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High-pressure phase transition in gallium phosphide: An x-ray-absorption spectroscopy study
J.P. Itié, A. Polian, C Jauberthie-Carillon, E Dartyge, A Fontaine, H Tolentino, G Tourillon
To cite this version:
J.P. Itié, A. Polian, C Jauberthie-Carillon, E Dartyge, A Fontaine, et al.. High-pressure phase tran- sition in gallium phosphide: An x-ray-absorption spectroscopy study. Physical Review B: Condensed Matter (1978-1997), American Physical Society, 1989. �hal-03059442�
PHYSICAL REVIEW B VOLUME 40, NUMBER 14 15NOVEMBER 1989-I
High-pressure phase transition in gallium phosphide:
An x-ray-absorption spectroscopy study
J.P.Itic, A.Polian, and C.Jauberthie-Carillon
Physique desMilieux Condenses, Universite Pierre et Marie Curie, Tour 13-4eetage, 4 place Jussieu, F-75252Paris CEDEX05, France
E.Dartyge, A.Fontaine, H.Tolentino, and G.Tourillon
Laboratoire pour lUtih'sation du Rayonnement Electromagnetique, Uniuersite de Paris—Sud, Sentiment 209D,F-91405Orsay CEDEX,France
(Received 6 February 1989)
High-pressure behavior ofGaP has been studied by x-ray-absorption spectroscopy in a diamond- anvil cell up to 36GPa. The room-temperature equation ofstate ofphase Ihas been determined.
The transition from a fourfold (zinc-blende) toa sixfold (P-tin) coordination scheme isobserved near 26GPa. Analysis ofthe data obtained on asample quickly depressurized from 36GPa shows that the transition isnot reversible and the recovered sample is mainly amorphous.
I. INTRODUCTION
Gallium phosphide is a semiconductor which crystal- lizes in the zinc-blende structure (Td space group). In this structure, each atom is tetrahedrally bonded to its first neighbors. Under pressure, a transition to ametallic centered tetragonal phase (g-tin) has been shown to occur in the 21 —25GPa range.
High-pressure properties of the zinc-blende structure (phase I) have been determined by Raman scattering, ' ultrasonic measurements, fundamental absorption, ' and refractive-index measurements. The transition to the metallic phase has been first observed by resistivity measurements'
"
and then by x-ray di8'raction. 'However, in spite of the number ofstudies performed on this compound, there are still some unknowns and controversies. In particular, even the room-temperature equation of state of the zinc-blende phase has not yet been determined, and the reversibility ofthe transition is still questioned. ' ' '
It has been shown that x-ray-absorption spectroscopy (XAS) and especially extended x-ray-absorption fine- structure spectroscopy (EXAFS) is sensitive to pressure and gives information about the local environment ofan atom: interatomic distances, number of neighbors in a shell, and the Debye-Wailer factor.'
In the present work we have studied the high-pressure behavior of GaP up to 36GPa at room temperature by energy-dispersive XAS in a diamond-anvil cell (DAC) at the K edge ofgallium (10367 eV). This experiment en- abled us to determine the equation of state of phase I (I' (25 GPa), and shows that at the transition, the galli- um coordination goes from a fourfold (tetrahedral) to a sixfold (octahedral-like) environment, results which are consistent with the 13-tin-like structure. Finally, analysis
ofthe results obtained on a sample quickly depressurized from 36to0 GPa shows that the transition isirreversible, the recovered sample being amorphous.
Section IIofthis paper is devoted to a brief presenta- tion of the experimental techniques used, namely the energy-dispersive x-ray-absorption spectrometer, as well as the physical parameters which can be determined by this technique. The results obtained on GaP are present- ed and discussed in Sec.III.
