Quaternary rare earth transition metal arsenide oxides RTAsO (T = Fe, Ru, Co) with ZrCuSiAs type structure

Article abstract of DOI:10.1016/S0925-8388(99)00802-6

The equiatomic quaternary compounds RFeAsO (R = La-Nd, Sm, Gd), RRuAsO (R = La-Nd, Sm, Gd-Dy) and RCoAsO (R = La-Nd) crystallize with the tetragonal ZrCuSiAs type structure, which was refined from single-crystal X-ray diffractometer data of PrFeAsO: P4/nmm, Z = 2, a = 398.5(1) pm, c = 859.5(3) pm, R = 0.058 for 167 structure factors and 12 variable parameters. All atomic positions are fully occupied. Chemical bonding in PrFeAsO can be rationalized with oxidation numbers corresponding to the formula Pr⁺³Fe⁺²As^-3O^-2. An electron count of 18 for the iron atoms can only be achieved if Fe-Fe interactions of 281.8 pm pm are considered as bonding.

Full text of DOI:10.1016/S0925-8388(99)00802-6

Journal of Alloys and Compounds 302 (2000) 70–74
L
www.elsevier.com/locate/jallcom
Quaternary rare earth transition metal arsenide oxides RTAsO
(T5Fe, Ru, Co) with ZrCuSiAs type structure
*
¨
P. Quebe, L.J. Terbuchte, W. Jeitschko
¨
¨
¨
Anorganisch-Chemisches Institut, Universitat Munster, Wilhelm-Klemm-Straße 8, D-48149 Munster, Germany
Received 18 November 1999; accepted 1 December 1999
Abstract
The equiatomic quaternary compounds RFeAsO (R5La–Nd, Sm, Gd), RRuAsO (R5La–Nd, Sm, Gd–Dy) and RCoAsO (R5La–Nd)
crystallize with the tetragonal ZrCuSiAs type structure, which was refined from single-crystal X-ray diffractometer data of PrFeAsO:
P4/nmm, Z52, a5398.5(1) pm, c5859.5(3) pm, R50.058 for 167 structure factors and 12 variable parameters. All atomic positions are
fully occupied. Chemical bonding in PrFeAsO can be rationalized with oxidation numbers corresponding to the formula
Pr13Fe12As23O22. An electron count of 18 for the iron atoms can only be achieved if Fe–Fe interactions of 281.8 pm are considered as
bonding.
2000 Elsevier Science S.A. All rights reserved.
Keywords: Rare earth compounds; Arsenide oxides; Single crystal X-ray diffraction; Structure determination
1. Introduction
In addition to quaternary compounds with ZrCuSiAs
type structure ternary compounds with this symmetry and
identical atomic positions are known, where the silicon and
the arsenic positions of ZrCuSiAs are occupied by the
same atomic species, e.g. the silicides and germanides
ZrCuSi2 and HfCuSi2 [17,18], ZrCuGe2 and HfCuGe2
[18], the uranium compounds UTPn2 (T5Fe, Co, Ni, Cu;
Pn5P, As, Sb, Bi) [19,20], the calcium bismuth compound
CaMnBi2 [21], the lanthanum transition metal antimonides
LaT12xSb2 (T5Mn, Co, Cu, Zn, Cd) with considerable
defects for the T sites [22–24] and various series of
additional rare earth transition metal antimonides where
the defects for the T sites frequently have not been
investigated: RMnSb2 [23,24], RFeSb2 [25], RCoSb2
[24,25], RNiSb2 [26,27], RPdSb2 [27], RCuSb2 [27],
RAgSb2 [28,29], RAuSb2 [24,27], RZnSb2 [23,24] and
RCdSb2 [23,24]. With arsenic only the copper series
RCuAs2 [28] is known and it seems that a large number of
bismuthides could be prepared, since the cerium com-
pounds CeTBi2 (T5Ni, Cu, Ag, Zn) have been reported
[30]. All together more than 200 representatives with this
structure have been characterized. Among the more inter-
esting properties of such compounds mixed or intermediate
valence can be expected for compounds containing cerium,
europium, ytterbium or uranium [30–32] as well as
interesting thermal and electrical transport properties of the
The quaternary equiatomic ZrCuSiAs type structure is
very simple with only eight atoms in the tetragonal cell. It
was first described as a ‘filled’ PbFCl type structure [1].
