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Crystal Structure of Novel NiÀZn Borides: First Observation of a BoronÀMetal Nested Cage Unit: B 20 Ni 6

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Crystal Structure of Novel NiÀZn Borides: First Observation of a BoronÀMetal Nested Cage Unit: B 20 Ni 6
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  Published:  July 19, 2011 r 2011 American Chemical Society  7669  dx.doi.org/10.1021/ic2007167 | Inorg. Chem.  2011, 50, 7669 – 7675 ARTICLEpubs.acs.org/IC Crystal Structure of Novel Ni  Zn Borides: First Observation of aBoron  Metal Nested Cage Unit: B 20 Ni 6 Zahida P. Malik, † Oksana Sologub, †  Andriy Grytsiv, † Gerald Giester, ‡ and Peter F. Rogl*  , † † Institute of Physical Chemistry, University of Vienna, W  € ahringerstrasse 42, A-1090 Vienna, Austria ‡ Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria b S  Supporting Information 1. INTRODUCTION The system Ce  Ni  Zn  Bispart ofthe multinaryMg-basedalloy system Mg  Zn  Mn(Ni)  RE used as high strength light- weight alloys for automotive applications (RE stands for a rareearthelement). 1,2  Althoughlittleisyetknownonthein fl uenceof nickel   boron additions in these multinary alloys, phase diagraminformation in terms of an isothermal section at 800   C and aliquidus projection has been provided for the Ni-rich corner of the Ni  Zn  B subsystem by Stadelmaier et al. 3 The  fi ndings of Stadelmaier et al. 3  5  were used in a review of the Ni  Zn  Bsystem by Bhan et al. 6 From the four ternary compoundsidenti fi ed, 3,4 only the crystal structure of the so-called  τ   phaseof the Cr 23 C 6  type (Ni 19.5 Zn 3.5 B 6 ) has been de fi ned from X-ray powder and single crystal rotation photographs. 5 Metal boridesare known to form a large variety of compounds, which arecharacterized by a large diversity of boron to boron bonds,reaching from isolated boron atoms in metal-rich compositionstoboronclustersin “ high ” -boroncompounds.Onthebasisoftheclassi fi cation of borides with respect to boron   boron aggrega-tionasafunctionoftheborontometalratio,B/M,boronclustersand frameworks have commonly been observed only at ratiosB/M > 4 and comprise a quite large variety of regular and/ordistorted boron clusters, cages, and frameworks such as (i) B 5 -pentagonalpyramidsinMgB 4  , 7 (ii)B 4 -squareunitsconnectedby  weak B  B bonds to a channel like framework in CrB 48 andMnB 49 hosting the metal atoms, (iii) B2 units connecting B 6 octahedrainSm 2 B 510 (Gd 2 B 511 -type)borides,(iv)corner-connected boron octahedra B 6  in hexaborides forming a cage acceptinglarge metal atoms, 12 (v) corner-linked B 12  icosahedra throughcarbon atoms in Mg 2 B 24 C, 13 (vi) interconnected B 12  cuboocta-hedra forming a B 24  cage centered by metal atoms indodecaborides, 14,15 (vii) eight supericosahedra [B 12 (B 12 ) 12 ]per unit cell of the hectoborides MB 6616,17 forming a framework that accepts metal atoms as well as additional borons. (viii)Icosahedra bonded via intericosahedral boron bonds create aseries of structures such as MgB 7  , Mg ∼ 5 B 44  , 18 and MgAlB 14 . 