PrRuSi2 and Nd(Rux )Si2, monoclinic variants of

Journal of Alloys and Compounds 368 (2004) 269–273
PrRuSi2 and Nd(Rux Ni1−x )Si2 , monoclinic variants
of the CeNiSi2 structure
P.S. Salamakha a,∗ , C. Rizzoli b , O.L. Sologub a,c , D. Belletti b ,
O.S. Protsyk d , A.P. Gonçalves a , M. Almeida a
a
c
Departamento de Qu`ımica, Instituto Tecnològico e Nuclear, P-2686-953 Sacavèm, Portugal
b Dipartimento di Chimica GIAF, Viale delle Scienze, 43100 Parma, Italy
Institut für Anorganische Chemie, Universität Wien, Währingerstrasse 42, A-1090 Wien, Austria
d L’viv Academy of Arts, Kubiyovych str. 30, L’viv, Ukraine
Received 12 August 2003; received in revised form 1 September 2003; accepted 1 September 2003
Abstract
The crystal structures of PrRuSi2 and Nd(Rux Ni1−x )Si2 , x = 0.75, 0.25 compounds were investigated by single crystal X-ray diffraction.
The PrRuSi2 compound belongs to the NdRuSi2 structure type, and the atomic coordinates of the Nd(Rux Ni1−x )Si2 compound are very close
to those presented for the ternary TmLi1−x Ge2 compound, which is isotypic with NdRuSi2 .
The interrelation between CaSb2 , PrRuSi2 , Nd(Rux Ni1−x )Si2 , CeNiSi2 and ZrSi2 structures is presented.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Intermetallics; Silicides; Rare earth; X-ray single crystal diffraction
1. Introduction
For the RMSi2 compounds where R is a rare earth metal
from La to Nd and M is one of 3- ,4- ,5d elements, the
CeNiSi2 type of structure was observed for Ni, Co, Rh
and Ir containing compounds [1–3] whereas for compounds
with manganese and iron the TbFeSi2 type of structure
was found [4]. The latter structure is formed as a result
of site exchange between transition metal and Si in the
BaAl4 -block of the CeNiSi2 structure. These two orthorhombic structures both with Cmcm space group are characterized as stacking variants of the BaAl4 - and AlB2 -slabs [5].
Later on, a monoclinic variant of the CeNiSi2 structure was
found for the RRuSi2 compounds (R = La, Ce [6] and
Nd [7]).
More recently, the existence of the new series of the
pseudo-ternary RM1 (M2 0.5 Si1.5 ) compounds with the
TbFeSi2 structure type was reported by Norlidah et al.
[8]. The partial replacement of Si by Cu, Pd or Pt in the
AlB2 -block does not change the initial structure for the
∗
Corresponding author.
E-mail address: [email protected] (P.S. Salamakha).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.09.001
RMnSi2 compounds while a structure transformation from
CeNiSi2 type to TbFeSi2 type was observed for RCoSi2
and RRhSi2 as a consequence of partial replacement of Si
by Cu atoms. It was also observed from the X-ray powder
diffraction and microprobe analysis of samples annealed for
20 days at 1273 K that the compounds with the composition
indicated above do not form in the R–Fe–Cu(Ni)–Si and
R–Ru–Cu(Ni)–Si systems.
The present paper contains the results of a crystal structure
investigation for the new ternary compound PrRuSi2 and
two crystals obtained from the Nd(Rux Ni1−x )Si2 specimens
using X-ray single crystal diffraction of the as cast samples.
2. Experimental details
Samples with the nominal compositions Pr25 Ru25 Si50 ,
Nd25 Ru20 Ni5 Si50 and Nd25 Ru5 Ni20 Si50 , weighing 1 g, were
prepared by arc melting on a water cooled copper hearth under argon atmosphere. The starting materials were high purity elements in the form of ingots (rare earths metals 99.9%,
transition metals 99.99%, silicon 99.999%). The melting
procedure was repeated at least three times in order to ensure
a better homogeneity, with total weight losses less than 1%.