Oxygen containing compounds with this structure were
reported in combination with sulfur, selenium and tel-
lurium as the other anionic component, e.g. the solid
electrolyte LaAgSO [2,3] with silver ions as conducting
species, the corresponding copper compound LaCuSO
[4,5], the series RCuSeO (R5Y, La, Sm, Gd [6] and
R5Nd, Gd, Dy, Bi [7,8]), BiCuSO [7], RCuSO (R5Nd,
Sm) [9] and LaCuTeO [9]. With oxygen and the pnictogens
as anionic components the following compounds were
reported: UCuPO [10], RFePO (R5La–Nd, Sm, Gd) [11],
RRuPO (R5La–Nd, Sm, Gd) [11], RCoPO (R5La–Nd,
Sm) [11], ThCuPO [12], ThCuAsO [12], RZnSbO (R5La–
Nd, Sm) [13], RMnPO (R5La–Nd, Sm, Gd–Dy) [14],
RMnAsO (R5Y, La–Nd, Sm, Gd–Dy, U) [14], RMnSbO
(R5La–Nd, Sm, Gd) [14], RZnPO (R5La, Ce) [15] and
RZnAsO (R5Y, La–Nd, Sm, Gd–Tm) [15]. Furthermore,
the sulfide and selenide fluorides BaCuSF and BaCuSeF
are known [16].
*Corresponding author. Tel.: 149-251-833-3121; fax: 149-251-833-
3136.
0925-8388/00/$ – see front matter
PII: S0925-8388(99)00802-6
2000 Elsevier Science S.A. All rights reserved.

P. Quebe et al. / Journal of Alloys and Compounds 302 (2000) 70–74
71
Table 1
more metallic compositions [31,32]. Here we report the
new quaternary series of rare earth transition metal arsen-
Lattice constants of the tetragonal compounds RTAsOa
Compound
a (pm)
c (pm)
c/a
V (nm3)
ide oxides RFeAsO, RRuAsO and RCoAsO.
LaFeAsO
CeFeAsO
PrFeAsO
NdFeAsO
SmFeAsO
GdFeAsO
LaRuAsO
CeRuAsO
PrRuAsO
NdRuAsO
SmRuAsO
GdRuAsO
TbRuAsO
DyRuAsO
LaCoAsO
CeCoAsO
PrCoAsO
NdCoAsO
403.8(1)
400.0(1)
398.5(1)
396.5(1)
394.0(1)
391.5(1)
411.9(1)
409.6(1)
408.5(1)
407.9(1)
405.0(2)
403.9(1)
402.7(1)
402.2(2)
405.4(1)
401.5(1)
400.5(1)
398.2(1)
875.3(6)
865.5(1)
859.5(3)
857.5(2)
849.6(3)
843.5(4)
848.8(1)
838.0(3)
833.7(1)
829.2(2)
819.1(7)
811.8(6)
807.8(1)
805.0(3)
847.2(3)
836.4(2)
834.4(2)
831.7(4)
2.168
2.164
2.157
2.163
2.156
2.155
2.061
2.046
2.041
2.033
2.022
2.010
2.006
2.001
2.090
2.083
2.083
2.089
0.1427
0.1385
0.1365
0.1348
0.1319
0.1293
0.1440
0.1406
0.1391
0.1380
0.1343
0.1324
0,1310
0.1302
0.1392
0.1348
0.1338
0.1319
2. Sample preparation and lattice constants
Most of the quaternary compounds reported here were
obtained first as byproducts of ternary samples without
intentional addition of oxygen. Subsequently many of
these compounds were prepared from quaternary samples,
where the oxygen was introduced by powders of the
¨
transition metal oxides: Fe2O3 (Riedel–de Haen, ‘‘rein’’),
RuO2 (Ventron, 99.9%) and Co3O4 (Merck, ‘‘rein’’). It is
not advisible to use oxides of the rare earth elements as a
source for oxygen since, because of their high stability,
they do not readily react with the other components of the
mixture. The rare earth elements were purchased in the
form of ingots with nominal purity 99.9%. Filings of these
were prepared under dry (Na) paraffin oil, which was
removed by dry n-hexane. The transition metals were
purchased as powders with nominal purities of 99.5% (Fe,
Co) and 99.9% (Ru). The arsenic pieces (.99%) were
purified by fractional crystallization. First the arsenic oxide
(As2O3) is sublimated under vacuum at 3508C, then the
elemental arsenic at 5508C.
