Magnetic and structural properties of pseudo-binary compounds CrAs 1-xPx

Article abstract of DOI:10.1016/j.jallcom.2004.04.055

Structural and magnetic properties of pseudo-binary compounds CrAs _1-xP_x (0≦x≦1) with the MnP-type structure have been studied by X-ray diffraction, magnetic susceptibility measurements, and differential thermal analysis. For the compounds with x=0 and 0.03, the lattice parameters a and c increase abruptly, and b and v (unit cell volume) decrease abruptly at 265 and 110K, respectively, which are considered to correspond to the Néel temperature (T_N), as temperature increases. Magnetic susceptibility of all the prepared compounds shows feasible temperature dependence at temperatures from 4.2 to 300K. The decrease of T_N with x is discussed based on the x dependence of lattice parameters.

Full text of DOI:10.1016/j.jallcom.2004.04.055

Journal of Alloys and Compounds 383 (2004) 189–194
Magnetic and structural properties of pseudo-binary
compounds CrAs P
1
−x x
∗
K. Kanaya, S. Abe, H. Yoshida , K. Kamigaki, T. Kaneko
Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan
Abstract
Structural and magnetic properties of pseudo-binary compounds CrAs1−x
P
x
(0ꢀxꢀ1) with the MnP-type structure have been studied by
X-ray diffraction, magnetic susceptibility measurements, and differential thermal analysis. For the compounds with x = 0 and 0.03, the lattice
parameters a and c increase abruptly, and b and v (unit cell volume) decrease abruptly at 265 and 110 K, respectively, which are considered
N
to correspond to the Néel temperature (T ), as temperature increases. Magnetic susceptibility of all the prepared compounds shows feasible
temperature dependence at temperatures from 4.2 to 300 K. The decrease of T with x is discussed based on the x dependence of lattice
N
parameters.
©
2004 Elsevier B.V. All rights reserved.
Keywords: CrAs1−xPx compounds; Néel temperature; Thermal expansion; Magnetic susceptibility; Thermal differential analysis
1
. Introduction
are discussed in the relation with the crystal parameters in
order to make clear the origin of their antiferromagnetism.
It is reported that the Néel temperature of CrAs0.95P0.05 is
246–260 K in [5] and that of CrAs0.98P0.02 150 K in [6],
respectively. The effect of the substitution of As by P on
the Néel temperature is very different from each other.
In this paper, the structural and magnetic properties of
pseudo-binary compounds CrAs1−xPx are studied in order to
make clear the origin of the occurrence of antiferromagnetic
order in the 3d transition metal pnictide with the MnP-type
crystal structure.
Pnictides CrAs and CrP are the compounds with the crys-
tal structure of a MnP-type as shown in Fig. 1. Neutron
diffraction studies for CrAs have been done by Kazama and
Watanabe [1], Boller and Kallel [2], and Selte et al. [3]. Ac-
cording to their results, CrAs is an antiferromagnet with the
Néel temperature (TN) of 248 K [1], 260 K [2], and 280 K
[3], respectively, the magnetic structure of which is a double
helical one propagating along the c-axis. Magnetic transi-
tion at TN occurs as the first-order transition accompanying
the abrupt changes of lattice parameters. However, it is re-
ported that its magnetic susceptibility has feasible temper-
ature dependence at temperatures from 4.2 to 1000 K and
does not show any remarkable anomaly at TN [1,2]. The
neutron diffraction study by Selte et al. [4] shows that no
magnetic order appears in CrP down to 1.5 K.
2. Sample preparations and experimental procedures
Samples were prepared in the following ways: powders
of Cr (99.9%), As (99.99%), and P (99.99%) were mixed in
There have been reports for magnetic properties of
pseudo-binary Cr pnictides with the MnP-type crystal
structure: CrAs1−xPx by Selte et al. [5], CrAs0.98Sb0.02
and Cr1−xMxAs (M = Ti, Fe, Co, Ni) by Suzuki and Ido
the desired proportions, sealed in an evacuated silica tube,
◦
(
1) kept for 3 days after slow heating to 300 C, (2) for 3
◦
days after slow heating to 600 C, and (3) for 7 days af-
ter slow heating to 800 C, and then cooled down slowly
◦
[
6,7], Cr1−xVxAs by Selte et al. [8], and CrAs1−xSbx by
in the furnace. The products thus obtained were crushed,
mixed to homogenize and then pressed into the pellets. The
heat-treatments of the pellets were done again following the
Kamigaki et al. [11], Yoshida et al. [10], and Suzuki and
Ido [9]. In all the above reports, their magnetic properties
◦
above three processes and finally heated for 7 days at 900 C,
∗
and then cooled in the furnace. The 10 samples with x = 0,
Corresponding author. Fax: +81-22-215-2231.
E-mail address: hyoshida@imr.tohoku.ac.jp (H. Yoshida).
0.03, 0.05, 0.1, 0.2, 0.3, 0.5, 0.6, and 1 were prepared with
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.04.055

