- Email: [email protected]
A new high-pressure polymorph of NiSb2
Intermetallics 8 (2000) 1399±1403
www.elsevier.com/locate/intermet
A new high-pressure polymorph of NiSb2 Hirotsugu Takizawa *, Kyota Uheda, Tadashi Endo Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan Received 15 June 2000; received in revised form 22 June 2000; accepted 22 June 2000
Abstract A new polymorph of NiSb2 was obtained under high-pressure/temperature condition of 6 GPa and 550±650 C. The crystal structure is orthorhombic with the space group Pbca (No. 61), isostructural with the low-temperature form of NiAs2 (pararammelsbergite). The crystallographic parameters were re®ned by the Rietveld analysis of the powder X-ray diraction data (a=0.62866(2) nm, b=0.63643(2) nm, c=1.23670(3) nm, Z=8). The structure can be regarded as an intermediate structure between the marcasite-type and the pyrite-type. The pararammelsbergite-type NiSb2 is a high-pressure form of NiSb2 and the density is slightly higher than that of ambient pressure form with the marcasite-type structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Crystallography; B. Phase transformations
1. Introduction Transition metal dichalcogenides and dipnictides (TX2) with the pyrite, marcasite, and arsenopyrite-type structures (see Fig. 1) have been widely studied on their structural relationship and chemical bonding schemes. The common characteristics of these closely related compounds are formations of X±X dimer (dumb-bells) and the mixed ionic and covalent character of the chemical bonds [1,2]. The X±X bond distances in these compounds are nearly equal to the sum of the single bond tetrahedral covalent radii. Transition metal atoms are octahedrally coordinated by six chalcogen or pnicogen atoms and possess T2 , T 3+ and T 4+ valence states in dichalcogenides, chalcopnictides, and dipnictides, respectively. It is known that two types of marcasite structures, normal marcasite-type and compressed marcasite-type (loellingite-type) structures, occur in the transition metal dipnictides and dichalcogenides [1±5]. The compounds with d 2±d 4 electron con®guration of the transition metal atoms crystallize into the compressed marcasite-type
* Corresponding author. Tel.: +81-22-217-7226; fax: +81-22-2177228. E-mail address: [email protected] (H. Takizawa).
structure which is characterized by the axial ratio c/a=0.53±0.57. On the other hand, the compounds with d 6±d 9 electron con®guration crystallize into the normal marcasite-type structure in which the axial ratio, c/a, is in the range 0.73±0.75. In the binary transition metal diantimonides, CrSb2, FeSb2, CoSb2, and NiSb2 are known to crystallize into the marcasite- and/or arsenopyrite-type structures [1,2]. MnSb2, which does not exist in the equilibrium Mn±Sb phase diagram, can be synthesized under high pressure and temperature condition of 6 GPa and 650 C. MnSb2 also has the marcasite-type structure [3]. In all cases, transition metal atoms adopt formal T 4+ valence states with 3d 2 (CrSb2) to 3d 6(NiSb2) electron con®gurations. In our previous study, a high-pressure phase transition from the compressed marcasite-type to CuAl2-type was found in CrSb2 [6,7]. The phase transition accompanies a change in chemical bonding from the mixed ionic and covalent character (compressed marcasite-type) to the metallic character (CuAl2-type). It is expected that a normal marcasite-type phase, e.g. NiSb2, would transforms to dense-packed structure under high pressure because the normal marcasite structure consisting of edge-sharing octahedra has rather large volume. In the present study, a new high-pressure form of NiSb2 was successfully obtained. This paper describes the crystal structure of the new high-pressure form.
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00091-1
1400
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
Fig. 1. Arrangement of the X±X dimer and the linkage of TX6 octahedra in the pyrite, marcasite, and arsenopyrite-type structures.