The iron containing compounds were obtained from
samples with the atomic ratio R:Fe:Fe2O3:As53:1:1:3.
These powders (300 mg) were added to a NaCl/KCl
mixture (1:1, 2 g), sealed into evacuated silica tubes and
annealed for 2 weeks at 8008C. It should be emphasized
that the NaCl/KCl flux only dissolves minor amounts of
the metallic components. Hence, it may be regarded as a
mineralizer, which enhances the formation of the quater-
nary compounds. After the annealing, the NaCl/KCl
matrix was dissolved in water at room temperature.
For the synthesis of the compounds containing
ruthenium or cobalt we prepared first the rare earth
arsenides RAs by reaction of the elemental components in
evacuated sealed silica tubes. The samples were placed in
the hot furnace which was closed thereafter. Thus, the
expected exothermic reaction was not observed. After
annealing for 2 days at 5008C the reaction products were
stored under argon or used immediately thereafter to
prepare the quaternary compounds. These were obtained
by solid state reaction of cold pressed-pellets of the ideal
compositions RAs:Ru:RuO2 and RAs:Co:Co3O4 in the
atomic ratios 2:1:1 and 4:1:1, respectively. The annealing
in evacuated silica tubes was for 1 week at 9508C. After
grinding the products to a fine powder and cold-pressing,
the samples were annealed again at 9508C for 2 weeks.
The quaternary compounds are stable in air for long
periods of time. They are black with metallic lustre.
Energy-dispersive X-ray fluorescence analyses in a scan-
ning electron microscope did not reveal any impurity
elements heavier than sodium. In previous investigations
a Standard deviations in the place values of the last listed digits are
given in parentheses throughout the paper.
of the isotypic phosphorous compounds RTPO (T5Fe, Ru,
Co) the oxygen content of the samples had been proven by
a windowless detector [11].
The samples were characterized by Guinier powder
diagrams using monochromatized Cu Ka1 radiation with
a-quartz (a5491.30 pm, c5540.46 pm) as an internal
standard. The lattice constants (Table 1) were obtained by
least-squares fits.
3. Structure refinement of PrFeAsO
A small single-crystal (6734437 mm3), isolated from
the polycrystalline sample prepared with the NaCl/KCl
flux, was used for the X-ray data collection on a four-circle
diffractometer (CAD4) using graphite monochromatized
Mo Ka radiation and a scintillation counter with pulse-
height discrimination. The scans were along the diffraction
streaks up to 2u5708 with background counts at both ends
of each scan and scan velocities optimized by fast pre-
scans. An absorption correction was applied from psi-scan
data. A total of 1145 reflections was measured in one half
of the reciprocal sphere, which resulted in 214 unique
reflections (internal residual on intensity values: Ri5
0.123). The lattice constants obtained on the four-circle
diffractometer (a5397.9(1) pm, c5858.0(2) pm) are
slightly smaller than those from the Guinier powder data
(Table 1), since they are affected by systematic errors due
to absorption.
The isotypy of the structure with that of tetragonal
ZrCuSiAs [1] was recognized from previous investigations
[10–15]. The high Laue symmetry had been confirmed
prior to the data averaging and the positional parameters of
PrFePO [11] were used as starting atomic parameters for

72
P. Quebe et al. / Journal of Alloys and Compounds 302 (2000) 70–74
Table 2
Atomic parameters of PrFeAsOa
Atom
P4/nmm
Occupancy
x
y
z
B
11 5B22
B33
Beq
Pr
Fe
As
O
2c
2b
2c
2a
101.2(5)
98(1)
100.4(9)
96(5)
1/4
3/4
1/4
3/4
1/4
1/4
1/4
1/4
0.1399(2)
1/2
0.6565(3)
0
0.03(4)
0.13(8)
0.31(6)
1.1(6)
0.71(5)
0.86(11)
0.68(10)
0.4(6)
0.26(4)
0.38(6)
0.44(4)
0.9(4)
a The occupancy parameters were obtained in a separate series of least-square cycles. In the final cycles the occupancy parameters were assumed to be
ideal. The anisotropic displacement parameters are of the form exp[20.25(h2a*2B11 1???)]. For symmetry reasons the values B12, B13 and B23 are all equal
to zero. The last column contains the equivalent isotropic displacement parameters Beq (31024, in units of pm2).
