Structures of the ternary iron germanium pnictides FeGe1-xPnx (Pn = P, As, Sb)

Article abstract of DOI:10.1016/S0925-8388(99)00653-2

The pseudo-binary FeGe-FePn (Pn = P, As, Sb) systems have been investigated by X-ray diffraction methods. The phosphide and arsenide systems form ternary solid solutions FeGe_1-xP_x (0.4≤x≤1.0) and FeGe_1-xAs_x (0.3≤x≤1.0) crystallizing in MnP-type structures. The antimonide system is more complex, forming intermetallic compounds that display incipient ordering of Sb₂ pairs as they are stuffed into the FeGe (x = 0) structure while Ge atoms are gradually removed. The structures of an intermediate member FeGe_0.80Sb_0.20 (x = 0.20) and of the limiting member FeGe_0.67Sb_0.33 (x = 0.33) in this progression were determined by single-crystal X-ray diffraction. Both compounds crystallize in hexagonal space groups: P6₃/mmc, a = 8.796(2), c = 8.004(2) angstroms, Z = 6 for Fe₃Ge_2.4Sb_0.6 (or FeGe_0.80Sb_0.20), and P6/mmm, a = 8.9885(5), c = 7.9043(6) angstroms, Z = 6 for Fe₃Ge₂Sb (or FeGe_0.67Sb_0.33). Beyond this point, ternary solid solutions FeGe_1-xSb_x (0.4≤x≤1.0) are formed, crystallizing in NiAs-type structures.

Full text of DOI:10.1016/S0925-8388(99)00653-2

Journal of Alloys and Compounds 298 (2000) 82–92
L
www.elsevier.com/locate/jallcom
Structures of the ternary iron germanium pnictides FeGe12xPnx
(Pn5P, As, Sb)
*
Allison M. Mills, Arthur Mar
Department of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2
Received 19 August 1999; accepted 19 August 1999
Abstract
The pseudo-binary FeGe–FePn (Pn5P, As, Sb) systems have been investigated by X-ray diffraction methods. The phosphide and
arsenide systems form ternary solid solutions FeGe12xPx (0.4 # x # 1.0) and FeGe12xAsx (0.3 # x # 1.0) crystallizing in MnP-type
structures. The antimonide system is more complex, forming intermetallic compounds that display incipient ordering of Sb2 pairs as they
are stuffed into the FeGe (x 5 0) structure while Ge atoms are gradually removed. The structures of an intermediate member
FeGe0.80Sb0.20 (x 5 0.20) and of the limiting member FeGe0.67Sb0.33 (x 5 0.33) in this progression were determined by single-crystal
˚
X-ray diffraction. Both compounds crystallize in hexagonal space groups: P63 /mmc, a 5 8.796(2), c 5 8.004(2) A, Z 5 6 for
˚
Fe3Ge2.4Sb0.6 (or FeGe0.80Sb0.20), and P6/mmm, a 5 8.9885(5), c 5 7.9043(6) A, Z 5 6 for Fe3Ge2Sb (or FeGe0.67Sb0.33). Beyond this
point, ternary solid solutions FeGe12xSbx (0.4 # x # 1.0) are formed, crystallizing in NiAs-type structures.
2000 Elsevier Science
S.A. All rights reserved.
Keywords: Crystal structure; Pnictide; Solid solution
1. Introduction
type structure is related to many other intermetallic struc-
tures, including those of important permanent magnet
materials such as SmCo5 and Sm2Co17 [9,10], further
studies seem warranted.
As part of a general search for new ternary pnictides
with useful physical properties, an investigation of the
pseudo-binary FeGe–FePn (Pn5P, As, Sb) systems was
undertaken. The parent binaries FeGe and FePn were
chosen in part because they display a variety of interesting
magnetic behaviour. We reasoned that the structures, and
hence properties, of any ternary phases formed would be
influenced by factors such as relative atomic sizes, elec-
tronegativities, and valence electron concentrations that are
easily manipulated in these systems through, for instance,
the elaboration of solid solutions FeGe12xPnx.
Of the three known polymorphs of iron monogermanide
FeGe [1], there exists a hexagonal one, which adopts the
relatively rare CoSn-type crystal structure [2,3] and which
exhibits a double-cone antiferromagnetic magnetic struc-
ture [4–6]. Substitutional studies involving FeGe are
sparse, with only the CoSn-type compound Fe2GaGe [7]
and the CoSn-related solid solutions Fe33Ge291xX62x (X5
Si, P) [8] having been reported to date. Since the CoSn-
The iron monopnictides FePn also display a range of
magnetic behaviour, from helical ferromagnetism in FeP
[11] and FeAs [12], which adopt the MnP-type structure
[13,14], to antiferromagnetism in FeSb [15], which adopts
the NiAs-type structure [16]. Numerous studies involving
substitution of the transition metal MxFe12xPn (M5Ti, V,
Cr, Mn, Co, Ni; Pn5P, As, Sb) have been carried out in
order to modify the properties of the parent binary FePn as
well as to effect transitions between the MnP- and NiAs-
type structures by changing the electron count [17–28]. In
contrast, surprisingly few studies have explored the effects
of anion substitution [29–33], even though it has been
demonstrated that structures and magnetic properties will
also be influenced by partial substitution of Pn with other
nonmetals, causing changes in the number of unpaired
metal electrons [34] and the metal–metal bond lengths
[35].
Structural studies on the CoGe12xPnx systems were
carried out earlier [36]; extended studies into the
FeGe12xPnx (Pn5P, As, Sb) systems are now reported
herein. The antimonide system is particularly rich, en-
*Corresponding author. Tel.: 11-780-492-5592; fax: 11-780-492-
8231.
E-mail address: arthur.mar@ualberta.ca (A. Mar)
0925-8388/00/$ – see front matter
PII: S0925-8388(99)00653-2
2000 Elsevier Science S.A. All rights reserved.

A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
83
compassing new intermetallic compounds whose structures
were determined at x 5 0.20 (FeGe0.80Sb0.20 or
standard) and analyzed with the Filmscan and Jade 3.0
software packages [37].
