A substitution of nickel by antimony:

Journal of Alloys and Compounds 270 (1998) 136–141

L

A substitution of nickel by antimony: Crystal and electronic structure of the new antimonide Hf 6 Ni 12x Sb 21x Holger Kleinke* ¨ Marburg, D-35032 Marburg, Germany Fb Chemie, Philipps-Universitat Received 30 January 1998; received in revised form 2 March 1998

Abstract The ternary antimonides Hf 6 M 12x Sb 21x (M5Fe, Co, Ni) were prepared by arc-melting of stoichiometric mixtures of Hf, HfSb 2 and M. According to the single crystal structure analyses, performed on Hf 6 NiSb 2 and Hf 6 Ni 0.76 Sb 2.24 , Hf 6 M 12x Sb 21x crystallizes in an ordered ¯ m, variant of the Fe 2 P structure type with the M and Sb atoms occupying the two P positions on 1b and 2c of space group P62 respectively (Zr 6 CoAl 2 type). The 3d metal atoms M can partially be replaced by antimony, leading to significant, anisotropic changes in the lattice dimensions which are a5765.6(1) pm, c5362.10(7) pm, V5183.81(5)310 6 pm 3 for Hf 6 NiSb 2 , and a5760.5(1) pm, c5372.40(7) pm, V5186.53(5)310 6 pm 3 for Hf 6 Ni 0.76 Sb 2.24 as determined by single crystal data. Calculations of the electronic structure ¨ of Hf 6 NiSb 2 using the Extended Huckel approximation show strong bonding Hf–Hf, Hf–Ni, and Hf–Sb interactions.  1998 Elsevier Science S.A. Keywords: Metal rich antimonides; Crystal structure; Electronic structure; Ni / Sb mixed occupancies

1. Introduction The binary hafnium antimonides are not well investigated. To date, no single crystal structure data on hafnium antimonides is available, their structures have always been characterized by comparison with analogous zirconium antimonides. However, although Hf 3 Sb [1], HfSb in both modifications [2], and HfSb 2 [3] are apparently isostructural to the corresponding zirconium compounds [4], other zirconium antimonides, namely Zr 2 Sb [5] and Zr 5 Sb 3 [6,7], seem to have no counterpart among the hafnium antimonides. The differences between the binary metal rich zirconium and hafnium tellurides are even larger: to date, a ‘Hf 3 Te’, corresponding to Zr 3 Te [8], is unknown, and the dimetal tellurides Zr 2 Te [9] and Hf 2 Te [10] crystallize in different structure types. Only very few hafnium antimonides have been found in the ternary systems which include a late 3d metal atom, i.e. HfCoSb (LiAlSi type) [11], HfNiSb (Co 2 Si) [12], HfNi 2 Sb (YPt 2 In) [13] and Hf 5 MSb 3 (M5Ni, Cu, Zn; Ti 5 Ga 4 structure) [14]. Two more structures were found in the zirconium system, namely a W5 Si 3 variant (Zr 5 M 0.5 Sb 2.5 , M5Fe, Co, Ni) and a Fe 2 P-type structure [15]. The latter phases were identified as minor products by means of *Corresponding author. Fax: 149-6421-28-8917. 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00524-6

X-ray powder diffraction and EDX investigations. According to these EDX results, a ‘Zr 6 Fe 0.7 Sb 2.3 ’ formed in the (ordered) Fe 2 P structure type which means the iron and antimony atoms share the Co site of the Zr 6 CoAl 2 [16] structure type. The question of a phase range was not tackled. The same structure type occurs among the zirconium tellurides but not the hafnium tellurides (Zr 6 MTe 2 with M5Mn, Fe, Co, Ni, Ru, Pt), where no mixed Fe / Te occupancies were observed [17]. Here we report on the results of the investigations of the Hf 6 M 12x Sb 21x series. In contrast to a hypothetical ‘Hf 6 MTe 2 ’, the new compounds Hf 6 MSb 2 can actually be prepared with M5Fe, Co, and Ni. Furthermore, we could confirm via single crystal analysis and EDX investigations that M can partially be replaced by Sb, whereas an incorporation of M on the Sb site was not observed. The phase range of Hf 6 Ni 12x Sb 21x is thus determined to be within 0#x#0.24. Attempts to synthesize isotypic compounds with M5V, Cr, Mn, or Cu failed and resulted instead in the formation of the W5 Si 3 variant [18] as reported for Zr 5 M 0.5 Sb 2.5 (with M5Fe, Co, Ni).

