Shock Waves (1998) 8: 173 176
Shock wave induced phase transition in Ä…-FePO4
K.D. Joshi1, N. Suresh1, G. Jyoti1, S.K. Kulshreshtha2, S.C. Gupta1, S.K. Sikka1
1
High Pressure Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
2
Chemistry Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India
E-mail : [e-mail]
Received 26 May 1997 / Accepted 1 September 1997
Abstract. Shock wave induced response of the berlinite form of FePO4 has been investigated up to 8.5 GPa.
The X-ray diffraction measurements on the shock recovered samples reveal transition to the mixture of an
amorphous phase and an orthorhombic phase around 5 GPa. The proportion of the amorphous material
in the recovered sample is found to decrease at higher pressure. The results are interpreted in terms of a
three-level free energy diagram for the crystal to amorphous transitions.
Key words: Shock wave, Phase transition, Berlinate FePO4, Amorphous
1 Introduction the presence of a broad peak which did not disappear till
10 GPa. The structure of the high pressure phase could
not be determined because of the paucity of the diffrac-
Many crystals have been observed to undergo crystalline
tion data.
to amorphous phase transitions under static and dynamic
Shock wave compression of materials is always accom-
compression [see Sharma and Sikka (1996) for review and
panied by shear, high temperature and defects. All these
references therein]. Among them quartz family of com-
factors could influence the nature as well as kinetics of a
pounds have been studied extensively because of their geo-
transformation. In the present work, we have examined
physical importance. Compounds belonging to this family
the behaviour of Ä…-FePO4 under shock loading in order to
have structures based on corner linked tetrahedral net-
compare it with that under static pressures.
work. This tetrahedral framework structure, which does
not favour close packing, has an important role in decid-
ing the various kinds of pressure induced phase transfor-
mations in these materials. Earlier studies indicate that 2 Experimental
the phase transformation and amorphization in these com-
pounds are related to the decrease in non bonded O O The sample was prepared from aqueous solution of acidic
distances due to reduction of T O T angle (T = Si, Al, ferric nitrate and diamonium hydrogen phosphate in sto-
P etc.) of tetrahedral network. These transformations are ichiometeric ratio and was precipitated by slowly adding
sensitive to shear component of stress, as is evident from approximately 1 M solution of ammonium hydroxide at
the results of shock wave Raman experiments that demon- 295 K with constant stirring. The precipitate was filtered
strated that the decrease in T O T angle under com- and repeatedly washed with water and oven dried at 395 K
pression is increased by the presence of non-hydrostatic for 24 hours. The powder so obtained was amorphous,
stresses (Gallivan et al. 1995). Also, shear stresses may which crystallized to trigonal phase after heating at 875 K
cause irreversibility in a transition that is reversible under for 24 hours (Gadgil et al. 1994). Samples from this pow-
hydrostatic conditions. Crystalline to amorphous phase der were prepared in the form of pellets with nominal di-
transition in berlinite form of AlPO4 is reversible or ir- ameter 13 mm and thickness 0.9 mm.
reversible depending on whether the applied stress is hy- Experiments were carried out using the gas gun with
drostatic or not (Gillet et al. 1995, Sankaran et al. 1990, bore size of 63 mm at our laboratory. The gun can acceler-
Krugger et al. 1990, Cordier et al. 1994). Recently, Chi- ate projectiles to the desired velocity in a well controlled
tra et al (1996) conducted static pressure experiments manner for dynamic compression of the samples (Gupta et
on the berlinite form of iron phosphate (Ä…-FePO4, space al. 1992). Four experiments have been conducted. In each
group P3121) in which Fe O(1) P and Fe O(2) P angles experiment, the sample was fitted into a matching hole in
are 139.2ć% and 137.5ć% respectively. On the basis of Ra- an aluminum circular disc of the same thickness. This disc,
man measurement in diamond anvil cell they reported a together with a 3 mm thick cover plate of type 304 stain-
phase transition around 3 GPa, which was confirmed by less steel (SS304) material, was emplaced in a threaded
the X-ray diffraction (XRD) measurements that showed steel capsule that was fixed in the center of the target
174 K.D. Joshi et al.: Shock wave induced phase transition in Ä…-FePO4
opment of these peaks is more clear in XRD pattern of
the sample pressurized up to 5.2 GPa (Fig. 2c). These new
diffraction peaks could be associated with an orthorhom-
bic phase having space group Cmcm and cell constants
a =5.227 Å, b =7.770 Å, c =6.322 Å (Kinomura 1976).