2. Experimental A stoichiometric mixture of nickel (>3 N) and antimony (>3 N) powders was heated at 620 C for 168 h in an evacuated silica tube. The reaction was repeated for several times with intermediate grinding to obtain a well-homogenized NiSb2. The normal marcasite-type sample, obtained in this way, was encapsulated into a hexagonal boron nitride (h-BN) crucible and subjected to high pressure and temperature condition using belt-type high-pressure equipment. The detailed cell assemblage of the belt-type equipment is described in our earlier paper [8]. The synthesis was carried out at 6 GPa and 550±650 C for 30 min and then the sample was quenched to room temperature prior to the release of the applied pressure. The obtained sample was analyzed by X-ray diraction technique using a monochromatic CuKa radiation. The crystal structure was re®ned by Rietveld analysis of the powder X-ray diraction data using the RIETAN program [9,10]. 3. Results and discussion The direct reactions of the elemental components under high-pressure conditions resulted in formation of a new polymorph of NiSb2. The sample contained, however, considerable amounts of NiSb and residual Ni and Sb. Single phase of the new polymorph was successfully obtained by high-pressure/temperature treatment of normal marcasite-type NiSb2 at 6 GPa and
550±650 C. Above this temperature range, the phase tended to decompose into NiSb (NiAs-type) and Sb. The obtained new phase is metastable at ambient condition and transforms to the normal marcasite-type phase when annealed at atmospheric pressure. The Table 1 Powder X-ray Rietveld re®nement for NiSb2 (high-pressure form) Chemical formula Formula weight Space group a (nm) b (nm) c (nm) V (nm3) Z Dcalc (103 kg/ m3) Powder color X-ray radiation Monochromator 2 range (deg) Step width (deg) No. of data points Counting time (s/step) Temperature ( C) No. of re¯ections No. of re®ned parameters No. of structural parameters
NiSb2 302.21 Pbca (No. 61) 0.62866(2) 0.63643(2) 1.23670(3) 0.49480(2) 8 8.114 Gray CuKa Graphite 10.00±130.00 0.03 4001 4 24 850 37 15
Reliability factors Rwp Rp RI RF S
9.13% 6.94% 2.62% 1.74% 1.3577
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
1401
Fig. 2. The observed, calculated, and the dierence X-ray diraction pro®les for NiSb2 (high-pressure form). Table 2 Atom coordinates for NiSb2 (high-pressure form) Atom
Site
Occ.
x
y
z
B (10ÿ2 nm2)
Ni Sb1 Sb2
8c 8c 8c
1.0 1.0 1.0
ÿ0.0068(7) 0.1280(3) 0.3517(3)
0.1691(7) 0.0400(2) 0.3134(2)
0.3711(3) 0.1842(1) 0.4339(1)
0.25(8) 0.35(4) 0.44(4)
phase transition from the normal marcasite-type to the new polymorph is reversible and reproducible, indicating that the new phase is a high-pressure polymorph of NiSb2. The X-ray diraction data of the high-pressure form was completely indexed on an orthorhombic unit cell with the lattice parameters a=0.6287(3) nm, b= 0.6364(3) nm, and c=1.236(2) nm. The axial ratios, c/a
and c/b, are very close to those of the orthorhombic pararammelsbergite (the low temperature polymorph of NiAs2; space group Pbca) [11], indicating that the compound is isostructural with NiAs2. The structure re®nement of the high-pressure form was carried out for 850 re¯ections, assuming that the space group was Pbca (No. 61). Table 1 shows the data collection and the re®nement conditions for the Rietveld analysis. The re®nement gave a good result with reliability factors of RI=2.62% and RF=1.74%, and S value (corresponding to the goodness-of-®t) of 1.3577. The observed, calculated, and the dierence plots of the X-ray diraction data are shown in Fig. 2. The re®ned atom coordinates are given in Table 2. Figs. 3 and 4 show the crystal structure and the nearest neighbor environments of the high pressure form. The
Fig. 3. Crystal structure of NiSb2 (high-pressure form).