2. Experimental details In order to minimize the loss of elemental antimony by

H. Kleinke / Journal of Alloys and Compounds 270 (1998) 136 – 141 Table 1 Lattice dimensions for Hf 6 M 12x Sb 21x Composition

a / pm

c / pm

V /(10 6 3pm 3 )

Hf 6 FeSb a2 ‘Hf 6 Co 1.5 Sb 1.5 ’ a,b Hf 6 CoSb a2 a Hf 6 Co 0.75 Sb 2.25 ‘Hf 6 Ni 1.25 Sb 1.75 ’ a,b Hf 6 NiSb a2 Hf 6 NiSb c2 a Hf 6 Ni 0.70 Sb 2.30 c Hf 6 Ni 0.76 Sb 2.24

767.2(2) 767.02(6) 767.5(2) 765.39(7) 765.2(1) 764.42(8) 765.6(1) 761.28(7) 760.5(1)

363.76(6) 360.67(4) 361.16(7) 364.43(4) 362.80(3) 364.1(2) 362.10(7) 369.69(6) 372.40(7)

185.42 183.76 184.24 184.89 183.97 184.25 183.81(5) 185.55 186.53(5)

a

Lattice dimensions determined from powder data, composition5starting composition. b Powder diagram also contained additional, unidentified reflections, composition of the Fe 2 P type phase thus uncertain. c Lattice dimensions and composition determined by single crystal data.

vaporization during the process of arc-melting, HfSb 2 was prepared first by annealing of stoichiometric mixtures of the elements hafnium (STREM, powder, 99.6% (including up to 2.2% zirconium)) and antimony (MERCK, powder) in a sealed silica tube at 6508C over a period of five days. The reaction was controlled by a Guinier diffractogram of the product which was identified as HfSb 2 . The reactions to the Fe 2 P structure type phases were carried out by arc-melting pressed pellets of different ratios of Hf, HfSb 2 and the 3d metal (Fe: STREM, powder, 99.999%; Co: ALFA, powder, -50 mesh, 99.8%; Ni: VENTRON, powder, -150 mesh, 99.9%) on a watercooled copper hearth under an argon flow of 3 l min 21 . The weight loss never exceeded 0.4%. The reactions of Hf, HfSb 2 and M5Fe, Co, Ni corresponding to the ratio of Hf: M: Sb56: 1: 2 all gave Hf 6 MSb 2 as single products, according to the results of the powder diagrams. The