1
90
K. Kanaya et al. / Journal of Alloys and Compounds 383 (2004) 189–194
Fig. 1. Crystal structure of a MnP-type.
the same procedures. The compounds CrAs1−xSbx (x = 0.2
and 0.3) with the MnP-type structure were also prepared in
the same way as mentioned above. X-ray diffraction analy-
sis showed that all the prepared samples are in a single phase
with the MnP-type crystal structure. The results of chemi-
cal analysis showed that the compositions of the prepared
samples coincide with the nominal compositions.
Powder X-ray diffraction measurements were carried out
at temperatures from 90 to 300 K. Magnetic susceptibility
was measured by using a magnetic balance at temperatures
from 4.2 to 300 K under magnetic fields from 2 to 11.7 kOe.
Differential thermal analysis (DTA) was done by a conven-
tional method.
3
. Experimental results
Fig. 2 shows the composition dependence of the lattice
Fig. 2. Composition dependence of a, b, c, and the unit cell volume v.
parameters a, b, c, and the unit cell volume v at room tem-
perature. As seen in the figure, the parameters a and b are
almost constant, but b decreases sharply to x = 0.05. In the
composition range of x > 0.2, a, b, and c decrease almost
linearly with x. The results agree well with those by Selte
et al. [6]. The unit cell volume v also decreases sharply with
x for x = 0.1 and linearly with x for x > 0.1.
at 110 K for x = 0.03. The transition temperature of 110 K
is considered to be the Néel temperature of CrAs0.97 P0.03,
since the temperature of 270 K mentioned above agrees with
the Néel temperature of CrAs. These abrupt changes of the
lattice parameters show that the transitions are of the first
order. The temperature dependence of the lattice parameters
for x = 0.05 does not show any remarkable anomalies in the
temperature range of this measurement, which shows that
no magnetic order occurs in CrAs0.95P0.05 down to 90 K.
The temperature dependence of the lattice parameters were
Fig. 3 shows the temperature dependence of the unit cell
volume v and the lattice parameters a, b, and c for the com-
pounds with x = 0, 0.03, and 0.05. With increase of tem-
perature, a and c increase, and b and v decrease abruptly at
2
70 K for x = 0, which corresponds to the Néel tempera-
ture. The abrupt changes of a, b, and c were also observed

K. Kanaya et al. / Journal of Alloys and Compounds 383 (2004) 189–194
191
Fig. 3. Temperature dependence of v, a, b, and c for CrAs1−xPx with x = 0, 0.03, and 0.05.
also measured for CrAs0.8P0.2 and CrAs0.6P0.4, in which
any anomalies were not observed at temperatures from 90
to 300 K.
Table 1 shows the change of the lattice parameters above
and below the transition temperature. The change is char-
√
acterized from that (c − 3b)/c changes its sign from neg-
ative value to positive one at the transition temperature as
√
−
3
−3
−3
follows: (c− 3b)/c = −8×10 (T < TN) and 35×10
−
3
(
T > TN) for CrAs and −5.7×10 (T < TN) and 59×10
(
T > TN) for CrAs0.97P0.03.
Magnetic measurements have been carried out at tem-
peratures from 4.2 to 300 K under magnetic field from 2
to 11.7 kOe by using a magnetic balance. Fig. 4 shows the
magnetization curves for the samples with x = 0.03, 0.2,
0
.3, 0.4, and 0.6 at 4.2 K, in which all the magnetizations
Table 1
ꢀa/a0
ꢀb/b0
ꢀc/c0
ꢀv/v0
−5.0 × 10− 3.0 × 10
3
−3
−3
−5.6 × 10
−8.8 × 10
−3
−3
1.9 × 10
3.8 × 10
−3
−3
CrAs
CrAs0.97P0.03 −3.5 × 10− 5.6 × 10
3
Fig. 4. Magnetization curves for CrAs1−xPx with 0.03, 0.2, 0.3, 0.4, and
0.6 at 4.2 K.
a0, b0, and c0 are the values just above TN.