19 (ix) Four icosahedra of boron atoms are linked by B  B bondsand carbon bridges in AlB 24 C 4 . 20  A linear C  B  C chain (B atthe center of symmetry) links B 12  icosahedra in Al 2.1 B 51 C 8 . 21,22 The present work intends to elucidate the hitherto unknowncrystal structures of the ternary Ni  Zn  B compounds. 2. EXPERIMENTAL SECTION Samples at a total amount of ca. 1 g each were prepared from Ni foil(AlfaAesar,purity>99.8mass%),zincgranules(AlfaAesar,purity>99.9mass %), and boron pieces (ChemPur, Karlsruhe, purity 98 mass %).Zinc drops were puri fi ed in an evacuated quartz tube by heating them50   C below the boiling temperature of Zn (907   C). Samples wereprepared from intimate blends of powders of arc melted NiB x  master Received:  April 7, 2011  ABSTRACT:  The crystal structures of three ternary Ni  Zn borides have beenelucidated by means of X-ray single-crystal di ff  raction (XSC) and X-ray powderdi ff  raction techniques (XPD) in combination with electron microprobe analyses(EMPA) de fi ning the Ni/Zn ratio. Ni 21 Zn 2 B 24  crystallizes in a unique structure type(space group  I  4/ mmm ;  a  = 0.72103(1) nm and  c  = 1.42842(5) nm;  R  F2 = 0.017), which contains characteristic isolated cages of B 20  units composed of two corrugatedoctogonal boron rings, which are linked at four positions via boron atoms. The B 20 units appear to have eight-membered rings on all six faces like the facesofa cube. Eachface is centered by a nickel atom. The six nickel atoms are arranged in the form of anoctahedron nested within the B 20  unit. Such a boron aggregation is unique and hasnever been encountered before in metal   boron chemistry. The crystal structure of Ni 12 ZnB 8  x  ( x  = 0.43; space group  Cmca  ,  a  = 1.05270(2) nm,  b  = 1.45236(3) nm,  c  =1.45537(3) nm;  R  F2 = 0.028) adopts the structure type of Ni 12  AlB 8  with  fi nite zigzagchains of   fi  ve boron atoms. The compound Ni 3 ZnB 2  crystallizes in a unique structure type (space group  C  2  /m  ,  a  = 0.95101(4) nm, b  = 0.28921(4) nm,  c  = 0.84366(3) nm,  β  = 101.097(3)   , and  R  F2 = 0.020) characterized by B 4  zigzag chain fragments with B  B bond lengths of 0.183  0.185 nm. The Ni 3 ZnB 2  structure is related to the Dy  3 Ni 2  type.  7670  dx.doi.org/10.1021/ic2007167 | Inorg. Chem.  2011, 50,  7669–7675 Inorganic Chemistry ARTICLE alloysand fi neZn fi lingsinpropercompositionalratios.Theblendswerecompacted at room temperature in a steel die without lubricants at apressureof800MPaandweresubsequentlysealedinquartztubesunder vacuum conditions. Samples were heated to 1150   C, kept at thistemperaturefor10min,cooledto800  Catarateof1  C/min,annealedat 800   C for 7 days, and subsequently quenched in water. X-raypowderdi ff  ractiondatawerecollectedfromeachalloyinas-castand annealed states employing a Guinier  Huber image plate system with monochromatic Cu K  R 1 radiation (8   < 2 θ  < 100  ). QuantitativeRietveld re fi nements of the X-ray powder di ff  raction data were per-formed with the FULLPROF program. 