CHIN. PHYS. LETT. Vol. 32, No. 5 (2015) 057201
Electrical Conduction in Deuterated Ammonium Dihydrogen Phosphate Crystals
with Different Degrees of Deuteration *
ZHU Li-Li(朱丽丽)1,2 , GAN Xiao-Yu(甘笑雨)1,2 , ZHANG Qing-Hua(张清华)3 , LIU Bao-An(刘宝安)1,2 ,
XU Ming-Xia(徐明霞)1,2 , ZHANG Li-Song(张立松)1,2 , XU Xin-Guang(许心光)1,2 ,
GU Qing-Tian(顾庆天)1,2** , SUN Xun(孙洵)1,2**
1
2
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100
Key Laboratory of Functional Crystal Materials and Device of Ministry of Education,
Shandong University, Jinan 250100
3
Chengdu Fine Optical Engineering Research Centre, Chengdu 610041
(Received 5 December 2014)
Conductivity measurements of deuterated ammonium dihydrogen phosphate (DADP) crystals with different
deuterated degrees are described. The conductivities increase with the deuterium content, and the value of the
𝑎-direction is larger than that of the 𝑐-direction. Compared with DKDP crystals, DADP crystals have larger
conductivities, which is partly due to the existence of A defects. The ac conductivity over the temperature range
25–170∘C has shown a knee in the curve of ln(𝜎𝑇 ) versus 𝑇 −1 . The conductivity activation energy calculated by
the slope of the high temperature region decreases with the deuterium content. The previously reported phase
transition is not seen.
PACS: 72.80.−r
DOI: 10.1088/0256-307X/32/5/057201
Ammonium dihydrogen phosphate (ADP) is an
important isomorph of potassium dihydrogen phosphate (KDP)-type crystal, widely used in the area
of nonlinear optical, electric-optics and fluorescence
analysis. Recently, ADP has drawn more attention for
its higher laser damage threshold, nonlinear optical coefficient and harmonic conversion efficiency.[1,2] Many
works have been carried out to investigate the properties of KDP and deuterated potassium dihydrogen
phosphate (DKDP) crystals.[3−5] The incorporation of
deuterium has sensitively changed some properties of
KDP crystal. However, the influence of deuterated
ADP crystal has not been well studied.
Electrical conductivity is an important parameter
for crystal while it is used as an electro-optic switch
device. The conductions in ADP and KDP are assumed to be ionic and the migrating particle is the
proton.[6−10] It is believed that the proton moves in
the three-dimensional hydrogen bond network, affecting the motion of neighboring protons. The interbond
and intrabond jump of the proton produces a vacancy
bond (L defect) and a doubly occupied bond (D defect). These defects are the main reason for conductance.
In the research of Murphy,[7] the slope of the conductivity curve at low temperature is different from
the high temperature part. These two straight lines
formed a knee, enabling the calculation of formation
energy and migration energy. However, Pollock et
al.[11] failed to find a knee and claimed that the formation and migration of defects are the same process in
ADP crystal. Harris et al.[8] discussed some previous
work and attributed the conduction to the L defect
and A defect (proton vacancy produced in the ammonium lattice). Rath et al.[12] studied the conductivity
and dielectric loss of doped ADP, attributed the increase in conductivity to the association of impurity
and the proton vacancy. Abdel-Kader[13] explained
the knee in ADP by phosphate group rotation and
proton jump above and below it. Some others argue
that the break is connected with the high temperature
phase transition.[14,15]
In view of the difference in previous data and
various interpretations, direct measurement of the
conductivity for ADP crystal is necessary. Since
ADP is a hydrogen bonded crystal containing ammonium groups, the measurement on deuterated analogue deuterated ammonium dihydrogen phosphate
(DADP) crystals is a useful way to investigate the
proton mechanism. This work presents more studies
on the conductivities of DADP crystals with regard to
their deuterium content and crystal structures.
DADP crystals are grown by the traditional
temperature-reduction method in an aqueous solution.
Extra pure NH4 H2 PO4 salt is dissolved in the heavy
water and deionized water to obtain solutions with
different deuterated degrees. The growth system consists of 5000 ml glass crystallizer placed in a controlled
* Supported by the National Natural Science Foundation of China under Grant Nos 51323002 and 51402173, the Independent
Innovation Foundation of Shandong University under Grant No 2012JC016, the Natural Science Foundation for Distinguished
Young Scholar of Shandong Province under Grant No JQ201218, and the Project of Key Laboratory of Neutron Physics of China
Academy Of Engineering Physics under Grant No 2014BB07.
** Corresponding author. Email: [email protected]; [email protected]
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]; [email protected]
© 2015 Chinese Physical Society and IOP Publishing Ltd
057201-1
CHIN. PHYS. LETT. Vol. 32, No. 5 (2015) 057201
ionic conduction, the equation of intrinsic conductivity can be expressed as
(︁
(︁ 𝑈 )︁
𝐸s )︁ 𝑞 2 𝛿 2 𝜈0
s
𝜎 = 𝑁 exp −
exp −
2𝑘𝑇 6𝑘𝑇
𝑘𝑇
(︁ 𝑊 )︁
s
,
𝜎𝑇 = 𝐴s exp −
𝑘𝑇
)
-1
Scm
-1
3.0
2.0
(
1.5
1.0
0.5
0.0
2.4
2.6
2.8
3
10 /
3.0
(K
3.2
3.4
-1
)
Fig. 1. The ac conductivity of ADP crystal as a function
of temperature along the 𝑐-direction at different frequencies.