270
P.S. Salamakha et al. / Journal of Alloys and Compounds 368 (2004) 269–273
Table 1
Parameters for the single crystals X-ray data collections and refinements
Compound
PrRuSi2
Nd(Ru0.75 Ni0.25 )0.8 Si2
Nd(Ru0.25 Ni0.75 )0.9 Si2
Diffractometer
Lattice parameters (Å)
a
b
c
β (◦ )
Cell volume (Å3 )
Calculated density (g cm−3 )
Linear absorption coefficient (mm−1 )
2θ max
h k l range
Enraf-Nonius CAD-4
Bruker SMART CCD
Philips PW1100
4.471 (2)
4.056 (2)
8.272 (5)
102.28 (4)
146.58 (2)
6.756
22.05
59.99
−6 ≤ h ≤ 6, −5 ≤ k ≤ 5,
−11 ≤ l ≤ 11
2578
476
332
26
0.0531, 0.1137
1.130
2.84/−3.86
4.151 (2)
4.057 (2)
8.514 (4)
104.07 (4)
139.08 (2)
6.514
23.61
59.66
−5 ≤ h ≤ 5, −5 ≤ k ≤ 5,
−7 ≤ l ≤ 9
994
308
297
28
0.0552, 0.1172
1.211
3.34/−4.23
4.122 (2)
4.058 (2)
8.501 (5)
103.78 (2)
138.10 (2)
6.447
24.77
69.98
0 ≤ h ≤ 6, −6 ≤ k ≤ 6,
−13 ≤ l ≤ 13
1209
673
472
28
0.0730, 0.1240
1.064
4.72/−5.21
Number of measured reflections
Number of unique reflections
Number of reflections with F0 > 4σ(F0 )
Number of refined parameters
R, wR2
Goodness of fit
Highest/lowest residual electron density(e/Å3 )
Table 2
Atomic coordinates and thermal parameters for the PrRuSi2 compound
Atom
Wyckoff
position
x
Pr
Ru
Si1
Si2
2
2
2
2
0.4122
0.1179
0.0336
0.6697
(e)
(e)
(e)
(e)
(3)
(4)
(18)
(17)
y
z
1/4
1/4
1/4
1/4
0.7988
0.3877
0.0899
0.4926
(2)
(3)
(11)
(10)
U11 × 102 (Å2 )
U22 × 102 (Å2 )
U33 × 102 (Å2 )
U13 × 102 (Å2 )
Ueq × 102 (Å2 )
0.52 (8)
0.49 (11)
0.7 (3)
0.3 (3)
0.38 (8)
0.36 (9)
0.4 (3)
0.7 (3)
1.52 (9)
1.22 (12)
1.7 (4)
1.4 (4)
0.40 (5)
0.41 (8)
0.8 (3)
0.7 (3)
0.78
0.66
0.85
0.78
(6)
(6)
(14)
(15)
U12 = U23 = 0.
Diffracted X-ray intensities were collected from the powdered samples on a Rigaku powder diffractometer (Bragg
Brentano goniometer, graphite crystal monochromator, normal focus Cu tube operated at 40 kV and 30 mA). The data
were recorded with a 2θ step size of 0.02◦ in a 2θ range of
20.00–100.00◦ and a counting time of 1 s at each step. Phase
identification and lattice parameters refinement were accomplished using the Powder Cell and WinPLOTR programs
[9,10].
Single crystals suitable for the X-ray measurements were
isolated from the surface of the alloys, glued on the top of a
glass fibre and mounted on the goniometer head. X-ray single crystal diffraction data for PrRuSi2 crystal were obtained
using a four circle diffractometer Enraf-Nonius CAD-4
with graphite monochromatized Mo K␣ radiation (λ =
0.71073 Å). In case of the Nd(Ru0.25 Ni0.75 )0.9 Si2 crystal,
a four circle diffractometer Philips PW1100 with graphite
monochromatized MoK␣ radiation (λ = 0.71073 Å) was
employed.
