Sc-Sc Bonding in the New Ternary Phosphide ScNiP

Article abstract of DOI:10.1006/jssc.1997.7704

The new phosphide ScNip can be synthesized by arc-melting of ScP and Ni, or by arc-melting of Sc with NiP. The lattice constants, as obtained from the bulk sample, area=6.3343(8) A,b=3.7375(7) A,c=7.0917(8), andV=167.89(4) A³. ScNiP crystallizes in the Co₂Si structure type. Although one might assign the trivalent state to Sc, corresponding to a formal ionic formula of Sc³⁺Ni^±0P^3-, the structure of ScNiP contains Sc-Sc bonds and shows weak metallic properties, as expected based on extended Hueckel calculations.

Full text of DOI:10.1006/jssc.1997.7704

JOURNAL OF SOLID STATE CHEMISTRY 137, 218—222 (1998)
ARTICLE NO. SC977704
Sc–Sc Bonding in the New Ternary Phosphide ScNiP
Holger Kleinke and Hugo F. Franzen
Ames Laboratory—U.S. Department of Energy, Iowa State University, Ames, Iowa 50011
Received June 11, 1997; accepted November 14, 1997
EXPERIMENTAL
The new phosphide ScNip can be synthesized by arc-melting of
Synthesis
Because of the high vapor pressure of elemental phos-
phorus the reactants did not include phosphorus in its
elemental form, but consisted of the phosphides and elemen-
tal metals. All phosphides used, i.e., ScP, FeP, CoP, and
NiP, were prepared from mixtures of the elements in the
ScP and Ni, or by arc-melting of Sc with NiP. The lattice
constants, as obtained from the bulk sample, are a ؍
6.3343(8) A, b ؍ 3.7375(7) A, c ؍ 7.0917(8), and V ؍ 167.89(4) Aꢀ.
ScNiP crystallizes in the Co Si structure type. Although one
ꢁ
might assign the trivalent state to Sc, corresponding to a formal
3؉
؎0
3
\
ionic formula of Sc Ni P , the structure of ScNiP contains
Sc—Sc bonds and shows weak metallic properties, as expected
based on extended Huckel calculations. ( 1998 Academic Press
Key Words: Scandium nickel phosphide; preparation; struc-
ture and bonding; pseudo gap; magnetism.
°
stoichiometric ratio of 1 : 1 in sealed silica tubes at 800 C in
a resistance furnace. The starting elements were obtained
from Alfa or Fisher (Fe: Alfa, powder, !22 mesh, 99.999%;
Co: Alfa, powder, !50#150 mesh, 99.9%; Ni: Fisher,
powder, purified; P: Alfa, powder, !100 mesh, red amorph-
ous, 99%).
ScCoP and ScNiP were synthesized by arc-melting of
pressed pellets of ScP and Co and Ni, respectively, under
a dynamic Argon flow. Analogous melting of ScP with Fe or
Sc with FeP both yielded ScP and Fe as main products. No
reflections of a ‘‘ScFeP,’’ isotypic to ScCoP and ScNiP, were
found in the experimentally obtained powder diagrams.
In contrast, the powder diagrams of ScCoP and ScNiP
could be indexed based on orthorhombic symmetry, using
the Guinier technique with silicon as internal standard and
CuKa radiation. The ideal lattice constants are listed in
Table 1. Our results for the cell dimensions of ScCoP are in
agreement with those given by Jeitschko et al. (6). The
volume of the unit cell of ScNiP is significantly greater than
that of ScCoP, despite the smaller Pauling radius of Ni
(r "1.154 A(r "1.162 A (7)) .
INTRODUCTION
Within the past two years, 12 new metal-rich ternary
phosphides have been uncovered using the early transition
metal atoms M"Zr, Hf, and the iron-group metal atoms
Mꢀ"Fe, Co, and Ni. A common feature in the structures of
the Zr or Hf rich phosphides is that the Mꢀ and P atoms are
both situated in (capped) trigonal prisms formed by the
M atoms, which build up a three-dimensional network. This
is true for the structures of Zr Ni P (1), M MꢀP (2, 3),
ꢂ
ꢁ ꢃ
ꢁ
Hf Co
P
(4), and Hf Ni
P (5), and in all these
ꢄ
ꢅ>V ꢀ\V
ꢄ
ꢅ>V ꢀ
structures, strong bonding character is observed for the
M—P, Mꢀ—P, M—M, and M—Mꢀ interactions. The two differ-
ent kinds of metal—metal bonds, M—M and M—Mꢀ interac-
tions, contribute significantly to the stability of these
refractory materials.