II. EXPERIMENT
The experiments were performed at the energy- dispersive x-ray-absorption port of Laboratoire pour 1' Utilisation du Rayonnement Electromagnetique (LURE) (Orsay, France). The main characteristics of the spec- trometer have been described elsewhere. ' The high- pressure cell was of Block-Piermarini type. Several runs have been performed on fine-grained powder with silicon oil as a pressure-transmitting medium, or single crystals with the 4:1 methanol-ethanol mixture as a pressure-transmitting medium. The diameter of the ex- perimental chamber (the hole drilled in the stainless-steel gasket) was 200)Mm in diameter and the initial thickness 50 pm. The pressure was measured outside the hutch, using the power five-ruby fluorescence scale.
The energy dispersive scheme is particularly suited for performing x-ray-absorption spectroscopy with aclassical DAC, i.e., measuring the transmission spectrum through the two anvils, for the following reasons: (i) With the new design, which utilizes an elliptically bent silicon crys- tal, the beam can be focused down to -400 pm in the horizontal plane (the vertical direction is only slitted).
The homogeneity ofthe energy distribution inside the po- lychromatic image ofthe source is almost perfect. With this geometry, an energy range of -400 eV above the edge was accessible at the Ga IC edge (Fig. 1). (ii) The whole spectrum is obtained at once in a short time (—1
s), which makes the alignment of the DAC (-200-pm
hole) easy, by looking at the video signal ofthe spectr'um on an oscilloscope. (iii) Since there is no mechanical
40 9709 1989 The American Physical Society
9710 J.P.ITIEet al.
GaP
P Po
l
100
I
200
P = i.s Gpa 300
E -Eo (ev)
FIG. 1. X-ray absorption spectrum ofGaP at the gallium K edge at 1.5Gpa.
where po is the atomic absorption, and p the measured cross section. In aone-electron, single-scattering approx- imation where the wave function associated with the pho- toelectron isaplane wave,
X(k)=
g,
lf,(k, ~)l»n[2k. R,+e,(k)], az,'
2k2 —28 /k. (k)
Xe e
where N. is the number ofneighbor atoms at the distance RJ, ~f,(k,rr)~ the backscattering amplitude, A, (k) the
electron mean free path, o.j the mean-square displace- ment between the absorbing atom and the jth back-
scattering atom (Debye-Wailer factor), and 4 (k)the to- tal phase shift due to the backscattering and the absorb- ing atom. The photoelectron wave vector kis given by
k=Pi '[2m(E Eo)]'— (2)
movement ofthe setup, there isno instability ofthe ener- gy scale or disalignment ofthe beam with respect to the aperture in the gasket: this last point is crucial for small samples.
As already mentioned, the x-ray beam goes through the diamond anvils and the geometry allows the use of
any type of DAC. Nevertheless, it has the drawback that the Bragg conditions are fulfilled for the monocrystalline diamond anvils for specific energies, resulting in a "hi- jacking" of these photons. These missing photons give rise to extra absorption peaks (glitches) in the spectrum.
A motor-driven stage is used to rotate the cell from out- side the hutch, in order to shift the Bragg peaks ofthe di- amonds as far as possible from the region ofinterest; the real-time observation ofthe transmission spectrum makes this adjustment relatively easy.
The absorption spectrum of GaP has been measured at the E edge of gallium (10367eV) using an optimized
Si[111]crystal as energy dispersive optics. Less than 10 min were necessary to obtain a good quality spectrum.
The energy calibration was made by measuring the ab- sorption spectrum of GaAs at room pressure, and by comparing this spectrum with data collected using a step-by-step scan.
Only spectra obtained on powder samples were ana- lyzed for the following reason: the overall shape ofthe absorption curves is similar for powder and for single crystals. With single crystalline samples it isnot possible to fill the whole experimental volume, whereas with powder this problem does not arise. Ifapart ofthe beam goes directly to the detector the white-line intensity is strongly reduced. Such an effect is also observed when holes are in the sample.
In the analysis ofthe spectra, the threshold energy is taken at the inAection point ofthe edge.
