Ab initio structure determination of new boride CePt3B, a distorted variant of CaTiO3

Article abstract of DOI:10.1016/S0925-8388(01)01913-2

X ray powder diffraction and electron probe micro-analysis were used to determine the crystal structure of a ternary boride CePt₃B, tetragonal in structure. The presence of two phases were identified through Rietveld refinement on the sample. Ce was found to be in a trivalent state in the compound. Magnetic susceptibility measurements also showed that the compound did not order magnetically at 2 K.

Full text of DOI:10.1016/S0925-8388(01)01913-2

Journal of Alloys and Compounds 337 (2002) 10–17
L
www.elsevier.com/locate/jallcom
Ab initio structure determination of new boride CePt B, a distorted variant
3
of CaTiO3
a,c ,
b
b
c
c
*
O.L. Sologub
, J.R. Hester , P.S. Salamakha , E. Leroy , C. Godart
a
Department of Quantum Matter, ADSM, 1-3-1 Kagamiyama, 739-8526 Higashi-Hiroshima, Japan
b
ANSTO/ANBF, KEK, Oho 1-1, Tsukuba, Ibaraki 305, Japan
c
CNRS-UPR209, ISCSA, 2–8 Rue Henri Dunant, 94320 Thiais, France
Received 4 October 2001; accepted 18 October 2001
Abstract
˚
˚
The crystal structure of the new ternary boride CePt B, tetragonal, space group P4mm (N99), a54.00433(18) A, c55.07547(7) A,
3
3
˚
Z51, V581.383(5) A was determined ab initio from X-ray powder diffraction using conventional and synchrotron radiation, optical
metallographic examination and electron probe micro-analysis (EPMA). Rietveld refinement on the sample with composition Ce Pd B
2
0
60 20
˚
revealed the presence of two phases, 95% of cubic CePd B , x50.6 (space group Pm-3m, CaTiO str. type, a54.19350(11) A) and 5% of
3
x
3
˚
CeB (space group Pm-3m, CaB str. type, a54.1389(2) A). Magnetic susceptibility measurements and X-ray absorption spectroscopy
6
6
show that Ce is in a trivalent state in CePt B and that the compound does not order magnetically at 2 K.
 2002 Elsevier Science B .V .
3
All rights reserved.
Keywords: Rare earth compounds; Crystal structure; X-ray diffraction; X-ray absorption spectroscopy; Magnetic measurements
˚
1
. Introduction
c57.8860(9) A [8]) and CePt B (unknown structure) have
3
been observed.
Reviews on the phase equilibria and crystal chemistry of
In this paper we present the results of phase analyses
ternary compounds in the R–M–B systems are available in
Refs. [1–4], however information on systems containing
Pd and Pt is still scarce. For palladium containing systems,
and crystal structure investigations for the CeM B (M5Pd,
Pt) alloys using X-ray conventional and synchrotron
3
powder diffraction. On the compound CePt B, we also
3
only the solubility of boron in RPd compounds have been
studied [5].
report on EPMA analysis, magnetic susceptibility and
X-ray absorption spectroscopy.
3
A few data have been recently reported on ternary
intermetallic cerium-based platinum borides. The crystal
structure for the as cast CePt B
ternary compound (new
2
22x
2
. Experimental details
˚
structure type, C222 space group, a53.1648(3) A, b5
˚ ˚
.4799(5) A, c57.8840(4) A) refined from X-ray single
5
Ternary samples Ce M B (M5Pd, Pt) were syn-
2
0
60 20
crystal data [6] has been published. S u¨ llow et al. reported
X-ray powder diffraction data and magnetic properties of
CePt B (CeCo B structure type, P6/mmm space group,
thesized by arc melting of the constituent elements under a
high purity argon atmosphere on a water-cooled copper
hearth. In order to ensure homogeneity, the arc-melted
button was turned over and re-melted several times. Weight
losses during melting were less than 1%. The samples were
wrapped in tantalum foils, sealed in evacuated silica tubes
and annealed at 1070 K for 2 weeks and then cooled down
to room temperature by submerging the tube in water.
Electron microprobe analysis was performed using
Cameca SX100 microprobe equipped with light element
analyzing multi-layered crystal with large d-spacing (LPC3
4
4
˚
˚
a55.446(4) A, c57.585(10) A) [7]. During Ce–Pt–B
phase diagram studies at 1070 K (O. Sologub, P.
Salamakha, C. Godart (2001) unpublished data), CePt B
˚
(new structure type, space group P6 22, a55.4898(5) A,
2
2
*
Corresponding author.
E-mail address: sologub@sci.hiroshima-u.ac.jp (O.L. Sologub).
made of Mo/B C for boron). The three elements were
4
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(01)01913-2

