Phase transformations of the ferromagnetic semiconductor Cd1-x MnxGeP2 at pressures of up to 5 GPa

Article abstract of DOI:10.1134/S0020168506080036

The electrical resistivity and Hall coefficient of Cd_1-x Mn _xGeP₂ (x=0-0.19) have been measured at 300 K and hydrostatic pressures of up to 5 GPa. The results indicate that CdGeP ₂ dissociates to CdP₂ and Ge at p = 3.2 GPa, while the incorporation of manganese stabilizes the crystal structure of CdGeP ₂, and Cd_0.81Mn_0.19GeP₂ undergoes a reversible phase transition at p = 3.5 GPa. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506080036

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 8, pp. 826–829. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.M. Novotortsev, A.Yu. Mollaev, I.K. Kamilov, R.K. Arslanov, U.Z. Zalibekov, S.F. Marenkin, S.A. Varnavskii, 2006, published in Neorganicheskie Mate-
rialy, 2006, Vol. 42, No. 8, pp. 916–918.
Phase Transformations
of the Ferromagnetic Semiconductor Cd Mn GeP
1
– x
x
2
at Pressures of up to 5 GPa
a
b
b
b
V. M. Novotortsev , A. Yu. Mollaev , I. K. Kamilov , R. K. Arslanov ,
b
a
a
U. Z. Zalibekov , S. F. Marenkin , and S. A. Varnavskii
a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
b
Institute of Physics, Dagestan Scientific Center, Russian Academy of Sciences,
ul. Yaragskogo 94, Makhachkala, 367003 Dagestan, Russia
e-mail: csq@mail.ru
Received October 21, 2005; in final form, February 28, 2006
Abstract—The electrical resistivity and Hall coefficient of Cd1 – xMn GeP (x = 0–0.19) have been measured
x
2
at 300 K and hydrostatic pressures of up to 5 GPa. The results indicate that CdGeP dissociates to CdP and Ge
2
2
at p = 3.2 GPa, while the incorporation of manganese stabilizes the crystal structure of CdGeP , and
2
Cd0.81Mn0.19GeP undergoes a reversible phase transition at p = 3.5 GPa.
2
DOI: 10.1134/S0020168506080036
INTRODUCTION
Cd1 – xMn GeP solid solutions are ferromagnetic
semiconductors with Curie temperatures TC above ampule: the temperature difference along the ampule
00 K and are potentially attractive as spintronic mate- was within 1°C. The samples were first heated to 450°C
Syntheses were performed in electrical furnaces
equipped with heating blocks which ensured a uniform
temperature profile along the entire length of the
x
2
3
rials [1, 2]. According to Demin et al. [2], their TC and held there for at least 48 h. Next, the temperature
increases with manganese content and reaches a maxi- was gradually raised to 800°C at a rate no faster than
mum at the solubility limit of Mn in CdGeP . One way 5°C/h and then maintained constant for at least 24 h.
2
The cooling rate was 5 to 10°C/s. The Mn content of the
samples was determined by atomic absorption. The
analytical data agreed well with the nominal composi-
tions.
of increasing the solubility of an impurity is by raising
the pressure. In connection with this, the purpose of this
work was to determine pressures that induce irrevers-
ible phase transformations of CdGeP2.
The x-ray diffraction (XRD) patterns (DRON-1 dif-
fractometer, Ni-filtered ëuK radiation, 2θ = 10°–90°)
α
EXPERIMENTAL
of all the samples were similar to those of CdGeP2,
without peaks attributable to manganese phosphides.
Comparison of peak positions in the XRD patterns of
our samples with PDF data revealed a shift of the peaks
to higher 2θ angles with increasing manganese content,
indicating a reduction in lattice parameters. Increasing
the Mn content from x = 0 to 0.09 and to 0.19 reduces
the a parameter from 5.741 to 5.738 and to 5.667 Å, in
agreement with the data reported by Medvedkin et al. [1].
We studied Cd1 – xMn GeP solid solutions with x =
x
2
0
–0.19. Samples for this investigation were synthesized
by reacting CdP and Ge powders prepared from single
2
crystals. Cadmium diphosphide crystals were grown as
described by Trukhan et al. [3]. The impurity content of
the high-purity single-crystal germanium used was
within 0.1 ppm by weight. The Mn-containing samples
were prepared using extrapure-grade manganese puri-
fied by double sublimation and V5 phosphorus. The
starting-mixture compositions corresponded to the
Electrical resistivity and Hall effect measurements
were carried out at hydrostatic pressures of up to 5 GPa
in a Toroid anvil cell mounted in a multiturn solenoid
hypothetical join CdGeP –MnGeP . The starting mix-
2
2
tures (45–50 g) were placed in silica ampules coated which generated magnetic fields H ≤ 300 kA/m. The
with pyrolytic carbon, which were then pumped down sample surfaces were ground and etched. The sample
–
2
to 10 Pa and sealed off.
dimensions were 3 × 0.8 × 0.8 mm. Point contacts were
8
26