1402
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
Fig. 4. Local environments of nickel and antimony atoms in NiSb2 (high-pressure form).
selected interatomic distances and bond angles are listed in Table 3, together with the values in the ambient pressure form (normal marcasite-type) given in the literature [5]. Ni atom is octahedrally coordinated by six antimony atoms at the average Ni±Sb distance of 0.2569 nm. This bond distance is very close to that of the ambient pressure form in which each nickel atom is in 3d 6 low-spin state. There are two crystallographic antimony sites, Sb1 and Sb2, in the high-pressure form. Table 3 Interatomic distances (nm) and bond angles ( ) in NiSb2 Pararammelsbergite-type (high-pressure)
Marcasite-type (ambient pressure)a
Distances Ni±Sb1
Distances 4 Ni±Sb 2 Ni±Sb
0.2569 0.2537
Sb±Sb
0.2882
Ni±Sb2 Sb1±Sb2 Angles Sb1±Ni±Sb1 Sb1±Ni±Sb2
Sb2±Ni±Sb2 Ni±Sb1±Ni Ni±Sb1±Sb2 Ni±Sb2±Ni Ni±Sb2±Sb1
a
0.2533(5) 0.2573(5) 0.2596(6) 0.2554(5) 0.2572(4) 0.2584(5) 0.2860(2) 87.3(1) 87.6(1) 98.6(1) 82.4(2) 87.4(2) 90.7(2) 95.5(2) 96.1(2) 98.0(2) 83.5(1) 83.7(1) 90.3(1) 115.5(1) 115.7(1) 119.6(1) 93.7(1) 102.4(1) 104.0(1) 96.5(1) 123.8(2) 127.3(1) 98.7(1) 99.3(1) 107.1(1)
Angles 4 Sb±Ni±Sb 4 Sb±Ni±Sb 2 Sb±Ni±Sb 2 Sb±Ni±Sb
86.8 93.2 83.3 96.7
2 Ni±Sb±Ni 1 Ni±Sb±Ni
124.3 96.7
2 Ni±Sb±Sb 1 Ni±Sb±Sb
103.5 101.5
The values are taken from Kjekshus et al. [5].
Both show a similar tetrahedral coordination environment; each antimony atom has three nickel atoms and one antimony atom at bonding distance. The Sb±Sb distance is 0.2860(2) nm, nearly the same as that of the ambient pressure form. This value is comparable to the distances within Sb±Sb dimers in other transition metal diantimonides with the pyrite, marcasite, and arsenopyrite-type structures [1,5]. The coordination environments of Ni and Sb atoms in the high-pressure form are nearly the same as those in the ambient pressure form. Distortion of the NiSb6 octahedron is, however, somewhat large in the highpressure form. The essential dierence between the high pressure and the ambient pressure forms is found in the linkage of NiSb6 octahedra. In the ambient pressure form, NiSb6 octahedra are arranged by sharing two edges with adjacent octahedra parallel to c-axis (Fig. 1). In contrast, NiSb6 octahedra are linked to adjacent octahedra by sharing one edge and four corners in the high-pressure form. This leads to eective packing of NiSb6 octahedra as compared to the ambient pressure form. The pressure-induced phase transition of NiSb2, from the normal marcasite-type to the pararammelsbergite-type, accompanies 1.6% volume decrease. The structure of the high-pressure form can be regarded as an intermediate structure between the marcasite-type and the pyrite-type structures. In the pyrite-type (Fig. 1), TX6 octahedra are connected with adjacent octahedra by sharing their corners. Consequently, packing that is more eective can be achieved in the pyrite-type structure. In fact, high-pressure phase transition from the marcasite-type to the pyrite-type was reported for analogous NiAs2 [12] and other binary phases such as FeSe2, FeTe2, CoTe2, and CuSe2 [13]. This leads to an assumption that the pararammelsbergite-type NiSb2 would possibly transform to the pyrite-type structure at higher pressure. The experiments up to 8 GPa, however, did not reveal a formation of the pyrite-type NiSb2. The magnetic susceptibility measurement revealed that the high-pressure form is diamagnetic, indicating the low spin 3d 6 electron con®guration of Ni4+. Fig. 5 shows the result of electrical resistivity measurement. The high-pressure form is a good metallic conductor although one may expect a semiconducting behavior for
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
1403
however, that the transition presumably cause a change in ligand ®eld because the transition accompanies a change of the linkage of NiSb6 octahedra from edge sharing to mixed edge and corner sharing networks. From this point of view, it is worth while comparing the physical properties of the pararammelsbergite-type NiSb2 with the normal marcasite-type form and the hypothetical pyrite-type form. Acknowledgements This work has been supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science, Sports, and Culture, Japan. Fig. 5. Temperature dependence of the electrical resistivity of NiSb2 (high-pressure form).
