High-pressure phase transitions in Zn
12xM
xSe
„M5Cd, Fe, and Mn…
Chih-Ming Lin and Der-San Chuu*
Institute of Electro-Physics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Ji-an Xu and Eugene Huang
Institute of Earth Sciences, Academia Sinica, NanKang P.O. Box 1-55, Taipei, Taiwan 11529, Republic of China
Wu-Ching Chou
Department of Physics, Chung Yuan Christian University, Chung-Li, Taiwan, Republic of China
Jing-Zhu Hu
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015
Jui-Hsiang Pei
National Hsinchu Teacher’s College, Hsinchu, Taiwan, Republic of China
~Received 12 May 1997; revised manuscript received 20 January 1998!
Energy-dispersive x-ray diffraction was employed to study pressure-induced phase transitions in Zn0.84Fe0.16Se, Zn0.9Cd0.1Se, and Zn0.76Mn0.24Se crystals up to 21.0, 23.3, and 24.3 GPa, respectively. Our
result shows that the B3 to B1 structure transitions for these crystals occurred at 11.460.5, 9.560.3, and 9.6 60.5 GPa, respectively. Compared to the phase-transition pressure (Pt) of ZnSe~14.4 GPa!, a reduction of
about 3–5 GPa is exhibited in these ternary compounds. This reduction in phase-transition pressureDPt in the
ternary compounds suggests that the fractional volume change, (DV/V0), of the B3-B1 phase admixture might
be the source of this reduction. Our results indicate that Pt~with respect to the phase transition pressure 14.4
GPa of ZnSe! is related to the fractional volume change (DV/V0) by the expression Pt[email protected]
(DV/V0)20.0281(DV/V0)2# GPa. @S0163-1829~98!05426-5#
ZnSe-based ternary compound semiconductors, which are random mixtures of ZnSe and magnetic or nonmagnetic ions, have attracted much attention due to the study of basic physi-cal properties such as the variation of long wavelength opti-cal phonon vibration modes versus the mole fraction of non-magnetic ions1and its ability to tune both the band gap and the lattice constant for application in optoelectronic devices. The diluted magnetic semiconductor2~DMS! Zn12xMnxSe is
one of the ZnSe ternary compound semiconductors, which have been reported to have many interesting physical prop-erties, such as intermediate-mode behavior3,4and an anoma-lous dependence of the band gap on the magnetic ion
composition.5 Some important results of the
pressure-induced phase transition of ZnSe ternary compound semi-conductors containing magnetic ions have been obtained;
e.g., Qadri et al.6 used the energy-dispersive
x-ray-diffraction ~EDXD! method to investigate the pressure ef-fects of the Zn0.83Fe0.17Se crystal, and Arora and co-workers and others applied a Raman scattering experiment to study the phase transitions of Zn12xFexSe,
4
Zn12xCoxSe,
7 and Zn12xMnxSe ~Refs. 8 and 9! under high pressure. It was
found that the existence of magnetic ions in the ZnSe crystal resulted in a reduction of the transition pressure. Such a re-duction was believed due to the hybridization of 3d orbitals into the tetrahedral bonds,6,10 while Qadri et al.11 reported that the phase transition pressure decreased when the lattice parameter increased. Usually, the semiconductor-metal tran-sition pressure ( Pt) identified by the EDXD result
corre-sponds to a change of the crystal structure which
accompa-nies the disappearance of the local magnetic ions and the
longitudinal optical~LO! phonon modes according to Raman
scattering results.4In this work, we study the phase transition of Zn12xMxSe, M5Cd, Fe, and Mn, crystals by the EDXD
method under high pressure.
