Semiconducting La2AuP3, the metallic conductor Ce2AuP3, and other rare-earth gold phosphides Ln2AuP3 with two closely related crystal structures

Article abstract of DOI:10.1002/1521-3749(200107)627:7<1699::AID-ZAAC1699>3.0.CO;2-V

The compounds Ln₂AuP₃ were synthesized by reaction of the elemental components in evacuated silica tubes. Their crystal structures were determined from single-crystal diffractometer data. The compounds with Ln = La, Ce, and Pr crystallize with an orthorhombic U₂NiC₃ type structure (Pnma, Z = 4). The structure refinement for Ce₂AuP₃ resulted in a = 774.14(6) pm, b = 421.11(4) pm, c = 1612.3(1) pm, R = 0.019 for 1410 structure factors and 38 variable parameters. For Pr₂AuP₃ a residual of R = 0.024 was obtained. Nd₂AuP₃ crystallizes with a monoclinic distortion of this structure: P2₁/c, Z = 4, a = 416.14(4) pm, b = 768.87(6) pm, c = 1647.1(2) pm, β = 104.06(1)°, R = 0.022 for 1361 F values and 56 variables. The near-neighbor coordinations of the two structures are nearly the same. In both structures the gold and phosphorus atoms form two-dimensionally infinite nets, where the gold atoms are tetrahedrally coordinated by phosphorus atoms with Au-P distances varying between 245.8 and 284.2 pm. Two thirds of the phosphorus atoms form pairs with single-bond distances varying between 217.7 and 218.9 pm. Thus, using oxidation numbers the structures can be rationalized with the formulas (Ln⁺³)₂[AuP₃]^-6 and (Ln⁺³)₂Au⁺¹(P₂)^-4P ^-3. Accordingly, La₂AuP₃ is a diamagnetic semiconductor. Pr₂AuP₃ is semi-conducting with an antiferromagnetic ground state, showing metamagnetism with a critical field of B_c = 0.5(±0.1) T. In contrast, the cerium compound is a metallic conductor, even though its cell volume indicates that the cerium atoms are essentially trivalent, as is also suggested by the ferro- or ferrimagnetic behavior of the compound. WILEY-VCH Verlag GmbH, 2001.

Full text of DOI:10.1002/1521-3749(200107)627:7<1699::AID-ZAAC1699>3.0.CO;2-V

Semiconducting La2AuP3, the Metallic Conductor Ce2AuP3,
and other Rare-Earth Gold Phosphides Ln2AuP3 with Two CloselyRelated
Crystal Structures
Marcus Eschen, Gunter Kotzyba, Bernd KuÈnnen, and Wolfgang Jeitschko*
MuÈnster, Anorganisch-Chemisches Institut der WestfaÈlischen Wilhelms-UniversitaÈt
Received November 21st, 2000.
Dedicated to Professor Robert SchoÈllhorn on the Occasion of his 65th Birthday
Abstract. The compounds Ln2AuP3 were synthesized by
reaction of the elemental components in evacuated silica
tubes. Their crystal structures were determined from single-
crystal diffractometer data. The compounds with Ln = La,
Ce, and Pr crystallize with an orthorhombic U2NiC3 type
structure +Pnma, Z = 4). The structure refinement for Ce2AuP3
coordinated by phosphorus atoms with Au±P distances vary-
ing between 245.8 and 284.2 pm. Two thirds of the phos-
phorus atoms form pairs with single-bond distances varying
between 217.7 and 218.9 pm. Thus, using oxidation numbers
the structures can be rationalized with the formulas
+Ln+3)2[AuP3]±6 and +Ln+3)2Au+1+P2)±4P±3. Accordingly,
La2AuP3 is a diamagnetic semiconductor. Pr2AuP3 is semi-
conducting with an antiferromagnetic ground state, showing
metamagnetism with a critical field of Bc = 0.5+± 0.1) T. In
contrast, the cerium compound is a metallic conductor, even
though its cell volume indicates that the cerium atoms are
essentially trivalent, as is also suggested by the ferro- or
ferrimagnetic behavior of the compound.