low spin 3d 6 electron con®guration as demonstrated in isoelectronic phases [1]. The observed diamagnetism and metallic conduction suggest a possible overlapping of the valence and conduction bands in the high-pressure form. This situation is similar to that of the ambient pressure form [14]. 4. Conclusion In the present study, a new polymorph of NiSb2 with the orthorhombic pararammelsbergite-type structure was obtained by pressure-induced phase transition of the ambient-pressure form. The octahedral coordination of low-spin (3d 6) Ni4+ and the interatomic distances remain unchanged via the phase transition. It is expected,
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Brostigen G, Kjekshus A. Acta Chem Scand 1970;24:2993. Holseth H, Kjekshus A. Acta Chem Scand 1970;24:3309. Takizawa H, Shimada M, Sato Y, Endo T. Mater Lett 1993;18:11. Hulliger F, Mooser E. J Phys Chem Solids 1965;26:429. Kjekshus A, Rakke T, Andresen AF. Acta Chem Scand 1974;A28:996. Takizawa H, Uheda K, Endo T, Shimada M. Rev High Pressure Sci Technol 1998;7:1043. Takizawa H, Uheda K, Endo TJ. Alloys Compounds 1999;287:145. Takizawa H, Sato T, Endo T, Shimada MJ. Solid State Chem 1987;68:234. Izumi F. In: Young RA, editor. The Rietveld method. Oxford: Oxford University Press, 1993. Chapter 13. Kim YI, Izumi F. J Ceram Soc Jpn 1994;102:401. Fleet ME. Am Mineral 1972;57:1. Donohue PC, Bither TA, Young HS. Inorg Chem 1968;7:998. Bither TA, Prewitt CT, Gillson JL, Bierstedt PE, Flippen RB, Young HS. Solid State Commun 1966;4:533. Holseth H, Kjekshus AJ. Less-Common Metals 1968;16:472.
www.elsevier.com/locate/intermet
A new high-pressure polymorph of NiSb2 Hirotsugu Takizawa *, Kyota Uheda, Tadashi Endo Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan Received 15 June 2000; received in revised form 22 June 2000; accepted 22 June 2000
Abstract A new polymorph of NiSb2 was obtained under high-pressure/temperature condition of 6 GPa and 550±650 C. The crystal structure is orthorhombic with the space group Pbca (No. 61), isostructural with the low-temperature form of NiAs2 (pararammelsbergite). The crystallographic parameters were re®ned by the Rietveld analysis of the powder X-ray diraction data (a=0.62866(2) nm, b=0.63643(2) nm, c=1.23670(3) nm, Z=8). The structure can be regarded as an intermediate structure between the marcasite-type and the pyrite-type. The pararammelsbergite-type NiSb2 is a high-pressure form of NiSb2 and the density is slightly higher than that of ambient pressure form with the marcasite-type structure. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Crystallography; B. Phase transformations
1. Introduction Transition metal dichalcogenides and dipnictides (TX2) with the pyrite, marcasite, and arsenopyrite-type structures (see Fig. 1) have been widely studied on their structural relationship and chemical bonding schemes. The common characteristics of these closely related compounds are formations of X±X dimer (dumb-bells) and the mixed ionic and covalent character of the chemical bonds [1,2]. The X±X bond distances in these compounds are nearly equal to the sum of the single bond tetrahedral covalent radii. Transition metal atoms are octahedrally coordinated by six chalcogen or pnicogen atoms and possess T2 , T 3+ and T 4+ valence states in dichalcogenides, chalcopnictides, and dipnictides, respectively. It is known that two types of marcasite structures, normal marcasite-type and compressed marcasite-type (loellingite-type) structures, occur in the transition metal dipnictides and dichalcogenides [1±5]. The compounds with d 2±d 4 electron con®guration of the transition metal atoms crystallize into the compressed marcasite-type
* Corresponding author. Tel.: +81-22-217-7226; fax: +81-22-2177228. E-mail address: [email protected] (H. Takizawa).