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reactions of the nominal compositions ‘Hf 6 Co 0.75 Sb 2.25 ’ and ‘Hf 6 Ni 0.7 Sb 2.3 ’ also yielded the Fe 2 P structure in quantitative yields. Indexing of the powder diagrams based on hexagonal symmetry, using silicon as internal standard, gave different lattice constants depending on the M / Sb ratio as listed in Table 1. The phases with a smaller M / Sb ratio have a larger volume and a larger c axis, but a slightly smaller a axis, compared to the stoichiometric Hf 6 MSb 2 phases. Reactions with a smaller Ni / Sb ratio of 1 / 5 did not lead to the formation of a Hf 6 Ni 12x Sb 21x phase. Carrying out reactions of the starting compositions ‘Hf 6 Co 1.5 Sb 1.5 ’ and ‘Hf 6 Ni 1.25 Sb 1.75 ’ in an attempt to replace antimony in part by cobalt or nickel resulted neither in significant changes of the lattice dimensions nor in production of a single product. The possibility of a replacement of Sb by Co or Ni thus remains questionable. EDX investigations of the three Co containing samples (see Table 1) showed varying Co / Sb ratios between approximately 1/3 (starting composition ‘Hf 6 Co 0.75 Sb 2.25 ’) and 1 / 2 (starting compositions ‘Hf 6 Co 1 Sb 2 ’ and ‘Hf 6 Co 1.5 Sb 1.5 ’). Single crystals were selected from the samples of the nominal compositions ‘Hf 6 Ni 1.25 Sb 1.75 ’ and ‘Hf 6 Ni 0.7 Sb 2.3 ’ for X-ray single crystal structure studies in order to investigate the phase range of Hf 6 Ni 12x Sb 21x and the structural differences arising from the different Ni / Sb ratios. Starting from the Zr 6 CoAl 2 model [16], both data sets could be refined to satisfying residual values. Crystallographic details may be found in Table 2, positional parameters and equivalent displacement factors are listed in Table 3. Interatomic distances are compared in Table 4. According to the results of the single crystal data, the Ni rich sample did not show a significant incorporation of

Table 2 Crystallographic data for Hf 6 NiSb 2 and Hf 6 Ni 0.76 Sb 2.24 Empirical formula Molar mass Temperature of data collection Crystal dimensions / mm 3 Space group Unit cell dimensions

Number of formula units, F(000) Calculated density Absorption coefficient Range of 2u No. of measured reflections No. of symmetrically independent reflections No. of observed reflections (I .2s (I)) No. of parameters refined R(F ), R w (F 2 ), goodness of fit Extinction coefficient Max., min. peak in final diff. map Absorption correction

Hf 6 NiSb 2 1373.15 g mol 21 295 K 0.0530.0130.008 ¯ m (No. 189) P62 a5765.6(1) pm c5362.10(7) pm V5183.81(5)310 6 pm 3 1,562 12.41 g cm 23 93.91 mm 21 48–60.58 1881 231 (R int 50.099) 221 14 0.027, 0.061, 1.20 0.0001(5) 2.78 e 2 /(10 6 pm 3 ), 22.06 e 2 /(10 6 pm 3 ) numerical (using XSHAPE / XRED from STOE)

Hf 6 Ni 0.76( 6 ) Sb 2.24 1388.28 g mol 21 0.0630.00830.007 a5760.5(1) pm c5372.40(7) pm V5186.53(5)310 6 pm 3 1,568 12.36 g cm 23 92.80 mm 21 1378 235 (R int 50.108) 217 17 0.041, 0.093, 1.16 0.0020(9) 4.79 e 2 (10 6 pm 3 ), 23.81 e 2 (10 6 pm 3 )

H. Kleinke / Journal of Alloys and Compounds 270 (1998) 136 – 141

138

Table 3 Positional parameters and equivalent displacement factors for Hf 6 NiSb a2 and Hf 6 Ni 0.76 Sb b2.24 Atom a

Hf1 Hf1 b Hf2 a Hf2 b Ni a Ni b,c Sb a Sb b c

notation

x

y

z

Ueq /(10 4 pm 2 )

3f 3f 3g 3g 1b 1b 2c 2c

0.3995(2) 0.4027(2) 0.7582(1) 0.7567(3) 0 0 1/3 1/3

0 0 0 0 0 0 2/3 2/3

0 0

0.0070(3) 0.0123(5) 0.0056(3) 0.008(2) 0.005(1) 0.018(3) 0.0045(5) 0.0092(7)

1 ] 2

0.545(3) 0 0 1 ] 2 1 ] 2

Occupancy: 76(6) % Ni and 24% Sb.