Table 3
Interatomic distances in PrFeAsOa
Pr:
4 O
232.7
331.7
368.1
370.5
Fe:
4 As
4 Fe
4 Pr
240.4
281.8
368.1
As:
4 Fe
4 Pr
4 O
240.4
331.7
356.2
O:
4 Pr
4 O
4 As
232.4
281.8
356.2
4 As
4 Fe
4 Pr
a All distances shorter than 380 pm are listed. The standard deviations are all 0.2 pm or less.
the structure refinement with a full-matrix least-squares
program [33] using atomic scattering factors, corrected for
anomalous dispersion, as provided by the program. The
weighting scheme accounted for the counting statistics,
and a parameter correcting for isotropic secondary extinc-
tion was optimized as a least-squares variable. There are
Z52 formula units in the cell of space group P4/nmm
for 167 structure factors greater than 2s. The weighted
residual was 0.146 for 214 F2 values and 12 variable
parameters. The highest residual electron densities in the
23
˚
final difference Fourier synthesis of 9.7 and 4.0 e A
were too close to the fully occupied Pr (61 pm) and As (37
pm) positions to be suited for additional atomic sites. The
final atomic parameters and interatomic distances are listed
in Tables 2 and 3.
(No. 129), and the calculated density is 7.03 g?cm23
.
To check the composition we refined occupancy values
in one series of least-squares cycles together with aniso-
tropic displacement parameters and a fixed scale factor.
The results are shown in the third column of Table 2. It
can be seen that all atoms have the ideal occupancy values
within three standard deviations. Therefore in the final
least-squares cycles we resumed to the ideal occupancy
values. A conventional residual of R50.058 was obtained
4. Discussion
The new ZrCuSiAs type compounds RFeAsO, RRuAsO
and RCoAsO are represented by their cell volumes in Fig.
1 together with the cell volumes of isotypic compounds
with related compositions reported earlier [11]. In general,
Fig. 1. Cell volumes of compounds RTPnO (T5Fe, Ru, Co; Pn5P, As) with ZrCuSiAs type structure.

P. Quebe et al. / Journal of Alloys and Compounds 302 (2000) 70–74
73
the cell volumes of the new arsenide oxides follow the
well known lanthanoid contraction of the trivalent rare
earth elements. For the series of arsenide oxides a slight
deviation from the smooth decrease, almost within the
error limits, occurs for CeCoAsO, which may be rational-
ized by an intermediate or mixed valence of cerium
(Ce13 / 14). In the series of compounds containing phos-
phorus, larger deviations occur for CeFePO and CeCoPO
indicating a larger proportion of Ce14. This may be
rationalized by the higher electronegativity of phosphorus
as compared to arsenic. The fact that CeRuPO fits rather
well into the plot of the compounds of the corresponding
trivalent rare earth elements seems to be an exception.
However, similar ‘‘exceptions’’ were observed for the cell
volumes of the polyphosphides and polyarsenides RT4P12
and RT4As12, where the deviations are large for T5Fe and
small for T5Ru and Os [34]. The fact that the lanthanum
compounds are formed for all of the investigated series
RTPO and RTAsO suggests that it might be possible to
prepare one or the other corresponding strontium or barium
compound.
In contrast, with the smaller rare earth elements, we
have been successful to prepare only TbRuAsO and
DyRuAsO. The corresponding compounds RFeAsO, brief-
ly mentioned as ternary ‘TbFeAs’ and ‘DyFeAs’ at a
conference [35], have not yet been reproduced as quater-
nary arsenide oxides. These compounds were obtained as
byproducts in small amounts and we have not reproduced
these results with the intentional addition of oxygen. In all
cases where the oxygen has been added intentionally in the
form of the binary transition metal oxides Fe2O3, RuO2
and Co3O4 there was excellent agreement (within four
combined standard deviations) between the lattice con-
stants of the ‘‘ternary’’ and quaternary samples. For that
reason we believe that the ZrCuSiAs (‘‘filled’’ PbFCl) type
compounds found in ‘‘ternary’’ samples were stabilized by
oxygen and not by nitrogen, although it seems possible
that the corresponding nitrides also could be prepared.