Fe3Ge2.4Sb0.6
)
and
x 5 0.33
(FeGe0.67Sb0.33
or
Mixtures of reactants with loading stoichiometry
FeGe12xPnx (0 # x # 1 in increments of 0.1; Pn5P, As,
Sb) were heated at 7008C for 4 days and then quenched to
room temperature. Solid solutions with the MnP-type
structure form over wide regions of homogeneity for the
phosphide FeGe12xPx (0.4 # x # 1.0) and arsenide
FeGe12xAsx (0.3 # x # 1.0) systems. A structure determi-
nation of a single crystal of FeGe0.15P0.85, grown in a tin
flux, confirms the MnP-type structure showing complete
disordering of Ge and P in the unique anion site [38]. In
contrast, solid solutions of the NiAs-type structure form
for the antimonide system FeGe12xSbx (0.4 # x # 1.0). No
evidence for superstructures was observed in the course of
indexing the powder patterns of the three solid solutions
above. The cell parameters refined with the use of the
program POLSQ [39] are given in Table 1.
Fe3Ge2Sb). The formation of these compounds provides
insight into the importance of Sb–Sb bonding to the
stability of these ‘‘stuffed’’ CoSn-type structures.
2. Experimental
2.1. Syntheses
Starting materials were the elemental powders: Fe
99.9% (Cerac), Ge 99.999% (Cerac), P 99.995% (Cerac),
As 99.99% (Alfa-Aesar), Sb 99.995% (Aldrich). Reactions
were carried out on a 0.250-g scale in evacuated fused-
silica tubes (8 cm length; 10 mm i.d.). Elemental com-
positions were determined by EDX (energy-dispersive X-
ray) analysis on a Hitachi F2700 scanning electron micro-
scope. X-ray powder patterns were collected on an Enraf-
Nonius FR552 Guinier camera (Cu Ka1 radiation; Si
Analysis of the reactions at the Ge-rich end of the
FeGe12xSbx (0 # x # 0.4) series indicated the presence of
phases whose powder patterns resemble that of hexagonal
FeGe. These were eventually identified as members of a
Table 1
Cell parameters for products obtained in FeGe12xPnx (Pn5P, As, Sb) reactions
3
˚
˚
˚
˚
Loading
composition
Structure type
of product
a (A)
b (A)
c (A)
V (A )
FeGe12xPx
FeGe0.6P0.4
FeGe0.5P0.5
FeGe0.4P0.6
FeGe0.3P0.7
FeGe0.2P0.8
FeGe0.1P0.9
FeP
MnP
MnP
MnP
MnP
MnP
MnP
MnP
5.310(2)
5.302(3)
5.300(2)
5.272(3)
5.250(3)
5.211(2)
5.191(3)
3.169(1)
3.178(2)
3.169(1)
3.163(1)
3.138(2)
3.116(1)
3.108(2)
5.915(2)
5.918(3)
5.913(2)
5.886(3)
5.853(3)
5.812(2)
5.794(3)
99.55(4)
99.72(7)
99.32(4)
98.15(6)
96.43(8)
94.36(5)
93.46(6)
FeGe12x Asx
FeGe0.7As0.3
FeGe0.6As0.4
FeGe0.5As0.5
FeGe0.4As0.6
FeGe0.3As0.7
FeGe0.2As0.8
FeGe0.1As0.9
FeAs
MnP
MnP
MnP
MnP
MnP
MnP
MnP
MnP
5.497(3)
5.509(3)
5.494(2)
5.495(2)
5.480(3)
5.468(3)
5.449(2)
5.436(2)
3.311(2)
3.318(2)
3.316(2)
3.321(2)
3.329(2)
3.344(2)
3.351(2)
3.370(2)
6.065(4)
6.076(3)
6.065(3)
6.060(2)
6.049(4)
6.040(3)
6.032(3)
6.028(3)
110.38(8)
111.07(7)
110.48(6)
110.60(6)
110.35(9)
110.45(8)
110.15(6)
110.43(7)
FeGe12xSbx
FeGea
FeGe0.9Sb0.1
FeGe0.8Sb0.2
FeGe0.7Sb0.3
FeGe0.6Sb0.4
CoSn
8.664(2)
8.786(2)
8.888(2)
8.948(2)
9.009(3)
4.004(2)
3.9993(6)
3.996(2)
4.025(2)
4.043(1)
4.0555(9)
4.076(1)
8.118(4)
8.012(3)
7.973(3)
7.939(3)
7.919(3)
5.027(3)
5.034(1)
5.052(3)
5.090(3)
5.116(3)
5.129(1)
5.135(2)
527.9(3)
535.4(3)
545.5(3)
550.5(3)
556.7(3)
69.81(6)
69.73(2)
69.86(6)
71.42(7)
72.41(4)
73.06(3)
73.87(4)
Stuffed CoSn
Stuffed CoSn
Stuffed CoSn
Stuffed CoSn
NiAs
NiAs
NiAs
NiAs
NiAs
FeGe0.5Sb0.5
FeGe0.4Sb0.6
FeGe0.3Sb0.7
FeGe0.2Sb0.8
FeGe0.1Sb0.9
FeSb
NiAs
NiAs
a The smaller unit cell of FeGe has been transformed to match the larger unit cells of the superstructures adopted by FeGe0.80Sb0.20 or FeGe0.67Sb0.33
(a 5Œ]3a9, c 5 2c9, and V5 6V9).

84
A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
family of CoSn-related structures containing stuffed Sb2
pairs. Reactions carried out at various temperatures be-
tween 600 and 9008C showed the persistence of these
compounds, and the refined cell parameters for the prod-
ucts obtained at 7008C are listed in Table 1. Single crystals
of an intermediate member FeGe0.80Sb0.20 and the limiting
member FeGe0.67Sb0.33 were grown in separate reactions.
