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A New Crystalline Phase of the Boron-Rich Metal-Boride Family: The Mg2B25 Species

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A New Crystalline Phase of the Boron-Rich Metal-Boride Family: The Mg2B25 Species
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  Solid State Sciences 8 (2006) 1202–1208www.elsevier.com/locate/ssscie A new crystalline phase of the boron-rich metal-boride family:the Mg 2 B 25  species Giovanni Giunchi a , ∗ , Luciana Malpezzi b , Norberto Masciocchi c a  Edison SpA R&D, Foro Buonaparte 31, 20121 Milano, Italy b  Dipartimento di Chimica, Materiali, Ingegneria Chimica “G. Natta”, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy c  Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy Received 26 October 2005; accepted 3 May 2006Available online 25 July 2006 Abstract A new complex magnesium boride, the Mg 2 B 25  species, has been prepared and its crystal structure determined from laboratory X-ray powderdiffraction data by the simulated annealing technique, followed by a conventional Rietveld refinement procedure. The title compound is R-centeredtrigonal (with  a = 11 . 0402 ( 3 )  Å and  c = 24 . 198 ( 1 )  Å, hexagonal setting), space group  R-3m , and is isomorphous with  β -boron and with therhombohedral compounds of the boron-rich metal-boride family. The magnesium atoms occupy interstitial sites with partial occupancies whichcan be interpreted with the aid of the analysis of “forbidden” interatomic Mg–Mg contacts. The Mg atom in the F site is vicariant with the B(4)atom and falls into a pseudometallocenic environment with the 10 nearest-neighbour B atoms; all other magnesium atoms are placed in latticecavities within corrugated slabs normal to  c , with Mg(D) and Mg(E) occupying sites already known to be populated in other metal borides, andMg(N) in a new one. These slabs present two equiprobable, but self excluding, configurations of ordered Mg atoms, which randomly stack alongthe  c -axis. Mg 2 B 25  shows a significant increase of the  c -axis of the rhombohedral lattice with respect to the other members of the family, whichwe attribute both to the larger metallic radius of Mg, vs. those of   3d   transition metal atoms and to the anisotropic stacking of the Mg atoms, and,possibly, to the presence of the rather stuffed Mg sublattices. © 2006 Elsevier Masson SAS. All rights reserved. Keywords:  Magnesium boride; Powder diffraction; Crystal structure; Disorder 1. Introduction The interest for a detailed description of the phase diagramof the Mg–B system has greatly increased with the recent dis-covery of the superconductivity of the MgB 2  [1]. Up to now theonly well characterized crystalline phases of this binary systemare the MgB 2  [2], MgB 4  [3], MgB 7  [4] and MgB 20  [5] species,even if Markowski et al. [6] had reported also the existence of the (still elusive) MgB 12  phase, for which a complete struc-ture determination is lacking. Among these crystalline phases,MgB 20  presents thepeculiaritytobea memberof thelargefam-ily of the boron-rich metal-borides, characterized by the preser-vation of the crystalline structure of the rhombohedral  β -boron,with the metal atoms either occupying lattice cavities or sub- * Corresponding author. Tel.: +39 02 62223194; fax: +39 02 62223074.  