Zn12xMxSe, M5Cd, Fe, and Mn, crystals grown by the
modified Bridgman method were grounded to 1mm size for
use in the EDXD measurement. A Mao-Bell-type diamond
anvil cell ~DAC! with T304 stainless steel gaskets which
were preindented to 15.0 GPa was used; the sample hole was
200 mm in diameter. The anvil parameters were 1/3 carats
with a 600 mm culet. Experimental details were described
earlier.12 For the DAC EDXD experiment, the
supercon-ductor wiggler synchrotron beam line X17C of the National
Synchrotron Light Source ~NSLS! of Brookhaven National
Laboratory was used. The beam size was 50350 mm2, a
germanium energy dispersive detector was set in the position where the diffracted angle (u) was 5°. So the relation of the energy of reflection, E, versus d spacings, d, is Ed571.137 keV Å. Methanol-ethanol 4:1 fluid was used as a hydrostatic
pressure medium and the internal gold standard13 was
em-ployed in the pressure determination in the EDXD measure-ment, respectively. The peak positions were read out by a peak search program provided by a VAX computer in the beam line X17C; the equation of state~EOS! data were fitted to the Murnaghan equation by use of a fitting program re-ported earlier.14
For the case of nonmagnetic impurities, a series of spectra of the Zn0.9Cd0.1Se loading run is shown in Fig. 1, which
PHYSICAL REVIEW B VOLUME 58, NUMBER 1 1 JULY 1998-I
PRB 58
contains the x-ray emission lines of Cd and the standard identified pressure lines of internal gold. Zn0.9Cd0.1Se has a
lattice parameter of 5.64960.001 Å from the EDXD
mea-surement at ambient pressure. Figure 1 shows that there are only three reflections~111!, ~220!, and ~311! of the B3
@zinc-blende ~ZB!# phase, by the relation of Ed571.137 keV Å;
those which appear are with d spacings 3.272, 2.002, and 1.714 Å, respectively, at ambient pressure. When the
pres-sure was increased to 9.5 GPa, the reflections ~200! and
~220! of the B1 @rocksalt ~RS!# phase appeared at the
high-energy side of the reflections ~111! and ~220! of the B3
phase, respectively. The d spacings at 9.5 GPa are 3.158, 1.939, and 1.646 Å for the B3 phase and 2.59 and 1.835 Å for the B1 phase, while the lattice parameters are 5.472
60.001 Å and 5.18560.001 Å for B3 and B1 phases,
re-spectively. The ambiguity pressure range of the B3 and B1 phases is from 9.5 to 11.0 GPa; in this pressure region a mixing of the B1 and B3 phases coexisted. The onset pres-sure of the transition from the B3 to B1 phase is 9.5 GPa; the reflections of the B3 phase of Zn0.9Cd0.1Se disappeared completely and only reflections of the B1 phase appeared apparently above 11.0 GPa. Since one can argue on thermo-dynamic grounds that the transition pressure should be nearer, or equal to, the onset pressure, therefore, the transi-tion pressure of B3 to B1 for Zn0.9Cd0.1Se is taken as 9.5 GPa. The B1 reflections~200! and ~220! were found to exist up to 23.3 GPa. For the case of the magnetic impurity ternary
compound of ZnSe, a series of Zn0.84Fe0.16Se and
Zn0.76Mn0.24Se loading run spectra are similar to those of Zn0.9Cd0.1Se. Zn0.84Fe0.16Se and Zn0.76Mn0.24Se have lattice
parameters 5.63960.001 and 5.70860.001 Å, respectively,
as obtained at ambient pressure from EDXD measurements. For the case of Zn0.84Fe0.16Se, only~111!, ~220!, and ~311! of the B3phase peaks appeared below 11.4 GPa. The d spac-ings at 11.4 GPa are 3.112, 2.556, and 1.802 Å for the B3 phase and are 1.861 and 1.634 Å for the B1 phase, respec-tively. The lattice parameters of the B3 and B1 phases were
5.35760.001 and 5.10460.001 Å at 11.4 GPa, respectively.
Above 11.4 GPa, peaks of the B3 phase disappeared and alternatively the peak of the B1 phase occurred apparently. The B1 peaks~200! and ~220! were found to exist up to 21.0 GPa. In the case of Zn0.76Mn0.24Se, only reflections ~111!,
~220!, and ~311! of the B3 phase with d spacings 3.302,
2.030, and 1.725 Å, respectively, appeared at ambient pres-sure. At 9.6 GPa, the reflections~200! and ~220! of the B1 phase appeared at the high-energy side of the reflections
~111! and ~220! of the B3 phase, respectively. The d
spac-ings at 9.6 GPa are 3.189, 1.946, and 1.668 Å for the B3 phase and 2.593 and 1.844 Å for the B1 phase. At 9.6 GPa the lattice parameter of the B3 phase is 5.519 Å and is 5.202 Å for the B1 phase. Above 9.6 GPa, the reflections of the
B3 phase of Zn0.76Mn0.24Se disappeared and the reflections of the B1phase occurred apparently. The B1reflections~200! and~220! were found to exist up to 24.3 GPa. Importantly, no two-phase coexistence ambiguity range occurred in the case of magnetic impurity ternary compound semiconductors of ZnSe. The structure phase-transition pressures were 11.4, 9.6, and 9.5 GPa for Zn0.84Fe0.16 Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se, respectively. By the way, the width of the tran-sition zone of Zn0.9Cd0.1Se is larger than in the other two samples.