O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
11
simultaneously measured with the following analytical
condition: 20 kV–20 nA for Ce La, Pt La and B Ka. The
can be explained by the fact that the surface state of the
sample and the standard can be slightly different (oxide
surface layer, conductive carbon layer), the B–Ka line
being easily absorbed by the surface layer and leads to an
overestimation of the boron content.
standards used for the calibration were CeAl for Ce, and
2
the pure elements for Pt and B. Typical beam sizes were
3
around 100 nm and the analyzed volume 1 mm . Back-
scattered electrons images (BSE) were recorded to evalu-
ate the homogeneity of the samples. The correction model
used for the quantification was the X-Phi model instead of
the PAP model because the X-Phi model is more accurate
for the quantification of light elements.
The laboratory X-ray powder diffraction data were
collected with a Bruker D8 diffractometer from a flat
rotating sample using Cu Ka radiation in the range 10–
4
. Structural investigations for the CePt B compound
3
4
.1. Ab initio structure solution from powder diffraction
Indexing the conventional X-ray powder diffraction data
of CePt B using the CRYSFIRE program package [10–14]
3
1
208 with a step width of 0.028 and a constant counting
suggested a tetragonal cell. A synchrotron powder pattern
was recorded to obtain better resolution. Use of the
synchrotron data (23 reflections, 2umax5408) increased the
figures of merit giving lattice parameters a54.0051(7),
c55.0760(8) A. Careful examination of the data and
application of a pattern decomposition method with cell
constraints (Le Bail method [15] by iterating the Rietveld
time of 15 s per step. Synchrotron data were collected at
KEK-PF BL20B [9] in an evacuated Debye–Scherrer
camera using a 0.3-mm diameter capillary and image
˚
plates spanning the range 5–858 at a wavelength of 0.75 A
˚
and effective step size of 0.018.
Magnetic measurements were performed in the tempera-
ture range 2–300 K using a MPMS SQUID (Quantum
Design) system.
XAS (X-ray absorption spectroscopy) was performed at
the French synchrotron radiation facility (LURE) in Orsay
using the X-ray beam of the DCI storage ring (working at
[16] decomposition formula) were applied; the structure
factors were extracted from the powder pattern with the
help of the PCW 2.3 program [17] and choosing the P4mm
space group [18].
The SHELXS-86 program [19] was used to locate the
heavy atom sites (Ce and Pt). After refining these heavy
atom coordinates using SHELXL-97 program package
1
.85 GeV and |320 mA) on the EXAFS D21 station. A
double Si 311 crystal was used as a monochromator. The
rejection of third-order harmonics was achieved with the
help of two parallel mirrors adjusted to cut off energies
higher than |9 keV. Experiments were carried out in the
[20], the boron site was located in the Fourier difference
synthesis. Using the extracted structure factors, the
SHELXL-97 refinement with complete structural model
gave R512%.
energy range 5640–5820 eV, which contains the L -edge
III
of Ce. Finely powdered samples were spread on an
adhesive Kapton tape and four such tapes were stacked
together for preparing a sample layer of thickness suffi-
cient enough to ensure a good signal. Moreover, this was
also helpful in eliminating, to a good extent, any sample-
free regions in the path of the radiation. X-ray absorption
spectra were measured at two fixed temperatures, 300 and
4.2. Rietveld refinement for CePt B samples from X-ray
powder diffraction
3
The structure of CePt B was refined from X-ray powder
3
data collected from two samples (a and b) using the atomic
parameters obtained above. Both samples contained small
amounts of CePt B and CePt B impurities with the most
1
0 K.
2
4
intense reflections occurring in the range 378,2u,418.
From the amplitude of the strongest impurity lines as
3
. Phase investigations for the CePt B compound
compared to the 100% intense line of CePt B, we can
3
3
estimate an impurity content of about 3–5%. Rietveld
refinement was performed using FULLPROF98 [21,22].
An experimentally determined Ka /Ka ratio of 0.5,
The BSE image reveals a biphasic material with very
fine elongated precipitates. The dimensions of the precipi-
tates were around 1 mm large and 10 mm long. For this
reason, the quantification is not easy and the beam volume
interaction must contain the matrix and the precipitate
leading to an inaccurate quantification of the precipitates.
Indeed, the composition of the matrix is homogeneous,
while the composition of the precipitates shows a tendency
to an increase of the platinum content. The average
composition of the matrix (after renormalization to one
cerium) is CePt2.97(9)B1.38(15). It can be noticed that the
boron content is higher than the nominal composition. This
1
2
cos u50.7998 for the monochromator polarization correc-
tion and a pseudo-Voigt profile shape function were used.
The background was refined with a polynomial function.
In the final cycles of the least-squares refinements, the
refined positional parameters and isotropic thermal param-
eters for the atoms were allowed to vary while the
occupancy parameter for the boron atom was held con-
stant. Further details on data collection and Rietveld
refinements are given in Table 1.
Experimental, calculated and difference X-ray diffrac-