E-mail address:  giovanni.giunchi@edison.it (G. Giunchi). stituting framework boron atoms. Recently, in the same family,also the ternary compounds of general Mg x Si y B 105  formula [7]have been included.In the present work we report on the crystal structure de-termination, by laboratory X-ray powder diffraction (XRPD)methods, of a new member of the metal-boride family: theMg 2 B 25  species. This new compound was isolated during ourstudies aimed to the synthesis of MgB 2  by boron infiltrationwithgaseousorliquidMg[8].Fromthestructuralpointofview,also Mg 2 B 25  preserves the rhombohedral lattice of the  β -boronstructures, as well as that of MgB 20 , but shows further enlargedlattice constants. All boron-rich metal-borides srcinated from β -boron present interstitial holes located in well defined posi-tions, commonly labeled as A, D, E and F sites [9]; the variousmembers of this metal-boride family, however, show rather dif-ferent stoichiometries, thanks to the differential occupation of these sites by the metal atoms. In the present compound we 1293-2558/$ – see front matter  © 2006 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.solidstatesciences.2006.05.007  G. Giunchi et al. / Solid State Sciences 8 (2006) 1202–1208  1203 have found, as occupied sites, the already known D, E and Fsites as well as a new interstitial cavity (N), which is near to thepreviously identified ( but empty ) F2 site [9].As later substantiated by our XRPD analysis, all the oc-cupancies of the magnesium atoms, as well as those of theB(4) and B(13) positions, are less than unity. These refinedvalues have been rationalized on the basis of physically ac-ceptable interatomic contacts, allowing us to interpret theMg(F)/B(4) vicariancy and to discuss the observed Mg(D,E,N)disorder through the existence of two geometrically congru-ent, equiprobable,  but differently oriented  , Mg sublattices inthe crystal.The study of the electronic features and of other chemical–physical properties of this new phase is not included here; how-ever, we like to remind the reader that Mg 2 B 25  occurs as anunavoidable (contaminant) minority phase during the prepa-ration of the MgB 2  superconducting materials (derived fromcrystalline  β -boron [10]), even after treatments at temperaturesas high as 900 ◦ C; therefore, the stability domain of Mg 2 B 25 can span a wide temperature range, thus calling for a detailedrevision of the phase diagram of the binary Mg/B system. 2. Experimental and data evaluation procedure 2.1. Sample preparation By reacting crystalline  β -boron powders with Mg vapour,at temperatures lower than 760 ◦ C in a sealed iron container,we isolated a new crystalline material, Mg 2 B 25 , hereafter  1 ,even if some (unreacted) Mg metal often, but not always, re-mains as a minority phase (as detected by XRPD). In thefollowing, the purest samples in our hands ( 1a  and  1b ) arepresented: their XRPD patterns, shown in Fig. 1, show thatthe most intense reflections fall at the following angular po-sitions: 2 θ( Cu K α ) = 9 . 9 ◦ ; 19.5 ◦ ; 20.5 ◦ ; 22.0 ◦ ; 24.9 ◦ ; 38.2 ◦ ;44.0 ◦ (most intense); 49.3 ◦ ; 51.2 ◦ . From the analysis of thestructured background trace, the presence of a portion of amor-phous material was also detected, but not quantified. An ICPanalysis of the sample  1a , which presents a very low amountof metallic Mg in the XRPD spectrum, gives a first indicationof the relative amount of B and Mg, with an B / Mg (atomic) = 11.7, and excludes a significant contamination by other metals.Thus, we employed this sample in the full structural determina-tion. 2.2. X-ray powder diffraction crystal structure determination (Very hard) powders of sample  1a  were thoroughly groundin an agate mortar, then carefully deposited in the hollow of an aluminium sample holder equipped with a quartz zero-background plate (supplied by  The Gem Dugout  , Swarth-more, PA). Diffraction data were collected with graphite-monochromatized Cu K α  radiation, in the 5–145 ◦ (2 θ  ) range,on a Bruker AXS D8 Advance  θ   : θ   diffractometer,   2 θ   = 0 . 02 ◦ ,  t  = 12 sstep − 1 . Generator settings: 40 kV, 40 mA, slits:DS 1.0 mm, AS 1.0 mm, RS = 0 . 2 mm. Peak search, profile Fig. 1. Raw XRPD traces of the new boron-rich magnesium boride phase, insamples  1a  and  1b . The minor differences are here attributed to texture andgranularity effects. fitting and subsequent indexing by TOPAS [11] indicated a trig-onal cell of approximately axes:  a = 11 . 