III. RKSUI.TSAND MSCUSSION
The oscillatory part ofthe absorption cross section is given by
where Eisthe energy ofthe incident photon, and Eo the threshold energy.
The above expressions imply that the photon extracts the photoelectron from the core level ofthe central atom (gallium in the present case). The wave associated with this photoelectron is then partly backscattered by the neighboring atoms, so that the final state of the dipolar transition is essentially a superposition of the outgoing wave and the backscattered wavelets, which give rise to the observed interferences. The plane-wave approxima- tion is not essential, but it is given here to simplify the presentation. In the actual analysis, phase shifts are ex- tracted from GaP at ambient pressure and therefore in- clude the spherical aspect of the photoelectronic wave.
At low energy, the mean free path of the electrons is large. Hence, a low-energy electron may be scattered by several neighbors, keeping an appreciable scattering am- plitude, which includes multiple-scattering events: ' it is the XANES region (x-ray absorption near-edge struc- ture), limited to approximately 50 eV above the absorp- tion edge, while the EXAFSpart is above this limit.
The y(k )is deduced from the experimental absorption spectrum by subtracting the base line, using a smoothing procedure. A Fourier transform of y(k)ismade in order to deduce information in real space. For every pressure, we used the same window for the Fourier-transform pro- cedure.
In order to determine the nearest-neighbor interatomic distance E. and the variation ofthe Debye-Wailer factor, the first step isto determine the backscattering amplitude
lfj.(k,~)l and the phase shift %~(k) of the Ga-P pairs.
This was done by measuring the EXAFS spectrum of GaP at ambient pressure; the structure and the distance between the various neighbors are known and the Debye-Wailer factor is taken to be O. Since a determina- tion of the absolute value ofthe Debye-Wailer factor is not possible, only its pressure dependence can be ob- tained. ~fj(k)~ and V~(k) are then adjusted in order to fit y(k) calculated using Eq. (1)to the inverse Fourier transform (IFT) ofthe first-neighbor peak.
HIGH PRESSUSUREPHASE TRANSITION IN GALLIUM. ..
- B'P
d=do I+
0
(3) fi the y(k) calculated using
a T ofthe first-neighbor peak
nd ste is to t e
~f (k)~ and 4~(k) to the IFT o e with two adjus a' stablee parameters Rand o..
'b d have obtained the Using the proceduure described, we av
te (EOS) of GaP up to 24 GPa (i.e.,just equation ofstate ~
1 hase transition) (Fig.
eto tetragona p ase his fi ure, the points represent e
hed EOS fitt d o th lt
I'=0,andB'o,i'ts pressure derivative. or acu ' 1, the Murnaghan EOSiswritten
—{1/3B')
I I
I I
GaP
~ ~
CL
' 2.3—
lg C9 LLl
XO
I-
CO
~ ~ EXpt.
Murnaghan
\~
2.2' 0
I
10 15
PRESSURE (GPa)
I
20
-nei hbors interatomic distanceein' GaP asa
FICx. 2. First-neig ors '
tthe experimental re- e. The oints represent t e exp
p
suts1 deducede uc from EXAFSanalysis an t e a best-fit Murnaghan equation ofs a e[ q.
sB =87.4GPa and B'=4.5GPa us- The best fit yields Bo=
ood agreement 0= ~ ~ These results are in goo
ing do=2.37 A. T e
to0.15GPa (Ref. 6)
=478 GP ) Thi i h fi
ic results measured up to
At higher pressure, g
a tetra onal metallic phase.
transition to a e g
b tion spectrum, espe-
1 seen on the a sorp ion sition is clear y
.3) This figure shows the ESran eg (Fig.
m ofa sample at our pres XANES spectrum o a
) The first two spectra
f h ' -bl d h to to the bottom .
