1. Shockinduced amorphization of qGeO2
N. Suresh, G. Jyoti, Satish C. Gupta, S. K. Sikka Sangeeta, and S. C. Sabharwal
Citation: Journal of Applied Physics 76, 1530 (1994); doi: 10.1063/1.357729
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2. Shock-induced amorphization of q-Ge02
i.
N. Suresh, G. Jyoti, Satish C. Gupta, and S. K. Sikka, &n’geeta
High Pressure Physics Division, Bhabha Atomic Research Centre,Bombay 400=085, India
S. C. Sabharwal’ -’
Te$t$al Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, i
Bombay 400 085, indin
{Received 18 February 1994; accepted for publication 7 April 1994)
__j ._
Polycrystalline samples ‘of the a-quartz phase of GeO, (q-Ge02), recovered from peak shock
compressions of 5,6.8; and 10 GPa, have been examined by x-ray diffraction, thermoluminescence,
and Raman techniques: The measurements show >that q-Ge02 irreversibly amorphizes above 6.8
GPa..-Theestimates of the shear band temperature suggest that -the mechanism of shock-induced
amorphization in q-GeO, is a solid-solid one, in contrast to’that in Sic?, quartz which has both
solid-solid and fusion-quenched components. .i/.
I. INTRODUCTION
Pressure-induced amorphization is currently an active
area of research. More- than 30 materials have been so far
observed to undergo. cry.stalline to amorphous phase transi-
tion under static pressures’ and five under shock compres-
sion (quart%‘-! anorthite,7 faylite,* graphite,? and
cristobalitei” see Ref. *11 for more details). Among these,
silica, because of its geophysical importance, has been the
most extensively studied material by both methods. o-quartz
is reported to amorphize on application of a static pressure.
between 15 and 35 GPa,‘2-1” and also under shock compres-
sion in the 15-40 GPa pressure range.?-” High resolution
electron microscopy measurements on shock-recovered
samples of quartz revealed two types of disordered materials,
one present in the transformation lamellae and the other pro-
duced along the microfaults The former has been inter-
preted as arising due to solid-solid transformation (diaplectic
glass) and the latter due to quenching of the molten material
~(fusion glass). Using electron microscopy, even shock (48
GPa) amorphized cristobalite (another crystalline phase of
SiOJ has been found to consist of both the diaplectic and
fusion glasses.” To compare the mechanism of crystal to
amorphous transition under static and dynamic pressures, it
would be interesting to study whether the melt-quenched ef-
fect could be suppressed in an analogous transition that oc-
curs at a lower pressure. In this regard a trigonal form of
GeOz (q-GeOJ is the ideal material to study as it has been
reported to amorphize in diamond cell experiments at about
6 GPa.““a
ing it to 1420 K. The molten charge, after soaking for half an
hour, was quenched to room temperature. The cell constants
of the resulting material (a =b =4.98 A, c=5.66 Aj ob-
tained from x-ray diffraction (XRD) measurements compare
well with those reported earlier for q-Gew02.“’
Shock compression of the samples was achieved by us-
ing the 63 mm bore gas gun at our laboratory.”A brief de-
scription of this system is as follows.z* It mainly consists-of
a breech, a barrel, and a target catcher system. The breech is
capable of holding a high pressure gas (nitrogen or helium)
and has a breech opening mechanism for allowing the gas to
flow behind the projectile within a few milliseconds. Wo
kinds of breeches are used: a wrap-around breech designed
for below 200 bars pressure and a double diaphragm breech
up to a pressure of 410 bars. Both planar and inclined im-
pacts (for compression shear measurements) can be obtained
up to a velocity of about 1 km/s.
The catcher system consists of two sections which are
separated from each other by a thick Mylar diaphragm. One
section (the target chamber) is permanently mounted to the
muzzle end of the barrel. The second section is a projectile-
and target-stopping system and it is here that we recover a
shock-loaded specimen. The supporting systems include
vacuum pumps for evacuating the barrel and target chamber
and a compressor for providing the required gas pressure. 4
remote control unit allows firing of the projectile in manual
or automode.
In this article, we present results of shock recovery ex-
periments on q-GeOz using a gas gun. Our analysis shows
that some metastable structure is developed for shock load-
ing of 6.8 GPa and for higher shock compressions it turns
amorphous.
II. EXPERIMENTAL
The trigonal phase of GeO, is a stable phase at high
temperature, however, this phase can be stabilized at room
temperature by quenching from melt.” Samples were pre-
pared using germanium oxide powder of 5N purity. The ma-
terial was completely molten in a platinum crucible by heat-
The samples were placed in a shock-recovery fixture as
shown in the Fig. 1. A specimen in the form of a rectangle,
with nominal dimensions 7 mmX6 mmX1 mm, was cut
from the melt-quenched material (bulk density almost the
same as the crystal density) and fitted into a matching hole in
an aluminium circular disc of the same thickness. The alu-
minium disc, along with a steel (SS 304) cover plate, was
positioned in a threaded steel (SS 304) capsule. This was
finaIly fixed on to a target ring that was mounted at the
muzzle end of the gun in the target chamber to achieve align-
ment of the sample parallel to the impactor.