06, and  c = 24 . 25 Å,M ( 12 ) = 77 . 5, and space group (from successful structure so-lution and refinement)  R-3m .Being isomorphous with a number of stuffed boron-richmetal-borides analogous to  β -boron, the structure requiredonly the location of the magnesium atoms, in the lattice in-terstices or in substitutional positions. An initial model builtfrom the atomic coordinates and occupancies presented byBrutti et al. [5] did not allow a satisfactory match between ob-served and calculated traces. Therefore, a new model for thelocation and occupancies of the Mg atoms was sought, us-ing the simulated annealing procedure implemented in TOPAS.The choice of the final model, the coordinates of which aresupplied below, stems therefore from the careful determina-tion of a new decoration (by different Mg atoms) of the ba-sic  β -boron structure, with occupation numbers derived fromthe analysis of “forbidden” intermetallic contacts, imposingthe partial site occupancies reported below. In the final refine-ment by the Rietveld method (as implemented in the TOPASsuite of programs), anti bumping restraints for B–B distanceswere set to 1.65 Å, in order to help convergence to a mean-ingful crystallochemical model and to ensure stability in thecomplex least-squares procedure. The heavily structured back-ground level was modelled as a polynomial function; a single,averageisotropicthermal-displacementmodelforallatomswasassumed in the final refinement procedure, the resulting value  1204  G. Giunchi et al. / Solid State Sciences 8 (2006) 1202–1208 being B = 0 . 54 ( 5 )  Å 2 value. Further details of the crystal struc-ture analysis can be found in Table 1. Table 2 contains the fractional atomic coordinates and the site occupation factorsfor all Mg and B atoms. Final agreement factors  R wp ,  R p  and R Bragg  are 0.128, 0.100 and 0.053, respectively. Fig. 2 showsthe final Rietveld refinement plot. The supplementary materialhas been deposited with the Fachinformationszentrum Karl-sruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (49)7247-808-666; e-mail: crysdata@fiz.karlsruhe.de) as supple-mentarymaterialNo.415555andcanbeobtainedbycontactingthe FIZ quoting the article details and the corresponding num-ber. 3. Results and discussion The XRPD trace of the new phase was indexed by a R-cen-tred trigonal unit cell (final refined values:  a = 11 . 0402 ( 3 )  Å, c = 24 . 198 ( 1 )  Å), which is very similar to those of pure rhom-bohedral  β -boron ( a = 10 . 9253 ( 5 )  Å,  c = 23 . 8103 ( 16 )  Å [9])and of MgB 20  ( a = 10 . 9830 ( 4 )  Å,  c = 24 . 1561 ( 15 )  Å [5]), buteven further inflated. This fact indicates that the new phase, Table 1Crystal data and other crystallographic analysis details for Mg 2 B 25 Compound Mg 2 B 25 f.w., gmol − 1 318.91Crystal system trigonalSpace group R-3m a , Å 11.0402(3) c , Å 24.198(1) V  , Å 3 2554.2(2) Z  12 ρ calc , gcm − 3 2.49 F( 000 )  1788 µ( Cu K α ) , cm − 1 20.1Diffractometer Bruker AXS D8 Advance T  , K 298Scan type  θ  : θ  Scan range, ◦ 9 < 2 θ < 145 N  data  6801 N  pks  664 R wp  0.128 R p  0.100 R Bragg  0.053gof 1.568E.s.d.’s in parentheses.Table 2Fractional atomic coordinates and occupancy factors for Mg 2 B 25 Atom Position  x/a y/b z/c  s.o.f.B1 36 (i)  0 . 17052 ( 54 )  0 . 16419 ( 60 )  0 . 17121 ( 32 )  1B2 36 (i)  0 . 31082 ( 85 )  0 . 29536 ( 67 )  0 . 13093 ( 28 )  1B3 36 (i)  0 . 25661 ( 80 )  0 . 21074 ( 71 )  0 . 41965 ( 33 )  1B4 36 (i)  0 . 2429 ( 10 )  0 . 25059 ( 88 )  0 . 34562 ( 46 )  0 . 8333B5 18 (h)  0 . 05446 ( 49 )  0 . 10892 ( 97 )  0 . 94323 ( 36 )  1B6 18 (h)  0 . 08460 ( 44 )  0 . 16920 ( 88 )  0 . 01360 ( 54 )  1B7 18 (h)  0 . 10818 ( 54 )  0 . 2164 ( 11 )  0 . 88663 ( 50 )  1B8 18 (h)  0 . 17152 ( 47 )  0 . 34304 ( 94 )  0 . 02960 ( 53 )  1B9 18 (h)  0 . 13109 ( 56 )  0 . 2622 ( 11 )  0 . 76760 ( 47 )  1B10 18 (h)  0 . 10638 ( 54 )  0 . 2128 ( 11 )  0 . 70082 ( 54 )  1B11 18 (h)  0 . 