(1.5 and 11.5 GPa) are those o t e zin-
ded b four phos- allium atom is surroun e y
where the ga ium
haracterized by three
1' " d o addi'o 1
the s ectrum is c arac s the "white line an gy p
h a feature is genera y
compounds When the pressure is d h'h
p
which reAects the shortening o t e in
h h f th h f th
ore striking is the c ange M
spectrum when t ep ase
p en at the ig es p
this experiment (36GPa) is definitely i eren
0 25
E — Eo (eV)
ES art ofthe spectrum at various pressures.
The spectrum at p=0GPa has been o taine a crease ofthe pressure from 36GPa.
ofthe s ectra. hestructures characteristic o
pp . Correlatively, there nment disappear.
t t ahedral enviro
.
d.. .
i.f...
;.. . .
h. firswhich is good enough to deduce in orrn shell ofneighbors only.
=0 in the same run t s ectrum obtained at P= in e
36GPa pressure) is different (i.e.,after quic relaxin the 3 ap
e hi h- ressure phases:
ks from those in
t but the two well-defined pea s the white line is present, but t e wo
'n hase Ihave disappeared.
observed in p as
bove one can extract
Fi .3are
m ofthe spectrap shown in ig.
in Fi .4,where curve (a) is ta en a t 0GP ft 1 i th
k i o h
ure the more intense pea is u sure. In this figure
r shell. Curves 4(a) and 4(b) exhibit alsoe at
f h hb
1 r er distance contributions due o ur
36 GPa ]
[curve 4(c)]c anand in the curve [curve o released at 0. As expected, t e posi
t distances during the g o p ifts toward s orter is
increase from 1.5 to
d' 1 i b th
hic distance is not iree y
fhkdd f
position ofthe IF peak because o t e e
si nificant pressure shift. Since there is no signi ca
e hase shift the peak shift gives a
p o p
asure of the pressure depen ence
6G.
distance. W en pressure is urt er inc
or eak shifts toward largera distances:
g o p
does not contra ic e this phenomenon do
e densification. Actua11y the coordina-
P"~ -
h d ~ o6
r is 4in the zinc-blende p ase an
1h h- h
ith a lar er interatomic dis- a in the tetragona ig-
~ ~
ing rise to a higher density wit a arger '
J.P.ITIEetal.
tance for the first neighbors.
Quantitative information is derived by fitting the IFT ofthe first-neighbor peak. Three examples are shown in Fig. 5, where the IFTis shown by the solid line and the calculated spectrum by the dots. At 4.9 C+Pa, i.e., in phase I (zinc blende), the fit is performed with four first neighbors. The fit is excellent. In the tetragonal high- pressure phase -there isa good quality fit by using six first neighbors: four phosphorus and two gallium atoms.
This fit is performed considering the two kinds ofatoms and utilizing the phase shift and the amplitude ofnormal GaP for the phosphorus neighbors. We have a peculiar situation, due to the Pand Ga atoms being first neighbors together. In the considered energy range, the back- scattering amplitude ofgallium is weak and the phase of the oscillation due to this atom is in opposition with P atoms. Such a situation is observed in Ni-As (Ref.26)or Zn-Al (Ref. 27) alloys as well. In order to extract the backscattering amplitude and the phase shift of Ga atoms, we chose to use the backscattering amplitude and the phase shift of As as determined from GaAs, where the first-neighbor distances are well separated, which is not the case forgallium metal, for example. It should not introduce a great difference, because the two atoms are almost neighbors in the Periodic Table. At 36CiPa, the best fit is obtained with four phosphorus at 2.41 A from the central atom and two gallium at 2.44A.