resulted
in
a = 774.14+6) pm,
b = 421.11+4) pm,
c =
1612.3+1) pm, R = 0.019 for 1410 structure factors and 38 vari-
able parameters. For Pr2AuP3 a residual of R = 0.024 was
obtained. Nd2AuP3 crystallizes with a monoclinic distortion
of this structure: P21/c, Z = 4, a = 416.14+4) pm, b =
768.87+6) pm, c = 1647.1+2) pm, b = 104.06+1)°, R = 0.022 for
1361 F values and 56 variables. The near-neighbor coordina-
tions of the two structures are nearly the same. In both struc-
tures the gold and phosphorus atoms form two-dimensio-
nally infinite nets, where the gold atoms are tetrahedrally
Keywords: Rare earth metals; Gold; Phosphides; Crystal
structure; Electrical conductivity; Magnetic properties
Der Halbleiter La2AuP3, der metallische Leiter Ce2AuP3 und andere
Seltenerd-Gold-Phosphide Ln2AuP3 mit zwei nahe verwandten Kristallstrukturen
InhaltsuÈbersicht. Die Verbindungen Ln2AuP3 wurden durch
Reaktion der Elemente in evakuierten Quarzglasampullen
hergestellt und ihre Kristallstrukturen anhand von Einkri-
stall-Diffraktometerdaten bestimmt. Die Verbindungen mit
Ln = La, Ce und Pr kristallisieren in der orthorhombischen
U2NiC3-Struktur +Pnma, Z = 4). Die Strukturverfeinerung
von Ce2AuP3 ergab: a = 774,14+6) pm, b = 421,11+4) pm,
c = 1612,3+1) pm, R = 0,019 fuÈr 1410Strukturfaktoren und
38 variable Parameter. FuÈr Pr2AuP3 wurde ein Restwert von
R = 0,024 bestimmt. Nd2AuP3 kristallisiert in einer mono-
klinen Verzerrungsvariante dieser Struktur: P21/c, Z = 4,
a = 416,14+4) pm, b = 768,87+6) pm, c = 1647,1+2) pm, b =
104,06+1)°, R = 0,022 fuÈr 1361 F-Werte und 56 Variable. Die
Atomumgebungen in den zwei Strukturen sind nahezu iden-
tisch. In beiden Strukturen bilden die Gold- und Phosphor-
atome ein zweidimensional-unendliches Netz, wobei die
Goldatome von Phosphoratomen tetraedrisch umgeben sind,
mit Au±P-AbstaÈnden von 245,8 bis 284,2 pm. Zwei Drittel
der Phosphoratome bilden jeweils Paare mit Einfach-
bindungsabstaÈnden zwischen 217,7 und 218,9 pm. Unter
Verwendung von Oxidationszahlen koÈnnen die Struk-
turen somit durch die Formeln +Ln+3)2[AuP3]±6 und
+Ln+3)2Au+1+P2)±4P±3 beschrieben werden. Dementspre-
chend ist La2AuP3 ein diamagnetischer Halbleiter. Pr2AuP3
ist halbleitend mit einem antiferromagnetischen Grund-
zustand, welcher Metamagnetismus mit einer kritischen
FeldstaÈrke von Bc = 0,5+± 0,1) T zeigt. Im Gegensatz dazu
hat die Cer-Verbindung metallische LeitfaÈhigkeit, obwohl ihr
Zellvolumen im wesentlichen dreiwertigem Cer entspricht,
was auch durch das ferro- oder ferrimagnetische Verhalten
der Verbindung nahegelegt wird.
Introduction
Only few ternary pnictides of the rare earth elements
with gold have been reported, most of them with anti-
mony as the pnictogen component. The series of
the cubic antimonides Ln3Au3Sb4 +Ln = Y, Nd, Sm,
Gd±Tm, Lu) with Y3Au3Sb4 type structure is known
* W. Jeitschko,
Anorganisch-Chemisches Institut, UniversitaÈt MuÈnster,
Wilhelm-Klemm-Straûe 8,
D-48149 MuÈnster, Germany
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708 Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0044±2313/01/6271699±1708 $ 17.50+.50/0 1699

M. Eschen, G. Kotzyba, B. KuÈnnen, W. Jeitschko
for some time [1]. Europium forms the hexagonal
equiatomic series EuAuPn +Pn = P, As, Sb) with an
ordered Ni2In +ZrBeSi) type structure [2±4]. YbAuSb
has the lower symmetry LiGaGe type structure [5, 6].
A LiGaGe type structure has also been found for the
bismuthide EuAuBi [5], while YbAuBi is dimorphic
with a LiGaGe type structure at high and a MgAgAs
type structure at low temperature [5]. CeAuSb is re-
ported to crystallize with a CaIn2 type structure [3].
The only other compounds seem to be the ones crys-
tallizing with the HfCuSi2 +ZrCuSiAs) type structure:
LnAuSb2 +Ln = La±Nd, Sm) [7, 8]. Of the presently
reported ternary phosphides Ln2AuP3 +Ln = La±Nd)
we have already given a brief account on the crystal
structure of the compounds with Ln = La±Pr [9].
Fig. 1 Cell volumes of the phosphides Ln2AuP3 +Ln =
La±Nd).
crystals of all compounds show metallic luster, like
crystals of elemental silicon. The powders are black.
They were characterized through their Guinier pat-
terns, recorded with monochromated CuKa1 radiation
using a-quartz +a = 491.30pm, c = 540.46 pm) as an
internal standard. To assure proper assignement of
indices the observed powder patterns were compared
with the calculated ones [10], eventually using the po-
sitional parameters as obtained from the single-crystal
investigations. The lattice constants +Table 1) were
refined by least-squares fits. The cell volumes show
the expected lanthanoid contraction +Fig. 1).
Sample Preparation and Lattice Constants
The rare earth metals were purchased in the form of ingots
+nominal purity 99.9%), stored under dry paraffin oil. They
were cut to pieces, washed in dry +Na) n-hexane, and arc-
melted to evaporate any hydrogen prior to the reactions.
Pieces of the resulting rare earth metal ingots and gold wire
+99.9%) were cold-pressed to pellets. These pellets were
placed in silica tubes together with pieces of semiconductor
grade red phosphorus +6 N, Hoechst, Werk Knapsack). The
silica tubes were heated under high vacuum to evaporate the
water content of the tubes, they were then flushed with puri-
fied argon, and sealed under high vacuum. The elemental
components had the ideal atomic ratio +2 : 1 : 3). These sam-
ples were placed in a cold furnace, heated to 400 °C within
20min, held at that temperature for 16 h, heated to 800 or
900 °C with a rate of 5 °C/h, further annealed at that tem-
perature for 100 h, and furnace-cooled. The silica tubes were
not visibly attacked by the samples. They were crushed in air.