Reaction of Fe, Ge, and Sb in the ratio 1:0.8:0.2 at 8008C
for 5 days, followed by cooling to 5008C over 4 days and
to 208C over 12 h, results in hexagonal prismatic crystals
of FeGe0.80Sb0.20 [anal. (mol%): Fe 49(1), Ge 40(1), Sb
11.0(3)% (average of two analyses)]. Similarly, reaction of
Fe, Ge, and Sb in the ratio 1:0.6:0.4 at 7008C for 4 days
followed by cooling to 208C over 12 h results in hexagonal
prismatic crystals of FeGe0.67Sb0.33 [anal. (mol%): Fe
50(1), Ge 34.2(7), Sb 15.6(3)% (average of four analy-
ses)].
2.2. Structure determination
Preliminary cell parameters for FeGe0.80Sb0.20 and
FeGe0.67Sb0.33 were determined from Weissenberg photo-
graphs, which revealed Laue symmetry 6/mmm in both
cases. The presence of six-fold rotational symmetry in
upper-level photographs rules out trigonal space groups.
Final cell parameters were determined from least-squares
analysis of the setting angles of 24 reflections centred on
an Enraf-Nonius CAD4 diffractometer in the range 208#
2u(Mo Ka)#398 for FeGe0.80Sb0.20 and 278#2u(Mo
Ka)#578 for FeGe0.67Sb0.33. Intensity data were collected
at 228C with the u–2u scan technique in the range 58#
2u(Mo Ka)#608 for FeGe0.80Sb0.20 and 58#2u(Mo Ka)#
708 for FeGe0.67Sb0.33. Crystal data and further details of
the data collections are given in Table 2. All calculations
were carried out using the SHELXTL (Version 5.1)
Table 2
Crystallographic data for FeGe0.80Sb0.20 and FeGe0.67Sb0.33
Formula
Fe3Ge2.4Sb0.6
Fe3Ge2Sb
(or FeGe0.80Sb0.20
405.10
)
(or FeGe0.67Sb0.33
429.41
)
Formula mass (amu)
Space group
a (A)
D46h –P63 /mmc (No. 194)
8.7958(18)a
D16h –P6/mmm (No. 191)
8.9885(5)a
˚
˚
c (A)
8.0040(18)a
536.3(2)
7.9043(6)a
553.05(6)
3
˚
V (A )
Z
6
6
T (8C)
22
22
Diffractometer
Enraf-Nonius CAD4
7.526
Enraf-Nonius CAD4
7.736
rcalc (g cm23
)
Crystal dimensions (mm)
Hexagonal prism,
0.2030.0730.06
Graphite-monochromated
Mo Ka, l 5 0.71073 A
351.9
0.0538–0.2266
u–2u
1.67
0.80 1 0.344 tan u
58#2u(Mo Ka)#608
2 12 # h # 12,
2 12 # k # 12,
2 11 # l # 11
5938
Hexagonal prism,
0.1330.0430.04
Graphite-monochromated
Mo Ka, l 5 0.71073 A
342.5
0.0949–0.3638
u–2u
1.67
0.80 1 0.344 tan u
58#2u(Mo Ka)#708
2 14 # h # 14,
2 14 # k # 14,
2 12 # l # 12
9738
Radiation
˚
˚
m(Mo Ka) (cm21
)
Transmission factorsb
Scan type
Scan speed (deg min21
Scan range (deg)
2u limits
)
Data collected
No. of data collected
No. of unique data,
including F2o ,0
No. of unique data,
with F2o . 2s(F2o)
No. of variablesc
R(F) for F2o . 2s(F2o)d
Rw(F2o)e
331 (Rint 5 0.162)
539 (Rint 5 0.190)
200
29
0.041
0.120
1.13
390
28
0.037
0.080
1.12
Goodness of fit
a Obtained from a refinement constrained so that a 5 b 5 908 and g 5 1208.
b An empirical face-indexed Gaussian-type absorption correction was applied, with the use of programs in the SHELXTL package.
c Including an extinction coefficient.
d R(F) 5 oiFou 2 uFci/ouFou.
e Rw(F2o) 5 [o[w(Fo2 2 Fc2)2]/owF4o]1 / 2; w21 5 [s2(F2o ) 1 (ap)2 1 bp], where p 5 [max(F2o,0) 1 2Fc2]/3. For FeGe0.80Sb0.20, a 5 0.0400, b 5 7.1719; for
FeGe0.67Sb0.33, a 5 0.0187, b 5 1.3813.

A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
85
package [40]. Conventional atomic scattering factors and
anomalous dispersion corrections were used [41]. Intensity
data were reduced and averaged, and face-indexed Gaus-
sian-type absorption corrections were applied in XPREP.
Initial atomic positions were located by direct methods
using XS, and refinements were performed by least-
squares methods using XL.
partially occupied. The Ge(2)/Sb(1) and Ge(3)/Sb(2)
displacement parameters were constrained to be identical
in order to obviate the high correlation between the
positional and displacement parameters of these closely-
spaced sites. Refinements allowing the occupancies of
successive atoms to vary freely resulted in essentially full
occupancy for Fe(1), Fe(2), and Ge(1), but considerably
less than 100% for the other atoms. In the final refinement,
the Fe(1), Fe(2), and Ge(1) occupancies were fixed at
100%, while those for the remaining atoms converged to
29.0(9)% for Ge(2), 49(1)% for Ge(3), 59.4(9)% for
Sb(1), and 22.2(6)% for Sb(2). The resulting formula can
be written as ‘‘Fe3Ge2.36Sb0.54’’ (with Z 5 6), in excellent
agreement with the elemental analysis and the nominal
loaded composition of ‘‘Fe3Ge2.4Sb0.6’’. Again, in order to
emphasize structural relations, we refer to this compound
interchangeably as ‘‘FeGe0.80Sb0.20’’ or ‘‘Fe3Ge2.4Sb0.6’’.