23 Single crystals were mechanically isolated from crushed alloys.Inspections onanAXS-GADDStexturegoniometerassuredhighcrystalquality, unit cell dimensions, and Laue symmetry of the specimens priorto the X-ray intensity data collections on a four-circle Nonius Kappadi ff  ractometer equipped with a CCD area detector employing graphitemonochromated Mo K  R  radiation (  λ  = 0.071069 nm). Orientationmatrices and unit cell parameters were derived using the programDENZO. 24 No absorption corrections were performed because of therather regular crystal shapes and small dimensions of the investigatedspecimens. The structures were solved by direct methods and werere fi ned with the SHELXL-97 program 25,26  within the Windows version WINGX. 27 The as-cast and annealed samples were polished using standardprocedures, and microstructures and compositions were examined by light optical microscopy (LOM) and scanning electron microscopy (SEM) via electron probe micro-analyses (EPMA) on a Zeiss Supra 55 VP equipped with an EDX system operated at 20 kV. For Ni/Zn ratios,the binary compound Ni 2 Zn 11  at the Zn-rich boundary (15 at.% Ni 28 ) was used as an EPMA standard. The di ff  erences between measured andnominal compositions were found to be less than  ( 1 atom %. 3. RESULTS AND DISCUSSION 3.1. Structural Chemistry.  3.1.1. Crystal Structure of Ni  21  Zn 2 B 24 —A Novel Structure Type with a Metal Nested Cage,B 20  Ni  6 .  A single crystal suitable for X-ray structure determination was selected from the alloy Ni 30 Zn 40 B 30 (inatom%) annealed at800   C for 7 days. The observed extinctions are consistent withthe body centered tetragonal space groups  I  4,  I  4,  I  4/ m  ,  I  422,  I  4 mm  ,  I  42 m  , and  I  4/ mmm . Structure solution was possible inspace group  I  42 m ; however, the search for missing symmetry prompted the space group  I  4  /mmm . Structure refinement in  I  4  / mmm  resulted in seven fully occupied metal atom positions andthree boron positions, yielding a structure formula Ni 21 Zn 2 B 24 . With anisotropic atomic displacement parameters (ADPs) forthe metal atoms and isotropic temperature factors for the boronsites, the final refinement converged to  R  F2 = 0.017 and residualelectron densities smaller than  ( 1.62 e  /Å  3 . Structure data aresummarized in Table 1, and the crystal structure is shown inFigures1and2.TheratioNi/Znobtainedfromtherefinementis91.22:8.77, in good agreement with the ratio 91.0:9.0 derivedfromEPMAonthebulksample.RietveldrefinementoftheX-ray powder spectrum confirmed the structure model.The crystal structure of Ni 21 Zn 2 B 24 — shown in Figure 1a inthree-dimensional view along [001] — is characterized by B 20 units (Figure 1a,c,d) made of two eight-membered corrugated boron rings consisting of B1 and B2 atoms. Two such rings arelinked at the four B2 positions via a B3 atom forming aB2  B3  B2 clamp and thereby building a cage around an empty Ni octahedron, [Ni 6 ] (Figure 1d), which is nested within the B 20 unit. The cage as well as the Ni octahedron is centered at site 2b(1/2,1/2,0). These cages appear to have eight-membered boron Table 1. Structural Data for Ni 21 Zn 2 B 24 alloy composition(atom %) a Ni 30 Zn 40 B 30 Ni/Zn atomic ratio,EPMA/re fi