The least square refinements of the 2θ values of 25
strong and well centered reflections from the various regions of reciprocal space in the range 15◦ ≤ 2␪ ≤ 30◦
were used to obtain the unit-cell parameters. Data for
the Nd(Ru0.75 Ni0.25 )0.8 Si2 crystal were collected with a
Bruker SMART CCD diffractometer using Mo K␣ radiation
(λ = 0.71073 Å). A total of 1300 frames were collected
with a ϕ of 0.3◦ and an exposition time of 40 s. Cell parameters of the Nd(Ru0.75 Ni0.25 )0.8 Si2 were obtained from
Table 3
Atomic coordinates and thermal parameters for the Nd(Ru0.75 Ni0.25 )0.8 Si2 and Nd(Ru0.25 Ni0.75 )0.9 Si2 crystals
Nd(Ru0.75 Ni0.25 )0.8 Si2
Nd(Ru0.25 Ni0.75 )0.9 Si2
Atom
Wyckoff
position
x
y
z
Ueq × 10, Å2
Atom
Wyckoff
position
x
y
z
Nd
M∗
Si1
Si2
2
2
2
2
0.3926 (8)
0.196 (2)
0.047 (4)
0.756 (4)
1/4
1/4
1/4
1/4
0.7873 (6)
0.3688 (18)
0.082 (3)
0.502 (3)
0.66
0.98
0.83
0.79
Nd
M∗∗
Si1
Si2
2
2
2
2
0.3938 (7)
0.1825 (16)
0.042 (3)
0.748 (3)
1/4
1/4
1/4
1/4
0.7875
0.3649
0.0823
0.5018
(e)
(e)
(e)
(e)
M∗ = 0.75Ru + 0.25Ni. M∗∗ = 0.25Ru + 0.75Ni.
(4)
(5)
(15)
(17)
(e)
(e)
(e)
(e)
Ueq × 10, Å2
(2)
(8)
(16)
(14)
0.79
1.26
0.85
0.91
(8)
(15)
(20)
(19)
P.S. Salamakha et al. / Journal of Alloys and Compounds 368 (2004) 269–273
the least-square analysis of the setting angles of 310 spots
using the SMART package [11]. Cell refinements and data
reductions were performed with the SMART and SAINT
packages [11].
The data sets were recorded at room temperature and
the intensities of the crystals were corrected for absorption (using psi scan for PrRuSi2 , and with assistance
of the programs ABSORB [12] and SADABS [11] for
Nd(Ru0.25 Ni0.75 )0.9 Si2 and Nd(Ru0.75 Ni0.25 )0.8 Si2 respectively), polarization and Lorentz effect. Further details
of data collections and structure refinements are listed in
Table 1.