The final refinements for FeGe0.67Sb0.33 and
FeGe0.80Sb0.20 lead to reasonable values for the anisotropic
displacement parameters and to featureless difference
electron density maps (Drmax 5 1.82, Drmin 5 2 2.16 e
The determination of space groups deserves some
]
discussion. Careful inspection of the hh2hl reflections with
l 5 2n 1 1 in the reciprocal space plots clearly reveals the
presence of a c-glide plane in FeGe0.80Sb0.20 [out of 300
such data, only two had I . 3s(I)] and the absence of a
c-glide plane in FeGe0.67Sb0.33 [out of 420 such data, 119
had I . 3s(I)]. Thus, P63 /mmc was chosen for
FeGe0.80Sb0.20
and P6/mmm
was chosen for
FeGe0.67Sb0.33. The choice of P6/mmm for FeGe0.67Sb0.33
is supported by its isotypy with CoGe0.67Sb0.33 [36] and its
relation to the structure of the parent binary FeGe (CoSn-
type) [1], both of which also crystallize in P6/mmm.
Attempted solution of the structure of FeGe0.80Sb0.20 in
P6/mmm was unsuccessful. Moreover, the excellent match
between the observed and calculated powder patterns for
FeGe0.80Sb0.20 eliminates any possibility that the differ-
ence in space group is an artifact of twinning problems in
the single crystal.
The structure determination of FeGe0.67Sb0.33 was the
more straightforward one. With the assumption that
FeGe0.67Sb0.33 is isostructural to CoGe0.67Sb0.33 [36], the
initial atomic positions were chosen from the latter, and
refinement proceeded smoothly. Since the Sb(1) site in
CoGe0.67Sb0.33 was found to deviate slightly from full
occupancy, refinements were conducted in FeGe0.67Sb0.33
allowing the occupancies of successive atoms to vary.
These resulted in essentially 100% occupancy for all atoms
except Sb(1) and Sb(2), which show only slight deviations
from full occupancies at 95.1(6) and 97.4(8)%, respective-
ly. The resulting formula can be written as
‘‘Fe3Ge2Sb0.96’’ (with Z 5 6), close to the ideal formula of
‘‘Fe3Ge2Sb’’ and in excellent agreement with the elemen-
tal analysis. For simplicity and for ease of comparison to
the parent binary FeGe, we use the formula
‘‘FeGe0.67Sb0.33’’ interchangeably with ‘‘Fe3Ge2Sb.’’
23
˚
A
A
for FeGe0.67Sb0.33; Drmax 5 2.65, Drmin 5 2 1.14 e
for FeGe0.80Sb0.20). (Some of the thermal ellipsoids
23
˚
are elongated along the c direction, consistent with what is
observed in the related structure of CoSn and explicable by
the unusual coordination of atoms within large hexagonal
channels [2].) The atomic positions were standardized with
the program STRUCTURE TIDY [42]. Final values of the
positional and displacement parameters are given in Table
3. Interatomic distances are listed in Table 4. Supple-
mentary material (powder diffraction data, anisotropic
displacement parameters, bond angles, structure am-
plitudes, and CIFs) is available from A.M.
3. Results
3.1. Solid solution ranges
Ternary solid solutions form for all three pnictide
systems FeGe12xPnx (Pn5P, As, Sb), with the phosphide
and arsenide systems displaying similar behaviour and the
antimonide system being unique. In the ranges 0.4 # x #
1.0 for FeGe12xPx and 0.3 # x # 1.0 for FeGe12xAsx,
single-phase products are obtained whose powder patterns
are consistent with the MnP-type structure. Extrapolating
from the single-crystal structure of a representative
member, FeGe0.15P0.85 [38], we presume that Ge and Pn
are completely disordered over the anion site for all
regions of solid solubility. Figs. 1 and 2 show plots of the
orthorhombic cell parameters derived from the powder
patterns for FeGe12xPx and FeGe12xAsx, respectively, as a
function of x. Starting from the pnicogen-rich end, FePn
(Pn5P, As) (x 5 1.0), substitution of Pn with Ge through
decreasing x causes the cell volume to expand in
The initial positions of all atoms in FeGe0.80Sb0.20
,
except Ge(2) and Ge(3), were found by direct methods,
and atomic assignments were made in analogy to
FeGe0.67Sb0.33 and FeGe. A first refinement revealed
1
2
] ]
considerable electron density at the sites (0,0,0) and (3 ,3 ,|
˚
0) located at the centres of iron hexagons, 0.52 A from
˚
Sb(2) and 0.60 A from Sb(1), respectively. These positions
were assigned to Ge atoms on the basis of reasonable
Fe–Ge bond lengths and the assumption that this structure
represents a member of a series FeGe12xSbx in which Sb2
pairs have partially replaced the Ge(1) atoms of FeGe. The
proximity of the positions of Ge(2) and Ge(3) to those of
Sb(1) and Sb(2), respectively, requires that these sites be

86
A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
Table 3
Positional and equivalent isotropic displacement parameters for FeGe0.80Sb0.20 and FeGe0.67Sb0.33
2
a
˚
Ueq (A )
Atom
Wyckoff
position
Occupancy
x
y
z
Fe3Ge2.4Sb0.6 (or FeGe0.80Sb0.20
)
Fe(1)
Fe(2)
Ge(1)
Ge(2)
Ge(3)
Sb(1)
Sb(2)
12k
6g
12j
4f
2a
4f
1
1
1
0.16563(13)
1/2
0.33972(17)
1/3
0
0.3312(3)
0
0.01113(13)
2/3
0