The samples Ln2AuP3 with Ln = La, Ce, and Pr were ob-
tained in single-phase form. The Guinier powder patterns of
the neodymium compound showed contamination with ele-
mental gold +diffuse diffraction lines) and NaCl-type NdP.
The latter is sensitive to air. The samples were routinely in-
vestigated with a scanning electron microscope. The ternary
phosphides crystallize in the form of elongated prisms.
Energy-dispersive X-ray analyses of these crystals did not
reveal any impurity elements heavier than sodium.
Electrical ConductivityMeasurements
Sintered, compact, polycrystalline pieces of the compounds
Ln2AuP3 with Ln = La, Ce, and Pr were investigated by the
four-probe van der Pauw method [11]. The samples had irre-
gular shapes with diameters varying between 0.2 and
0.5 mm. For the contacts a well-conducting silver glue was
used, which hardens by evaporation of the solvent, as
described earlier [12]. The contacted samples were slowly
pushed into and subsequently pulled out of a Dewar which
was partially filled by liquid helium, while the temperatures
and the resistivities were measured continuously. Each com-
pound was investigated at least twice. In addition, for the
cerium compound we also investigated an elongated single-
crystal. The results were completely reproducible. Because
of the uncertainty in estimating the geometry of the con-
tacted samples, the absolute values of the specific resis-
The ternary compounds are stable on air for long peri-
ods of time, also in powdered form. Well-developed
Table 1 Lattice constants of the orthorhombic rare-earth metal gold phosphides Ln2AuP3 +Ln = La, Ce, Pr) and the corre-
a)
sponding monoclinic neodymium compound Nd2AuP3
Compound
sp. gr.
a/pm
b/pm
c/pm
a/°
b/°
V/nm3
La2AuP3
Ce2AuP3
Pr2AuP3
Nd2AuP3
Pnma
Pnma
Pnma
P21/c
P21/n
783.62+8)
774.14+6)
770.92+7)
416.14+4)
768.87+6)
425.40+4)
421.11+4)
418.75+4)
768.87+6)
416.14+4)
1627.5+2)
1612.3+1)
1605.6+2)
1647.1+2)
1597.8+2)
±
±
0.54253+2)
0.52561+2)
0.51832+2)
0.51121+3)
0.51121+3)
±
±
±
±
±
104.06+1)
±
b)
Nd2AuP3
89.43+1)
a)
Standard deviations are listed in parentheses in the place values of the least significant digits throughout the paper.
b)
Lattice constants of Nd2AuP3 in a setting P21/n with analogous lattice constants as obtained for the related orthorhombic compounds. The transforma-
tion matrix from the nonstandard P21/n setting with a as the unique axis to the standard P21/c setting with b as the unique axis is 010/100/011.
1700
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708

La2AuP3, Ce2AuP3, and other Rare-Earth Gold Phosphides
be rationalized by a small admixture of Ce+4 in the
compound, which can also be concluded from the
magnetic properties +see below), even though the plot
of the cell volumes +Fig. 1) does not give any indica-
tion for Ce+4. On the other hand, phosphides and
polyphosphides of cerium frequently show mixed or
intermediate valent behavior [13, 14, and references
therein], and only few mobile electrons are needed at
the Fermi surface to result in metallic behavior. The
discontinuity of the electrical resistivity of Ce2AuP3 at
37 ± 3 K corresponds within the error limits to the
onset of the magnetic order of this ferromagnetic
compound.
tivities are correct only within a factor of two. The relative
values at different temperatures, however, are much more
reliable.
The electrical resistivities q of the three phosphides
Ln2AuP3 are plotted as a function of temperature
+Ln = Ce) or reciprocal temperature +Ln = La, Pr) in
the upper parts of Figs. 2±4. The lanthanum and
the praseodymium compounds show semiconducting
behavior: their resistivities q decrease with increasing
temperature. For these two compounds we have
plotted the resistivity values as functions of the reci-
procal temperature in order to determine the band
gaps Ea from the slopes of the q vs. 1/T plots at low
temperature, according to the Arrhenius equation
q = q0 exp Ea/2 kT or lnq ± lnq0 = Ea/2 kT. The resulting
activation energies are all rather small and most likely
correspond to various impurity levels. For La2AuP3
two such activation energies could be discerned with
Ea = 0 .0 2 eV andEa = 0.09 eV +Fig. 2) for each of the
two investigated samples. For the sample of Pr2AuP3
the +probably non-intrinsic) band gap amounts to
Ea = 0 .0 7 eV.
In contrast, the cerium compound Ce2AuP3 shows
metallic conductivity: the absolute values of the elec-
trical resistivities are much smaller than those of the
semiconductors La2AuP3 and Pr2AuP3, and also the
resistivity of the cerium compound increases with
increasing temperature. This differing behavior of the
cerium compound was observed both for a polycrys-
talline sample as well as for a single crystal +the same
one as used for the structure determination). It may
Magnetic Properties
The magnetic susceptibilities of the three compounds
Ln2AuP3 +Ln = La±Pr) were investigated with a SQUID
magnetometer +Quantum Design, Inc.) between 2 K and
room temperature. Polycrystalline samples of between 5 and
20mg were enclosed in thin-walled silica tubes of about
2 mm diameter. They were investigated with magnetic flux
densities of up to ± 5.5 T. In general, the samples were
cooled in zero magnetic field and slowly heated in various
magnetic fields. To check for ferromagnetic impurities the
samples were also investigated with high magnetic flux den-
sities. Only small amounts of ferromagnetic impurities were
detected this way in our samples, and no extrapolations to
infinite magnetic flux densities were necessary.