The samples were shock loaded by the impact of a flyer
plate attached to the nose of the projectile. In a particular
experiment, the projectile was accelerated to the chosen ve-
locity by suitably selecting the breech pressure. The samples
1530 J. Appl. Phys. 76 (3), 1 August 1994 0021-8979/94/76(3)/i 530/5/$6.00 Q 1994 American Institute of Physics
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3. PROJECTILE
-.
FIG. 1. Schematics of the target assembly (not to scale).
reached the final pressure by a reverberating shock wave
between the front steel cover plate and the steel holder. Just
before the impact, the velocity of the projectile was mea-
sured by recording the time of flight between four consecu-
tive pairs of pins shorted by the projectile. The pressure in
the sample was computed with the use of a hydrodynamic
computer code in conjunction with the measured projectile
velocity. The peak pressures in the three experiments re-
ported here were 5, 6.8, and 10 GPa.
The shock-recovered samples were characterized using
x-ray diffraction and Raman techniques. Thermolumines-
cence (TL) glow curves of the. material before and after
shock treatment were also recorded over the temperature
range 30-400 “C. The experimental setup used for the mea-
surements is described elsewhere.“” Samples were irradiated
at room temperature in a 6oCogamma chamber for an expo-
sure of 50 C kg-‘.
Ill. RESULTS AND DlSCUSSlON
The XRD measurements on the shock-recovered
samples are displayed in Fig. -2. The pattern of the sample
recovered from 5 GPa is identical to the one at ambient con-
ditions, and contains sharp peaks (loo), (lOl), (llO), (102),
(ill), and (200) at d spacings.consistent with the quartz
structure of Ge0a.a’ For 6.8 GPa, the XRD pattern contains
all the original peaks riding on. an amorphous background.
The peaks are, however, much broadened indicating that lat-
tice strains are present and/or the particle size is reduced.
Moreover, the pattern contains ,two additional peaks (d spac-
ings 4.04 8, and 3.11 ;i> which do not correspond to any of
the known polymorphs of GeO,. Finally, the pattern for the
10 GPa sample shows a broad hump implying the glassy
state of the material. The first glass peak is at q-1.5 A-’
(a) ”
5.0 GPa
114
--
-=-=I
P---cl 30
1 8
36 L6
c4 28
FIG. 2. X-ray powder diffraction patterns of q-GeOa shock recovered from
(a) 5 GPa, (b) 6.8 GPa, and (c) 10 GPa. XRD measurementswere made in
step scan mode (step size 0.05”) at room temperature using .a DIANO 2000
series diffractometer with standard x-ray generator operated with a Cu tube
at 40 kv and -26 mA.
(28=21.7”) which is in close agreement with the 4 value
corresponding’to the first peak in the structureVfactor of the
vitreous GeO, (Ref. 23) and also that of the pressure amor-
phized GeO, under static conditions.” The persistence of the
weak peaks of the ambient phase suggest that amorphization
is not yet complete.
J. Appl. Phys., Vol. 76, No. 3, 1 August 1994 Suresh et al. 1531
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4. 10 GPo
TEMPERATUREC’C 1
FIG. 3. TL glow curves for q-GeO, samples: (a) O-ambient q-GeOz, tb)
O-5 GPa sample, and (c) X-6.8 GPa sample.
The TL glow curves obtained for the initial q-GeO, ma-
terial, and for those recovered from peak shock compressions
of 5 and 6.8 GPa are shown in Fig. 3. As shown by plot (a)
for the ambient q-Ge02, only one broad peak with some
structure and peaking about 140 “C is observed here. No
significant change in its position, though a reduction in the
TL output, is noted in the glow curve of the 5 GPa sample
shown by plot (bj. In the case of the 6.8 GPa sample, while
the TL output is further reduced, an additional broad TL
glow peak around 350 “C is also developed [plot (c)l. For the
10 GPa case, we could not record any TL output. These
results together with XRD measurements show that the
broad TL peak recorded for the initial material is character-
istic of the q-GeO,. Increasing shock compression leads to a
decreasein the TL output and finally it disappears, indicating
amorphization at 10 GPa. The development of an additional
TL peak at high temperature in the 6.8 GPa sample suggests
the formation of some metastable structure between the
quartz form and the amorphous state. The metastable nature
of the structure developing at 6.8 GPa is borne out by the
fact that heating of the sample up to 400 “C during its
maiden heating run to record the TL glow curve suppressed
the emission at 350 “C in the subsequent heating.