0.00506(14)
0
1/4
0.5032(14)
0
0.0723(4)
0.0750(7)
0.0091(5)
0.0087(6)
0.0111(5)
0.0099(7)
0.0057(17)
0.0099(7)
0.0057(17)
0.290(9)
0.49(1)
0.594(9)
0.222(6)
1/3
0
2/3
0
4e
Fe3Ge2Sb (or FeGe0.67Sb0.33
)
Fe(1)
Fe(2)
Ge(1)
Ge(2)
Sb(1)
Sb(2)
12o
6i
6k
6j
4h
2e
1
1
1
1
0.16690(10)
1/2
0.31452(12)
0.34802(14)
1/3
0.3338(2)
0
0
0
2/3
0
0.24422(11)
0.26089(19)
1/2
0.0060(2)
0.0073(3)
0.0071(2)
0.0080(3)
0.0067(2)
0.0064(3)
0
0.951(6)
0.974(8)
0.31686(12)
0.17931(17)
0
a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
FeGe12xPx but to remain relatively constant in
FeGe12xAsx. These trends are consistent with Ge being
larger than P, but similar in size to As. The cell expansion
is isotropic on decreasing x in FeGe12xPx, as seen in the
increases in a, b, and c (Fig. 1), with relative expansions of
|2% each to account for the 6% increase in cell volume on
going from FeP to FeGe0.6P0.4. In contrast, the apparent
sions of 1.1% in a and 0.6% in c on going from FeAs to
FeGe0.7As0.3 (Fig. 2). In the MnP structure, which adopts
an orthorhombic distortion of the more symmetric hexa-
gonal NiAs structure, the metal atoms are shifted to form
two close metal–metal contacts in the bc plane in addition
to the two contacts in the ac plane that are remnants of the
metal–metal bonding network in the NiAs structure
[13,43]. The anions are also shifted to form nonmetal–
nonmetal contacts in the ab plane. These distortions result
in zigzag chains of metal atoms along the a direction, and
similar chains of metal and of nonmetal atoms along the b
direction. The interpretation of the trend in cell parameters
constancy in the cell volume on decreasing
x in
FeGe12xAsx results from the near cancellation of the
relative contraction of 1.8% in b with the relative expan-
Table 4
˚
Selected interatomic distances (A) in FeGe0.80Sb0.20 and FeGe0.67Sb0.33
FeGe0.80Sb0.20
Fe(1)–Ge(1)
Fe(1)–Ge(1)
Fe(1)–Ge(3)
Fe(1)–Fe(1)
Fe(1)–Fe(2)
Fe(1)–Ge(2)
Fe(1)–Sb(2)
Fe(1)–Sb(2)
Fe(2)–Ge(1)
Fe(2)–Ge(2)
Fe(2)–Fe(1)
Fe(2)–Sb(1)
2.4741(12) (32)
2.4949(12) (32)
2.524(2)
2.525(2) (32)
2.5474(11) (32)
2.556(2)
2.585(2)
2.603(2)
2.4777(9) (34)
2.5393(5) (32)
2.5474(11) (34)
2.6042(9) (32)
Ge(1)–Fe(1)
Ge(1)–Fe(2)
Ge(1)–Fe(1)
Ge(1)–Ge(1)
Ge(1)–Ge(1)
Ge(2)–Fe(2)
Ge(2)–Fe(1)
Ge(3)–Fe(1)
Sb(1)–Fe(2)
Sb(1)–Fe(1)
Sb(1)–Sb(1)
Sb(2)–Fe(1)
Sb(2)–Fe(1)
Sb(2)–Sb(2)
2.4741(12) (32)
2.4777(9) (32)
2.4949(12) (32)
2.792(2)
2.917(3)
2.5393(5) (33)
2.556(2) (33)
2.524(2) (36)
2.6042(9) (33)
2.611(2) (33)
2.845(6)
2.585(2) (33)
2.603(2) (33)
2.801(12)
FeGe0.67Sb0.33
Fe(1)–Ge(1)
Fe(1)–Ge(2)
Fe(1)–Fe(2)
Fe(1)–Fe(1)
Fe(1)–Sb(2)
Fe(1)–Sb(1)
Fe(2)–Ge(2)
Fe(2)–Ge(1)
Fe(2)–Fe(1)
Fe(2)–Sb(1)
2.4714(8) (32)
2.4870(8) (32)
2.5963(8) (32)
2.5984(16) (32)
2.6486(16)
Ge(1)–Fe(1)
Ge(1)–Fe(2)
Ge(1)–Ge(1)
Ge(2)–Fe(2)
Ge(2)–Fe(1)
Ge(2)–Ge(2)
Sb(1)–Fe(2)
Sb(1)–Fe(1)
Sb(1)–Sb(1)
Sb(2)–Fe(1)
Sb(2)–Sb(2)
2.4714(8) (34)
2.5203(13) (32)
2.8270(11) (32)
2.4735(14) (32)
2.4870(8) (34)
2.732(3)
2.6322(3) (33)
2.6539(16) (33)
2.895(2)
2.6539(16)
2.4735(14) (32)
2.5203(13) (32)
2.5963(8) (34)
2.6322(3) (32)
2.6486(16) (36)
2.835(3)
Fig. 1. Plot of the orthorhombic cell parameters for FeGe12xPx. Ternary
solid solutions of the MnP-type structure form for 0.4 # x # 1.0.

A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
87
Fig. 3. Plot of the hexagonal cell parameters for CoSn-related type
structures (0 # x # 0.4) and NiAs-type structures (0.4 # x # 1.0) in
FeGe12xSbx. The cell parameters for FeGe (x 5 0) have been transformed
to correspond to those of the ternary FeGe12xSbx (x # 0.4) superstruc-
tures.
Fig. 2. Plot of the orthorhombic cell parameters for FeGe12xAsx. Ternary
solid solutions of the MnP-type structure form for 0.3 # x # 1.0.
in FeGe12xAsx, then, is that as Ge atoms replace As atoms,
there is a slight strengthening of the nonmetal–nonmetal
bonding along the b direction, accompanied by a weaken-
ing of the Fe–Fe bonding in the zigzag chains along the a
direction.
In the Ge-rich end of the FeGe12xPx (0 , x , 0.4) and
FeGe12xAsx (0 , x , 0.3) systems, products form whose
powder patterns are very complicated and do not appear to
match either those of the known binary iron germanides or
pnictides, or of a hexagonal Fe33Ge291xP62x phase iden-
tified recently [8]. Attempts to index these patterns by trial
and error were unsuccessful, and it is possible that these
are not single-phase products or are of low symmetry. At
x 5 0, the hexagonal FeGe phase is obtained.
ters (Fig. 3) are readily understood, with a, c, and V
contracting as smaller Ge atoms replace Sb atoms in FeSb
with decreasing x, and reaching a plateau at x 5 | 0.5. The
formula FeGe0.5Sb0.5 thus corresponds approximately to
the limiting composition for the NiAs-type phase. At
x 5 0.4, a miscibility gap occurs, and a new phase appears
which is more closely related to the CoSn-type binary
FeGe.