Since the scattering power of these light elements is small,
the observed Guinier powder patterns of these quaternary
compounds agree well with patterns calculated, assuming
both the filled or unfilled PbFCl type structure.
the iron atoms obtain the oxidation number 12, according
to the formula Pr13Fe12As23O22. Thus, each iron atom
possesses six electrons (a ‘‘d6-system’’) which are not
involved in interactions with the atoms of the other
elements.
Each arsenic atom has four iron neighbours, and since
we assume its octet to be filled, we could count two
electrons for each of the four As–Fe interactions. How-
ever, if we look at the coordination of the arsenic atoms
(Fig. 2) we see that the four iron neighbours of an arsenic
atom are all at one side; at the opposite side the arsenic
atom is coordinated by praseodymium atoms. Thus, a
count of two electrons for each As–Fe interaction is at best
an upper limit. Conversely, we can also count these
bonding electrons at the iron atoms: at most two for each
of the four tetrahedral Fe–As bonds. Hence, we may
obtain an electron count of 14 for each iron atom: (at most)
8 electrons from the 4 Fe–As interactions plus 6 electrons
of the d6-system. We note that each iron has, in addition to
the 4 strongly bonded arsenic atoms, 4 iron neighbours in
square-planar arrangement with the rather long Fe–Fe
distance of 281.8 pm. Nevertheless, we can assume some
Fe–Fe bonding. The electrons needed for these interactions
come from the d6-system. Since the electrons of these
bonding Fe–Fe interactions are now counting at both iron
atoms of an Fe–Fe bond, the total electron count for each
Chemical bonding in the quaternary pnictide oxides can
be rationalized by simple models using PrFeAsO as an
example. Since the praseodymium atoms are relatively far
apart from each other (Table 3) we exclude any substantial
Pr–Pr bonding and we count the valence electrons of the
praseodymium atoms at the more electronegative arsenic
and oxygen atoms. In analogy, there are no short As–As,
As–O or O–O interactions and therefore, in agreement
with the octet rule, we arrive at the oxidation numbers 23
and 22 for the arsenic and oxygen atoms, respectively.
This does not contradict more or less covalent Fe–As or
Fe–O interactions. The corresponding bonding electrons
are only counted at the arsenic and oxygen atoms. The
important result of this rationalization lies in the fact that
Fig. 2. Crystal structure and near-neighbour coordinations of the tetra-
gonal ZrCuSiAs type compound PrFeAsO. The site symmetries of the
central atoms of the coordination polyhedra are indicated in parentheses.

74
P. Quebe et al. / Journal of Alloys and Compounds 302 (2000) 70–74
[11] B.I. Zimmer, W. Jeitschko, J.H. Albering, R. Glaum, M. Reehuis, J.
Alloys Comp. 229 (1995) 238.
[12] J.H. Albering, W. Jeitschko, Z. Naturforsch. 51b (1996) 257.
[13] P. Wollesen, J.W. Kaiser, W. Jeitschko, Z. Naturforsch. 52b (1997)
1467.
iron atom is greater than 14, possibly as high as 18, the
inert gas shell.
Acknowledgements
[14] A.T. Nientiedt, W. Jeitschko, P.G. Pollmeier, M. Brylak, Z. Natur-
forsch. 52b (1997) 560.
[15] A.T. Nientiedt, W. Jeitschko, Inorg. Chem. 37 (1998) 386.
[16] W.J. Zhu, Y.Z. Huang, F. Wu, C. Dong, H. Chen, Z.X. Zhao, Mater.
Res. Bull. 29 (1994) 505.
[17] H. Sprenger, J. Less-Common Met. 34 (1974) 39.
[18] L.S. Andrukhiv, L.A. Lysenko, Ya.P. Yarmolyuk, E.I. Gladyshevskii,
Dopov. Akad. Nauk Ukr. RSR, Ser. A (1975) 645.