Moreover, the peaks are riding over hump like background
indicating the presence of amorphous material. This sug-
gests that the trigonal phase has irreversibly transformed
partly to orthorhombic crystalline phase and partly to the
amorphous phase around 5 GPa. The coexistence of the
high pressure crystalline phase and the amorphous phase
is quite unusual.
It is interesting to compare the above results with
those of the static pressure measurements of Chitra et al.
(1996) performed concurrently in our laboratory. In the in-
situ XRD measurements on the sample above 3 GPa, Chi-
Fig. 1. Pressure history in the FePO4 sample calculated using
tra et al. observed the disappearance of the strong (012)
a two dimensional hydrodynamic code. The projectile velocity
original peak (of the trigonal structure) with emergence of
in this case is 0.448 km/s
a weaker peak close to it and appearance of a new broad
peak near (110) original peak. This indicated a transition
to new crystalline phase. However, because of paucity of
ring. Upon the impact of the flyer plate, attached to the
the diffraction data, the new structure could not be de-
nose of the projectile accelerated to the chosen velocity,
termined. On comparing our XRD pattern with that of
the sample reached the final pressure by a reverberating
Chitra et al., we found that the new unidentified peaks
shock wave between the front steel cover plate and the
seen under static pressure belonged to the orthorhombic
steel capsule. Just prior to impact, the projectile velocity
structure observed in the shocked samples, and suggested
was measured by recording the time interval between the
that the unidentified high pressure structure reported by
electrical pulses generated due to shorting of suitably po-
Chitra et al. could be orthorhombic. This was confirmed
sitioned four pairs of brass pins by the moving projectile
by the later XRD measurements (Chitra et al. 1996) on
body.
the pressure quenched samples prepared by compressing
The pressure history in the sample was estimated by
above 3 GPa between tungsten carbide anvils in a 100 ton
performing numerical simulations using a two dimensional
press.
hydrodynamic code with the measured projectile veloc-
The XRD pattern of the sample shock recovered from
ity and estimated Hugoniot of FePO4 as input. The mea-
8.5 GPa (Fig. 2d) shows that in comparison with that of
sured value of bulk modulus B (= 25 GPa) (Chitra et
the 5.2 GPa pattern the amorphous background has de-
al. 1996) and the assumed value of its pressure derivative
B (= 4) was used for constructing the Hugoniot of FePO4 creased, the peaks belonging to the orthorhombic struc-
ture are much weaker and the peaks corresponding to the
in the c, s form {c = (B/Á)1/2 and s =(B +1)/4}. In
parent trigonal phase are sharper. Two experiments gave
the numerical simulations both the loading and unloading
almost similar results. This behaviour of Ä…-FePO4 is ab-
were assumed to be occurring along this Hugoniot. The
normal and is in contrast to that of q-GeO2 (Suresh et al.
peak pressures for the four experiments corresponding to
1994) where the proportion of the amorphous component
the measured projectile velocities of 0.11, 0.19, 0.27 and
in the shock retrieved samples increased with the peak
0.45 km/sec were estimated to be 2(Ä… 0.5), 3.5(Ä… 0.5),
loading stress.