nement91:9/91.2:8.8formula from re fi nement Ni 21 Zn 2 B 24 structure type Ni 21 Zn 2 B 24 space group  I  4  /mmm ; No. 139 b θ  range; sets; frames;time/frame2.85 <  θ  < 36.27; 7; 403; 200 s a  [nm] 0.72103(1) c  [nm] 1.42842(5) volume (nm 3 ) 74.261Z 2data collection/  λ  (nm) Mo K  R  radiation/0.071069cryst size [  μ m] 20  30  45mosaicity 0.59re fl ns in re fi nement 493  F  o  > 4 σ  ( F  o ) of 561number of variables 34 R  F2 =  ∑ | F  o2  F  c2 |/ ∑  F  o2 0.017 R  Int  0.064GOF 0.685extinction (Zachariasen) 0.00070(5)Ni1 in 16 n  (0,  y  ,  z ); occ.  y  = 0.30020(5),  z  = 0.10062(3);1.00(1) U  11 ; c U  22 ;  U  33 ; U  23 ;  U  13  =  U  12 = 00.0042(2); 0.0059(2);0.0063(2);  0.0010(1)Ni2 in 8  j  ( x  , 1/2, 0); occ.  x  = 0.24032(8); 1.00(1) U  11 ;  U  22 ;  U  33 ; U  23  =  U  13  =  U  12  = 00.0068(2); 0.0042(2); 0.0049(2)Ni3 in 8  f   (1/4, 1/4, 1/4); occ. 1.00(1) U  11  =  U  22 ;  U  33 ;  U  23  =  U  13 ;  U  12  0.0113(2); 0.0053(2);  0.0003(1); 0.0051(2)Ni4 in 4 d   (0, 1/2, 1/4); occ. 1.00(1) U  11  =  U  22 ;  U  33 ; U  23  =  U  13  =  U  12  = 00.0048(2); 0.0043(3)Ni5 in 2 a  (0, 0, 0); occ. 1.00(1) U  11  =  U  22 ;  U  33 ; U  23  =  U  13  =  U  12  = 00.0034(2); 0.007(4)Ni6 in 4 e  (0, 0,  z ); occ.  z  = 0.36216(5); 1.00(1) U  11  =  U  22 ;  U  33 ; U  23  =  U  13  =  U  12  = 00.0043(2); 0.0069(3)Zn1 in 4 e  (0, 0,  z ); occ.  z  = 0.19029(5); 1.00(1) U  11  =  U  22 ;  U  33 ; U  23  =  U  13  =  U  12  = 00.0090(2); 0.0073(3)B1 in 16 n  (0,  y  ,  z ); occ.;  U  iso d   y  = 0.2984(4);  z  = 0.3453(2)1.00(1); 0.0061(5)B2 in 16 m  ( x  ,  x  ,  z ); occ.;  U  iso  x  = 0.2929(3),  z  = 0.1059(2)1.00(1); 0.0052(5)B3 in 16 l  ( x  ,  x  , 0); occ.;  U  iso  x  = 0.2025(4); 1.00(1);0.0052(6)residual electron density; max;min in [electrons/nm 3 ]  10001.62;  1.02 a Nominalcompositionofthealloyfromwhichasinglecrystalwasisolated. b Crystal structure data are standardized using the program StructureTidy. 36  c  Anisotropic atomic displacement parameters  U  ij  in [10 2 nm 2 ]. d  Isotropic atomic displacement parameters  U  iso  in [10 2 nm 2 ].  7671  dx.doi.org/10.1021/ic2007167 | Inorg. Chem.  2011, 50,  7669–7675 Inorganic Chemistry ARTICLE rings onallsixsideslikethe faces ofacube.Thecages arestackedalong [100] directly on top of each other. The boron   borondistances within the ring as well as the B2  B3 distances are within 0.177 nm of the sum of the covalent radii of two boronatoms ( R  B  = 0.088 nm). The distances between the center of thecage and the surrounding Ni atoms are  d  2b  Ni2  = 0.187 nm and d  2b  Ni6  = 0.196 nm and are too short for metal  metal bonds.It may be noted here that stabilities of hyper-coordinatedd-block metal atoms centered in planar boron rings MB n  ( n  = 7,8, 9, and 10) have recently been explored by density-functiontheory(DFT)computations. 