3.2. Structures of FeGe0.80Sb0.20 and FeGe0.67Sb0.33
In the antimonide system FeGe12xSbx, two major re-
gimes are found, as depicted in Fig. 3. In the Ge-rich
region, 0 # x # 0.4, a series of ternary compounds forms
which adopt superstructures related to the CoSn-type, to be
described in more detail later. In the Sb-rich region, 0.4 #
x # 1.0, ternary solid solutions with the NiAs-type struc-
ture form, which may be regarded as resulting from partial
substitution of Ge for Sb in the parent binary FeSb (x 5
1.0). However, the NiAs-type phase is formed along with
significant amounts of FeSb2. This is not unexpected given
that at 7008C in the binary Fe–Sb system, FeSb2 coexists
with Fe11xSb (a nonstoichiometric metal-rich phase with
extra Fe atoms entering vacant sites in the ideal NiAs-type
structure leading towards the Ni2In-type structure) [44].
The compositions of the ternary phases in this region,
therefore, cannot be directly related to the loading com-
position. Nevertheless, general trends in the cell parame-
A series of structures is elaborated in the region 0 # x #
0.4 of the FeGe12xSbx system, with FeGe at one extreme,
FeGe0.67Sb0.33 at the other extreme, and FeGe0.80Sb0.20 as
a representative intermediate structure. All three can be
decomposed into a common framework of composition
´
‘‘Fe3Ge2’’ comprising 3636 (kagome) nets of Fe atoms
and 63 honeycomb nets of Ge atoms, as shown in Fig. 4.
When stacked in an alternating fashion along the c
direction (Fig. 4a), the overlapping Fe and Ge nets leave
1
2
] ]
open channels along (0,0,z) and ( 3 , 3 ,z). These channels
can be occupied by single Ge atoms at the centre of
hexagons in the plane of the 3636 Fe nets (sites marked by
3 or 1 in Fig. 4c), or by Sb2 pairs whose centre of mass
lies at the centre of hexagons in the plane of the 63 Ge nets
(sites marked by h, n, «, or , in Fig. 4b,d). The differing
occupancy of these sites distinguishes the three structures

88
A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
of FeGe, FeGe0.80Sb0.20, and FeGe0.67Sb0.33 from each
other, as listed in Table 5 and shown in Fig. 5.
In FeGe, all the planar hexagonal sites within the 3636
Fe nets (3, 1) are completely occupied by the Ge(1)
atoms (Fig. 5a). (These sites are really equivalent in FeGe,
whose smaller unit cell can be transformed to those of
FeGe0.80Sb0.20 and FeGe0.67Sb0.33 via a 5 2a9 1 b9, b 5 2
a9 1 b9, and c 5 2c9.) At the other extreme, in
FeGe0.67Sb0.33, the (0,0,z) sites in one 63 Ge net (n) and
3
1
2
] ]
the ( 3 , 3 ,z) sites in the other 6 Ge net («) are essentially
completely occupied by Sb2 pairs in an ordered fashion
(Fig. 5c). The ordering of these Sb2 pairs within the two
kinds of Ge nets gives rise to the superstructure and the
larger unit cell (with six times the volume of the FeGe unit
cell) observed in FeGe0.67Sb0.33. It is also possible to
regard the FeGe0.67Sb0.33 structure (Fig. 5c) as resulting
from Sb atoms replacing the Ge(1) atoms in FeGe (Fig.
5a), followed by a shift of these atoms above and below
the Fe nets. This point of view reinforces the composition-
al similarity of FeGe (‘‘Fe3Ge2’’ framework1Ge atoms5
‘‘Fe3Ge3’’) and FeGe0.67Sb0.33 (‘‘Fe3Ge2’’ framework1
shifted Sb atoms5‘‘Fe3Ge2Sb’’). However, we prefer the
description of an Sb2 pair because bonding considerations
suggest that this is an inseparable entity and because it
facilitates the description of the intermediate structures
between FeGe and FeGe0.67Sb0.33. In FeGe0.80Sb0.20, all
possible sites within the Fe nets are occupied by Ge atoms
and all possible sites within the Ge nets are occupied by
Sb2 pairs, but at the partial occupancies shown in Fig. 5b.
Both single Ge atoms and Sb2 pairs fill the channels of the
‘‘Fe3Ge2’’ framework in a disordered manner in this
structure. Although the partial occupancy of Sb(1) and
Sb(2) can also imply the presence of single Sb atoms on a
local scale, we favour an interpretation in which the
insertion of Sb2 pairs gradually excludes the presence of
Ge(2) or Ge(3) atoms directly above and below in the
sandwiching Fe nets. Continuing this process of insertion
of Sb2 pairs concomitant with removal of Ge atoms leads
ultimately to the limiting structure of FeGe0.67Sb0.33
.
1
2
] ]
The insertion of Sb2 pairs into the (0,0,z) and ( 3 , 3 ,z)
channels introduces distortions in the Fe and Ge nets, as
can be discerned by examining the distances listed in Table
5. Within the 3636 nets, each Fe atom is bonded to four
neighbouring Fe atoms in the same plane. In FeGe, the Fe
nets are perfectly flat and undistorted, with all Fe–Fe
˚
distances equal at 2.5013(2) A [1]. With increasing Sb
˚
content, the Fe–Fe bonds lengthen to 2.525(2)–2.547(1) A
˚
in FeGe0.80Sb0.20 and further to 2.5963(8)–2.598(2) A in
FeGe0.67Sb0.33, and the Fe nets become ruffled. Within the
63 nets that alternate in stacking with the Fe nets, each Ge
atom is bonded to three neighbouring Ge atoms in the
same plane. In FeGe, the Ge nets are again flat and
3
´
Fig. 4. (a) Stacking of 3636 (kagome) Fe nets and 6 Ge nets in an
1
2
] ]
aaa9a fashion leads to open channels along (0,0,z) and ( 3 , 3 ,z) that can
˚
undistorted, with all Ge–Ge distances equal at 2.8883(2) A
be filled by Ge atoms in the Fe net, or Sb2 pairs skewering the Ge net.