The variations of the interplanar distances dhkl ~Å! shown in Fig. 2 for Zn0.9Cd0.1Se. Zn0.84Fe0.16Se and Zn0.76Mn0.24Se FIG. 1. A series spectra of Zn0.9Cd0.1Se at various pressure
re-corded in a loading run which contains the x-ray-emission lines of Cd Ka1 and Cd Kb1and the standard identified pressure lines of
Au~111!.
FIG. 2. The variation of dhkl~Å! of Zn0.9Cd0.1Se with pressure
~GPa! for the B3 and B1 phases.
have the same relations as that of Zn0.9Cd0.1Se. All the inter-planar distances decreased as the pressure was increased for both B3 and B1 phases. Figure 3 shows the EOS relations as a function of pressure for Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se, respectively. V0 is the volume at ambient pres-sure. The arrows labeled A, B, and C at 11.4, 9.6, and 9.5 GPa indicate transition pressures of the B3 to B1 phase for Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se, respec-tively. The data for both phases were fitted to the Murnaghan equation by a fitting process of Xu et al.14The values of K0, the isothermal bulk modulus at ambient pressure, and K08, the pressure derivative of the isothermal bulk modulus evaluated at ambient pressure, of the Murnaghan equation for these three samples in the region below and above the phase tran-sitions~B3 and B1 phases! obtained from the fitting process are listed in Table I. The values of K08 were consistent with the slops of d spacings for B3 and B1 in the loading run spectra. It shows that all samples in the pressure region (B1) above the phase transition were less compressible than that in the pressure region (B3) below the phase transition.
The phase-transition pressure Ptof ZnSe was found to be
14.4 GPa.4As we mentioned above, our results show that the
Ptis decreased as the impurity ion Fe, Cd, or Mn was mixed
to form the ternary compounds ZnFeSe, ZnCdSe, or
ZnMnSe, respectively. For example, the Pt’s of
Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se are 11.4, 9.6, and 9.5 GPa, respectively. The reason for the reduction of the phase transition pressure was extensively studied by many authors for a decade.6,10,15,16 Qadri et al.6 and Ves
et al.15 investigated the variation of Pt of Zn12xMnxSe and
Zn12xFexSe with the impurity concentration x of Mn and Fe.
They concluded that the decreasing of the B3 to B1phase transition pressure was strongly dependent on the increasing of the impurity concentration. In a later work of Qadri
et al.,16 the pressure effect on the phase transition of Cd12xMnxTe was investigated, and a similar result was
ob-tained. Maheswaranathan et al. studied Cd0.52Zn0.48Te and Cd12xMnxTe with 0<x<0.52 ~Ref. 10! using a
photoemis-sion method; they indicated that the substitution of Zn by Cd in the zinc-blende lattice made the lattice more stable than the substitution of Zn by Mn. They found that Mn, but not Zn, weakened the zinc-blende crystal structure and made it less stable under pressure and suggested that 3d orbitals of Mn ions but not Zn ions hybridize into tetrahedral bonds because the 3d electrons are less tightly bound in Mn than in Zn. And they also found that Cd and Zn d levels do not
hybride with s p3 bonding orbitals. Therefore, they
con-cluded that the cause of the reduction of the phase-transition pressure was attributed mainly to the hybridization of the Mn or Fe 3d orbitals into tetrahedral bonds in the Mn or Fe
ternary alloys. Furthermore, Qadri et al.11 and
Ma-heswaranathan et al.10 indicated that in the ~Zn,M!Se
~M5Fe, Mn! system the lattice parameter increased and the
transition pressure decreased as the magnetic ion was mixed into ZnSe. The covalent radius of Zn is 1.25 Å, which is larger than the 1.17 Å of Fe and 1.17 Å of Mn, respectively. Thus, the reduction of the phase-transition pressure Pt was
also ascribed to the decrease of the covalent radius of the impurity ion as Fe or Mn substituted the Zn ions.