The lanthanum compound La2AuP3, where the lantha-
num atoms are not expected to carry magnetic
moments, because of the empty f shell, shows +nearly)
temperature-independent diamagnetic behavior +Fig. 2).
The upturn of the magnetic susceptibility v at low
temperatures is frequently observed for diamagnetic
or Pauli-paramagnetic samples, and is usually ascribed
to +very small amounts of) paramagnetic impurities
+ªCurie tailº). Nevertheless, we have evaluated
these data with the modified Curie-Weiss law
v = v0 + C/+T ± H). For the temperature-independent
term we obtained
a
susceptibility of v0 =
±1.76+± 0.02) ´ 10±9 m3/mol. From the temperature-
dependent term we derived a magnetic moment of
lexp = 0.06+± 0.01) lB/mol. This value is indeed very
small as compared to the value of leff = 1.73 lB
expected for one localized unpaired d electron. Thus,
the gold atoms do not carry magnetic moments, as
would be expected for Au+I) atoms.
The reciprocal susceptibilities 1/v of Ce2AuP3 and
Pr2AuP3 +Figs. 3 and 4) follow the Curie-Weiss law.
The magnetic moments derived from the linear por-
tions of the 1/v vs. T plots above 150K amount to
lexp = 2.50+± 0.02) lB/Ce and lexp = 3.68+± 0.02) lB/Pr,
respectively. The corresponding Weiss constants
amount to H = 9+± 3) K for Ce2AuP3 and H = 3+± 3) K
for Pr2AuP3. The magnetic moments are in good
agreement with the theoretical values of leff = 2.54 lB/
Ce3+ and leff = 3.58 lB/Pr3+ expected for the trivalent
Fig. 2 Temperature dependence of the electrical resistivity
q and the magnetic susceptibility v of La2AuP3.
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708
1701

M. Eschen, G. Kotzyba, B. KuÈnnen, W. Jeitschko
Fig. 4 Electrical resistivity and reciprocal susceptibility of
Pr2AuP3.
Fig. 3 Electrical and magnetic properties of Ce2AuP3. In
the upper diagram the electrical resistivity is plotted as a
function of temperature. The other diagrams show the reci-
procal susceptibility and the magnetic susceptibility at low
temperature, measured at low magnetic flux density
+0.002 T). The susceptibility at low temperature was deter-
mined twice: by cooling the sample in zero field +zfc) and in
the magnetic field +fc). For the latter curve the derivative is
also shown +ªkink point determinationº).
rare earth atoms. On close inspection, it can be seen
that the temperature-dependence of the reciprocal
susceptibility of the cerium compound shows a small
deviation from linearity. Such deviations are typical
for mixed-valent +3/+4 behavior of cerium [14±18],
and this small deviation from the Curie-Weiss law cor-
relates with the metallic conductivity of this com-
pound.
Fig. 5 Magnetization curves of ferromagnetic Ce2AuP3 and
At low temperature the reciprocal susceptibility 1/v
of Ce2AuP3 suggests ferromagnetic behavior, which
would be in agreement with the positive Weiss con-
stant H = 9+± 3) K. This seems to be confirmed by the
magnetization curve of Ce2AuP3 +upper part of Fig. 5)
which shows a hysteresis as it is typical for a ferro-
magnet. However, at 5.5 T +the highest magnetic flux
metamagnetic Pr2AuP3.
density obtainable with our magnetometer) a mag-
netic moment of 0.78+± 0.02) lB/Ce is obtained, which
amounts to only 36.4% of the theoretical saturation
magnetization of 2.14 lB/Ce3+ +calculated from the re-
1702
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708

La2AuP3, Ce2AuP3, and other Rare-Earth Gold Phosphides
a)
Table 2 Crystal data of Ce2AuP3, Pr2AuP3, and Nd2AuP3
lation lsm+calc) = g ´ J lB). Thus, a second step in the
magnetization +as is known for metamagnets) seems
possible, in which case the ground state would be
ferrimagnetic. An accurate Curie temperature for
Ce2AuP3 was obtained from a susceptibility determi-
nation of a field-cooled sample as shown in the bot-
tom part of Fig. 3. The derivative of this curve re-
sulted in TC = 31+± 1) K. This value is in reasonable
agreement with the discontinuity of the electrical
resistivity curve which on cooling has an onset at
37+± 3) K +top diagram of Fig. 3).
At low temperature the reciprocal susceptibility of
Pr2AuP3 +lower part of Fig. 4) shows an upturn, as is ty-
pical for antiferromagnetic order, with a minimum cor-
responding to a NeÂel temperature of TN = 8.7+± 0.6) K.
The magnetization curve +lower part of Fig. 5) recorded
at a temperature of 2 K is typical for a metamagnet.
The spins are oriented antiparallel up to a magnetic flux
density of about 0.5 T. With increasing magnetic field
above 0.5 T the magnetic moments begin to reorient,
reaching a total moment of lsm+exp) = 1.52+± 0.02) lB/Pr
at 5.5 T. This value amounts to less than 50% of the
theoretical saturation magnetization of lsm+calc) = 3.2 lB/
Pr. Hence, again +like in Ce2AuP3) a second meta-
magnetic step seems possible.