PHASE TRANSFORMATIONS OF THE FERROMAGNETIC SEMICONDUCTOR
827
ρ/ρ , R /R
ρ/ρ , R /R
0 H H0
0
H
H0
1
01
1
2
3
4
1
00
1
2
3
4
1
1
0–1
1
00
0–2
0
1
2
3
4
5
Pressure, GPa
1
0–1
Fig. 2. (1, 2) Resistivity and (3, 4) Hall coefficient of
Cd0.91Mn0.09GeP2 as functions of (2, 4) increasing and
(1, 3) decreasing pressure.
0
1
2
3
4
5
Pressure, GPa
Fig. 1. (1, 2) Resistivity and (3, 4) Hall coefficient of
lier results [4], we conclude that CdGeP dissociates at
2
CdGeP as functions of (2, 4) increasing and (1, 3) decreas-
2
3
.2 GPa, as supported by XRD data: after unloading,
ing pressure.
the XRD pattern of the sample shows peaks from CdP2
and Ge.
made by Sn. The uncertainties in resistivity ρ, Hall
Figure 2 shows the resistivity and Hall coefficient of
Cd0.91Mn0.09GeP2 as functions of increasing and
decreasing pressure. The resistivity gradually decreases
with increasing pressure below ~3.3 GPa and then
drops sharply, by almost two orders of magnitude. At
coefficient R , and pressure p were within ±3, 3.5, and
H
3
%, respectively.
RESULTS AND DISCUSSION
The electrical properties of our samples are summa-
rized in the table. All of the samples were p-type. With
increasing Mn content, the carrier concentration in the
samples and their electrical conductivity increase, with
no type conversion.
p > 3.5 GPa, ρ varies little. The behavior of R is simi-
H
lar to that of resistivity. The reverse (unloading) ρ(p)
and R (p) curves of Cd0.91Mn0.09GeP differ in shape
H
2
from those of CdGeP . During unloading, the phase
2
transformation of the Cd0.91Mn0.09GeP sample occurs
2
at p = 2.3 GPa. Since the ρ(p) and R (p) measured
H
Figure 1 shows the resistivity (curves 1, 2) and Hall
Transport properties of Cd1 – xMn GeP samples (p-type con-
x
2
coefficient (curves 3, 4) of CdGeP as functions of
2
ductivity)
Composition
CdGeP2
increasing (curves 2, 4) and decreasing (curves 1, 3)
pressure. The resistivity of CdGeP decreases only
2
R ,
H
ρ,
µ,
N, cm–3
gradually with increasing pressure below 3.2 GPa and
then drops sharply, by almost one order of magnitude.
Starting at p = 4 GPa, ρ varies little with pressure. The
2
3
Ω cm
cm /(V s)
cm /C
8.5 × 1016 27.5
73.1
20
3
2.6
6.6
4.2
behavior of R is similar to that of resistivity. The ρ(p)
H
and R (p) data obtained during loading and unloading Cd0.91Mn0.09GeP2
3.12 × 1017 3.02
2.08 × 1018 0.72
H
differ markedly. During unloading, the ρ and R of
H
Cd0.81Mn0.19GeP2
CdGeP are linear functions of p. By analogy with ear-
2
INORGANIC MATERIALS Vol. 42 No. 8 2006