29  WhereastheB 20 unitsare fl anked by squares of Ni1 atoms parallel to (100) and (010) (seeFigure 1b), a square net formed by Ni3 and Ni4 atoms at adistance of 0.255 nm is attached to B1 (Ni4) and to B1 and B2(Ni3). The unit cell contains two blocks of [Ni 6 ] nested B 20 units,whicharelinkedviaB2  B3bondstoahexa-cappedsquareprism around Ni5. These blocks are connected by shared in fi niteplanar nets formed of intercrossed   Ni4  Ni3  Ni4   chains(Figure 1b). Along the  z direction, the structure canbe viewed ascomposed of Ni-atom chimneys, which accommodate the col-umns formed by alternating [Ni 6 B 20 ] and Ni5[Ni 8 Zn 2 B 4 ] units(Figure 1c). The atomic environment for every atom site isdepicted in Figure 2. Boron atoms are in tricapped trigonalprismatic coordination (CN = 9), in which we encounter sevenmetal atoms (B1 and B3) or six nearest metal neighbors (B2).Due to the smaller a ffi nity of Zn to B with respect to Ni  B, thecoordination  fi gures around the boron atoms include only Niatoms. The range of metal   boron distances from 0.199 to 0.228is typical for nickel borides. 30 Metal atoms have coordinationnumbers ranging from 10 to 16, which in some cases form irregu-larly shaped coordination  fi gures. For instance, the coordinationpolyhedron around the Ni2 atom is a slightly distorted Archi-median antiprism of metal atoms with a zigzag chain of eight boron atoms around its waist. Similarly, Zn is at the center of a bicapped Archimedian antiprism of metal atoms. Ni3 residesinside a hexa-capped square prism revealing distorted rhombicand triangular faces. The atomic environment of Ni4 is adistorted cuboctahedron. Ni5 is at the center of a square prismof eight Niatoms, with the six faces capped by four B and two Znatoms. Ni6 atoms reside in a basket formed by a square Ni atom baseand azigzagringofeight boronatomscapped by aZn atom.It should be emphasized that the B 20  units are a unique boron aggregation that has never been encountered before inmetal   boron chemistry. 31 In view of boride classi fi cation, the crystal structure of Ni 21 Zn 2 B 24  ,althoughitsboron/metalratioisonlyslightlyhigherthan 1, is a new example of the combination of a boron cage witha nested empty metal octahedron forming a B 20 Ni 6  unit linkingthe electron de fi cient boron atoms in the cage to the metalframework of the crystal structure. It is somewhat surprising thatthe nested metal octahedron is formed by Ni atoms, whichthemselvesneedto fi lltheirdshell.Inorder tofullyelucidate thisparticularly interesting bonding situation, a current investigationfocuses on the physicochemical properties of Ni 21 Zn 2 B 24  incombination with a DFT calculation of the electronic structure. 3.1.2. Crystal Structure of Ni  12  ZnB 8   x   (x = 0.43) with Ni  12  AlB 8 Structure Type.  A single crystal was selected from a crushed as-cast sample, Ni 58.34 Zn 4.86 B 36.79  (in atom %). Systematic extinc-tions for a C-centered orthorhombic unit cell ( a  =1.05270(2) nm,  b  = 1.45236(3) nm, and  c  = 1.45537(3) nm)resulted in two possible space group types,  C  2 cb  (standardsetting  Aba2 ) and  Cmca . Structure solution with direct methods was successful in centrosymmetric  Cmca  , the space group of highest symmetry. Refinement — employing anisotropic atomicdisplacement parameters (ADPs) for the metal atoms andisotropic temperature factors for the boron atoms — convergedto R  F2 =0.028withresidualelectrondensitieslessthan ( 2.0e  /Å  3 . With Zn and Ni atoms in 13 independent and fully occupiedpositionsandninesitesforboronatoms,ofwhichthesitesforB8and B9 were occupied at 70% and 65%, respectively, therefinement yielded the composition Ni 12 ZnB 8  x  ( x  = 0.43).The ratio Ni/Zn = 92.3:7.7 is in accordance with the value of 91.6:8.4foundbyEPMA.Unitcellparameters,crystalsymmetry,and atom distribution reveal isotypism with the structure type Figure 1.  