05628 ( 52 )  0 . 1126 ( 10 )  0 . 32565 ( 50 )  1B12 18 (h)  0 . 08962 ( 53 )  0 . 1792 ( 11 )  0 . 40277 ( 53 )  1B13 18 (h)  0 . 05467 ( 92 )  0 . 1093 ( 18 )  0 . 55301 ( 80 )  0 . 5B14 6 (c)  0 0 0 . 3869 ( 11 )  1B15 3 (b)  0 0 0 . 5 1MgN 18 (h)  0 . 12343 ( 30 )  0 . 24686 ( 60 )  0 . 25261 ( 32 )  0 . 5MgD 18 (h)  0 . 20087 ( 31 )  0 . 40174 ( 62 )  0 . 17673 ( 32 )  0 . 5MgE 6 (c)  0 0 0 . 23533 ( 58 )  0 . 5MgF 18 (f)  0 . 3745 ( 19 )  0 0 0 . 1667E.s.d.’s in parentheses.Fig. 2. Rietveld plot for powders of   1a , with peak markers and difference plot at the bottom.  G. Giunchi et al. / Solid State Sciences 8 (2006) 1202–1208  1205Table 3Site occupancy factors for MB x  phasesAtom  β -Boron Mg 2 B 25  LiB 13  MgB 20  MnB 23  CuB 28  SiB 36  CrB 41  FeB 49 B(1) 1 . 00 1 . 00 1 . 00 1 . 00 1 . 00 1 . 00 0 . 87 1 . 00 1 . 00B(4) 1 . 00 0 . 83 1 . 00 0 . 92 1 . 00 1 . 00 1 . 00 1 . 00 1 . 00B(13) 0 . 73 0 . 50 0 . 64 0 . 59 0 . 65 0 . 61 0 . 74 0 . 72 0 . 73B(16) 0 . 25 0 . 00 0 . 10 0 . 00 0 . 00 0 . 20 0 . 00 0 . 00 0 . 00M(1) – 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00 0 . 13 0 . 00 0 . 00M(A1) – 0 . 00 0 . 00 0 . 00 0 . 26 0 . 06 0 . 46 0 . 72 0 . 51M(A2) – 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00 0 . 05 0 . 00 0 . 00M(D) – 0 . 50 1 . 00 0 . 49 0 . 43 0 . 43 0 . 00 0 . 18 0 . 19M(E) – 0 . 50 1 . 00 0 . 88 0 . 66 0 . 50 0 . 00 0 . 00 0 . 00M(F) – 0 . 17 0 . 00 0 . 08 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00M(N) – 0 . 50 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00 0 . 00M(A1) near 0.000, 0.000, 0.134; M(A2) near 0.108, 0.216, 0.100.Fig. 3. Schematic drawing of the B 2 (B 5 ) 2  fragment (a) built aroundthe [B(4)–B(4)] dumbbell (in blue) and the alternative  pseudometallocenic Mg(B 5 ) 2  moiety (b), obtained by the occupation of site F (Mg in green). MgB x , may be ascribed to the well known series of the boron-rich metal-borides [12] with a magnesium content higher thanin MgB 20 . A preliminary evaluation has given 12  < x <  13,while the structural model refinement described above (withfree s.o.f.’s for the Mg atomic positions of the purest sample), 1a , indicates  x ∼ 12 . 5. Even if there is an unavoidable uncer-tainty in the measurement of the  x  parameter, the structuralmodel derived from our XRPD trace and from the analysis of theinteratomiccontactsamongdisorderedMgandBions(thor-oughly discussed in the following section), allowed us to safelyattribute the Mg 2 B 25  composition to the new phase (vide in-fra).The refined coordinates and s.o.f.’s for the atoms reported inTable 2 can be easily interpreted in terms of a  β -boron struc-ture, where a number of boron icosahedra are either fused orlinked by  exohedral  B–B bonds (as for the B(4)–B(4) cou-ple). The reader is referred to the work by J.L. Hoard andco-workers [13] for the description of the rather complex struc-ture of   β -boron; this structure was found, through the years,capable to sustain the presence of small quantities of “dop-ing” metals, either in substitutional or interstitial mode. A listof samples of the MB x  family, possessing the basic rhombo-hedral  β -boron structure and of known interstitial site partialoccupancy, has been reported in Table 3. Thus, the title com-pound, Mg 2 B 25 , nicely fits within this group, although pos- Table 4Chemically relevant Mg–B and B–B bond distances (Å), inincreasing orderMagnesium–Boron (individual values)Mg(D)–B(13) 2 . 13 ( 2 ) Mg(D)–B(12) 2 . 26 ( 1 ) Mg(D)–B(2) 2 . 34 ( 1 )  2 × Mg(D)–B(3) 2 . 37 ( 1 ) Mg(D)–B(2 ′ ) 2 . 340 ( 6 ) Mg(D)–B(13 ′ ) 2 . 44 ( 1 )  2 × Mg(D)–B(3 ′ ) 2 . 44 ( 1 )  2 × Mg(D)–B(1) 2 . 476 ( 9 )  2 × Mg(E)–B(1) 2 . 41 ( 1 )  6 × Mg(E)–B(11) 2 . 44 ( 1 )  3 × Mg(F)–B(8) 2 . 21 ( 1 )  2 × Mg(F)–B(10) 2 . 352 ( 7 )  2 × Mg(F)–B(3) 2 . 412 ( 9 )  2 × Mg(F)–B(4) 2 . 81 ( 2 )  2 × Mg(F)–B(6) 2 . 87 ( 2 )  2 × Mg(N)–B(11) 2 . 18 ( 1 ) Mg(N)–B(12) 2 . 