5.2(Ä… 0.5) and 8.5 GPa(Ä… 0.5), respectively. A typical cal-
As the Cmcm phase is the equilibrium phase under
culated pressure profile for the sample shock loaded to
pressure for this family of phosphate compounds (Sharma
8.5 GPa is shown in Fig. 1. As shown in the figure, the
et al. 1995), the above results could be interpreted in
sample reached the final pressure after a few reverbera-
terms of a three level free energy diagram. Figure 3 shows
tions and its peak pressure lasted for around 1 2 µs. The
a schematic diagram of the free energy versus the reaction
shock recovered samples were characterized using powder
coordinate, where Gc1, Ga and Gc2 are the free energies of
x-ray diffraction.
the initial crystalline phase c1 just before the transforma-
tion, amorphous phase a, and the high pressure phase c2,
respectively; "ga, "gc2 and "gc1c2 (not shown in figure)
3 Results and discussion
are the barrier heights for c1 to a, a to c2 and c1 to c2
transitions, respectively. The compressed berlinite phase
The XRD measurements on the shock recovered samples
(c1) energetically prefers to transform to the Cmcm phase
are displayed in Fig. 2. The XRD pattern of the sample
(c2) above 3.5 GPa. However the kinetics impedes this and
recovered from 2 GPa is identical to that at ambient con- instead a transformation to the amorphous (a) phase oc-
ditions (Fig. 2a). However, for 3.5 GPa sample, the diffrac- curs. This then transforms to the equilibrium phase. This
tion pattern exhibits new peaks along with the broadening
is similar to the behaviour of quartz SiO2, where the c a
of the peaks of the original phase (Fig. 2b). The devel-
K.D. Joshi et al.: Shock wave induced phase transition in Ä…-FePO4 175
ab
cd
Fig. 2a d. X-ray diffraction pattern of FePO4 sample a under ambient condition and those recovered from b 3.5 GPa c 5.2 GPa
d 8.5 GPa. The reflections (hkl) belong to the trigonal structure whereas (hkl)" correspond to the orthorhombic structure
transition occurs at 21 GPa and the traces of the equilib-
rium stishovite phase are detected at 70 GPa (Hazen et al.
1979). The fact that both the transitions in FePO4 occur
in a small pressure interval suggests that the energy bar-
riers "ga and "gc2 are of similar magnitude. However, we
cannot rule out the direct c1 to c2 transformation which
depends on the barrier "gc1c2. Now, in the case of 8.5 GPa
experiment, although higher component of both the amor-
phous phase and orthorhombic phase would have formed,
these might have not been retained on unloading due to
the reverse transformation caused by residual temperature
as our sample has lot of porosity. This leads to the pres-
ence of very small components of the amorphous as well
as higher pressure structure.
4 Summary
Fig. 3a,b. A schematic diagram showing a free energy as a
function of reaction coordinate b the three level free energy
We find the onset of shock induced amorphization in Ä…-
diagram for crystal to amorphous transition
FePO4 around 3 GPa along with transition to an orthor-
176 K.D. Joshi et al.: Shock wave induced phase transition in Ä…-FePO4
hombic phase. However, unlike under static high pressures Gillet P, Badro J, Varrel B, Macmillan PF (1995) High pres-
where the amount of amorphous material continues to in- sure behavior in Ä…-AlPO4: Amorphization and the memory
crease on increasing the pressure, under shock compres- glass effect, Phys. Rev. B. 51: 11262
Gupta SC, Agarwal RG, Gyanchandani JS, Roy S, Suresh N,
sion proportion of the amorphous material has substan-
Sikka SK, Kakodkar A, Chidambaram R (1992) A single
tially decreased at 8.5 GPa, because of the reversion due
stage gas gun for shock wave studies. In: Schmidt SC, Dick
to higher post shock temperature. Our results support the
RD, Forbes JW, Tasker DG (eds) Shock Compression of
validity of the three level free energy diagram for interpret-
Condensed Matter-1991, Elsevier, Amsterdam, pp. 839
ing c a transitions.
Hazen RM, Finger LW (1979) Polyhedral tilting : A common
type of pure displacive phase transition and its relationship
Acknowledgement. We thank M. Chitra for providing the
to analcite at high pressure, Phase Trans.1, 1; Compara-
static pressure results prior to publication.
tive crystal chemistry, Am. Sci. 72: 143 (1984); Crystals
at high pressure Sci. Am. 252: 110 (1985); Moffat WG,
Pearsall GW, Wulff J (1980) The Structure and Properties
of Materials. Wiely Eastern Limited, New Delhi, Vol. 1,
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