3. Results
The phase analyses of the X-ray powder patterns revealed
that the PrRuSi2 sample was almost single phase and small
amounts of secondary phases (5% vol.) were observed in the
neodymium containing samples. Automatic indexing of the
powder diffraction data using the TREOR [13] and DICVOL
Table 4
Selected interatomic distances (up to ∼3.5 Å)
PrRuSi2
Nd(Ru0.75 Ni0.25 )0.8 Si2
Nd(Ru0.25 Ni0.75 )0.9 Si2
Atom
d (Å)
Atom
d (Å)
Atom
d (Å)
Pr–Si2
Pr–2Si2
Pr–2Si1
Pr–2Si1
Pr–Si1
Pr–2Ru
Pr–Si1
Pr–Ru
Pr–2Ru
2.997
3.112
3.117
3.183
3.225
3.249
3.267
3.375
3.505
(8)
(7)
(6)
(7)
(9)
(3)
(10)
(4)
(3)
Nd–2Si1
Nd–2Si1
Nd–Si2
Nd–Si2
Nd–2M
Nd–Si2
Nd–M
Nd–Si1
Nd–Si1
Nd–2M
3.084 (14)
3.111 (13)
3.12 (2)
3.134 (18)
3.146 (8)
3.16 (2)
3.206 (9)
3.18 (2)
3.22 (2)
3.457 (17)
Nd–2Si1
Nd–2Si1
Nd–2Si2
Nd–Si2
Nd–2M
Nd–Si2
Nd–M
Nd–Si1
Nd–Si1
Nd–2M
3.086
3.086
3.125
3.126
3.145
3.150
3.157
3.181
3.200
3.493
(9)
(10)
(9)
(11)
(5)
(11)
(5)
(13)
(13)
(8)
Ru–Si2
Ru–2Si2
Ru–Si1
Ru–Si2
Ru–2Ru
Ru–2Pr
Ru–Pr
Ru–2Pr
2.347
2.365
2.411
2.435
3.084
3.249
3.375
3.505
(8)
(5)
(10)
(8)
(4)
(3)
(4)
(3)
M–2Si2
M–Si2
M–Si1
M–Si2
M–2Nd
M–2Nd
M–Nd
2.293 (14)
2.325 (19)
2.37 (3)
2.372 (18)
3.146 (8)
3.206 (9)
3.457 (17)
M–2Si2
M–Si1
M–Si2
M–Si2
M–2Nd
M–2Nd
M–Nd
2.316
2.328
2.343
2.354
3.145
3.157
3.493
(6)
(15)
(12)
(12)
(5)
(5)
(8)
Si1 –Ru
Si1 –2Si1
Si1 –2Pr
Si1 –2Pr
Si1 –Pr
Si1 –Pr
2.411
2.496
3.117
3.183
3.225
3.267
(10)
(10)
(6)
(7)
(9)
(10)
Si1 –M
Si1 –2Si1
Si1 –2Nd
Si1 –2Nd
Si1 –Nd
Si1 –Nd
2.37 (3)
2.44 (3)
3.084 (14)
3.111 (13)
3.18 (2)
3.22 (2)
Si1 –M
–2Si1
–2Nd
–2Nd
–Nd
–Nd
2.328
2.442
3.086
3.086
3.181
3.200
(15)
(15)
(9)
(10)
(13)
(13)
Si2 –Ru
Si2 –2Ru
Si2 –Ru
Si2 –2Si2
Si2 –Pr
Si2 –2Pr
2.347
2.365
2.435
2.551
2.997
3.112
(8)
(5)
(8)
(9)
(8)
(7)
Si2 –2M
Si2 –M
Si2 –M
Si2 –2Si2
Si2 –2Si2
Si2 –Nd
Si2 –2Nd
Si2 –Nd
2.293 (14)
2.325 (19)
2.372 (18)
2.890 (10)
2.915 (10)
3.12 (2)
3.134 (18)
3.157 (20)
Si2 –2M
Si2 –M
Si2 –M
Si2 –2Si2
Si2 –2Si2
Si2 –2Nd
Si2 –Nd
Si2 –Nd
2.316
2.343
2.354
2.880
2.905
3.125
3.126
3.150
(6)
(12)
(12)
(10)
(10)
(9)
(11)
(11)
271
[14] programs suggested monoclinic unit cells with close
lattice parameters for all three samples.
For the analyses of the X-ray single crystal data and for
the structure refinement, the WinGX1.64 program package
[15] was used. The atomic parameters of the NdRuSi2 structure type (space group P21 /m [16]) were taken as starting
positions for all crystals. The structures were refined by
a full-matrix least squares program using atomic scattering factors provided by the program package SHELXL-97
[17]. The weighting schemes included a term, which accounted for the counting statistics, and the parameter correcting for isotropic secondary extinction was optimized.
The anisotropic displacement parameters for all atoms were
refined. The final residuals are presented in Table 1.