1
92
K. Kanaya et al. / Journal of Alloys and Compounds 383 (2004) 189–194
Fig. 7. Thermal differential analysis for CrAs.
Fig. 5. Temperature dependence of magnetic susceptibilities for
CrAs1−xPx with 0.05, 0.2, 0.3, 0.4, and 0.6.
of susceptibilities for x = 0 and 0.03, together with those
for x = 0.05 and 0.6. It is noted that the susceptibilities for
x = 0 and 0.03 are smaller than that for 0.05, although the
composition became smaller. And then the susceptibilities
for x = 0 and 0.03 appear to decrease slightly at lower tem-
peratures, but, the susceptibility peak characteristic of usual
antiferromagnet is not observed at TN.
Differential thermal analysis was carried out for the sam-
ples with x = 0 and 0.03 in order to make clear the mag-
netic transition temperature. As shown in Fig. 7, the result
of DTA shows that the transitions occur at 261 K on cool-
ing and 268 K on heating for CrAs, respectively. The tran-
sition temperatures on heating almost agree with those of
anomalies observed in the temperature dependence of lattice
parameters on heating. The result for CrAs0.93P0.03 shows
that the transitions also occur at 84 K on cooling and 106 K
on heating, respectively. The existence of thermal hysteresis
observed above indicates that the transitions are of the first
order, which correspond the abrupt changes of the lattice
parameters at transition.
increase linearly with field. As seen in the figure, magnetic
susceptibilities become large with decrease of x from x =
0
.6 to 0.2. But it should be noted that the susceptibility for
x = 0.03 becomes small again in spite of further decrease
of x. Fig. 5 shows the temperature dependence of magnetic
susceptibilities of the samples with x from 0.05 to 0.6. The
values of susceptibilities increase with decrease of x in this
composition range. The susceptibilities for x = 0.4 and 0.6
are almost independent of temperature, but those for x =
0
.05, 0.2, and 0.3 increase at low temperatures ascribing to
the impurity effect. Fig. 6 shows the temperature dependence
4. Discussions
In the lattice parameters a, b, and c versus composition x
curves at room temperature shown in Fig. 2, aext, bext, and
cext which are obtained by extrapolating to x = 0, the linear
parts of a, b, and c–x curves for x > 0.2 are aext = 5.66 Å,
bext = 3.407 Å, and cext = 6.218 Å, respectively. The ob-
served lattice parameters for x = 0 are a = 5.66 Å, b
=
ob
ob
3
.474 Å, and c = 6.203 Å, respectively. And then the val-
ob
ues of (a −aext)/aext, (b −bext)/bext, and (c −cext)/cext
ob
ob
ob
−
3
−3
−3
,
are obtained to be −2.6×10 , 19×10 , and −2.4×10
respectively. The results show that b increases rapidly in the
Fig. 6. Temperature dependence of magnetic susceptibilities for
CrAs1−xPx with x = 0, 0.03, 0.05, and 0.6.

K. Kanaya et al. / Journal of Alloys and Compounds 383 (2004) 189–194
193
range of low x. On the other hand, the temperature depen-
dence of the lattice parameters for x = 0 and 0.03 show that
a and c decrease and b increases with setting of magnetic
order below TN. The ratio of the changes of b is eight times
larger than those of a and c. These results suggest that the
rapid increase of b in the range of low x is closely connected
to the occurrence of magnetic order. Similar relation be-
tween the magnetic order and lattice parameters is observed
in Cr1−xVxAs by Selte et al. [8]. In the case of Cr1−xVxAs,
a and c increase with x and their x dependence is opposite
to those in CrAs1−xPx, but b decreases at first rapidly with
x and has a similar x dependence with that in CrAs1−xPx.
Thus, only b has the same x dependence in both compounds.
According to the results by Selte et al., an antiferromagnetic
order occurs for x < 0.5 in Cr1−xVxAs. It is considered
that the occurrence of antiferromagnetism in those Cr com-
pounds with the MnP-type structure has a close relation with
the rapid increase of b from the results mentioned above,
since the a lattice parameters at room temperature are the
values in a paramagnetic state free from the large exchange
striction occurring in the antiferromagnetic state.
Fig. 8. Temperature dependence of magnetic susceptibilities for
CrAs1−xPx with x = 0 and 0.03 and CrAs1−xSbx with x = 0.2 and 0.3.
As shown in Fig. 6, the temperature dependence of mag-
netic susceptibilities of CrAs, CrAs0.97P0.03, CrAs0.8Sb0.2,
and CrAs0.7Sb0.3 compounds are characterized by the slight
increase of susceptibility on heating through TN and the ab-
sence of susceptibility peaks at TN. It is known that the crys-
tal structure of both pseudo-binary compounds changes into
the NiAs-type structure from the MnP-type one at high tem-
peratures, in which the distance between Cr atoms become
larger. It is considered that the increase of susceptibility on
heating is ascribed to the approach to the NiAs-type struc-
ture with larger unit cell volume. On the other hand, it is
well known that the susceptibility peak does not appear at
TN in an itinerant electron antiferromagnet Cr being a spin
density wave. Lack of susceptibility peak at TN for CrAs and
CrAs0.97P0.03 in Fig. 8 is considered their magnetic property
to be ascribed to itinerant electron magnetism.
which no magnetic order occurs. As mentioned above, bc
for the CrAs1−xPx compounds is ∼3.42 Å corresponding to
x ∼ 0.05; and then bc for Cr1−xVxAs is also estimated to
be ∼3.43 Å [8].
As described above, the substitution of As with P de-
creases the unit cell volume v of the compounds. The Néel
temperature of CrAs0.97P0.03 can be estimated using the
pressure dependence of TN of CrAs, if the decrease of TN
with P addition can be assumed to be only due to the de-
Selte et al. have reported that the Néel temperature of
CrAs0.95P0.05 is 246–260 K, a little lower than that of CrAs.
In the present study, however, any anomaly corresponding
to TN was not observed in the temperature dependence of
lattice parameters at temperatures from 100 to 300 K for
CrAs0.95P0.05. On the other hand, anomalies were observed
at 110 K in the thermal expansions of lattice parameters
and at 104 K in DTA, respectively, for CrAs0.97P0.03. Fig. 9
shows the Néel temperatures for CrAs1−xPx with x = 0.02,
0
.03, and 0.05 by Suzuki and Ido [6], the present study,
and Selte et al. [5], respectively. As shown in the figure,
the Néel temperatures for x = 0, 0.02, and 0.03 are almost
on a straight line, in which TN appears to become 0 around
x ∼ 0.05. There is a large discrepancy between the effects
of substitution of As with P on TN by Selte et al. and the
above results. Once, Suzuki and Ido [6] have pointed out that
the appearance of antiferromagnetism in Cr pnictides with
the MnP-type structure connects strongly with the value of
b and there exists a critical b value of bc = 3.38 Å, below
Fig. 9. Composition dependence of the N e´ el temperature (TN) for
CrAs1−xPx: x = 0 and 0.03 [present results], 0.02 [6], and 0.05 [5].