The third spectrum of Fig. 5 shows the fit obtained on
the same sample after releasing the pressure. In this fit, the number offirst neighbors and the distance were free parameters. The best fit is obtained with 3.1phosphorus as first neighbors at a distance of 2.39 A (the nearest- neighbor's distance in GaP phase Iis 2.36 A). The com- parison ofthe absorption spectrum ofthe recovered sam- ple with that of an amorphous GaP sample (Fig. 6) shows a great similarity. In the EXAFSrange, the oscil- lations coincide, but in the XANES spectrum of the recovered sample, little ofthe ascendent crystalline struc- ture remains, but it is heavily damped, resembling that of the amorphous sample. The conclusion from this com- parison is that when pressurization is carried far above the phase-transition pressure, the recovered sample is al- most entirely amorphous. Using x-ray diffraction, Hu et al.' did not find any evidence for amorphization on a sample recovered at room pressure after a pressurization up to 29.6GPa. In fact, these authors obtained the x-ray
0-
GaP
Co O~c
D
0
0-
i
2
I
4
0 2 4 k(a.u.}
DlsTANGE (A)
FIG.4. Amplitude ofthe Fourier transform ofthe EXAFS oscillations at various pressures. (a),1.5 GPa,'(b), 11.5GPa; (c), 36 GPa; (d),0 GPaafter a rapid decrease from 36GPa.
FIG. 5. First-neighbors filtered EXAFS oscillations at vari- ous pressures. The continuous line is the experimental spec- trum and the dotted line is the fit using Eq. (1). 4.9Gpa: four phosphorus neighbors at 2.34 A. 36 GPa: four phosphorus neighbors at 2.41 A and two gallium neighbors at 2.44 A. 0 GPa (recovered sample): 3.1phosphorus neighbors at2.39A.0
HIGH-PRESSURE PHASE TRANSITION IN GALLIUM. . . 9713
T0
KQ
CO
Kl
F ~r ~, ~v 0 0 A4 ~~~ 0~0~~
amorphous
~0~0~ ~ ~too
pres$ur/zed 200
E —Eo (eV}
FIG.6. Comparison ofthe EXAFSspectra ofthe recovered sample (dotted line) and an amorphous sample (Ref.28) (solid line). The vertical scale isnot the same in order toavoid the su- perposition ofthe curves.
pattern characteristic ofphase I,except that all the peaks were weaker than in the original spectra ofphase I. This change was attributed to an irreversible decrease ofthe thickness of the gasket (and therefore of the sample) when high pressure isapplied. Nevertheless, it should be recalled that at 2S GPa they found that only 75% ofthe sample was in the high-pressure phase. Since the pres- sure was not raised to a significantly much higher pres- sure (29.6 GPa), it is likely that in their experiment the transition was not completed. In fact, this could explain why they found phase I in their x-ray pattern of the
recovered sample. A more recent zero-pressure electon- diffraction study carried out on a sample pressurized to 28.4GPa and x-ray-difFraction study on the sample pres- surized to 32 GPa show the presence of a mixture of amorphous and polycrystalline GaP. The transmission spectra on asample quenched from 32.5GPa is similar to that ofannealed amorphous GaP.
IV. CONCLUSION
GaP has been studied as afunction ofpressure in a dia- mond anvil cell by x-ray-absorption spectroscopy up to 36GPa. The analysis ofthe spectra shows the following.
(i) Ityields the equation ofstate in the zinc-blende phase up to the phase transition. (ii)The transition from phase
I, where the gallium atoms are fourfold coordinated, to the 13-tin structure, where it is sixfold coordinated, is clearly seen on the EXAFS and on the XANES part of the spectrum, which is very sensitive to a change in the local environment of the absorbing atom. (iii) The sam- ple recovered at @=0after pressurization to 36 GPa is mainly amorphous. The long-range order of the recovered sample appears to be dependent on the max- imum pressure reached.
ACKNOWLEDGMENTS
Physique des Milieux Condenses is Unite Associe no.
782 of Centre National de Recherche Scientifique. La- bor atoire pour 1' Utilisation du Rayonnement Electromagnetique (Orsay, France) is associated with Centre National de la Recherche Scientifique (Paris, France), Ministere de 1'Education Nationale de la Jeunesse et des Sports (MENJS), and Commissariat a 1'Energie Atomique. This work has been supported by MENJS under Contract No. 8850171.
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