nickel on the antimony position, whereas the Ni site is statistically occupied by 76(6)% Ni and 24% Sb in case of the Ni poor sample. The Flack parameters [19] of both data sets were calculated to be almost zero (Hf 6 NiSb 2 : x52 0.02(5); Hf 6 Ni 0.76 Sb 2.24 : x520.08(7)). In the latter case, final refinements allowing a mixed Ni / Sb occupancy on the Ni site significantly improved the conventional residual value R(F ) from 0.0431 to 0.0418. Using the Hamilton test [20], the hypothesis that the Ni site is solely occupied by nickel can be rejected at the 0.5% level. Any hypothetical superstructure reflections would have been observed, since an IPDS diffractometer (STOE) was used for the data collection. With the unit cell experimentally determined, no long-range ordering of the Ni and Sb atoms on the Ni site is possible because there is only one Ni site per unit cell. The unusually high U 33 parameter of the Hf2 atom in this structure model (about seven times higher than U 11 and U 22 ), however, indicates that Hf2 6 prism is locally elongated in direction of the c axis when filled with an Sb atom. Refinements of the Hf2 position in a split position above and below the mirror plane perpendicular to the c axis resulted in a shift of the Hf2 atom of 16(2) pm in the c direction, uniform displacement parameters, and a low-

ered R(F ) value of 0.0410. Removing the mirror plane perpendicular to [001] by reducing the symmetry to that of space group P31 m, allowing different occupancy factors and different deviations from the original position for the two Hf2 sites, resulted in the same structure model within the standard deviations. Thus, the structure model with the ¯ m is considered to be the best split position of Hf2 in P62 one. The empirical formula was thus determined to be Hf 6 Ni 0.76( 6) Sb 2.24 which corresponds within the standard deviations to the initial composition ‘Hf 6 Ni 0.7 Sb 2.3 ’. It is concluded, in good agreement with the differences in the lattice dimensions and the EDX results of the samples containing Co, that Hf 6 M 12x Sb 21x exhibits a significant phase range, with x between 0 and 0.24 in case of the Hf / Ni / Sb system.

3. Results and discussion The structure of Hf 6 NiSb 2 can be described based on the Hf tetrakaidecahedra surrounding the Ni and Sb atoms, respectively, as depicted in Fig. 1. The Hf 9 Ni tetrakaidecahedra form chains centered at the origin condensed via opposite triangular faces. These chains are interconnected via short Hf–Hf contacts of 328.9(1) pm to a three-dimensional network. This is, on the other hand, connected with the Hf 9 Sb tetrakaidecahedra by sharing common Hf atoms. Regarding the Hf polyhedra around Ni and Sb as three-capped trigonal prisms, the capping Hf atoms (Hf2) of the Hf 6 Sb prisms are situated at the corners of the Hf 6 Ni prisms and vice versa. From this point of view, the chains of trigonal Hf1 6 Sb prisms are condensed via all three edges parallel to the c axis. The occurrence of capped trigonal Hf 6 Ni prisms is a common feature in intermetallics like HfNi [21], which is also observed in the structure of some phosphides and

Table 4 c Interatomic distances and MOPs a for Hf 6 NiSb 2b and Hf 6 Ni 0.76 Sb 2.24 Bond

No.

Length b / pm

Length c / pm

MOP b

Hf1–Hf1 Hf1–Hf2 Hf1–Hf2 Hf1–Hf1 Hf1–Ni Hf1–Sb Hf2–Hf2 Hf2–Hf1 Hf2–Hf1 Hf2–Hf2 Hf2–Ni Hf2–Sb Ni–Hf1 Ni–Hf2 Sb–Hf1 Sb–Hf2

43 43 23 23 13 43 23 43 23 23 23 23 33 63 63 33

405.3(1) 322.45(9) 328.9(1) 362.10(7) 305.9(1) 295.88(5) 320.6(2) 322.5(1) 328.9(1) 362.10(7) 258.93(8) 296.52(8) 305.9(1) 258.93(8) 295.88(5) 296.52(8)