Potential sites within these nets viewed down the c direction are shown in
(b), (c), and (d), and are occupied as listed in Table 5 to result in the
structures of FeGe, FeGe0.80Sb0.20, and FeGe0.67Sb0.33 (Fig. 5).
[1]. Insertion of Sb2 pairs has a dramatic effect in
distorting these Ge nets to form short and long Ge(1)–
˚
Ge(1) interactions of 2.792(2) and 2.917(3)–3.086(3) A,

A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
89
Table 5
Site occupations (in 3636 Fe and 63 Ge nets) and homoatomic bond distances (A) in FeGe0.80Sb0.20 and FeGe0.67Sb0.33
˚
FeGe
FeGe0.80Sb0.20
(or Fe3Ge2.4Sb0.6
FeGe0.67Sb0.33
(or Fe3Ge2Sb)
)
Sites along (0,0,z)a
In Ge net (h)
In Fe net (3)
In Ge net (n)
a
–
Ge
–
Sb2 pairs
Ge
Sb2 pairs
–
–
Sb2 pairs
1
2
] ]
Sites along ( 3 ,3 ,z)
In Ge net («)
In Fe net (1)
In Ge net (,)
–
Ge
–
Sb2 pairs
Ge
Sb2 pairs
Sb2 pairs
–
–
d(Fe–Fe)
d(Ge–Ge)
d(Sb–Sb)
2.5013(2)
2.8883(2)
–
2.525(2)–2.547(1)
2.792(2)–3.086(3)
2.801(1)–2.845(6)
2.5963(8)–2.598(2)
2.732(3)–3.334(4)
2.835(3)–2.895(2)
a Symbols representing these sites are defined in Fig. 4.
˚
respectively, in FeGe0.80Sb0.20. Finally, at the extreme of
FeGe0.67Sb0.33, the Ge nets have distorted to become two
types: the Ge(1) net which segregates into isolated six-
membered rings (intra-ring 2.827(1) vs. inter-ring 3.334(4)
[2.835(2)–2.895(2) A] imply strong bonding interactions,
equivalent to a single bond such as found in elemental
˚
a-Sb [2.908 A] [45] or in the Sb2 pairs of FeSb2 [2.887(1)
A] [46]. Each filling atom (Sb or Ge) is additionally
coordinated to a hexagon of Fe atoms at Fe–Sb distances
of 2.585(2)–2.611(2) A in FeGe0.80Sb0.20 and 2.6322(3)–
2.654(2) A in FeGe0.67Sb0.33, or at shorter Fe–Ge dis-
tances of 2.524(2)–2.556(2) A in FeGe0.80Sb0.20. These
˚
˚
A distances), and the Ge(2) net which segregates into pairs
˚
˚
(short 2.732(3) vs. long 3.128(3) A distances). These two
˚
nets are also distinguishable because the Ge(1) net con-
1
2
˚
] ]
tains the Sb(1) pairs, occupying the ( 3 , 3 ,z) sites which are
twice as dense as the (0,0,z) sites, while the Ge(2) net
contains the Sb(2) pairs.
bond lengths are similar to those found in other com-
˚
pounds (e.g., Fe–Sb distances of 2.68 A in FeSb [16] and
˚
Although these structures have been represented by a
stacking of layers, it is important to realize that bonding
extends between the layers as well, in the c direction. Each
Fe atom in the 3636 net is bonded to four Ge atoms at
2.5762(6)–2.616(1) A in FeSb2 [46]; Fe–Ge(1) distance
˚
of 2.5013(2) A in FeGe [1]).
The trends in the cell parameters shown in Fig. 3 in the
Ge-rich end of FeGe12xSbx are now understandable.
Starting from FeGe (x 5 0) and increasing x, the effect of
replacing larger Sb atoms for Ge atoms produces an
overall increase in cell volume. However, this volume
increase occurs mainly because insertion of Sb2 pairs
spreads apart the nets along a, to permit reasonable Fe–Sb
bonds to form. Note that the c-axis actually contracts on
increasing x, implying strengthened bonding along this
direction, consistent with the crystal structures.
˚
2.474(1)–2.495(1) A in FeGe0.80Sb0.20 and 2.471(1)–
˚
2.520(1) A in FeGe0.67Sb0.33 in the adjacent Ge nets above
and below. These distances are reasonable, when compared
˚
to the 2.4892(3) A Fe–Ge(2) bonds found in FeGe [1].
Alternatively, the Ge atoms in the 63 nets can be regarded
as being coordinated in a trigonal prismatic fashion to six
Fe atoms. The Sb–Sb distances in the Sb2 pairs of
˚
FeGe0.80Sb0.20 [2.801(1)–2.845(6) A] and FeGe0.67Sb0.33
Fig. 5. Comparison of the structures of (a) FeGe, (b) FeGe0.80Sb0.20, and (c) FeGe0.67Sb0.33, viewed perpendicular to the c direction. Small open circles are
Fe, medium lightly-shaded circles are Ge, and large dark circles are Sb. Partial occupancies of the filling Ge atoms and Sb2 pairs are indicated.

90
A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
4. Discussion
metal–metal bonding levels favours the MnP-type struc-
ture in this range of electron count [43].
4.1. MnP- and NiAs-type phases in FeGe12xPnx
The binary transition-metal monopnictides commonly
adopt the hexagonal NiAs-type structure or its ortho-
rhombic MnP- or NiP-type derivatives [13,47], and many
workers have classified these structural differences on the
basis of simple criteria such as atomic sizes, elec-
tronegativities, and valence electron counts [19,43,48–53].