,*
!ꢀ
In order to obtain single crystals suitable for a single
crystal structure investigation, the arc-melted sample con-
taining ScNiP was annealed in an induction furnace under
No comparable compounds are known so far for the
group 3 elements. In general, the question is, are the metal
atoms Sc, Y, and La, despite the lower electron counts
compared to Zr and Hf, and in the case of Sc the relatively
small 3d orbitals, capable of forming similar pnictides which
contain M—M bonds. The highest Sc content (33.3 at.%) has
been found in ScCoP, which was prepared using a tin flux.
As determined by powder diffraction, ScCoP occurs in the
\ꢆ
a dynamic vacuum of +2 ; 10 mbar at a maximum
°
temperature of 1350 C over a period of 5 h.
Structure Determination
A needle-like single crystal from the annealed ScNiP
Co Si structure type (6). Here we report on the isostructural sample was selected for the structure determination. A four-
ꢁ
ScNiP. We discuss the bonding and physical properties as circle diffractometer with a rotating anode was used for
well as reasons for the nonexistence of a ‘‘ScFeP.’’
the data collection (RIGAKU AFC6R, MoKa). The cell
218
0022-4596/98 $25.00
Copyright ( 1998 by Academic Press
All rights of reproduction in any form reserved.

Sc—Sc BONDING IN ScNiP
219
TABLE 1
TABLE 3
Atomic Positions and Temperature Factors [A2] for ScNiPa
ꢀ
Compound
a/A
b/A
c/A
»/A
Atom
x
z
B
/A@
U
/Aꢁ
U
/Aꢁ
ꢁꢁ
U
/Aꢁ
U
/Aꢁ
%ꢃ
ꢅꢅ
ꢀꢀ
ꢅꢀ
?
ScCoP
6.2706(8)
6.268(2)
6.3343(8)
3.7479(6)
3.750(1)
3.7375(7)
7.054(1)
7.050(3)
7.0917(8)
165.79(4)
165.7
167.89(4)
Sc
Ni
P
0.0221(4) 0.6853(3) 0.4(1)
0.1423(3) 0.0619(2) 0.51(7) 0.009(1) 0.0049(9) 0.0057(8)
0.2653(5) 0.3813(4) 0.3(1) 0.004(1) 0.003(2) 0.005(1)
0.005(1) 0.003(1) 0.005(1) !0.000(1)
@
ScCoP
ScNiP
0.0016(7)
0.001(1)
?
This work.
Ref. (6).
?All atoms on Wyckoff position 4c, y"1/4, U "U "0.
ꢅꢁ
ꢁꢀ
@
@B "(8nꢁ/3)+ + U a*a*a a .
%ꢃ
G
H
GH G
H
G H
Magnetic Properties
constants were refined using the setting angles of 19 careful-
°
°
ly centered reflections in the range of 16.89 (2h(21.95 .
The data were corrected for Lorentz and polarization ef-
fects. Two octants were measured and later merged. An
empirical absorption correction (( scan) was applied.
The systematic extinctions (k#l"2n#1 for all 0kl and
h"2n#1 for all hk0 reflections) restricted the possible
Magnetic data were obtained for the annealed bulk
sample of ScNiP (37.3 mg). Temperature dependent meas-
urements were made at 3 Tesla in the temperature range of
6—297 K on a Quantum Design MPMS SQUID mag-
netometer. The data were corrected for the diamagnetic
atom cores.
space groups to Pnma or Pna2 . The structure model, as
obtained from the direct methods (SHELXS (8)), could be
refined in the higher symmetry (Pnma). Reducing the sym-
ꢅ
RESULTS AND DISCUSSION
metry by removing the mirror plane (PnmaPPna2 ) nei-
ꢅ
ScNiP is isotypic to TiMꢀP (Mꢀ"Fe, Co, Ni) (10, 11),
ZrMꢀP (Mꢀ"Fe, Co) (10, 12), and HfMꢀP (Mꢀ"Fe, Co, Ni),
ther improved the residual factors nor resulted in any
significant changes of the structure model. All structure
refinements were performed using the TEXSAN program
whereas ZrNiP occurs in the Ni In structure type (13).