401.3(1) 316.4(6) / 335.5(6) 318.1(6) / 337.1(6) 372.40(7) 306.3(2) 297.22(6) 320.5(3) 316.4(6) / 335.5(6) 318.1(6) / 337.1(6) 372.40(7) 250.9(8) / 274.6(8) 294.3(2) 306.3(2) 250.9(8) / 274.6(8) 297.22(6) 294.3(2)

0.032 0.255 0.171 0.071 0.087 0.300 0.262 0.255 0.171 0.128 0.232 0.289 0.087 0.232 0.300 0.289

a

MOP5Mulliken overlap population. ] ] ]

Fig. 1. Projection of the structure of Hf 6 NiSb 2 along [001]. Large, white circles: Hf; medium, white: Sb; small, black: Ni. Only Hf–Hf bonds are shown.

H. Kleinke / Journal of Alloys and Compounds 270 (1998) 136 – 141

tellurides, namely HfNi x P [22], Hf 2 NiP [23], Hf 5 Ni 11x P3 [24], and Hf 8 NiTe 6 [25]. However, a striking difference between these phosphides and the antimonide Hf 6 NiSb 2 and the tellurides is that the Hf 6 Ni prisms are always capped by P atoms, but never by Sb or Te atoms. On the other hand, the Te atoms sheathe the Hf framework, producing large cavities, whereas both kind of pnictogen atoms, P as well as Sb, are found in trigonal Hf 6 prisms in these Hf rich phases. The Hf–Ni bonds within the prism (Hf2–Ni) are much shorter than the bonds between Ni and the capping Hf atoms (Hf1) (d Hf 2 – Ni 5258.93(8) pm (63), d Hf 1 – Ni 5 305.9(1) pm (33)). In contrast to this, the Hf–Sb distances do not differ that much within the Hf 9 Sb tetrakaidecahedron (d Hf 1 – Sb 5295.88(5) pm (63), d Hf 2 – Sb 5296.52(8) pm (33)). These Hf–Sb distances are slightly longer than the sum of Pauling’s single bond radii [26] (r Hf 1r Sb 5144.2 pm1139.0 pm5283.2 pm) which is also true for the smaller Hf–Ni distance (r Hf 1r Ni 5144.2 pm1115.4 pm5 259.6 pm). The latter distance is comparable to those found in hafnium nickel phosphides (Hf 5 Ni 11x P3 : 253(1) pm# d Hf – Ni #283.7(3) pm; Hf 2 NiP: 257.3(5) pm#d Hf – Ni # 292.2(7) pm). In all cases, bonding character of the Hf–Ni interactions can be assumed according to Lewis’ acid / base concept, considering the valence-electron poor Hf atom as acid and the valence-electron rich Ni atom as base. Since no single crystal data is available for hafnium antimonides, the Hf–Sb interactions are compared with Zr–Sb interactions, which should have similar lengths because of the similar metallic radii of the Zr and Hf atoms (145.4 pm vs. 144.2 pm). The Zr–Sb bond lengths vary from 284–305 pm in the structure of ZrSb [4] and from 291.44(7) pm to 296.65(6) pm in the structure of ZrNiSb [12], showing that a Hf–Sb bond length of ca. 296 pm is normal in the structure of a Hf rich antimonide. Regarding Hf as the most cationic and Sb as the most anionic component in this structure, strong Hf–Sb bonding can be expected. The different sizes of nickel and antimony occur along with different distances to the surrounding Hf atoms. As a consequence, the Hf–Hf separations within the triangular faces of the Hf 6 Ni prism (320.6(2) pm) are much shorter than those of the Hf 6 Sb prism (405.3(1) pm). The shorter Hf–Hf distance, being the shortest in the structure of Hf 6 NiSb 2 , is comparable to the short Hf–Hf bonds in the hexagonal modification of elemental hafnium (between 313 pm and 320 pm). Slightly longer Hf–Hf distances occur between the Hf atoms of the Hf 6 Sb and the Hf 6 Ni prisms (322.45(9) pm and 328.9(1) pm), which should still have bonding character. Shorter Hf–Hf bonds exist in the structures of the related tellurides Hf 5 FeTe 3 (between 305.0(3) pm and 316.4(2) pm) [25] and Hf 8 MnTe 6 (between 315.96(2) pm and 318.1(1) pm) [27] which also consist of Hf 9 M tetrakaidecahedra (M5Mn, Fe). The Hf– Hf separations parallel to the c axis of 362.10(7) pm also may have some bonding character. Apparently, the Hf atoms are not completely oxidized by the Ni and Sb atoms,

