26] for the sodium compound (similarly to what we knowabout the modifications of elemental boron) and cube-oc-tahedra for the zirconium and the uranium compounds [27,28]. In Fig. 5, Na2B29 is compared with an EEL spectrumof β-boron (Na2B29 contains both icosahedral and inter-stitial boron atoms. The edge is expected to be less resolvedthan it would be the case for α-boron). The similarity ofthe shape of the edges is quite obvious. It is different fromthat of the dodecaborides of zirconium and uranium, whichis shown in Fig. 6. Here, the presence of boron atoms in adifferent geometric arrangement is reflected by a very sim-ple and characteristic fine structure of the BK edge. It isalso remarkable that the onset of the spectra is different fordifferent structures like that of KB5C (maximum of peak Aat 190.0 eV), Na2B29 (192 eV), and ZrB12 (195 eV).

CaB2C2, LaB2C2, and CeB2C2

We would like to end this chapter by returning to a secondgroup of layered structures, this time not containing sheetsof hexagons like in the diborides mentioned above, but 482

nets of boron and carbon atoms. The diboridedicarbides ofcalcium, lanthanum, and cerium have been described as be-

Z. Anorg. Allg. Chem. 2002, 628, 2691�26962694

Fig. 5 Experimental ELNES for Na2B29 and β-boron.

Fig. 6 Experimental ELNES for ZrB12 and UB12.

ing isotypic with respect to their B/C ordering scheme. Thishas been shown to be not true [29�31], and it has beenespecially difficult to prove the different suggestions madein the literature, since boron and carbon atoms in an ex-tended array are almost indistinguishable by X-ray or neu-tron scattering methods. However, by analyzing the finestructure of the BK edges in EEL spectra of the three com-pounds (Fig. 7), it becomes evident that the boron atomsexperience a different environment in each compound. Ob-viously, the crystal structures of CaB2C2, LaB2C2, andCeB2C2 are not isotypic. This is supported by theoreticalcalculations of the ELNES for the three compounds inthree different structural models, taking into account thedifferent electronical status due to the character of the threedifferent metal components. This will be described in moredetail elsewhere [32].


Fig. 7 Experimental ELNES for CaB2C2, LaB2C2, and CeB2C2.

Density of states, theoretical, and experimentalenergy-loss near-edge structures

The ionization edge of an element in the electron energy-loss spectrum reflects the density of states above the Fermilevel. To calculate these DOS on the basis of the densityfunctional theory leads to a description for the ground stateof an atom. The EEL spectra however result from exci-tations of electrons into higher states. Therefore, for the cal-culation of theoretical ELNES the DOS curve has to bemodified by including transition probabilities into excitedelectronic states. If we do so for the spectra of SrB6 andBaB6, we re-discover variations of the fine structure thatwere already observed experimentally (see above). For thetheoretical edges, Fig. 8 shows that in fact one of the bandsis much broader for BaB6 than it is for the strontium com-pound. It is tempting now to go back to the partial DOSfor B(p) and Ca(p), Sr(d) and Ba(d) contributions, and usethem to explain the details of the theoretical spectra. Evenif this cannot be done without being extremely careful dueto the theoretical restrictions for transferring a picturegained from ground state calculations to a situation, that isinfluenced by excited states, we found it very convincing toobserve differences in the partial DOS of the calcium andstrontium compound on the one hand and the barium com-pound on the other. For the latter, the total DOS above theFermi level is much more influenced by the metal atomscompared with the boron atom contribution [33]. These de-tails will have to be investigated further in future.

Z. Anorg. Allg. Chem. 2002, 628, 2691�2696 2695

Fig. 8 Theoretical ELNES for SrB6 and BaB6.

Conclusions

Electron energy-loss spectroscopy has proven to be a versa-tile instrument to probe for structural features in boron-containing compounds. Here, for the first time an initial“spectra library” has been presented for boron-rich com-pounds, mainly metal borides. The BK edges in these spec-tra can serve as a coordination fingerprint, since their shapehas proven to be highly dependent on the geometrical ar-rangement of polyhedral frameworks and layered struc-tures. Even the boron/carbon ordering in boridecarbidescan be established using such a sensitive probe. The exper-imental access is supported extremely well by “Full Poten-tial” calculations based on the density functional theory. Itis highly desirable to extend this kind of investigation toother compounds that contain light elements like boron, forexample borates [34�38], borophosphates [39, 40], ormaybe carbides [41, 42], especially when it appears neces-sary to supplement a picture that, owing to the intrinsiclimitations of each analytical method, would otherwise beincomplete. This may prove to be helpful even when single-crystal X-ray diffraction experiments have been performed.Slight deviations in symmetry and electronical status willbe found in the EEL spectra. Either an octahedron is a“real” octahedron, or it will show.

Experimental Part

Samples of boron-rich solids were synthesized by high-temperaturereactions in tantalum or iron tubes or in an arc-bow as describedearlier. Elemental boron was purchased from H.C. Starck(β-rhombohedral modification). Initial characterization was doneby X-ray powder diffraction and EDX analysis. For EEL spec-troscopy, a PEELS 666 instrument (Gatan) with a YAG scintil-lation detector was used at a TEM CM 30-ST (Philips) operating


at 300 kV (LaB6 filament). Dispersion at the spectrometer waschosen to be 0.1 eV per channel. The half width of the zero losspeak was smaller than 1 eV, usually 0.9 eV (resolution of the spec-tra). The intensity I of electrons transmitted (arbitrary units) wasrecorded against the energy loss ∆E (eV). Thin samples were pre-pared by suspending small particles on a lacey carbon foil on acopper grid. A background correction was performed by fitting anenergy-dependent function to an edge-free region of the spectrumand applying this function to the whole spectrum. Multiple scat-tering contributions were reduced by Fourier deconvolution. Nosample deterioration due to the electron beam was observed.The maxima of the bands have to be referenced against the zero-loss peak. This was done for over-all spectra of each compoundbefore each data collection. However, the data collection of theELNES spectra cannot be performed with sufficiently high resolu-tion if the zero-loss peak is recorded in parallel. Since we attemptedto receive spectra of the highest possible resolution, we did notreference the spectra shown in this publication. Therefore, absolutefigures for the peak maxima and shoulders seldom will be given inthe text, but differences between peak maxima are discussedwhere appropriate.It is important to note that, for a certain compound, the ELNESof these edges always look the same, no matter what sample ofthe corresponding compound or what grain within the sample waslooked at. A series of spectra using samples of different batches wasrecorded for each compound, and the variation of shapes observedwithin a sample was found to be much smaller than the differencesbetween spectra of different compounds.Calculations of the electronic structure of crystalline solids wereperformed using the program package WIEN97 [12], a full-poten-tial linearized augmented plane wave (FLAPW) method based onthe density functional theory. Density of states for the unoccupiedbands above Fermi level were calculated for the ground state andhad to be modified by using the program TELNES [13].

Acknowledgements. We would like to thank Dr. W. Mertin for valu-able help and the Deutsche Forschungsgemeinschaft and the Fondsder Chemischen Industrie for financial support.

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Probing for Structural Features of Boron-rich Solids with EELS