ꢁ
The structure of ScNiP can be described based on the
ꢁ
package (9). Refinements against F with anisotropic
trigonal prisms around the P atoms, which are interconnec-
ted via common edges to form puckered sheets parallel to
[100], as emphasized via thick lines in Fig. 1. The Sc atoms
form four corners and one cap, and the Co atoms form two
corners and two caps of each trigonal prism. The sheets of
prisms are directly connected to form a three-dimensional
network, because the caps of the prisms of one sheet form
the corners of the prisms of the neighboring sheet.
temperature factors for all sites converged quickly to
ꢁ
ꢁ
R(F )"0.061,R (F )"0.064, goodness of fit"1.09
U
(R(F)"0.036). More crystallographic details are listed in
Table 2, atomic positions and temperature factors in Table
3, and selected bond distances for ScNiP in Table 4.
TABLE 2
Selected Crystallographic Data for ScNiP
The structure of ScNiP can also be described based on the
coordination polyhedra around the metal atoms. It consists
Empirical formula
Molecular weight
ScNiP
134.63g/mol
of edge-condensed (distorted) square ScP pyramids, with
ꢄ
Temperature of data collection
Crystal dimensions
Space group
295K
the Ni atoms in tetrahedral voids of the P sublattice (Fig. 2).
0.06 mm;0.002 mm;0.001mm
Pnma (No. 62)
One further P atom, situated at a much larger distance from
Unit cell dimensions
a"6.327(2)A, b"3.728(2)A,
c"7.081(3) A, »"167.0(2)Aꢀ
Number of formula units
Calculated density
4
TABLE 4
5.353g/cmꢀ
Selected Interatomic Distances [A] < 3.8 A for ScNiP
Absorption coefficient
F(000)
Scan mode, scan width
Scan speed
15.76mm\ꢅ
256
u—2h, (1.26#0.34 tanh)°
4.0°/min (in u, 3 rescans)
Central
atom
Central
atom
Neighbor
Distance No.
Neighbor
Distance No.
Range of 2h
4°—60°
571
324 (R "0.114)
138
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Ni
Ni
Ni
Ni
P
3.232(4) 2;
3.293(2) 2;
3.729(2) 2;
2.773(3)
2.786(2) 2;
2.958(2) 2;
2.973(3)
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Sc
Sc
Sc
Sc
Ni
Ni
P
2.773(3)
No. of measured reflections
No. of independent reflections
No. of observed reflections (I'3p(I))
No. of parameters refined
2.786(2) 2;
2.958(2) 2;
2.973(3)
2.736(3) 2;
3.729(2) 2;
2.335(2) 2;
2.391(4)
*ꢁꢂ
20
R(Fꢁ), R (Fꢁ), goodness of fit
0.061, 0.064, 1.09
0.25705;10\ꢄ
1.29 e\/Aꢀ, !1.64 e\/Aꢀ
( scan
U
Extinction coefficient
2.646(4)
P
P
Max., min. peak in final diff. map
Absorption correction
P
P
P
2.647(3) 2;
2.685(3) 2;
3.472(4)
2.419(4)
Min., max. transmission
0.73, 0.99

220
KLEINKE AND FRANZEN
Sc—P interactions (2.7 A should have bonding character
with Pauling bond orders between 0.66 and 0.57, as ex-
pected based on the facts that Sc is the most electropositive
and P the most electronegative element in ScNiP. On the
other hand, the bond order of the long Sc—P distance of
3.472(4) A (0.04) is rather small. The Ni—P distances, being
the shortest interatomic separations in this structure, also
have significant bond orders (ranging from 0.73 for
r
"2.335(2) A to 0.53 for r
,*\.
Besides Sc—P and Ni—P bonds, there are also relatively
" 2.419(4) A).
,*\.
short Sc—Sc, Sc—Ni, and Ni—Ni interactions between adjac-
ent layers of trigonal Sc Ni P prisms. The Ni—Ni distance
ꢃ
ꢁ
between the prisms of 2.736(3) A is much shorter than that
within the prism (3.729(2) A). Similarly, the shortest Sc—Sc
interaction (3.232(4) A) is found between two Sc Ni P
ꢃ
ꢁ
prisms, whereas the shortest one within a Sc Ni P prism is
ꢃ
ꢁ
slightly longer (3.293(2) A). However, the relatively shortest
metal—metal bonds in ScNiP, i.e., those with the highest
bond order, are those between Sc and Ni. We calculated the
Pauling bond orders of all the different metal—metal bonds
for a first estimation of the bond strengths of the different
metal—metal bonds. The highest Pauling bond order (0.51) is
calculated for the Sc—Ni bond of 2.773(3) A, the other Sc—Ni
bond orders of 0.48, 0.25, and 0.23 are also a hint of the
occurrence of Sc—Ni bonding. This bonding corresponds to
the Lewis acid/base concept, considering the electron-poor
metal Sc as the acid and the electron-rich metal Ni as the
base. However, the two Sc—Sc distances (3.3 A also have
significant bond orders of 0.26 and 0.20, respectively. It is
concluded, based on these values, that a significant electron
density at the Sc atoms is available for a positive overlap
population between the Sc atoms, because otherwise a re-
pulsive Coulomb interaction would preclude such a close
distance.