1
2
O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
Table 1
Parameters for data collection and structural refinements of various samples of CePt B compound
3
Space group; Z
Sample; radiation source
Cell parameters (A)
P4mm; 1
a, X-ray
4.0031(3)
b, X-ray
4.0056(1)
5.0710(2)
81.367(7)
Cu Ka
10–120
0.02
c, synchrotron
4.00433(18)
5.07547(7)
81.383(5)
0.749706(4)
5–85
˚
5
.0736(4)
3
˚
Volume (A )
Wavelengths (A)
Data range (degrees, 2u )
Counting step (degrees, 2u )
Number of reflections
81.30 (1)
Cu Ka
10–120
0.02
˚
0.01
186
112
112
Number of refined parameters
Zero point (degrees, 2u )
Asymmetry parameters
26
26
33
20.150(5)
0.07(3)
0.002(3)
0.11(2)
0.02(1)
1.19(4)
20.032(2)
0.0024(17)
0
.01(1)
h (pseudo-Voigt)
g , g , g
1.12(5)
0.58(3)
1
2
3
0.0361(10)
2
0.000422(1)
Half width parameters
U
V
W
0.21(7)
0.21(5)
20.01(1)
0.27(4)
0.02(3)
0.019(6)
0.30(3)
20.019(11)
0.0050(11)
Rietveld reliability factors (%)
RP
RWP
11.0
15.5
12.09
1.65
4.77
8.53
12.2
8.34
2.16
4.97
2.81
3.70
8.49
0.18
0.93
R
x
E
2
RB
RF
3.21
3.55
Structure refinement program
FULLPROF98
FULLPROF98
LHPM
tion patterns for the CePt B compound are presented in
3
Fig. 1.
4
.3. Synchrotron data reduction
The incident wavelength and an inter-image plate scale
factor were determined by performing a Rietveld refine-
ment on a NIST standard Si640b powder pattern. Back-
ground was refined using a 12-term Chebyshev type II
polynomial, and effective capillary radius was fixed at a
reasonable value. A pseudo-Voigt peak shape was used,
with the mixing parameter g modeled as g 5g 1g 2u 1
1
2
2
g (2u ) . Otherwise, data refinement proceeded as for the
3
laboratory data, using the program LHPM [23]. Ex-
perimental, calculated and difference X-ray patterns are
given in Fig. 2.
The atomic coordinates, which correspond to their
standardized form according to STIDY [24], thermal
parameters and inter-atomic distances are shown in Tables
2
and 3.
4
.4. Atomic coordination
The crystal structure of the CePt B compound and
3
coordination polyhedra for Ce, Pt1, Pt2 and B atoms are
presented in Fig. 3. Each atom of Ce is surrounded by a
polyhedron of 12 Pt atoms (Fig. 3b). Coordination poly-
Fig. 1. Experimental, calculated and difference X-ray diffraction patterns
for two samples (a and b) of CePt B compound (laboratory diffractome-
3
ter).