structure which is characterized by the axial ratio c/a=0.53±0.57. On the other hand, the compounds with d 6±d 9 electron con®guration crystallize into the normal marcasite-type structure in which the axial ratio, c/a, is in the range 0.73±0.75. In the binary transition metal diantimonides, CrSb2, FeSb2, CoSb2, and NiSb2 are known to crystallize into the marcasite- and/or arsenopyrite-type structures [1,2]. MnSb2, which does not exist in the equilibrium Mn±Sb phase diagram, can be synthesized under high pressure and temperature condition of 6 GPa and 650 C. MnSb2 also has the marcasite-type structure [3]. In all cases, transition metal atoms adopt formal T 4+ valence states with 3d 2 (CrSb2) to 3d 6(NiSb2) electron con®gurations. In our previous study, a high-pressure phase transition from the compressed marcasite-type to CuAl2-type was found in CrSb2 [6,7]. The phase transition accompanies a change in chemical bonding from the mixed ionic and covalent character (compressed marcasite-type) to the metallic character (CuAl2-type). It is expected that a normal marcasite-type phase, e.g. NiSb2, would transforms to dense-packed structure under high pressure because the normal marcasite structure consisting of edge-sharing octahedra has rather large volume. In the present study, a new high-pressure form of NiSb2 was successfully obtained. This paper describes the crystal structure of the new high-pressure form.
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00091-1
1400
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
Fig. 1. Arrangement of the X±X dimer and the linkage of TX6 octahedra in the pyrite, marcasite, and arsenopyrite-type structures.
2. Experimental A stoichiometric mixture of nickel (>3 N) and antimony (>3 N) powders was heated at 620 C for 168 h in an evacuated silica tube. The reaction was repeated for several times with intermediate grinding to obtain a well-homogenized NiSb2. The normal marcasite-type sample, obtained in this way, was encapsulated into a hexagonal boron nitride (h-BN) crucible and subjected to high pressure and temperature condition using belt-type high-pressure equipment. The detailed cell assemblage of the belt-type equipment is described in our earlier paper [8]. The synthesis was carried out at 6 GPa and 550±650 C for 30 min and then the sample was quenched to room temperature prior to the release of the applied pressure. The obtained sample was analyzed by X-ray diraction technique using a monochromatic CuKa radiation. The crystal structure was re®ned by Rietveld analysis of the powder X-ray diraction data using the RIETAN program [9,10]. 3. Results and discussion The direct reactions of the elemental components under high-pressure conditions resulted in formation of a new polymorph of NiSb2. The sample contained, however, considerable amounts of NiSb and residual Ni and Sb. Single phase of the new polymorph was successfully obtained by high-pressure/temperature treatment of normal marcasite-type NiSb2 at 6 GPa and
550±650 C. Above this temperature range, the phase tended to decompose into NiSb (NiAs-type) and Sb. The obtained new phase is metastable at ambient condition and transforms to the normal marcasite-type phase when annealed at atmospheric pressure. The Table 1 Powder X-ray Rietveld re®nement for NiSb2 (high-pressure form) Chemical formula Formula weight Space group a (nm) b (nm) c (nm) V (nm3) Z Dcalc (103 kg/ m3) Powder color X-ray radiation Monochromator 2 range (deg) Step width (deg) No. of data points Counting time (s/step) Temperature ( C) No. of re¯ections No. of re®ned parameters No. of structural parameters
NiSb2 302.21 Pbca (No. 61) 0.62866(2) 0.63643(2) 1.23670(3) 0.49480(2) 8 8.114 Gray CuKa Graphite 10.00±130.00 0.03 4001 4 24 850 37 15
Reliability factors Rwp Rp RI RF S
9.13% 6.94% 2.62% 1.74% 1.3577
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
1401
Fig. 2. The observed, calculated, and the dierence X-ray diraction pro®les for NiSb2 (high-pressure form). Table 2 Atom coordinates for NiSb2 (high-pressure form) Atom
Site
Occ.