FIG. 1. Structure of ScNiP in a projection along [010]. Small, black
circles: P; medium, white: P; large, white: Sc atoms. Horizontal: c axis. Only
metal—metal bonds are shown.
the central Sc atom (3.472(4) A, compared to the five other
Sc—P distances between 2.646(4) A and 2.685(3) A), com-
pletes the Sc coordination sphere to a strongly distorted
octahedron. A calculation of the Pauling bond orders, using
Pauling’s equation d(n) " d(1)!0.6 log n, with n " bond
order, r "1.439 A and r "1.100 A (7), shows that the
1#
.
In order to further investigate the character of the differ-
ent metal—metal interactions, we calculated the electronic
structure of ScNiP using the extended Huckel approxima-
tion. We performed a charge iteration on the metal atoms in
ScNiP, taking the P parameters from standard sources (14).
The charge-consistent metal parameters as well as the
P parameters can be found in Table 5.
To a first approximation, the density of states of ScNiP
consists of three parts in the energy window shown in Fig.
3 (between !16 eV and !8 eV). Below this energy window,
the 3s states of P occur, and at higher energies the 3p
orbitals of P (between !16 eV and !12.5 eV) predominate.
Between these states and the next peak, which has mainly Ni
d and also via covalent mixing some Sc d character, is
a significant gap. The sharp Ni peak overlaps only slightly
with the much broader Sc d orbitals exactly at the Fermi
level of !9.97 eV. As a consequence, the Fermi level lies in
a pseudo gap, i.e., only a very small number of states is
found at the Fermi level. Thus, weak metallic properties can
be assumed.
FIG. 2. Structure of ScNiP in a projection along [010]. Small, black
circles: Co; medium, white: P; large, white: Sc atoms. Horizontal: c axis.
Only metal—P bonds are shown.

Sc—Sc BONDING IN ScNiP
221
TABLE 5
Parameters Used for the EH Calculations
Orbital
H /eV
1
c
1
c
GG
ꢅ
ꢅ
ꢁ
ꢁ
Sc, 4s
Sc, 4p
Sc, 3d
Ni, 4s
Ni, 4p
Ni, 3d
P, 3s
!9.00
!4.32
!9.42
!8.67
!4.20
!11.06
!18.60
!12.50
1.30
1.30
4.35
1.93
1.93
5.75
1.88
1.63
0.4228
0.5862
1.70
0.7276
0.5845
2.200
P, 3p
In an attempt to confirm the metallic character of ScNiP,
we measured the magnetic susceptibilities between 6 K and
297 K of the annealed bulk sample of ScNiP. Figure 4 shows
the magnetic susceptibility (circles) as a function of the
temperature. The small value of the magnetic susceptibility
FIG. 4. Temperature dependence of the magnetic susceptibilities for
ScNiP. Circles: raw data after the diamagnetic core correction; crosses: fit
of the paramagnetic impurity; diamonds: difference between raw data and
the contribution of the paramagnetic impurity.
\ꢄ
of room temperature of 9.1 ; 10 emu/mol suggests Pauli
paramagnetism. The increase of the magnetic signal at lower
temperature points to a paramagnetic impurity, following
dependent contribution of the Curie-Weiss impurity is
shown as crosses in Fig. 4. The resulting values for s, shown
as diamonds in Fig. 4, are basically temperature indepen-
dent, which confirms the validity of the fitting equation
used. The so corrected susceptibility at room temperature of
the Curie-Weiss law s "C/(¹!h). With this supposi-
ꢄ!ꢅ!