The Raman spectrum of q-GeO, at ambient conditions is
shown in Fig. 4. The spectrum with the strongest band at 441
cm-r is identical to that reported by Wolf et aLI For the
sample unloaded from 5 GPa, there is no noticeable differ-
ence in the Raman spectrum compared to that of the initial
material. For the 6.8 GPa sample, the Raman .bands are
broadened, indicating some disordering. This result is similar
to that for the sample decompressed from 6.3 GPa in static
measurements.r7 However, unlike that in static pressure ex-
periments, the peak of the 441 cm-r band does not shift to
higher frequency. The spectrum does-not show the additional
Raman bands expected from the new metastable structure
suggested by XRD and TL measurements. This may be due
to the small proportion of the new phase, the signals of
which are overwhelmed by that of the parent phase. Further,
the Raman bands for the 10 GPa samples are very broad,
i532 J. Appl. Phys., Vol. 76, No. 3, 1 August 1994
6.8 GPa
AMBIENT
100 200 300 400 500 600 700 800
WAVE NUMBER (cm-")
FIG. 4. Raman spectra of q-Ge02: (a) at ambient conditions, tb) after shock
recovery from 6.8 GPa, and (c) from 10 GPa. Raman measurements were
conducted using a 514.5 nm line of an argon ion laser (see Ref. 29) operated
at a power of 80 mW. The spot size was 50 pm. The spectra were recorded
without a polarization analyzer (Ref. 30).
suggestive of amorphization. No new Raman lines are ob-
served.
We could not do electron microscopy measurements on
shock-recovered q-GeO, as the retrieved specimens were
pulverized. Therefore, we are unable to determine the nature
of the disordered material (diaplectic and fusion glass com-
ponents). However, we present below an estimate of the
shear band temperatures in GeO, in order to assessthe pos-
sibility of the existence of the fusion glass in the shock-
recovered samples.
The concept of shear banding during shock deformation
is now well established.‘4-28 During shock wave loading ma-
terial undergoes heterogeneous deformation. The plastic en-
ergy dissipated during deformation is initially deposited in
the localized zones, called shear bands, largely as heat,
which is then distributed through thermal conduction to the
surrounding material which is not so severely deformed.
Thus, depending on the rate of the energy dissipation and
thermal diffusion, the temperature of the shear bands can
momentarily rise appreciably above that of the bulk material.
During the thermal equilibration of the shear bands with the
bulk of the material, the cooling rate can be as high as 1Or1
K/sz4 Considering the situation in which the local tempera-
ture exceeds the melting temperature, the shear band material
would melt. For substances known to exhibit fused glass
Suresh et al.
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5. TABLE I. Material properties for GeOz and SiO, .
Density (Mg/m3)
Gruneisen parameter
Intercept (km/s)
Slope
Thermal diffusivity (mz/s)
Specific heat (J/kg K)
Melting temperature (K)
GeO2 SiO,
PO 4.29 2.63
Y 0.66 0.70
CO 3.86 3.68
S 1.25 2.12
40x10-7 12x10-7
0.562x lo3 1.245X10”
1388 1710
performed a parametric study by varying the strain rate val-
ues from 10” to lo8 s-r and d values of 2 and 5 ,xm.‘8
Ins Figs. 5(a) and 5(b), we have plotted the calculated
temperatures for GeO, and SiO,, respectively. Figure 5(b)
depicts that the shear band temperature in SiO, exceeds its
melting point in lo-16 GPa pressure range. This range is
close to the pressures at which it begins to show presence of
fusion glass.5 By contrast, the shear band temperatures in
GeOz are much smaller than in SiO, for all shock pressures.
At 10 GPa, the local temperatures for all cases are smaller
than the melting temperature at ambient (at 10 GPa, the melt-
ing temperature is even higher).” This suggests that the
fused GeOz will not form up to this shock pressure. ,The
lower local temperature reached near the transition pressure
in GeO, as compared to that in SiO, is for two reasons: First,
the transition in GeOZ occurs at a lower shock pressure and
second, the thermal diffusivity in GeO, is three times as
large compared to that of SiOa.
phases the molten mass may be trapped as vitreous material
due to very high cooling rates.
According to the theory of Grady and Asay,z4 the shear
band temperature may be written as
Tl= Toe YE-+
SC;d
$l2,1/2
-x(~XPZ ’ (l)
where, To is the initial temperature, E and E the strain and
strain rate, respectively, and d the spacings between the ad-
jacent shear bands. C, is the bulk sound velocity, S the in-
tercept of the linear shock velocity-particle velocity relation,
C, the specific heat, x the thermal diffusivity, and y the
Gruneisen constant.
The thermomechanical parameters used in Eq. (1) for
both SiOZ and GeOz are listed in Table I. As the wave profile
measurements for strain rate and electron microscopy data
for shear band spacings on Ge02 are not available, we have
A : 1ssntropic Tempos
a i d = 2Lnn. i - I.0 *
c : d = ImL 2 - 1Q6.-1
= 2pm. ; = 1o’P-1
- 107.-l
(b)
FIG. 5. Calculated shear band and isentropic temperatures as a function of
shock pressure: (a) GeO, and (b) SiO,.
IV. CONCLUSIONS
We have shown that shock-induced amorphization in
quartz phase of GeO, occurs above 6.8 GPa. A new uniden-
tified metastable phase is detected in the samples unloaded
from the shock compression of 6.8 GPa. Estimates of the
shear band temperatures suggest the absence of fusion glass
in shock-recovered q-GeO, and that the mechanism of
shock-induced amorphization in q-Ge02 is a solid-solid one.
ACKNOWLEDGMENTS
We thank Dr. Ram Prasad and Dr. N. C. Soni for XRD
and Dr. S. K. Deb for Raman measurements.
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