The standardized atomic coordinates [18] and interatomic
distances for the PrRuSi2 compound are given in Tables 2
and 4 respectively. There is no evidence of vacancies or
mixed occupation on any of the sites.
During the structure refinement of the Nd(Ru0.75 Ni0.25 )0.8
Si2 and Nd(Ru0.25 Ni0.75 )0.9 Si2 crystals, it was determined
that the occupation factors for atomic positions which are
filled by a statistical mixture of transition metals are 80
and 90% respectively. The final structural data for the
Nd(Ru0.75 Ni0.25 )0.8 Si2 and Nd(Ru0.25 Ni0.75 )0.9 Si2 crystals
Fig. 1. Projections of the structures of PrRuSi2 and Nd(Rux Ni1−x )Si2
along [0 1 0] and NdNiSi2 along [0 0 1].
272
P.S. Salamakha et al. / Journal of Alloys and Compounds 368 (2004) 269–273
are given in Table 1, the atomic coordinates and interatomic
distances are presented in Tables 3 and 4.
4. Discussion
The PrRuSi2 compound belongs to the NdRuSi2 structure
type, and the atomic coordinates of the Nd(Rux Ni1−x )Si2
compound are very close to those presented for the ternary
TmLi1−x Ge2 compound [19], which is isotypic with
NdRuSi2 . Similarly to the TmLi1−x Ge2 compound, partial
vacancies on the transitions metal site and, with the assistance of the program PLATON, the pseudo-orthorhom-
bic symmetry were observed for the Nd(Rux Ni1−x )Si2
compound.
Projections of the PrRuSi2 and Nd(Rux Ni1−x )Si2 structures along [010] and NdNiSi2 along [001] are shown
in Fig. 1. As one can see, the distortion of the NdNiSi2
(CeNiSi2 structure type) are more pronounced in the structure of PrRuSi2 than in the Nd(Rux Ni1−x )Si2 one. The
interatomic distances between the zigzag linked atoms of
the transition metals are 3.084 and 4.069 Å for PrRuSi2 and
3.657 and 3.660 Å for Nd(Rux Ni1−x )Si2 as compared with
dNi–Ni = 3.649 Å for the NdNiSi2 compound.
The PrRuSi2 and Nd(Rux Ni1−x )Si2 structures, two monoclinic variants of the CeNiSi2 structure, are the members
Fig. 2. Interconnections of the tetragonal antiprisms and trigonal prisms in CaSb2 , PrRuSi2 , Nd(Rux Ni1−x )Si2 , CeNiSi2 and ZrSi2 structures.
P.S. Salamakha et al. / Journal of Alloys and Compounds 368 (2004) 269–273
of a structural series of compounds which is characterized
by intergrowth of BaAl4 and AlB2 slabs.
On the other hand, these structures are filled-up derivatives of the binary CaSb2 structure which itself is a stacking
variant of distorted unfilled tetragonal anti-prisms and filled
trigonal prisms. The CeNiSi2 structure is a filled-up derivative of the binary ZrSi2 structure which is a stacking variant
of unfilled tetragonal anti-prisms and filled trigonal prisms.
Interconnections between the tetragonal anti-prisms and
trigonal prisms in the CaSb2 , PrRuSi2 , Nd(Rux Ni1−x )Si2 ,
CeNiSi2 and ZrSi2 structures are shown in Fig. 2.
We wish to mention that an X-ray powder diffraction and
microprobe analysis would be worthwhile to perform on annealed samples in order to determine the concentration limits
of the existence of the Nd(Rux Ni1−x )Si2 homogeneity range.
Acknowledgements
The FCT Grant for the research work of P.S. at the Institute of Nuclear Technology, Sacavèm, Portugal is highly
appreciated. O.S. is grateful to the Austrian FWF for a Lise
Meitner Grant (Project No. M635) and a NATO Fellowship
in Portugal.
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