x
y
z
B (10ÿ2 nm2)
Ni Sb1 Sb2
8c 8c 8c
1.0 1.0 1.0
ÿ0.0068(7) 0.1280(3) 0.3517(3)
0.1691(7) 0.0400(2) 0.3134(2)
0.3711(3) 0.1842(1) 0.4339(1)
0.25(8) 0.35(4) 0.44(4)
phase transition from the normal marcasite-type to the new polymorph is reversible and reproducible, indicating that the new phase is a high-pressure polymorph of NiSb2. The X-ray diraction data of the high-pressure form was completely indexed on an orthorhombic unit cell with the lattice parameters a=0.6287(3) nm, b= 0.6364(3) nm, and c=1.236(2) nm. The axial ratios, c/a
and c/b, are very close to those of the orthorhombic pararammelsbergite (the low temperature polymorph of NiAs2; space group Pbca) [11], indicating that the compound is isostructural with NiAs2. The structure re®nement of the high-pressure form was carried out for 850 re¯ections, assuming that the space group was Pbca (No. 61). Table 1 shows the data collection and the re®nement conditions for the Rietveld analysis. The re®nement gave a good result with reliability factors of RI=2.62% and RF=1.74%, and S value (corresponding to the goodness-of-®t) of 1.3577. The observed, calculated, and the dierence plots of the X-ray diraction data are shown in Fig. 2. The re®ned atom coordinates are given in Table 2. Figs. 3 and 4 show the crystal structure and the nearest neighbor environments of the high pressure form. The
Fig. 3. Crystal structure of NiSb2 (high-pressure form).
1402
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
Fig. 4. Local environments of nickel and antimony atoms in NiSb2 (high-pressure form).
selected interatomic distances and bond angles are listed in Table 3, together with the values in the ambient pressure form (normal marcasite-type) given in the literature [5]. Ni atom is octahedrally coordinated by six antimony atoms at the average Ni±Sb distance of 0.2569 nm. This bond distance is very close to that of the ambient pressure form in which each nickel atom is in 3d 6 low-spin state. There are two crystallographic antimony sites, Sb1 and Sb2, in the high-pressure form. Table 3 Interatomic distances (nm) and bond angles ( ) in NiSb2 Pararammelsbergite-type (high-pressure)
Marcasite-type (ambient pressure)a
Distances Ni±Sb1
Distances 4 Ni±Sb 2 Ni±Sb
0.2569 0.2537
Sb±Sb
0.2882
Ni±Sb2 Sb1±Sb2 Angles Sb1±Ni±Sb1 Sb1±Ni±Sb2
Sb2±Ni±Sb2 Ni±Sb1±Ni Ni±Sb1±Sb2 Ni±Sb2±Ni Ni±Sb2±Sb1
a
0.2533(5) 0.2573(5) 0.2596(6) 0.2554(5) 0.2572(4) 0.2584(5) 0.2860(2) 87.3(1) 87.6(1) 98.6(1) 82.4(2) 87.4(2) 90.7(2) 95.5(2) 96.1(2) 98.0(2) 83.5(1) 83.7(1) 90.3(1) 115.5(1) 115.7(1) 119.6(1) 93.7(1) 102.4(1) 104.0(1) 96.5(1) 123.8(2) 127.3(1) 98.7(1) 99.3(1) 107.1(1)
Angles 4 Sb±Ni±Sb 4 Sb±Ni±Sb 2 Sb±Ni±Sb 2 Sb±Ni±Sb
86.8 93.2 83.3 96.7
2 Ni±Sb±Ni 1 Ni±Sb±Ni
124.3 96.7
2 Ni±Sb±Sb 1 Ni±Sb±Sb
103.5 101.5
The values are taken from Kjekshus et al. [5].