Compound
Ce2AuP3
570.1
Pr2AuP3
571.7
Nd2AuP3
578.4
Formula mass
Space group
Pnma +No. 62) Pnma +No. 62) P21/c +No. 14)
Formula units/cell, Z
Pearson symbol
4
4
4
oP24
oP24
7.33
25 ´ 25 ´ 15080
1.38
62
mP24
7.52
Calculated density/g/cm3 7.21
Crystal size/lm3
Max/min transmission
2h range up to/°
Range in hkl
Total number of
reflections
40 ´ 40 ´ 500
1.26
8065
±7, ±13, 0±25 ±11, ±6, ±24
566074506472
´ 80 ´ 25
1.51
±6, ±11, ±23
Independent reflections
1516
1042
0.045
1618
0.053
Internal residual, Ri +F 2) 0.035
Reflections with Fo > 2r
Variables
1410959
38
1361
38
56
R +no. of F )
0.019 +1410)
0.047 +1516)
4.2
0.024 +959)
0.064 +1042)
4.1
0.022 +1361)
0.053 +1618)
2.7
Rw +no. of F 2)
Highest residual
3
Ê
peaks/e/A
a)
The residuals are defined as R = R||Fo| ± |Fc||/R|Fo|;
Rw = {R[w+Fo2 ± Fc2)2]/R[w+Fo2 )2]}1/2 where w = 1/[r2+F2o) + +aP)2 + bP] and
P is [2 F2c + Max+F2o, 0)]/3.
Table 3 Atomic coordinates and equivalent isotropic dis-
placement parameters +104 ´ pm2) of Ce2AuP3, Pr2AuP3, and
a)
Nd2AuP3
Atom
x
y
z
Beq
Ce2AuP3 +Pnma)
Crystal Structures
1
Ce1
Ce2
Au
P1
P2
P3
4 c
4 c
4 c
4 c
4 c
4 c
0.09753+3)
0.23538+3)
0.00988+2)
0.0311+1)
0.2026+1)
0.3842+1)
/
0.79484+2)
0.52370+2)
0.35088+1)
0.09385+8)
0.20088+8)
0.91088+8)
0.602+4)
0.673+5)
1.134+4)
0.70+1)
0.98+2)
0.68+1)
4
4
4
4
4
4
1
1
1
1
1
/
/
/
/
/
Small crystals were isolated from the crushed samples and
investigated for their suitability for the structure determina-
tion on the basis of Laue transmission diffraction patterns.
The X-ray intensity data were collected on a four-circle
Enraf-Nonius CAD4 diffractometer using graphite-mono-
chromated MoKa radiation and a scintillation counter with
pulse-height discrimination. The background was determined
on both ends of each h/2h scan. Empirical absorption correc-
tions were applied on the basis of psi scan data. Further de-
tails are summarized in Table 2.
Pr2AuP3 +Pnma)
1
1
1
1
1
1
Pr1
Pr2
Au
P1
P2
P3
4 c
4 c
4 c
4 c
4 c
4 c
0.09974+5)
0.23377+5)
0.01290+4)
0.0320+2)
0.2031+2)
0.3842+2)
/
4
/
4
/
4
/
4
/
4
/
4
0.79638+2)
0.52387+2)
0.34843+2)
0.0933+1)
0.2013+1)
0.9120+1)
0.619+9)
0.704+9)
1.173+9)
0.72+2)
1.18+3)
0.65+2)
Nd2AuP3 +P21/c)
The structures were determined and refined using the pro-
gram systems SHELXTL PLUS [19] and SHELX-97 [20]. The
centrosymmetric orthorhombic space group Pnma was sug-
gested by the program software for the structures of Ce2AuP3
and Pr2AuP3, which turned out to be correct during the struc-
ture refinements. For Nd2AuP3 the centrosymmetric monocli-
nic space group P21/n was suggested, which was standardized
to the setting P21/c by the program STRUCTURE TIDY [21].
The metal atoms of both structures were located from ana-
lyses of the Patterson functions, and the phosphorus atoms
were found from difference Fourier syntheses. The structures
were refined by a full-matrix least-squares program, using
atomic scattering factors, corrected for anomalous dispersion,
as supplied by the program. To check for the correct composi-
tions we refined occupancy values together with anisotropic
thermal parameters for all positions. These varied between
0.97+2) for the position of the P2 atom and 1.00+1) for the
Au1 position of Ce2AuP3, between 0.99+1) for Pr2 and 1.01+2)
for P3 of Pr2AuP3, and between 1.00+1) for Nd1 and 1.02+2)
for P2 of Nd2AuP3. Thus, the deviations from the full occu-
pancy were insignificant and we resumed to the ideal values
for the final refinement cycles.
Nd1
Nd2
Au
P1
P2
P3
4 e
4 e
4 e
4 e
4 e
4 e
0.44179+8)
0.25769+9)
0.08948+7)
0.1811+4)
0.0054+5)
0.3455+4)
0.39579+5)
0.76778+4)
0.51981+4)
0.0337+2)
0.2052+2)
0.1156+2)
0.20226+2)
0.02448+2)
0.34444+2)
0.4074+1)
0.2997+1)
0.0865+1)
0.629+7)
0.691+7)
1.222+7)
0.71+2)
0.91+3)
0.64+2)
a)
The positional parameters were standardized using the program
STRUCTURE TIDY [21].