O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
13
Fig. 2. Experimental, calculated and difference X-ray diffraction patterns
for CePt B compound (synchrotron).
3
Table 2
Atomic coordinates and thermal parameters for CePt B compound
3
2
˚
Atom
Site
x
y
z
B (A )
Sample (A): conventional X-ray data
a
Ce
Pt1
Pt2
B
1a
2c
1b
1b
0
0
0.5
0.5
0
0.0
0.587(85)
0.429(49)
0.607(75)
0.5
0.5
0.5
0.5132(13)
0.1174(11)
0.688(17)
a
1.0
Fig. 3. Crystal structure of the CePt B compound (a) and coordination
3
polyhedra for Ce (b), Pt1 (c), Pt2 (d) and B (e) atoms.
Sample (B): conventional X-ray data
a
Ce
Pt1
Pt2
B
1a
2c
1b
1b
0
0
0.5
0.5
0
0.0
0.793(61)
0.542(33)
0.626(51)
0.5
0.5
0.5
0.5134(10)
0.1177(8)
0.672(12)
5
. Physical properties of the CePt B compound
3
a
1.0
Fig. 4 shows the magnetic susceptibility and its inverse
Sample (C): synchrotron data
a
versus temperature for CePt B sample annealed at 1070 K
3
Ce
Pt1
Pt2
B
1a
2c
1b
1b
0
0
0.5
0.5
0
0.0
0.59(6)
0.33(3)
0.38(4)
3(1)
measured with two magnetic fields (200 and 500 G). In the
high temperature range (|40–300 K), a Curie–Weiss law
behaviour corresponding to Ce in a trivalent state is
observed. Good fits are obtained with a modified Curie
Weiss law:
0.5
0.5
0.5
0.5209(11)
0.1201(11)
0.68(3)
a
Fixed parameter.
hedra of Pt1 are distorted tetragonal prisms Pt B (Fig.
6
2
C
T 2u
3
c). Pt2 atoms are placed inside tetragonal antiprisms
x 5 x 1
]
p
o
formed by Ce and Pt1 atoms, (Ce Pt ) with additional
4
4
boron atoms situated opposite the face formed by cerium
and (or) platinum atoms (Fig. 3d). The coordination
polyhedron for B is a tetragonal pyramid formed by five
platinum atoms (Fig. 3e). No contact between boron atoms
has been observed.
with negative paramagnetic temperature of u |235 K and
an effective moment of 2.58 mB.
p
Fig. 5 shows the LIII absorption edge of CePt B
3
measured at 300 and 10 K. One peak appears at around
3
1
5
723 eV, corresponding to Ce in agreement with mag-
netic measurements results.
Table 3
˚
Selected interatomic distances d (A) and coordination numbers of atoms
(
CN) for the CePt B compound
3
6
. Structural investigation for the CePd B sample
3
˚
˚
Atom
d (A)
CN
Atom
d (A)
CN
10
Rietveld refinement on the CePd B alloy obtained under
Ce–4 Pt2
2.893
3.154
3.259
Pt2–B
–4 Pt1
–B
2.232
2.817
2.838
2.893
3
–
–
4 Pt1
4 Pt1
12
the synthesis conditions of our experiment revealed the
presence of two phases: 95% of CePd B , x50.6 (CaTiO
structure type) and 5% of CeB (CaB structure type) with
close values of the lattice parameter, a54.19254(14) A˚
and a54.1386(3) A, respectively. The refinement of the
structure from X-ray powder diffraction data was per-
3
x
3
–
4 Ce
6
6
Pt1–2 B
2.154
2.817
2.832
B–4 Pt1
–Pt2
2.151
2.232
5
–
–
2 Pt2
4 Pt1
8
˚

1
4
O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
Fig. 4. x versus T and 1/x versus T for the CePt B compound at 200 and 500 G.
3
formed using FULLPROF98 [21]. Refinement of the
synchrotron data confirmed the conclusions drawn from
the laboratory data. Experimental, calculated and differ-
6 Pd atoms (Fig. 6d). No contact between boron atoms has
been observed (see Tables 5 and 6).
ence X-ray diffraction patterns for the CePd B sample are
3
presented in Figs. 6 and 7 for laboratory and synchrotron
results, respectively. Rietveld refinement proceeded as for
7. Discussion
the CePt B compound. Further details are given in Table
The binary CePd compound crystallizes with an AuCu
3
3
3
˚
4
.
structure type (Pm-3m space group, a54.1278 A) [25].
When boron is placed at the 1b site (0.5 0.5 0.5), a
perovskite CaTiO3 structure type with slightly enlarged
lattice parameter should be produced. The CePd B (x5
6
.1. Atomic coordination
3
x
The crystal structure of the CePd B (x50.6) compound
0.6) structure can be considered as arising from such an
3
x
and the coordination polyhedra for Ce, Pd and B atoms are
presented in Fig. 8. The coordination polyhedron of Ce is a
cubo-octahedron formed by 12 Pd atoms (Fig. 6b). Each
atom of Pd is surrounded by octahedral Ce B (Fig. 6c).
insertion of boron atoms into the AuCu structure and also
3
as a defect modification of the CaTiO structure. These
3
4
2
The B coordination polyhedron is an octahedron formed by
Fig. 6. Experimental, calculated and difference X-ray diffraction patterns
for the CePd B sample (laboratory diffractometer). Two rows of vertical
3
lines show the Bragg reflection positions for CePd B , x50.6 (upper) and
3
x
Fig. 5. XAS spectra for the CePt B compound at 10 and 300 K.
CeB6 (lower).
3