139

Table 5 Parameters used for EH calculations on Hf6NiSb2 Orbital

Hii / eV

ß1

Hf,6s Hf,6p Hf,5d Ni,4s Ni,4p Ni,3d Sb,5s Sb,5p

27.86 24.34 27.73 27.51 23.50 29.57 218.80 211.70

2.21 2.17 4.36 1.925 1.925 5.75 2.32 2.00

c1

ß2

c2

0.6967

1.709

0.5322

0.5862

2.200

0.5845

so that some 5d electrons are available for the formation of Hf–Hf bonds. Since the Hf–Hf bonds ,330 pm form a three-dimensional network, three-dimensional metallic properties are most likely. The calculations of the electronic structure of Hf 6 NiSb 2 , ¨ carried out using the Extended Huckel approximation [28– 30], confirm the expectation of a metallic character. The Sb parameters were taken from standard sources [31], and the parameters for the metal atoms Hf and Ni were obtained from alternating charge iterations on Hf 6 NiSb 2 (Table 5). The calculated DOS curves (Fig. 2) show a relatively high number of states at the Fermi level (28.246 eV), which consists mainly of Hf d states. The 3d states of Ni are located in a sharp peak well below the Fermi level (around 29.8 eV), and a second Ni peak, having basically s character, is found between 210.2 eV and 211 eV. The 5p states of Sb occur principally between 212 eV and 215 eV. The existence of Hf contributions at these regions shows covalent mixing of the Hf orbitals with the Ni and Sb states. If one assigns the electrons of the Hf–Sb and Hf–Ni interactions completely to the more electronegative element, i.e. Ni and Sb, an assignment of oxidation states leads to (Hf 11.33 ) 6 (Ni 22 )(Sb 23 ) 2 , considering all Sb orbi-

Fig. 2. Densities of states for Hf 6 NiSb 2 . Dashed, horizontal line: Fermi level. Solid line: total DOS, dashed: Hf; dash–dotted: Ni; dotted: Sb contribution.

140

H. Kleinke / Journal of Alloys and Compounds 270 (1998) 136 – 141

tals as well as the d and s orbitals of Ni as filled. Thus, 2.67 electrons remain per Hf atom for the formation of Hf–Hf bonds which sum up to six (Hf1) and eight (Hf2) bonds ,330 pm per Hf atom. The comparable oxidation states of nickel and antimony enable the partial replacement of Ni by Sb despite of the size differences. Crystal orbital overlap populations were also calculated to gain more insight into the different characters of the different interatomic interactions. The COOP curves of the Hf–Hf, Hf–Ni and Hf–Sb interactions are shown in Fig. 3. Mainly bonding states are found for all three different kinds of bonds. Since all bonding Hf–Ni and Hf–Sb states are filled, these interactions are optimized for valenceelectron numbers above 35 e 2 per formula unit. The energetically lowest lying antibonding states of these interactions occur above 21 eV, well above the energy window shown. Only Hf–Hf crystal orbitals contribute significantly to the region at the Fermi level, which become antibonding at an electron concentration of 54 e 2 per formula unit (the valence–electron concentration of Hf 6 NiSb 2 is 44 e 2). It can be concluded that, in theory, more and more bonding Hf–Hf interactions will be filled by raising the valence-electron numbers from 35 to 54, whereas the Hf–Ni and Hf–Sb interactions remain almost unchanged. This observation might explain why this structure type is found for such different compounds as Zr 6 CoAl 2 , Zr 6 MTe 2 (with M5Mn–Ru, Pt), and Hf 6 M 12x Sb 21x (M5Fe, Co, Ni). The Mulliken overlap populations (MOP) calculated for the Hf–Hf interactions show that the shorter bonds (,330 pm) are about as strong (MOPs between 0.171 and 0.262 electrons per bond) as the bonds in elemental hexagonal hafnium (MOPs between 0.249 and 0.275). Still bonding character is calculated for the Hf–Hf distances parallel to the c axis with lengths of 362.10(7) pm (MOP(Hf1–