We thank Mrs. U. Ch. Rodewald for the competent
collection of the diffractometer data and Mr. K. Wagner for
investigating our samples on the scanning electron micro-
¨
scope. We also thank Dr. G. Hofer (Heraeus
ˆ
Quarzschmelze) and the Rhone-Poulenc company for
´
´
[19] J. Ste¸pien-Damm, D. Kaczorowski, R. Troc, J. Less-Common Met.
generous gifts of silica tubes and rare earth elements. This
work has been supported by the Deutsche Forschungs-
gemeinschaft, the Fonds der Chemischen Industrie and the
International Centre for Diffraction Data.
132 (1987) 15.
[20] D. Kaczorowski, J. Alloys Comp. 186 (1992) 333.
¨
[21] E. Brechtel, G. Cordier, H. Schafer, Z. Naturforsch. 35b (1980) 1.
¨
[22] G. Cordier, H. Schafer, P. Woll, Z. Naturforsch. 40b (1985) 1097.
[23] O. Sologub, K. Hiebl, P. Rogl, O.I. Bodak, J. Alloys Comp. 227
(1995) 40.
[24] P. Wollesen, W. Jeitschko, M. Brylak, L. Dietrich, J. Alloys Comp.
245 (1996) L5.
References
[25] A. Leithe-Jasper, P. Rogl, J. Alloys Comp. 203 (1994) 133.
[26] Yu.V. Pankevich, V.K. Pecharskii, O.I. Bodak, Izv. Akad. Nauk
SSSR, Met. (1983) 227.
[1] V. Johnson, W. Jeitschko, J. Solid State Chem. 11 (1974) 161.
[2] M. Palazzi, C. Carcaly, J. Flahaut, J. Solid State Chem. 35 (1980)
150.
¨
[27] O. Sologub, K. Hiebl, P. Rogl, H. Noel, O.I. Bodak, J. Alloys Comp.
[3] M. Palazzi, S. Jaulmes, Acta Crystallogr. B37 (1981) 1337.
[4] M. Palazzi, C.R. Acad. Sci. Paris 292 (1981) 789.
[5] Y. Takano, C. Ogawa, Y. Miyahara, H. Ozaki, K. Sekizawa, J.
Alloys Comp. 249 (1997) 221.
210 (1994) 153.
¨
[28] M. Brylak, M.H. Moller, W. Jeitschko, J. Solid State Chem. 115
(1995) 305.
¨
[29] O. Sologub, H. Noel, A. Leithe-Jasper, P. Rogl, O.I. Bodak, J. Solid
[6] W.J. Zhu, Y.Z. Huang, C. Dong, Z.X. Zhao, Mater. Res. Bull. 29
(1994) 143.
State Chem. 115 (1995) 441.
[30] H. Flandorfer, O. Sologub, C. Godart, K. Hiebl, A. Leithe-Jasper, P.
¨
[7] A.M. Kusainova, P.S. Berdonosov, L.G. Aksel’rud, L.N. Kholod-
kovskaya, V.A. Dolgikh, B.A. Popovkin, J. Solid State Chem. 112
(1994) 189.
[8] P.S. Berdonosov, A.M. Kusainova, L.N. Kholodkovskaya, V.A.
Dolgikh, L.G. Aksel’rud, B.A. Popovkin, J. Solid State Chem. 118
(1995) 74.
Rogl, H. Noel, Solid State Commun. 97 (1996) 561.
[31] Y. Muro, N. Takeda, M. Ishikawa, J. Alloys Comp. 257 (1997) 23.
[32] K.D. Myers, S.L. Bud’ko, I.R. Fisher, Z. Islam, H. Kleinke, A.H.
Lacerda, P.C. Canfield, J. Magn. Magn. Mater. 205 (1999) 27.
[33] G.M. Sheldrick, SHELX-97, a Program Package for the Solution
¨
¨
and Refinement of Crystal Structures, Universitat Gottingen, Ger-
[9] B.A. Popovkin, A.M. Kusainova, V.A. Dolgikh, L.G. Aksel’rud,
Russian J. Inorg. Chem. 43 (1998) 1471.
many, 1997.
[34] Ch.B.H. Evers, W. Jeitschko, L. Boonk, D.J. Braun, Th. Ebel, U.D.
Scholz, J. Alloys Comp. 224 (1995) 184.
¨
[10] D. Kaczorowski, J.H. Albering, H. Noel, W. Jeitschko, J. Alloys
¨
[35] L.J. Terbuchte, W. Jeitschko, Z. Kristallogr. 186 (1989) 291.
Comp. 216 (1994) 117.