O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
15
Fig. 7. Experimental, calculated and difference X-ray diffraction patterns
for the CePd B sample (synchrotron). Two rows of vertical lines show the
3
Bragg reflection positions for CePd B , x50.6 (lower) and CeB (upper).
3
x
6
results correlate well with a suggestion that formation of
ternary RM B phases with CaTiO structure is extremely
3
3
rare due to dimensional factors [2].
There are no reports of a binary Ce–Pt [26] phase with
Fig. 8. Crystal structure of the CePd B (x50.6) compound (a) and
3
x
coordination polyhedra for Ce (b), Pd (c) and B (d) atoms.
Table 4
Parameters for data collection and structural refinements of CePd B alloy
3
Sample; radiation source
Phase
CePd B, X-ray
CePd B, synchrotron
3
3
CePd B , x50.6, (95%)
CeB6 (5%)
Pm-3m
4.1395(13)
70.93(4)
CePd B , x50.6, (95%)
CeB (5%)
Pm-3m
4.1389(2)
70.902(7)
3
x
3
x
6
Space group; Z
Pm-3m
Pm-3m
˚
Cell parameters (A)
4.1936(5)
73.75(1)
Cu Ka
18–100
0.02
4.19350(11)
73.745(3)
0.749706(4)
5–85
0.01
70
3
˚
Volume (A )
˚
Wavelengths (A)
Data range (degrees, 2u )
Counting step (degrees, 2u )
Number of reflections
34
30
68
Number of refined parameters
Zero point (degrees, 2u )
Asymmetry parameters
31
32
20.031(6)
20.07(4)
20.0080(13)
0.0092(8)
2
0.00(1)
h (pseudo-Voigt)
g , g , g
1.12(8)
0.93(3)
1
2
3
0
.0065(13)
2
0.000048(13)
Halfwidth parameters
U
V
0.19(8)
0.13(5)
0.093(8)
0.047(4)
W
20.019(8)
20.0039(4)
Rietveld reliability factors (%)
RP
RWP
RE
10.4
16.1
11.49
5.35
6.92
13.71
2
x
1.96
3.70
3.97
0.26
2.23
RB
RF
10.0
6.62
2.81
Structure refinement program
FULLPROF98
LHPM

1
6
O.L. Sologub et al. / Journal of Alloys and Compounds 337 (2002) 10–17
Table 5
and B with z50.5 cannot be positioned in the same plane
normal to the XY plane, therefore the Pt2 and B atoms shift
along c. That is, as a result of a lattice structural deforma-
tion, an internal deformation takes place leading to the
disappearance of the mirror plane perpendicular to the
Atomic coordinates and thermal parameters for the phases in CePd B
sample
3
2
˚
Atom
W.N.
x
y
z
B (A )
Occupancy (%)
Phase 1–CePd B : X-ray diffraction
3
x
Ce
Pd
B
1a
3c
1b
0
0
0.5
0
0.5
0.5
0
0.5
0.5
0.59(9)
0.73(5)
100
100
60(9)
4
-fold axis and changing the space group from P4/mmm to
P4mm in CePt B (Fig. 9).
a
3
1.00
Phase 2–CeB : X-ray diffraction
6
Ce
B
1a
6f
0
0
0.5
0
0.5
0.5(3)
1.00
100
100
a
0.215(15)
Acknowledgements
a
Fixed parameter.
Part of this work was performed at the Australian
National Beamline Facility with support from the Aus-
tralian Synchrotron Research Program, which is funded by
the Commonwealth of Australia under the Major National
Research Facilities Program. The research work of O.S.
was supported by CNRS-NATO and JSPS Fellowships.
Table 6
Selected interatomic distances d (A) and coordination numbers of atoms
˚
(
CN) for the CePd B (x50.6) compound
3 x
˚
˚
Atom
d (A)
CN
Atom
d (A)
CN
6
Ce–12 Pd
B–6 Pd
2.965
2.097
12
6
Pd–2 B
–4 Ce
2.097
2.965
References
AuCu type. The CePt B structure observed here is related
to both of the structures discussed above. As a conse-
quence of the increasing c lattice parameter in the CaTiO3
3
3
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˚
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