Crystal structure of Ni 21 Zn 2 B 24 . (a) Perspective view along[100] with anisotropic displacement parameters (for metal atoms) fromsingle crystal re fi nement. (b) In fi nite Ni layer formed of   Ni3  Ni4  Ni3 atoms parallel to (001). (c) Ni 21 Zn 2 B 24  structure asan arrangement of (i) B 20  units nesting empty octahedra [Ni 6 ] and (ii)Ni5[Ni 8 Zn 2 B 4 ] polyhedra (for better visualization, only the slab within1/4 <  z  <3/4 is shown). (d) Enlarged view of B 20  units in combination with empty octahedra [Ni 6 ]. (e) Ni 4  squares around the B 20  unit. Figure 2.  Coordination polyhedra of atoms in Ni 21 Zn 2 B 24 .  7672  dx.doi.org/10.1021/ic2007167 | Inorg. Chem.  2011, 50,  7669–7675 Inorganic Chemistry ARTICLE Table 2. Structural Data for Ni 12 ZnB 8  x  ( x  = 0.43) alloy composition (atom %) a Ni 58.34 Zn 4.86 B 36.79 Ni/Zn atomic ratio, EPMA/re fi nement 91.6:8.4/92.3:7.7formula from re fi nement Ni 12 ZnB 8  x  ( x  = 0.43)structure type Ni 12  AlB 8 space group  Cmca ; No. 64  b θ  range; sets; frames; time/frame 2.77 <  θ  < 34.99; 7; 516; 200 s/frame a ;  b ;  c  [nm] 1.05270(2); 1.45236(3); 1.45537(3) volume (nm 3 ) 222.511(2) Z   1data collection/  λ  (nm) Mo K  R  radiation/0.071069cryst size [  μ m] 25  30  40mosaicity 0.48re fl ns in re fi nement 1823 F  o  > 4 σ  ( F  o ) of 2554number of variables 129 R  F2 =  ∑ | F  o2  F  c2 |/ ∑  F  o2 0.028 R  Int  0.060GOF 1.037extinction (Zachariasen) 0.00017(1)Ni1 in 8  f   (0,  y  ,  z ); occ.  y  = 0.42251(4),  z  = 0.08096(4); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13  =  U  12  = 0 0.0083(3); 0.0102(3); 0.0092(3);  0.0022(2)Ni2 in 16  g   ( x  ,  y  ,  z ); occ.  x  = 0.37254(5),  y  = 0.06824(3),  z  = 0.06440(3); 1.00(1) U  11 ; c U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0112(2); 0.0096(2); 0.0104(2); 0.0008(2); 0.0004(2); 0.0005(1)Ni3 in 8  f   (0,  y  ,  z ); occ.  y  = 0.13668(4),  z  = 0.12413(4); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13  =  U  12  = 0 0.0092(3); 0.0081(3); 0.0074(3); 0.0007(2)Ni4 in 16 g ( x  ,  y  ,  z ); occ.  x  = 0.13373(4),  y  = 0.42087(3),  z  = 0.22086(3); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0087(2); 0.0099(2); 0.0075(2);  0.0006(2);  0.0003(2); 0.0014(2)Ni5 in 16 g ( x  ,  y  ,  z ); occ.  x  = 0.16955(4),  y  = 0.00959(3),  z  = 0.13227(3); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0096(2); 0.0080(2); 0.0088(2); 0.0008(2); 0.0014(2); 0.0003(2)Ni6 in 8  f   (0,  y  ,  z ); occ.  y  = 0.38296(4),  z  = 0.36934(4); 1.00(1) U  11 ;  U  22  =  U  33 ;  U  23 ;  U  13  =  U  12  = 0 0.0089(3); 0.0078(3);  0.0002(2)Ni7 in 16  g   ( x  ,  y  ,  z ); occ.  x  = 0.16960(4),  