Arora and Sakuntala,8in the investigation of the relation-ship of Pt versus x in the ternary system Zn12xMnxSe,
ob-served that an apparently different behavior existed in the higher-impurity-concentration ternary in contrast to the re-sult of the lower-x ternary investigated by Qadri et al.6and Ves et al.15. Thus, the transition pressure did not manifest a strong dependence on the Mn concentration. Furthermore, our results show that a reduction of transition pressure was
also present in the nonmagnetic ternary compound
Zn0.9Cd0.1Se. This is a manifestation of the fact that the hy-bridization of the 3d orbitals of the magnetic ion Fe or Mn might not be the main reason for the reduction of the transi-tion pressure in the ternary alloys of ZnSe. It is also known the covalent radius of Cd is 1.48 Å which is larger than that
~1.25 Å! of Zn. Thus, the reduction of the phase-transition
FIG. 3. V/V0 vs pressure for the B3 and B1 phases of
Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se, respectively. The
symbols A, B, and C indicate the transition pressures of the B3 to
B1 phase for Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se,
re-spectively.
TABLE I. The values of K0 and K08 for Zn0.84Fe0.16Se,
Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se under and above the phase
transi-tion ~B3 and B1 phases! obtained from the fitting process by Xu
et al.~Ref. 14!. K0is the isothermal bulk modulus at zero pressure,
and K08 is the pressure derivative of the isothermal bulk modulus
evaluated at zero pressure.
Sample Phase K0~GPa! K08
Zn0.84Fe0.16Se B3 58.8560.22 4.1260.19 B1 80.1561.69 3.6760.91 Zn0.76Mn0.24Se B3 60.4860.26 4.3760.16 B1 70.8661.61 3.6460.87 Zn0.9Cd0.1Se B3 60.2360.29 4.3260.18 B1 97.0261.74 3.9760.85 18 BRIEF REPORTS PRB 58
pressure caused by the decrease of the covalent radius of the impurity ion in the ZnMSe system proposed by Qadri et al. could not be correct.
To explain the reduction of the phase-transition pressure of the impurity mixing semiconductor, let us consider the volume change of the unit cell for phase transition from the
B3 to B1 phase in Table II. One can note that the increase of
the percentage of the reduction of phase-transition pressures with respect to 14.4 GPa of ZnSe relates prominently to the increasing percentage of the reduction of the volume changes for our three samples while phase transition from B3 to B1 occurred. The percentage of the reduction of the volume changes is the ratio of the B1 volume to the B3 volume
times 100% at Pt. The above measurements indicate that
decreasing in the phase-transition pressure Pt for a phase
transition from the B3 to B1 phase can be related to the increase of the percentage of the reduction of the volume
changes (DV/V0), by the expression [email protected]
(DV/V0)20.0281(DV/V0)2# in GPa for our cases of the
Zn12xMnxSe, M5Cd, Fe, and Mn system. Therefore, the
greater the percentage of the reduction of the volume de-crease of the B3 to B1 phase transition is, the greater the percentage of the reduction of the phase-transition pressure can be obtained in ZnSe-based ternary compound
semicon-ductors. Hence, the fractional volume changes (DV/V0),
while at the B3 to B1 phase transition might be the source of the reduction of the phase transition pressure from B3 to B1 for ZnSe compound semiconductors of random kinds of im-purity ions.
In summary, our EDXD data showed that the bulk modu-lus for Zn0.84Fe0.16Se, Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se is
58.8560.22, 60.4860.32, and 60.2360.29 GPa before
phase transition and the pressure derivative is 4.1260.16, 4.3760.21, and 4.3260.18, respectively. The greater the in-crease of the fractional volume changes while at the phase-transition~ B3 to B1! region, the greater the decrease of the reduction in the semiconductor-metal phase-transition pres-sure can be obtained. No apparent effect of 3d electronic hybridization can be observed in our works. We suggest that the effect of increasing the fractional volume change of ZnSe-based ternary semiconductors with any kind of impu-rity ions may be the main reason to reduce the stability of the
B3 phase under the application of pressure.
This work was supported by the National Science Coun-cil, Taiwan under Grant Nos. NSC87-2112-M-009-009 at NCTU, NSC86-2112-M-033-012 at CYCU, and NSC 85-2111-M-001-002 at IES.
*Author to whom correspondence should be addressed.
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Zn0.76Mn0.24Se, and Zn0.9Cd0.1Se, respectively.
Sample
(DV/V0): the
fractional volume changes from
the B3 to B1 phase ~with respect to the B3
volume while at Pt!
(DPt/14.4): percentage
of the reduction of the B3 to B1 phase-transition pressure~GPa!
~with respect to 14.4 GPa of ZnSe! Pt: B3 to B1 phase-transition pressure ~GPa! Zn0.84Fe0.16Se 13.5% 20.8% 11.460.5 Zn0.76Mn0.24Se 16.3% 33.3% 9.660.5 Zn0.9Cd0.1Se 16.1% 34.0% 9.560.3 PRB 58 BRIEF REPORTS 19