The results of the structure determinations are sum-
marized in the Tables 2±4. The anisotropic displace-
ment parameters and the structure factor tables have
been deposited.1) The crystal structures and the near-
neighbor environments are shown in Figs. 6±9.
1)
They may be obtained from the Fachinformationszentrum
Karlsruhe, D-76344 Eggenstein-Leopoldshafen +Germany),
by quoting the Registry Numbers: CSD-411550+Ce 2AuP3),
CSD-411551 +Pr2AuP3), and CSD-411552 +Nd2AuP3).
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708
1703

M. Eschen, G. Kotzyba, B. KuÈnnen, W. Jeitschko
Table 4 Interatomic distances in the isotypic orthorhombic structures of Ln2AuP3 +Ln = Ce/Pr) and in the monoclinic struc-
a)
ture of Nd2AuP3
Ce2AuP3/Pr2AuP3
Ln1: P3
2 P1
290.3/287.3
294.0/292.5
302.1/300.4
313.6/313.6
326.3/324.7
217.7/217.9
294.0/292.5
297.2/297.1
299.7/298.1
Ln2: P3
2 P3
291.6/288.5
293.3/290.5
297.2/297.1
299.7/298.1
328.8/329.2
348.2/349.3
358.1/356.9
217.7/217.9
252.1/251.8
284.2/278.02
302.1/300.4
313.6/313.6
358.1/356.9)
Au:
P3:
2 P3
P2
P2
2 Ln1
Ln2
2 Ln2
2 Au
Ln1
Ln2
Ln2
245.8/246.1
252.1/251.8
284.2/278.0
326.3/324.7
328.8/329.2
348.2/349.3
245.8/246.1
290.3/287.3
291.6/288.5
293.3/290.5
2 P2
2 P2
2 Au
P2
P1
2 P1
Au
P1:
2 Au
2 P2
P1
2 Ln1
Ln2
2 Ln2
P2:
Au
Au
2 Ln1
2 Ln1
+2 Ln2
Nd2AuP3
Nd1: P3
P1
284.0Nd2: P3
286.8
289.7
297.2
301.6
307.2
319.5
325.5
331.5
360.7)
367.2)
218.9
286.8
294.6
297.1
297.2
298.3
285.5
Au:
286.7
288.8
294.6
297.1
298.3
330.8
337.0Nd2
346.7
359.8
370.3
218.9
252.8
271.5
289.7
301.6
307.2
331.5
337.0
370.3
P3
246.3
P3
P3
P1
P1
P1
Au
P2
Au
Au
P2
P3
P2
P2
Nd1
Nd1
Nd2
247.2
252.8
271.5
319.6
325.5
330.8
P2
P1
P2
P2
Au
Au
P2
+Au
+Au
346.7
Nd2
+Nd1
+Nd1
Au
359.8
360.7)
367.2)
246.3
247.2
284.0
285.5
286.7
288.8
P1:
P2
P2:
P1
P3:
Nd1
Nd2
Nd2
Nd1
Nd2
Au
Au
Nd1
Nd1
Nd1
Nd1
Nd2
Nd2
Au
Nd1
Nd2
Nd2
Nd2
a) These distances were computed with the lattice constants obtained from the Guinier powder data. Standard deviations are all equal or less than 0.2 pm.
All distances shorter than 370pm + Ln±Ln, Ln±Au, Ln±P, Au±Au, Au±P) and 350pm +P±P) respectively, are listed. Atoms at distances listed here in par-
entheses are not shown as neighbors in Fig. 8.
Discussion
be seen in the lower part of Fig. 7. The P1 atoms have
no gold neighbors. Together with the P2 atoms they
form P1±P2 pairs. Therefore, the structures may be
viewed as consisting of two-dimensionally infinite
layers, now again with the composition AuP3. These
layers are folded in such a way that they enclose the
Ln1 atoms. The Ln2 atoms are located between these
layers.
In Fig. 8 we show the near-neighbor environments
of both structures. The arrangements of the coordina-
tion polyhedra are such that corresponding near-
neighbor interactions can be compared. The structure
of Pr2AuP3 +and isotypic Ce2AuP3, which has similar
interatomic distances) has the higher symmetry and
its near-neighbor coordinations can more easily be dis-
cussed. The Pr1 atoms have 7 phosphorus and 2 gold
atoms, while the Pr2 atoms have 6 phosphorus +not
counting the two P2 neighbors at 357 pm) and 3 gold
neighbors. Thus, both have the coordination number 9.
However, the Pr±P bonds are stronger than the Pr±Au
bonds, as can be seen by a comparison of the average
Pr±P distances. They amount to 300.0 pm for Pr1 and
293.8 pm for Pr2, thus reflecting the larger number of
The crystal structure of the new compounds Ln2AuP3
+Ln = La, Ce, Pr) is formally isotypic with orthorhom-
bic U2NiC3 [22]. It is well known that many binary
transition metal carbides TC and rare earth phos-
phides LnP crystallize with NaCl-type structure. How-
ever, for ternary carbides and phosphides isotypy is
rare. The only other example known to us are the
compounds LnZn3P3 and PrCd3P3 which are isotypic
with ScAl3C3 and UAl3C3 [23].