1
94
K. Kanaya et al. / Journal of Alloys and Compounds 383 (2004) 189–194
crease of v. The decrease ratio of v of CrAs0.97P0.03 to CrAs
is (vx=0.03 − vx=0)/vx=0 = −0.013. At present, there is
no report on a volume compressibility of CrAs. Thus, the
pressure necessary to decrease v by −1.3% is estimated to
be about 10 kbar if we use the compressibility k = 1.34 ×
References
[1] N. Kazama, H. Watanabe, J. Phys. Soc. Japan 30 (1971) 1319.
[
[
2] H. Boller, A. Kallel, Solid State Commun. 9 (1971) 1666.
3] K. Selte, A. Kjekshus, W.E. Jamison, A.F. Andressen, J.E. Enge-
bretsen, Acta Chem. Scand. 25 (1971) 1703.
−3
1
0
per kbar. And the pressure change of TN for CrAs is
[4] K. Selte, A. Kjekshus, A.F. Andressen, Acta Chem. Scand. 26 (1972)
dTN/dp = −12 K/kbar [10] and −15 K/kbar [12], respec-
tively. From these results, TN of CrAs0.97P0.03 is estimated
to be 120–150 K, which is a little higher than the observed
value.
It is desirable that the relation between the occurrence
of magnetic order and the crystallographic parameters will
be clarified experimentally by the measurements of pressure
change of the lattice parameters, especially of b, in CrAs by
X-ray diffraction under pressure.
4
188.
[5] K. Selte, H. Hjersing, A. Kjekshus, A.F. Andressen, P. Fisher, Acta
Chem. Scand. A29 (1975) 695.
6] T. Suzuki, H. Ido, J. Appl. Phys. 73 (1993) 5688.
7] T. Suzuki, H. Ido, J. Appl. Phys. 69 (1991) 4624.
8] K. Selte, H. Hjersing, A. Kjekshus, A.F. Andressen, Acta Chem.
Scand. A29 (1975) 312.
[
[
[
[9] A. Kallel, H. Boller, E.F. Bertaut, J. Phys. Chem. Solids 35 (1974)
139.
1
[
10] H. Yoshida, T. Kaneko, M. Shono, S. Abe, M. Ohashi, J. Magn.
Magn. Mater. 15–18 (1980) 1147.
[11] K. Kamigaki, M. Shono, H. Yoshida, S. Abe, T. Kaneko, J. Magn.
Magn. Mater. 70 (1987) 218.
Acknowledgements
[12] E.A. Zavadskii, I.A. Sibarova, Sov. Phys. Solid State 18 (1976) 1009.
The authors wish to express their sincere thanks to Pro-
fessor T. Suzuki and H. Ido for their valuable discussions.