Both show a similar tetrahedral coordination environment; each antimony atom has three nickel atoms and one antimony atom at bonding distance. The Sb±Sb distance is 0.2860(2) nm, nearly the same as that of the ambient pressure form. This value is comparable to the distances within Sb±Sb dimers in other transition metal diantimonides with the pyrite, marcasite, and arsenopyrite-type structures [1,5]. The coordination environments of Ni and Sb atoms in the high-pressure form are nearly the same as those in the ambient pressure form. Distortion of the NiSb6 octahedron is, however, somewhat large in the highpressure form. The essential dierence between the high pressure and the ambient pressure forms is found in the linkage of NiSb6 octahedra. In the ambient pressure form, NiSb6 octahedra are arranged by sharing two edges with adjacent octahedra parallel to c-axis (Fig. 1). In contrast, NiSb6 octahedra are linked to adjacent octahedra by sharing one edge and four corners in the high-pressure form. This leads to eective packing of NiSb6 octahedra as compared to the ambient pressure form. The pressure-induced phase transition of NiSb2, from the normal marcasite-type to the pararammelsbergite-type, accompanies 1.6% volume decrease. The structure of the high-pressure form can be regarded as an intermediate structure between the marcasite-type and the pyrite-type structures. In the pyrite-type (Fig. 1), TX6 octahedra are connected with adjacent octahedra by sharing their corners. Consequently, packing that is more eective can be achieved in the pyrite-type structure. In fact, high-pressure phase transition from the marcasite-type to the pyrite-type was reported for analogous NiAs2 [12] and other binary phases such as FeSe2, FeTe2, CoTe2, and CuSe2 [13]. This leads to an assumption that the pararammelsbergite-type NiSb2 would possibly transform to the pyrite-type structure at higher pressure. The experiments up to 8 GPa, however, did not reveal a formation of the pyrite-type NiSb2. The magnetic susceptibility measurement revealed that the high-pressure form is diamagnetic, indicating the low spin 3d 6 electron con®guration of Ni4+. Fig. 5 shows the result of electrical resistivity measurement. The high-pressure form is a good metallic conductor although one may expect a semiconducting behavior for
H. Takizawa et al. / Intermetallics 8 (2000) 1399±1403
1403
however, that the transition presumably cause a change in ligand ®eld because the transition accompanies a change of the linkage of NiSb6 octahedra from edge sharing to mixed edge and corner sharing networks. From this point of view, it is worth while comparing the physical properties of the pararammelsbergite-type NiSb2 with the normal marcasite-type form and the hypothetical pyrite-type form. Acknowledgements This work has been supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science, Sports, and Culture, Japan. Fig. 5. Temperature dependence of the electrical resistivity of NiSb2 (high-pressure form).
low spin 3d 6 electron con®guration as demonstrated in isoelectronic phases [1]. The observed diamagnetism and metallic conduction suggest a possible overlapping of the valence and conduction bands in the high-pressure form. This situation is similar to that of the ambient pressure form [14]. 4. Conclusion In the present study, a new polymorph of NiSb2 with the orthorhombic pararammelsbergite-type structure was obtained by pressure-induced phase transition of the ambient-pressure form. The octahedral coordination of low-spin (3d 6) Ni4+ and the interatomic distances remain unchanged via the phase transition. It is expected,
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Brostigen G, Kjekshus A. Acta Chem Scand 1970;24:2993. Holseth H, Kjekshus A. Acta Chem Scand 1970;24:3309. Takizawa H, Shimada M, Sato Y, Endo T. Mater Lett 1993;18:11. Hulliger F, Mooser E. J Phys Chem Solids 1965;26:429. Kjekshus A, Rakke T, Andresen AF. Acta Chem Scand 1974;A28:996. Takizawa H, Uheda K, Endo T, Shimada M. Rev High Pressure Sci Technol 1998;7:1043. Takizawa H, Uheda K, Endo TJ. Alloys Compounds 1999;287:145. Takizawa H, Sato T, Endo T, Shimada MJ. Solid State Chem 1987;68:234. Izumi F. In: Young RA, editor. The Rietveld method. Oxford: Oxford University Press, 1993. Chapter 13. Kim YI, Izumi F. J Ceram Soc Jpn 1994;102:401. Fleet ME. Am Mineral 1972;57:1. Donohue PC, Bither TA, Young HS. Inorg Chem 1968;7:998. Bither TA, Prewitt CT, Gillson JL, Bierstedt PE, Flippen RB, Young HS. Solid State Commun 1966;4:533. Holseth H, Kjekshus AJ. Less-Common Metals 1968;16:472.