The fourth new compound reported here, Nd2AuP3,
crystallizes with a monoclinic distorted version of the
structure established for Pr2AuP3 +and Ce2AuP3). The
atomic labels of both structures correspond to each
other. In both structures the gold atoms are tetra-
hedrally coordinated by phosphorus atoms. These
AuP4 tetrahedra form chains via common P2 corners,
resulting in the composition AuP3, as can be seen in
the upper part of Fig. 7. The AuP3 chains are further
condensed via common P3 atoms, thus forming two-
dimensionally infinite sheets of corner-sharing AuP4
tetrahedra, resulting in the composition AuP2, as can
1704
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708

La2AuP3, Ce2AuP3, and other Rare-Earth Gold Phosphides
Fig. 6 Crystal structures of orthorhombic Pr2AuP3 and
monoclinic Nd2AuP3. As can be seen, both structures are
very similar. The standardized unit cells are outlined on the
left-hand side of the drawings. In Pr2AuP3 all atoms are situ-
ated on mirror planes extending perpendicular to the y axis.
The atoms on such a mirror plane are viewed in a direction
perpendicular to the plane on the right-hand side of the fig-
ure. In Nd2AuP3 this mirror plane is lost. However, as can
be seen on the left-hand side, the displacements of the atoms
from this pseudo-mirror plane are small.
Fig. 7 Projection of the crystal structures of Pr2AuP3 and
Nd2AuP3 emphasizing the AuP4 tetrahedra and the network
of the gold and phosphorus atoms. The atomic arrangement
of both structures is very similar. In the drawing of the
praseodymium compound a chain of gold and phosphorus
atoms is outlined. In both of the structures these chains are
condensed via P3 atoms to form two-dimensionally infinite
nets. Such a net is outlined in the structure of the neo-
dymium compound.
phosphorus neighbors of the Pr1 atoms. The indivi-
dual Pr1±P and Pr2±P distances cover the relatively
narrow range from 287.3 to 313.6 pm. The gold atoms
have distorted tetrahedral phosphorus coordination
with Au±P distances varying between 246.1 and
278.0pm; the tetrahedral angles vary between
100.51+1) and 116.62+2)°. The P1 and P2 atoms are
paired with a P1±P2 bond distance of 217.9 pm, only
slightly shorter than the average P±P distances of
220.9, 221.5, and 222.8 pm in the white, violet, and
black modifications of elemental phosphorus [24], re-
spectively. In addition to the one phosphorus neighbor
the P1 atom has 5 praseodymium neighbors, thus
forming a very distorted octahedron. In contrast, the
P2 atom forms, in addition to the P2±P1 bond, two
P2±Au bonds +at 251.8 and 278.0pm) in perfectly pla-
nar arrangement +these atoms are all situated on a
mirror plane). In addition, the P2 atom has 2 Pr neigh-
bors at 300 pm above and below the mirror plane. The
P3 atoms have two gold neighbors at 246.1 pm +bond
angle Au±P±Au: 116.62+2)°) and four Pr2 neighbors.
Together these six atoms form a distorted octahedron.
In general, the interatomic distances in Ce2AuP3
are very similar to the corresponding distances in
Pr2AuP3. However, a close look reveals some interest-
ing details. The strong Au±P and P±P bonds in Ce2AuP3
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708
1705

M. Eschen, G. Kotzyba, B. KuÈnnen, W. Jeitschko
pound +Table 4), while the Ln1±Au distances show the
expected difference. Possibly this might again be ratio-
nalized by packing effects for these less important
interactions. In any case we cannot offer any other
rationalization.
The differences between the structures of Pr2AuP3
and isotypic Ce2AuP3 on the one hand, and that of
Nd2AuP3 on the other, are all related to the lower
symmetry of the neodymium compound. The space
groups of the two different structures are translatio-
nengleich +they have the same translational symmetry)
as can be seen by comparing the nonstandard setting
P21/n 1 1, the space group of Nd2AuP3, with that of
Pnma +full symbol P21/n 21/m 21/a), found for Pr2AuP3
+and Ce2AuP3) +Table 1). The space group P21/n 1 1 is
a maximal non-isomorphic subgroup of the space
group P21/n 21/m 21/a adopted by Pr2AuP3. The neo-
dymium compound has lost the mirror plane of Pnma.
The distortions due to this loss of symmetry are rela-
tively small. It can be assumed that on heating, the
neodymium compound will transform to the higher-
symmetric structure of Pr2AuP3. The corresponding
displacive phase transition can be classified as ferro-
elastic, since this transition is associated with a change
of the crystal system +from orthorhombic to mono-
clinic). We have not searched for this expected phase
transition of Nd2AuP3. We were fortunate to have
selected a single-domain crystal for our investigation
of Nd2AuP3, since twinning can be expected for the
lower-symmetric form. This crystal was isolated from
a sample which had been annealed at 800 °C and this
temperature might already be below that of the
expected phase transition. The monoclinic angle of
89.43+1)° deviates only little from 90°, however, some
atomic displacements are easily visible on the left-
hand side of Fig. 6. These displacements correspond to
deviations of atomic positions from the mirror plane
which is present in the structure of the higher-sym-
metric praseodymium compound.
