High (pressure, temperature) phase diagrams of ZnO and AlN from ...

High (pressure, temperature) phase diagrams of ZnO and AlN from
second harmonic generation measurements
Lkhamsuren Bayarjargal and Björn Winkler
Citation: Appl. Phys. Lett. 100, 021909 (2012); doi: 10.1063/1.3676057
View online: http://dx.doi.org/10.1063/1.3676057
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i2
Published by the American Institute of Physics.
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High (pressure, temperature) phase diagrams of ZnO and AlN from second
harmonic generation measurements
Lkhamsuren Bayarjargal a) and Björn Winkler
Institut für Geowissenschaften, Goethe-Universität, Altenhöferallee 1, D-60438 Frankfurt a. M., Germany
(Received 3 November 2011; accepted 20 December 2011; published online 11 January 2012)
The pressure-induced B4 ! B1 structural phase boundaries of ZnO and AlN have been determined
with the second harmonic generation (SHG) technique at high temperature. The SHG
measurements of AlN show that between 15.6 and 18 GPa, the phase boundary has a negative slope
of nearly 627 K/GPa, and that below 15.6 GPa, the slope is significantly smaller ( 77 K/GPa).
ZnO has a phase boundary with a negative slope of nearly 1427 K/GPa around 5.3–6 GPa and
228 K/GPa below 5 GPa. The phase transition pressure of AlN is sensitive to deviatoric stress and
varies from 18 to 24.5 GPa. VC 2012 American Institute of Physics. [doi:10.1063/1.3676057]
Laser heating experiments in a diamond anvil cell
(DAC) allow to study materials under extreme conditions,
but in situ measurements to delineate phase boundaries are
still challenging. Typical in situ characterization methods for
DAC are x-ray diffraction, inelastic x-ray scattering, x-ray
absorption spectroscopy, Mössbauer, and various optical
spectroscopic methods (Raman, IR, and UV). In contrast to
diffraction and these spectroscopic techniques, second harmonic
generation (SHG) has been used rather seldom for
high pressure experiments. 1–3 At ambient conditions, SHG
has been established as a versatile probe of crystallographic
symmetry. 4 In particular, SHG powder measurements are
extremely sensitive to detect the absence of an inversion center
in crystalline structures 5 and phase transitions at high
temperature. 6 In a pioneering high (pressure, temperature)
study, the presence of SHG was used to confirm the absence
of a center of symmetry in a high pressure and high temperature
phase of CO 2, 7 but systematic studies have been presented
only at high pressure for quartz, 1 ZnSe, 2 and ZnO. 3
However, the SHG method has not been used for delineation
of phase boundaries at high (p,T).
AlN and ZnO are isostructural to wurtzite at ambient
conditions, i.e., the hexagonal, acentric B4 structure type. At
ambient temperature and high pressure, the B4 phase transforms
into the rocksalt structure (B1). 8–14 The wurtzite-torocksalt
structural phase boundary of AlN was studied
through ab initio calculations 15 and by multi anvil press
experiments. 16 However, the five experimental data points
obtained earlier in a very limited (p,T) range 16 are insufficient
to delineate the phase boundary of AlN at high (p,T).
No further experimental studies of the phase boundary of
AlN have been reported so far at high (p,T).
The phase boundary of the B4 ! B1 transition of ZnO
has a slope with dP/dT 0 up to 1273 K, 10 while earlier a
negative slope dP/dT ¼ 5 10 3 GPa/K of the phase
boundary had been reported from 600 to 1000 K. 10 Based on
atomistic simulations, the melting temperature increases
from 2400 to 3400 K with increasing pressure. 17 Hence,
there is a large region in (p,T) space where the position of
the phase boundary is unknown. SHG experiments on AlN
and ZnO benefit from the very large SHG coefficients (11.7
a) Electronic mail: Bayarjargal@kristall.uni-frankfurt.de.
APPLIED PHYSICS LETTERS 100, 021909 (2012)
and 5.6 pm/V for AlN and ZnO, respectively) which are
20–40 times larger than quartz, 18 and here, we show that the
(p,T) phase diagram of these systems can efficiently be studied
by SHG measurements. The onset of the phase transition
causes a decrease of the SHG signal, and the transition is
complete once the SHG signal has vanished.
For our experiment, we used compacted powder samples
of ZnO (99% purity, Merck) and AlN (99% purity, Strem).
The samples were loaded into holes of 110–140 lm diameter
in tungsten gaskets preindented to thicknesses of 38–46 lm
in Boehler-Almax diamond anvil cells. 19 KCl and Ne were
used as a pressure medium for laser heating experiments,
and KCl was used also to thermally insulate the sample from
the diamond. Pressure was determined using the ruby fluorescence
method. For the pressure measurements, a blue
laser (k ¼ 473 nm, 80 mW power) is employed. The accuracy
of the pressure measurements is better than 0.2 GPa. The
pressure range of laser heating experiments was defined by
the pressure before and after laser heating. A CO2 laser
(Coherent, Diamond K-250, TEM00) was employed for heating
the sample. The laser heating part of our experimental
set-up is described in detail in Ref. 20. The temperatures of
the heated area are determined by the two-colour pyrometer
method, Planck and Wien fit. The accuracy of temperature
measurement is around 6150 K. For the in-situ SHG measurements,
a pulsed Nd:YAG laser (Continuum, Surelite,
k ¼ 1064 nm, 10 Hz, 6 ns) was integrated into the CO 2 laser
heating system. With a harmonic separator and with a monochromator,
the fundamental infrared light was separated
from the generated second harmonic. The generated SHG
signal was collected with a photo-multiplier (R2949, Hamamatsu)
and oscilloscope. In the annealing experiments, the
pressure was increased up to an initial pressure without heating.