32 ( 2 ) Mg(N)–B(1) 2 . 34 ( 1 )  2 × Mg(N)–B(9) 2 . 485 ( 7 )  2 × Mg(N)–B(10) 2 . 485 ( 7 )  2 × Boron–boron (ranges) for each crystallographically inde-pendent boron atom; typical e.s.d’s 0.01 ÅB1 1 . 71–2 . 04B2 1 . 78–2 . 04B3 1 . 75–1 . 87B4 1 . 67–2 . 02B5 1 . 71–1 . 80B6 1 . 71–1 . 80B7 1 . 71–1 . 84B8 1 . 67–1 . 87B9 1 . 68–1 . 93B10 1 . 68–1 . 88B11 1 . 83–1 . 97B12 1 . 75–2 . 02B13 1 . 66–1 . 84B14 1 . 76–1 . 83B15 1 . 66–1 . 66E.s.d.’s in parentheses ( nx  refers to the presence of   n  equiv-alent bond distances with symmetry related atoms and theprimedatomsarereferredtodistinctsymmetryequivalents). sessing a rather high Mg percentage with most cavities filledby magnesium atoms. As in many other metal-borides, alsoin Mg 2 B 25  the B(4) atom position is only partially occupied  1206  G. Giunchi et al. / Solid State Sciences 8 (2006) 1202–1208 Fig. 4. Schematic drawing of the (slightly corrugated) magnesium atoms sublattice [Mg(D), Mg(E), Mg(N)] located at  z ∼ 0 . 25. The trace of the trigonal cell is alsodrawn with solid lines (srcin in the bottom left corner). Mg(E) in 6c (0, 0, 0.23), pink; Mg(D), blue, and Mg(N), yellow, both in 18h, (0.20, 0.40, 0.18) and (0.12,0.25, 0.25), respectively. All symmetry equivalents with 0 . 1  < z <  0 . 4 (in R-3m) are shown. Note that Mg–Mg contacts of adjacent atoms fall well below 2.5 Å,thus indicating that they lie in mutually excluding crystallographic sites (vide infra). (s.o.f. of 0.833), depending whether or not an  interstitial  Mgatom [Mg(F)], with s.o.f. of 0.167, is present. The short B(4)–Mg(F) distance (only 0.85 Å) indeed requires the presence of two mutually excluding,  alternative  cases: either B(4) caps apentagonal B 5  ring of a B 12  icosahedron (1.67(1)–2.02(1) Åaway) and is linked to its symmetry equivalent atom in “radial”direction [B(4)–B(4) 1.69(2) Å], or a Mg(F) atom sits in a pen-tagonal antiprismatic cavity defined by the very same  five  (2 × )boron atoms [Mg(F)–B bond distances in the 2.21(1)–2.87(2) Årange].The two situations are shown for simplicity in Figs. 3a and b.The shortest Mg(F) contact with the other magnesium atomsin the structure is 2.53(1) Å; taking into account the metallicradius of Mg (ca. 1.60 Å), this requires a (conditioned) disor-dering scheme or sequence in the occupancy factors of the othermagnesium atoms (as described below).Slightly more complex is the description of the coordinationenvironments of the other magnesium atoms, labelled as Mg(D)and Mg(E) (in agreement with previously reported structurerefinements and labelling), and Mg(N) (N for  new ), the latterbeing located in a cavity near 0.12, 0.24, 0.25, never found tobe populated by the dopant atoms in the other known metal-borides.In Mg 2 B 25 , Mg(N) is surrounded by 8 boron atoms withMg–B distances below 2.5 Å, and shares short (  forbidden ) con-tacts with the other magnesium atoms ( < 2 . 6 Å). Similarly,Mg(D) and Mg(E) possess several short ( < 2 . 5 Å)  bonding contacts with framework boron atoms, but also a few short( < 2 . 6 Å) Mg–Mg contacts. The relevant Mg(D,E,N)–B inter-atomic distances (as well as all the B–B bond distances ranges)arereportedinTable4andthedispositionoftheMg(D),Mg(E),Mg(N) atoms, each one having a fractional s.o.f.’s, is visualizedin Fig. 4.The model presented here is based upon the following con-siderations: Fig. 5. Schematic drawing of the two  congruent, though displaced  , Mg sublat-tices (layers A and B) which alternate with equal probability along the  c -axis. (a) The location in the cell of the Mg(D), Mg(E) and Mg(N)atoms allows to evidence (slightly corrugated) slabs, nor-mal to [001], well separated from each other, built by  false Mg–Mg contacts ( < 2 . 5 Å).(b) Within each slab, two adjacent Mg atoms  cannot   coexist.Only Mg atoms which are  not first nearest neighbours  canbe  simultaneously  present.
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