y  = 0.13245(3),  z  = 0.00178(3); 1.00(1) U  11  =  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0096(2); 0.0088(2); 0.0017(2); 0.0005(2); 0.0015(2)Ni8 in 16 g ( x  ,  y  ,  z ); occ.  x  = 0.13588(4),  y  = 0.28315(3),  z  = 0.08587(3); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0089(2); 0.0079(2); 0.0106(2);  0.00003(15);  0.0011(2);  0.0002(2)Ni9 in 16  g   ( x  ,  y  ,  z ); occ.  x  = 0.36810(4),  y  = 0.23615(3),  z  = 0.10344(3); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0108(2); 0.0073(2); 0.0088(2); 0.0005(2); 0.0019(2); 0.0008(2)Ni10 in 16  g   ( x  ,  y  ,  z ); occ.  x  = 0.13440(4),  y  = 0.11128(3),  z  = 0.26854(3); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13 ;  U  12  0.0120(2); 0.0093(2); 0.0081(2);  0.0004(2); 0.0005(2);  0.0016(2)Ni11 in 8 e  (1/4,  y  , 1/4); occ.  y  = 0.26391(4); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  13 ;  U  23  =  U  12  = 0 0.0090(3); 0.0075(3); 0.0078(3); 0.00058(2)Zn1 in 8  f   (0,  y  ,  z ); occ.  y  = 0 . 26794(4),  z  = 0.23828(4); 1.00(1) U  11 ;  U  22 ;  U  33 ;  U  23 ;  U  13  =  U  12  = 0 0.0098(3); 0.0111(3); 0.0117(3);  0.0025(2)Zn2 in 4 a  (0, 0, 0); occ. 1.00(1) U  11 ;  U  22  =  U  33 ;  U  23 ;  U  13  =  U  12  = 0 0.0097(4); 0.0096(4); 0.0011(3)B1 in 8  f   (0,  y  ,  z ); occ.;  U  iso d   y  = 0.0249(4),  z  = 0.3280(4); 1.00(1); 0.012(1)B2 in 8  f   (0,  y  ,  z ); occ.;  U  iso  y  = 0.0113(4),  z  = 0.2053(4); 1.00(1); 0.0082(10)B3 in 16  g   ( x  ,  y  ,  z ); occ.;  U  iso  x  = 0.1906(4),  y  = 0.4256(3),  z  = 0.0792(3); 1.00(1); 0.0093(7)B4 in 8  f   (0,  y  ,  z ); occ.;  U  iso  y  = 0.1727(4),  z  = 0.4824(4); 1.00(1); 0.011(1)B5 in 8  f   (0,  y  ,  z ); occ.;  U  iso  y  = 0.2958(4),  z  = 0.4969(4); 1.00(1); 0.010(1)B6 in 16  g   ( x  ,  y  ,  z ); occ.;  U  iso  x  = 0.2987(4),  y  = 0.3624(3),  z  = 0.1514(3); 1.00(1); 0.0092(7)B7 in 16  g   ( x  ,  y  ,  z ); occ.;  U  iso  x  = 0.2115(4),  y  = 0.1546(3),  z  = 0.1426(3); 1.00(1); 0.0121(8)B8 in 8 d   ( x  , 1/2, 0); occ.;  U  iso  x  = 0.2722(7); 0.70(2); 0.007(2)B9 in 8  f   (0,  y  ,  z ); occ.;  U  iso  y  = 0.1428(6),  z  = 0.3615(5); 0.65(2); 0.008(2)residual electron density; max; min in [electrons/nm 3 ]  1000 2.01;  1.49 a Nominal composition of the alloyfrom whicha single crystal wasisolated.  b Crystal structure data are standardized using theprogram Structure Tidy. 36 c  Anisotropic atomic displacement parameters  U  ij  in [10 2 nm 2 ].  d  Isotropic atomic displacement parameters  U  iso  in [10 2 nm 2 ].  7673  dx.doi.org/10.1021/ic2007167 | Inorg. Chem.  2011, 50,  7669–7675 Inorganic Chemistry ARTICLE Ni 12  AlB 8 . 