At that pressure, we heated the sample up to the defined
temperature. Then the sample was quenched to ambient temperature.
The SHG signal of the heated samples was collected
with an experimental setup which has been described
in detail elsewhere. 3
The pressure dependence of the SHG intensity of AlN
was studied up to a pressure of 35 GPa at ambient temperature
in different pressure media. The evolution of the SHG
intensity with increasing pressure is shown in Fig. 1.
On increasing pressure, the SHG intensity of AlN increases
0003-6951/2012/100(2)/021909/3/$30.00 100, 021909-1
VC 2012 American Institute of Physics
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021909-2 L. Bayarjargal and B. Winkler Appl. Phys. Lett. 100, 021909 (2012)
SHG intensity [arb. units]
SHG intensity [arb. units]
100
80
60
40
20
0
0 5 10 15 20 25 30
Pressure [GPa]
120
100
80
60
40
20
0
0 5 10 15 20 25 30 35
Pressure [GPa]
up to 16 GPa in a nonhydrostatic pressure medium (KCl) and
drops to zero at around 26 GPa. At 16 GPa, the B4 phase
starts to transform into the B1 phase. In the nonhydrostatic
pressure medium, both phases coexist between 16 and
26 GPa, presumably, as the transition is kinetically hindered.
In contrast, the phase transition of AlN measured in a quasihydrostatic
pressure medium (Ne) shows an increase of the
SHG intensity up to 22.5 GPa and drops up to 27.5 GPa. The
pressure range from the onset to the completion of the phase
transition is around 5 GPa, twice smaller than the pressure
range observed in KCl. The coexistence range or the transition
rate probably depends on the rate with which pressure is
increased but this was not studied here. The transition pressures
24.5 GPa and 18 GPa were taken as the pressure where
the SHG intensity had decreased by 50% in nonhydrostatic
and quasihydrostatic pressure medium, respectively. These
observations can explain the discrepancies reported in the literature
concerning the phase transition pressure (22.9 GPa
and 14-20 GPa from x-ray diffraction, 13,14 16.9 GPa from
visual observation, 21
B4 B1
(a) AlN in KCl
(b) AlN in Ne
B4 B1
FIG. 1. Pressure dependence of the SHG intensity on pressure increase. The
two graphs show the experiments in two different pressure media KCl (a)
and Ne (b). The vertical dashed gray line presents the pressure chosen as the
transition pressure of the B4 ! B1 phase transition. The dashed dark line
represents a fit of the pressure dependence of the SHG signal.
and 22 GPa from shock wave
experiments 22 ).
Figure 2 shows the phase diagram for AlN based upon
the measurements described here, five data points from an
earlier experimental study 16 and a previous theoretical
study. 15 In our experiments, the phase boundary is significantly
shifted to higher pressures with respect to the phase
boundary which had been derived using density functional
theory and the quasiharmonic approximation. 15 The phase
boundary was derived by fitting two straight lines to the
mean (p,T) points representing the B1 or B4 phase and joining
them smoothly at their intersection. The SHG measurements
show that between 15.6 and 18 GPa the phase
boundary of AlN has a negative slope of nearly 627 K/GPa
and below 15.6 GPa of 77 K/GPa. While melting
points 23,24 of AlN have been measured at pressures

021909-3 L. Bayarjargal and B. Winkler Appl. Phys. Lett. 100, 021909 (2012)
whereas for AlN, relatively long times of more than 15 min
were required. We found that ZnO has a phase boundary
with a negative slope of nearly 1427 K/GPa around
5.3-6 GPa and 228 K/GPa below 5 GPa. This is consistent
with an earlier study 10,25 at lower temperatures. It is known
that depending on the pressure media ZnO can go through an
acentric tetragonal intermediate iT (I4mm) phase or a centrosymmetric
hexagonal intermediate iH (P63/mmc) phase at
ambient temperature. 3,26 Previous SHG experiments in a
nonhydrostatic pressure medium show a decrease of the intensity
of the SHG signal between 6 and 9 GPa. This
decrease of the SHG intensity reveals the presence of the
centrosymmetric iH phase. On the other hand, the positive
slope of the intensity of the SHG signal in a hydrostatic pressure
medium is due to the formation of an acentric tetragonal
intermediate phase iT. 3 Both phase assemblies B4 þ iH or
B4 þ iH coexist over a pressure range between 6 and 9 GPa
at ambient temperature. Above 9 GPa, the B1 phase is stable
and remains stable to 200 GPa. 12 Depending on the pressure
medium, the B4 ! B1 transition pressure 9 GPa can
increase up to 11 GPa (in a hydrostatic medium). 3
In the present work, we have shown that SHG is an efficient
and accurate tool to determine the (p,T) phase boundaries
for phases undergoing a transition between a
centrosymmetric and an acentric phase. Our results of ZnO
are compatible with previous works and extend the phase
transition line to higher temperatures. The phase boundary of
AlN is significantly shifted to higher pressures with respect
to the phase boundary which had been derived using atomistic
models. 15
Financial support from the DFG, Germany, within
SPP1236 (Projects Ba4020, Wi1232) and the FOKUS program
of the Goethe University is gratefully acknowledged.
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