Fig. 3. COOP curves for Hf 6 NiSb 2 . Dashed, horizontal line: Fermi level. Solid line: Hf–Hf; dashed: Hf–Ni; dotted: Hf–Sb interactions. Right part of the diagram: bonding (1), left part: antibonding interactions (2).

Hf1)50.071; MOP(Hf1–Hf1)50.128), whereas the overlap populations of the Hf–Hf separations with the triangular faces of the Hf1Sb 6 prism can probably be neglected (MOP50.032). The large differences between the Hf1–Ni and Hf2–Ni bond lengths correspond to similar differences in the bond strengths (MOP(Hf1–Ni)50.087 vs. MOP(Hf2–Ni)50.232), whereas no significant differences are found between the different Hf–Sb bonds (MOP(Hf1– Sb)50.300 vs. MOP(Hf2–Sb)50.289), in agreement with their almost identical lengths. A detailed comparison of the structures of Hf 6 NiSb 2 and Hf 6 Ni 0.76 Sb 2.24 is instructive. One expects based on the different radii of the Ni and Sb atoms (r Ni 5115.2 pm, r Sb 5139 pm), that an incorporation of Sb on the Ni site would lead to an expansion of the three-capped Hf 6 Y prism (Y5Ni site with Sb incorporation) and thus to an enlargement of all lattice dimensions. However, since the Hf1–Ni bonds which are located in the a,b plane are long enough (305.9(1) pm) for a Hf–Sb separation, the a axis doesn’t have to be expanded for an elongation of the Hf1–Y distance. On the other hand, the short Hf2–Ni bonds of Hf 6 NiSb 2 (258.93(8) pm) are too short for Hf–Sb bonds. An elongation of this bond is realized by a longer c axis (372.40(7) pm vs. 362.10(7) pm) but a smaller a axis (760.5(1) pm vs. 765.6(1) pm, with a5 b because of the hexagonal symmetry), although the Hf2–Y bond has an impact on the a axis, being directed from the origin of the unit cell to (0.2433(3) a, 0.2433(3) b, ]21 c). In addition, the occurrence of the split position of Hf2 is most likely a consequence of the different sizes of Ni and Sb: in case the Hf2 6 Y prism is centered by the smaller Ni atom, Hf2 probably fills the site closer to the center, whereas in case of an Sb centered Hf2 6 prism, Hf2 should sit on the position with the larger Hf2–Y distance. The Hf2–Y distances vary between 250.9(8) pm and 274.6(8) pm; the former, which is comparable to the shortest Hf–Ni bond (253(1) pm) in Hf 5 Ni 11x P3 , is a reasonable Hf–Ni distance, and the latter is still shorter than the sum of the Pauling radii of hafnium and antimony (283.2 pm). The averaged Hf2–Y distance is clearly longer than the Hf2–Y bond in Hf 6 NiSb 2 (262.8 pm vs. 258.93(8) pm). A small shift of the Hf2 position in the a,b plane away from the Ni / Sb site at the origin also helps in achieving the elongated Hf2–Y distance (from (0.2418(1), 0, ]12 ) in Hf 6 NiSb 2 to (0.2433(3), 0, ]12 ) in Hf 6 Ni 0.76 Sb 2.24 ). A similar shift is observed for the Hf1 atom (from (0.3995(2), 0, 0) in Hf 6 NiSb 2 to (0.4027(2), 0, 0) in Hf 6 Ni 0.76 Sb 2.24 ), so that the Hf1–Y distance is slightly longer in case of Hf 6 Ni 0.76 Sb 2.24 (306.3(2) pm vs. 305.9(1) pm) despite of the smaller a axis. The atomic positions of the Ni and Sb sites are constant relative to the unit cell, being fixed on special positions. The differences in the Hf positions and lattice dimensions equalize each other with respect to the averaged distances between the Hf atoms and the pure Sb site, which are 296.1 pm in the structure of Hf 6 NiSb 2 and 296.2 pm in