32 Crystallographic data including occupancy and ther-mal parameters for individual atomic positions are summarized inTable 2. Figure 3 portrays the crystal structure of Ni 12 ZnB 8  x  inthree-dimensional view along [100]. As described earlier, 32 the structure type of Ni 12  AlB 8  ischaracterized by a stacking of two layer units alternating along[100]. Both layers are characterized by   fi  ve-membered boronzigzag chains running along [011] and [01  1]. Whereas the boron zigzag orientation is in the  bc  plane for the layer at  x  = 0,the zigzag orientation for the layer  x  = 0.5 changes to the plane a  b √  2. Besides the B  B chains, isolated boron atoms exist inthe layer at  x  = 0.5.B  B distances in the chain (B 6  B 3  B 8  B 3  B 6 ) parallel tothe  bc  plane are uniformly 0.180 nm, but B  B distances in thechain (B 5  B 4  B 9  B 1  B 2 )parallel to the a  b √  2planerangefrom 0.178 to 0.181 nm, all close to the sum of covalent boronradii. The site for B9 is occupied by 65% only and is further bonded to 6Ni + 1Zn + 2B. Whereas Ni1 centers a cube of eightNi atoms capped by four boron atoms, Ni2 has a waist of boronatoms B3  B8  B3 and B1  B9  B4 (both from the middle partof two di ff  erent  fi  ve-membered zigzag chains); above and below are squares of metal atoms, one of which is capped by a B7 atom(isolated boron atom in the unit cell). Ni3, Ni6, and Ni11 are atthe centers of a rather distorted cube with Ni atoms at eightcorners, and all six faces are capped (by 2Zn + 4B atoms).Polyhedra of Ni4, Ni5, Ni7, Ni8, Ni9, and Ni10 form triangularandsquarefaceswith fi  veboronand10metalatoms.Ni11has10metal and four B atoms in its polyhedron. Zn1 and Zn2 reside incuboctahedra with additional boron atoms. For Zn1, one squareis capped by one B atom, while for Zn2, two boron atoms capsquares (see Figure 4a). For boron coordination, see Figure 4b. While B1, B3, B4, B8, and B9 form tricapped trigonal prisms bondingto two boron atoms at atotal coordination number of 9,atomsB2,B5,B6,andB7areinsidesquareantiprisms,whereonesquarefaceiscappedbyaboronatomforB2,B5,and B6butbyametal atom in the case of B7. Isolated boron atoms (B7) intetrakaidecahedral metal coordination in combination with boronchain fragments are consistent with the low boron to metal ratioin the structure (B/M = 8:13) as a typical feature seen in low- boron structure types. 3 3.1.3. Crystal Structure of Ni  3  ZnB 2 —A Novel Structure Type. Single crystals of Ni 3 ZnB 2  were grown from an alloy of composi-tion Ni 23 Zn 67 B 10  (in atom %) melted at 1150   C, slowly cooledat 0.5   C/min to 970   C, and water quenched. The excess of Zn was dissolved in dilute HCl. The observed extinctions areconsistent with the C-centered monoclinic space groups  C  2  /m  , C  2, and  Cm . Structure refinement in  C  2  /m  resulted in four fully occupied metal atom positions and two boronpositions, yieldinga new and unique structure type, Ni 3 ZnB 2 . With anisotropicatomicdisplacementparameters(ADPs)forthemetalatomsandisotropic temperature factors for the boron atoms, the finalrefinement converged to  R  F2 = 0.020 and residual electron Figure4.  (a) Coordination polyhedra aroundNiand ZnatomsinNi 12 ZnB 8  x  ( x  = 0.43).(b)Coordination polyhedra around Batomsin Ni 12 ZnB 8  x ( x  = 0.43). Figure 3.  Unit cell of Ni 12 ZnB 8  x  ( x  = 0.43), showing (a) two types of isolated  fi  ve-membered boron zigzag chains, (b) a B6  B3  B8  B3  B6chain with a B  B equidistance of 0.180 nm, and (c) a B5  B4  B9  B1  B2 chain with di ff  erent B  B distances.
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