tion, the raw data have been fitted to the equation
s
"s #s, with s "observed molar susceptibility
ꢀ"ꢆ
ꢄ!ꢅ! ꢀ"ꢆ
and s"molar susceptibility of ScNiP. According to this fit,
\ꢄ
8.66;10 emu/mol, is almost equal to the susceptibility as
\ꢀ
the Curie-Weiss constant is determined to C"1.22;10
measured and corrected for the diamagnetic cores (9.1;
\ꢅ
emu mol K, and h"!9.716 K, and the impurity is esti-
\ꢄ
10 emu/mol). These small positive values are typical for
mated to be 5.7 at.% for a compound with a total spin
magnetic moment of S"1/2, corresponding to one un-
paired localized electron per formula unit. The temperature
Pauli paramagnetic compounds, which means that there are
some itinerant electrons present. Since the magnetic suscep-
tibility of a Pauli paramagnet is proportional to the density
of states at the Fermi level, we conclude that there is a small,
but significant, density of states at the Fermi level as cal-
culated using the extended Huckel approximation, and that
the metallic character of ScNiP is confirmed by our experi-
ment.
The assignment of the different peaks discussed above
points to filled 3s and 3p states of P and also to filled Ni
d states, which corresponds to a formal ionic formula of
ꢀ> !ꢇ ꢀ\
Sc Ni
P
. However, the significant Sc contributions to
the peaks in the DOS below the Fermi level, caused by
covalent mixing of the Sc states with the Ni, as well as the
ꢀ>
P states, prove that Sc is not fully oxidized to Sc . This
enables Sc to form the observed Sc—Sc bonds. This supposi-
tion is confirmed by the calculated crystal orbital overlap
populations, as shown in Fig. 5. The Sc—Sc interactions
(solid line in Fig. 5) have bonding character between
!16 eV and !9 eV. The Fermi level of ScNiP lies just
below a region where many bonding Sc—Sc states occur,
which means that raising the Fermi level would result in
stronger Sc—Sc bonding.
Figure 5 also shows the Sc—Ni (short dashes), Sc—P (long
dashes), Ni—Ni (dash-dotted line), and Ni—P (dotted line)
tom to top). Solid line: total DOS; dashed: Sc; dotted line: Ni contributions. interactions. Each of these interactions is summed up to
FIG. 3. Densities of states for ScNiP. Dashed horizontal lines: Fermi
levels for vec"64 e\, vec"68 e\ and vec"72 e\, respectively (from bot-

222
KLEINKE AND FRANZEN
However, ScCoP does crystallize in the same structure
type as ScNiP. Although the radius of nickel is smaller than
that of cobalt (1.154 A vs 1.162 A), the unit cell of ScNiP is
larger than the cell of ScCoP. This can be understood based
on the filling of antibonding Sc—Mꢀ, Mꢀ—Mꢀ, and Mꢀ—P states
when Co is replaced by Ni. The fact, that the b axis of ScNiP
is smaller than the ScCoP b axis, is explained by the occur-
rence of (weak) Sc—Sc bonds parallel to [010], which are
stronger in case of ScNiP. The above mentioned Sc—Mꢀ,
Mꢀ—Mꢀ, and Mꢀ—P are not parallel to [010] and lie partly in
the ac-plane so that their influence is largest on the a and
c axes.
ACKNOWLEDGMENTS
H.K. thanks the DFG for the financial support of this work. The Ames
Laboratory is operated by Iowa State University for the U.S. Department
of Energy under Contract W-7405-Eng-82 and is part of the Institute for
Physical Research and Technology consortium of fundamental and applied
research centers.
FIG. 5. Summed crystal orbital overlap populations for ScNiP. Hori-
zontal lines: Fermi levels for vec"64 e\, vec"68 e\, and vec"72 e\,
respectively (from bottom to top). Left: antibonding; right: bonding. Solid
line: Sc—Sc; dashed line: Sc—Ni; long dashes: Sc—P; dashed-dotted line:
Ni—Ni; dotted line: Ni—P interactions.
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Sc—Ni states are filled at the Fermi level of ScNiP. From
these results, it is not obvious why an isostructural ‘‘ScFeP’’
did not form in our experiments. A (theoretical) substitution
of Ni by Co or Fe would yield a lower valence electron
concentration and thus lower lying Fermi levels as indicated
by the horizontal dashed lines in Fig. 5. Going from ScNiP
to hypothetical ‘‘ScFeP,’’ the bonding character of the Sc—Sc
and Sc—P bonds decreases, whereas on the other hand the
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concludes that a structure type like Co Si with short Sc—Sc
ꢁ
bonds is not favored for compounds with a low electron
count. A comparison with the related Ti—Mꢀ—P system
underlines this supposition, because TiMnP, being isoelec-
tronic with the hypothetical ‘‘ScFeP,’’ is found to form the
Fe P structure type (15).
ꢁ