H. Kleinke / Journal of Alloys and Compounds 270 (1998) 136 – 141

Hf 6 Ni 0.76 Sb 2.24 . The split position of Hf2 occurs with two shorter (316.4(6) pm and 318.1(6) pm) and two longer Hf1–Hf2 distances (335.5(6) pm and 337.1(6) pm), compared to the structure of Hf 6 NiSb 2 with the two Hf1–Hf2 bond lengths of 322.5(1) pm and 328.9(1) pm. However, there are only minor differences in the averaged Hf–Hf bonds ,340 pm, ranging from 320.6(2) pm to 328.9(1) pm (Hf 6 NiSb 2 ) and from 320.5(3) pm to 327.6 pm (Hf 6 Ni 0.76 Sb 2.24 ). It is concluded that the smaller a axis of Hf 6 Ni 0.76 Sb 2.24 is the result of the need for constant Hf–Sb and Hf–Hf distances, whereas the differences in the Hf positions and the larger c axis are due to the larger size of antimony, compared to Ni. The fact that the Hf–Sb and Hf–Hf bond lengths are more or less independent of the Ni / Sb ratio implies that nickel and antimony have a similar electron deficiency. As a consequence, the impossibility of a substitution of antimony by nickel in the structure of Hf 6 NiSb 2 should be caused by steric effects, especially under consideration of the various compounds with different valence-electron numbers crystallizing in the Zr 6 CoAl 2 structure type. One might suggest that a (partial) replacement of Sb by Ni would occur with a loss of significant Hf–Sb bonding in exchange for only weak Hf–Ni bonds because of the averaged length of the Hf–Sb bonds of 296 pm which hampers a large overlap between a Hf and a Ni atom.

Acknowledgements I am grateful to Professor Dr. B. Harbrecht for his generous support and interest in this research. This work was financially supported by the ‘‘Deutsche Forschungsgemeinschaft’’. I am also indebted to the ‘‘Fonds der ¨ Chemischen Industrie’’ and the ‘‘Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie’’ for a Liebig-Fellowship.

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4. Conclusion The new compounds Hf 6 M 12x Sb 21x (with M5Fe, Co, Ni) have been uncovered and identified as crystallizing in an ordered variant of the Fe 2 P structure type. According to the results of calculations of the electronic structure of Hf 6 NiSb 2 , Hf 6 MSb 2 should be a metallic compound with strong Hf–Hf, Hf–M, and Hf–Sb bonds. In case of Hf 6 Ni 12x Sb 21x , the phase range has been determined to be within 0#x#0.24. Decreasing the Ni / Sb ratio occurs with a larger c axis and a larger cell volume, but a smaller a axis, compared to the stoichiometric phase Hf 6 NiSb 2 . With x.0, the Ni site is partially occupied by antimony, leading to the occurrence of a split position of Hf2 which forms a short Hf–Ni bond (258.93(8) pm) in Hf 6 NiSb 2 . As a consequence, two different Hf2–(Ni / Sb) bonds (250.9(8) pm and 274.6(8) pm) are found in the structure of Hf 6 Ni 0.76 Sb 2.24 .

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