8
28
NOVOTORTSEV et al.
ρ/ρ , R /R
before and after the loading–unloading cycle differ
0
H
H0
02
slightly, the phase transition seems to be accompanied
by partial dissociation of the sample.
1
1
1
1
2
3
4
Figure 3 shows the resistivity and Hall coefficient of
Cd0.81Mn0.19GeP2 as functions of increasing and
decreasing pressure. Both the resistivity and Hall coef-
ficient slightly increase with pressure before reaching a
maximum at p = 3.5 GPa and then drop sharply, by
almost six and two orders of magnitude, respectively.
01
00
Note that the ρ and R measured before and after the
H
loading–unloading cycle coincide. During unloading,
the phase transition occurs at p = 2.3 GPa. Thus, this
sample undergoes a reversible structural phase transi-
tion at p = 3.5 GPa, as supported by XRD data: after
1
1
1
1
0–1
unloading, the XRD pattern of Cd0.81Mn0.19GeP2
0–2
showed only peaks from CdGeP2.
Comparison of the resistivity and Hall coefficient
versus pressure data for CdGeP2 (Fig. 1),
Cd0.91Mn0.09GeP2 (Fig. 2), and Cd0.81Mn0.19GeP2
0–3
(Fig. 3) indicates that the incorporation of Mn extends
the pressure stability range of the material: CdGeP dis-
0–4
2
0
1
2
3
4
5
sociates at 3.2 GPa, Cd0.91Mn0.09GeP partially dissoci-
2
Pressure, GPa
ates at 3.3 GPa, and Cd0.81Mn0.19GeP undergoes a
2
reversible structural phase transition at 3.5 GPa. This is
supported by XRD data: as the Mn content increases,
the XRD patterns of the samples show, in addition to
peaks from CdP and Ge, those from CdGeP .
Fig. 3. (1, 2) Resistivity and (3, 4) Hall coefficient of
Cd0.81Mn0.19GeP2 as functions of (2, 4) increasing and
(1, 3) decreasing pressure.
2
2
To analyze the variation in the volume fraction of
C1
the parent phase, C , we considered a mixed-phase sys-
1
tem–effective medium model [5]. It can be seen in
Fig. 4 that, after one loading–unloading cycle, C is 0.1
1.0
0.8
0.6
0.4
0.2
0
1
in CdGeP , 0.5 in Cd0.91Mn0.09GeP , and 1 (fully
2
2
1
2
3
reversible process) in Cd0.81Mn0.19GeP2.
CONCLUSIONS
The present ρ(p), R (p), and XRD data indicate that
H
CdGeP dissociates to CdP and Ge at p = 3.2 GPa. The
2
2
incorporation of Mn stabilizes the crystal structure of
CdGeP , and Cd0.81Mn0.19GeP undergoes a reversible
2
2
phase transition at p = 3.5 GPa.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project nos. 05-03-33068 and
0
1
2
3
4
5
Pressure, GPa
0
5-02-16608) and the Presidium of the Russian Acad-
emy of Sciences (project Physics and Mechanics of
Heavily Compressed Matter and the Internal Structure
of the Earth and Other Planets).
Fig. 4. Volume fraction of the parent phase as a function of
increasing and decreasing pressure for (1) CdGeP2,
(
2) Cd0.91Mn0.09GeP , and (3) Cd0.81Mn0.19GeP2.
2
INORGANIC MATERIALS Vol. 42 No. 8 2006

PHASE TRANSFORMATIONS OF THE FERROMAGNETIC SEMICONDUCTOR
829
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INORGANIC MATERIALS Vol. 42 No. 8 2006