These schemes also succeed when extended to mixed-
4.2. CoSn-related phases in FeGe12xSbx
FeGe is the only germanide to adopt the hexagonal
CoSn-type structure which is encountered almost exclu-
sively in binary compounds of the first-row transition
metals and the heavier group 13 and 14 elements (In, Tl,
Sn, Pb) [55]. The hexagonal FeGe phase is formed only
within a narrow temperature range (630–7408C), trans-
forming to cubic and monoclinic polymorphs at lower and
higher temperatures, respectively [1]. Since the compounds
FeGe0.80Sb0.20 and FeGe0.67Sb0.33 could be synthesized
over a wider temperature range (at least 600–9008C), the
substitution of Sb for Ge appears to promote the stabiliza-
tion of the CoSn-type structure. Because the Sb atoms are
presumably too large to fill the hexagonal planar sites
within the 3636 Fe net left vacant when the filling Ge
atoms leave, they instead form pairs that reside in the
hexagonal channels. This suggests that it may be feasible
to insert other large atoms such as Sn, and experiments are
underway to verify this.
Unlike most intermetallic compounds which are char-
acterized by closest packing and dense arrangements of
atoms, the CoSn-type structure is unusual in featuring
large voids at the centres of hexagons in the 63 nonmetal
nets which can be potentially filled by guest atoms [2,54].
Thus, inserting large electropositive atoms R into such
vacant sites in FeGe results in the family of compounds
RFe6Ge6 (R5Li, Mg, Sc, Y, Pr, Nd, Sm, Gd–Lu, Ti, Zr,
Hf, Nb, Mn) [56–61]. The Ge atoms previously in the
plane of the 3636 Fe net are displaced above and below so
as to form Ge2 pairs, analogous to the Sb2 pairs in
FeGe0.67Sb0.33. Indeed, the compound LiFe6Ge6 would be
isostructural to FeGe0.67Sb0.33 were it not for the inserted
Li atom: LiFe6Ge6 ;hFe6(Ge4Sb2) [56]. There also exists
a Mg-filled member, MgFe6Ge6, but with a different
ordering of Ge2 pairs [57]. We may assume, following the
Zintl concept [62,63], that the electropositive atom R
donates its valence electrons to the FeGe network (i.e.,
Mg21[Fe6Ge6]22) to produce an equivalent effect to that
achieved by substitution of a nonmetal with more valence
electrons (such as Sb) for the filling Ge atoms in FeGe.
The ordering of the Ge2 or Sb2 pairs may still depend on
subtle size effects, but it is clear that electronic factors are
probably relevant in the pairing of atoms in these com-
pounds. Sb2 pairs are normally found in Zintl compounds
where electron counting rules, based on full electron
transfer from the electropositive component to each Sb
atom which attains an octet, can account well for the
bonding [64,65]. That these pairs persist in an intermetallic
structure such as the CoSn-type indicates the importance of
Sb–Sb bonding, which must make a significant contribu-
tion towards the stability of these compounds. Since the
9
metal solid solutions M12xMx Pn. However, it is not
obvious if they can be applied equally well to mixed-anion
systems, since partial substitution of a pnicogen with
another nonmetal introduces more profound effects
(changes in average anion size, electronegativity, average
number of electrons in nonmetal valence p-orbitals) than
can be realized in mixed-metal systems, where the d-
electron count is primarily changed. Moreover, in the
systems under consideration, we would not expect com-
plete substitution of Ge for Pn while retaining a MnP- or
NiAs-type structure because FeGe adopts a different
structure.
Although several kinds of structural maps have been
developed that separate the binary pnictides into the MnP-
and NiAs-type domains [48,49,52,53], the simplest is the
Mooser–Pearson plot, which uses the average principal
¯
quantum number of the valence shell n and the difference
in electronegativities of cation and anion Dx as coordi-
¯
nates [52,53]. With higher n and lower Dx, directional
bonding becomes less important, so it is not surprising that
¯
FeSb (n 5 4.5, Dx 5 0.1) adopts the more symmetric
¯
NiAs-type structure while FeP (n 5 3.5, Dx 5 0.4) and
¯
FeAs (n 5 4, Dx 5 0.3) adopt the distorted MnP-type
¯
structure. Ge substitution for P increases n and reduces Dx
in FeP, and Ge substitution for As only reduces Dx in
FeAs. Neither of these changes are sufficient to draw these
systems out of the region of the MnP-type structure, as
observed experimentally. On the other hand, Ge substitu-
¯
tion for Sb mainly reduces n in FeSb, with the expectation
that, at some point, a new phase may form which features
directional bonding. However, the structure formed is not
the predicted MnP-type, but rather a CoSn-related type for
FeGe12xSbx (0 # x # 0.4). Calculation of the electron
localization function for CoSn has indicated that bonding
within the 63 Sn layers is directional and covalent [54],
and we assume that this holds as well for FeGe0.80Sb0.20
and FeGe0.67Sb0.33
.
For transition-metal monophosphides and less reliably
for the monoarsenides, a valence electron count below 11
favours the NiAs-type and from 11 to 14, the MnP-type
structure [19,43]. Accordingly, all members of FeGe12xPx
(0.4 # x # 1.0) and FeGe12xAsx (0.3 # x # 1.0), having
electron counts from 12.3 to 13, fall within the MnP-type
region. Band structure calculations have substantiated this
empirical relationship by showing that occupation of

A.M. Mills, A. Mar / Journal of Alloys and Compounds 298 (2000) 82–92
91
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system FeSn12xSbx (0 # x # 0.2) was reported to adopt the
undistorted CoSn-type structure, it should be re-examined
in light of this proposal [66]. Furthermore, the relationship
of FeGe0.67Sb0.33 to LiFe6Ge6 suggests that a variety of
electropositive atoms R can also be inserted into a
FeGe0.67Sb0.33 host, and preliminary experiments towards
this aim confirm this. Finally, given the range of magnetic
properties exhibited by both FeGe and their filled RFe6Ge6
derivatives, we anticipate equally interesting behaviour for
FeGe12xPnx.
˚
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Acknowledgements
This work was supported by the Natural Sciences and
Engineering Research Council of Canada and the Universi-
ty of Alberta. We thank Dr. Robert McDonald (Faculty
Service Officer, X-ray Crystallography Laboratory) for
data collection and Ms. Christina Barker (Department of
Chemical and Materials Engineering) for assistance in
EDX measurements.
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