Fig. 8 A comparison of the near-neighbor environments in
the structures of Pr2AuP3 +upper half) and Nd2AuP3 +lower
half of the drawing). All atoms are situated on mirror planes
in Pr2AuP3; in Nd2AuP3 all atoms occupy the general posi-
tion. Single-digit numbers correspond to the atom designa-
tions. Interatomic distances +pm) are indicated by three-digit
numbers.
and Pr2AuP3 +corresponding distances in parentheses)
are practically the same, as could be expected; Au±P3:
245.8 +246.1) pm, Au±P2: 252.1 +251.8) pm, P1±P2:
217.7 +217.9) pm. In contrast, the somewhat weaker
Au±P2 bond of 284.2 pm in Ce2AuP3 is considerably
longer than the corresponding bond of 278.0pm in
Pr2AuP3. We ascribe this difference to a packing
effect, which can result in relative large differences
for weaker bonds, whereas the strongest bonds be-
tween corresponding atoms are expected to remain
the same. The average Ln±P distances are also of
some interest. They amount to 301.4 +300.0) pm for
the Ln1 atom and to 295.8 +293.8) pm for the Ln2
atom. Thus, the Ce1 atom is relatively smaller +as
compared to the corresponding distances in the pra-
seodymium compound) than the Ce2 atom. Since
the physical properties +especially the electrical con-
ductivity) suggest some partial tetravalence of the cer-
ium atoms in Ce2AuP3, we ascribe a slightly higher
valence to the Ce1 atoms than to the Ce2 atoms,
although both atoms must have valencies close to
three, as is indicated not only by the volume plot
+Fig. 1) but also by the magnetic behavior of this com-
pound. Interestingly, the Ln2±Au distances are slightly
smaller in the cerium compound as compared to the
corresponding distances in the praseodymium com-
As might have been expected, the major differences
in interatomic distances between the structures of
Pr2AuP3 and Nd2AuP3 are all associated with the
lanthanoid atoms; only small differences occur for the
Au±P and P±P distances. These differences can be
rationalized as being due to the smaller size of the
neodymium atoms as compared to that of the praseo-
dymium atoms. Distances Ln±P which are equal in
Pr2AuP3 +due to its mirror plane) become unequal in
Nd2AuP3. Thus, in the neodymium compound some
Nd±P distances are contracted and others are ex-
panded. In this way the neodymium atoms obtain
lower effective coordination numbers. We list the
examples with the greatest differences. The reader
may follow this comparison by looking at the near
neighbor coordinations +Fig. 8). The two equal Pr2±P2
distances of 357 pm correspond to the unequal Nd2±P2
distances of 337 and 370pm +a difference D of 33 pm),
1706
Z. Anorg. Allg. Chem. 2001, 627, 1699±1708

La2AuP3, Ce2AuP3, and other Rare-Earth Gold Phosphides
ent character of a bond. Since there are no bonding
Ln±Ln interactions we can assume the oxidation num-
ber +3 for the La, Pr, and Nd atoms. We then arrive
at the formulas +Ln+3)2[AuP3]±6 and +Ln+3)2Au+1-
+P1±P2)±4+P3)±3 for the compounds with Ln = La, Pr,
and Nd. For a corresponding band structure we can
assume that the valence band is filled and separated
from the conduction band by a band gap. In agree-
ment with these plausible assumptions the lanthanum
and the praseodymium compounds were found to be
semiconducting. In contrast, the cerium compound is
a metallic conductor, thus indicating that the cerium
atoms are +at least) partially tetravalent. We have
already discussed above that the magnetic properties
of Ce2AuP3 indicate a predominance of the trivalent
state of the cerium atoms in this compound. Only a
very small number of electrons needs to be present at
the Fermi level of an unfilled band to result in metal-
lic conductivity.
Fig. 9 Comparison of the lanthanoid environments of the
phosphorus pairs in Pr2AuP3 and Nd2AuP3. In both com-
pounds the P2 pairs are situated in monocapped trigonal
prisms of lanthanoid atoms. The Ln±P distances +pm) show-
ing the largest differences in the two compounds are empha-
sized by thick lines. In addition to the Ln atoms the P2 pairs
have two gold atoms as neighbors.
two Pr1±P2 distances of 314 pm correspond to
Nd1±P2 distances of 302 and 332 pm +D = 30pm), two
Pr1±P2 distances of 300 pm correspond to Nd1±P2 dis-
tances of 290and 370 pm + D = 17 pm). Finally, the
Ln±Au distances also show a differentiation: the two
equal Pr2±Au distances of 349 pm correspond to dis-
tances of 347 and 360pm + D = 13 pm) in the neo-
dymium compound.
Acknowledgments. We thank Dipl.-Ing. U. Ch. Rodewald and
Dr. M. H. MoÈller for the competent collection of the four-
circle diffractometer data and Mr. K. Wagner for the investi-
gations at the scanning electron microscope. We are further-
more indebted to Dr. W. Gerhartz +Degussa), the Hoechst
AG, Werk Knapsack, and Dr. G. HoÈfer +Heraeus Quarz-
schmelze, Hanau) for generous gifts of gold wire, ultrapure
red phosphorus, and silica tubes, respectively. This work was
also supported by the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie, and the International
Centre for Diffraction Data.
From the viewpoint of the phosphorus atoms the
differences between the two structures are less under-
standable. The P1 and P3 atoms of the two structures
show no great differences in their environments. In
Fig. 9 we show the environments of the P2 pairs in
both structures. It can be seen that the major differ-
ences in P±Ln distances all occur for the P2 atom,
which seems to have moved to the left-hand side of its
surrounding Nd6 prism when compared to the corre-
sponding Pr6 prism. If the Ln6 prisms were rigid
groups, one could have expected that the lower-sym-
metric phosphorus environment should occur for the
larger Ln6 prism.
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