The expression for conductivity is
∑︁
𝜎=
𝑛𝑖 𝑞𝑖 𝜇𝑖 ,
-8
W
) (
-10
-12
2.2
2.4
2.6
2.8
3
10 /
(K
3.0
-1
)
3.2
3.4
Fig. 2. A typical result of the ac conductivity in DADP
crystals (𝑥 = 0.7, 𝑐-direction, at 1 kHz). All the crystals
show the same trend as this one.
2.5
mW
Electrical conductivity
1 kHz
20 kHz
40 kHz
100 kHz
-6
-1
Scm
-1
SK)
where the first part is determined by the intrinsic defect, and the second part is determined by the impurity. Linear relationship of ln(𝜎𝑇 ) against 1/𝑇 for
measured DADP crystals is obtained in this work.
Each curve consists of two regions, which means that a
‘knee’ does exist in the conductivity plot. This behavior of conductivity is the character of a dielectric with
impurity. A typical result in the whole temperature
range is shown in Fig. 2.
4.0
3.5
(2)
where 𝑊s is the activation energy of conductivity, including formation energy 𝐸s and migration energy 𝑈s .
The expression for impurity ion conductivity can be
written in the same way. Thus the conductivity plots
can be expressed by two parts,
(︁ 𝑊 )︁
(︁ 𝑊 )︁
s
2
𝜎𝑇 = 𝐴s exp −
+ 𝐴2 exp −
, (3)
𝑘𝑇
𝑘𝑇
ln(
temperature water bath. The solution is filtered by
0.22 µm micro-porous membrane before overheating.
A 5 cm × 5 cm × 1 cm 𝑧-cut seed is also overheated
before being put into the solution. The crystallization
procedure is operated within a temperature range of
35–50∘C and the temperature reduction rate is 0.05–
0.1∘C/day. The forward-stop-backward rotation mode
is adopted with the speed of 30 rpm. All the crystals
are transparent without visible macroscopic defects.
The square plate specimens in 8 × 8 × 2 mm3 are
cut from the same section in those crystals. Each
crystal has specimens along 𝑎 and 𝑐 directions, which
are coated with silver on the surface respectively.
The alternating current (ac) measurements are made
along 𝑎 and 𝑐 directions from 25 to 170∘C in the frequency range 50 Hz–1 MHz by using an Agilent 4294A
precision impedance analyzer. The heating rate is
0.5∘C/min to avoid the inhomogeneous distribution of
temperature in the crystal. The temperature of each
point is maintained for 10 min to reach the stability.
Figure 1 shows the electrical conductivity of ADP
crystal calculated from the measured resistivity as a
function of temperature. The ac conductivity slightly
decreases with temperature at the beginning and
sharply increases after about 400 K. It can be seen that
the ac conductivity increases with the frequency. In
the low temperature range, the conductivity shows a
relatively large variation on frequency, while it hardly
changes with frequency in the high temperature part.
In other words, the conductivity in the left part is
highly temperature dependent while the right part is
more dependent on frequency. For all the crystals
measured, the conductivity shows the same tendency
with ADP crystal.
(1)
𝑖
where 𝑛 is the carrier concentration, 𝑞 is the charge
of each carrier, and 𝜇 is the mobility. For KDP-type
crystals, the conduction in them is considered as being due to the similar ionic conduction mechanism. In
The plots of ln 𝜎 versus 𝑇 −1 for DADP crystals
from 130 to 170∘C are shown in Fig. 3. In the deuterium composition range of 0–0.7, the electrical conductivity increases with the deuterium content. The
conductivity along the 𝑎-direction is higher than that
of the 𝑐-direction. The results of the 𝑏-direction are
the same as the one of the 𝑎-direction and they are
not shown here. DKDP crystals, which have a similar crystal structure with DADP crystals, show the
same trend of conductivity.[16] The basic growth units
−
of KDP and ADP crystals are K+ , NH+
4 and H2 PO4
groups. Each PO4 group is connected with the adjacent four groups by the hydrogen bond, forming an
open hydrogen-bond network (Fig. 4), which is mainly
distributed in a plane that is almost perpendicular to
the 𝑐-direction. Hence the migration of the proton
along the 𝑎-direction is stronger than that along the
𝑐-direction. Our results, that conductivity along the
057201-2
CHIN. PHYS. LETT. Vol. 32, No. 5 (2015) 057201
𝑎-direction is higher than the 𝑐-direction, are proof of
this consideration from crystal structure.
The migration of the proton, including intra-bond
and inter-bond jump in the network, produces ionization defects and L or D defects. The combined motion
of them contributes to conduction in KDP crystal.
The conduction in ADP crystal, however, possesses
an additional hydrogen-bond, connecting the hydrogen in the NH+
4 ion to oxygen in the phosphate group
(Fig. 4). Such a proton vacancy existing in the ammonium lattice is called the A defect.[8] It can be seen
that the concentration of proton vacancies produced
in ADP crystal is more than that in KDP. Moreover, the tightness and melting point of KDP are
higher than ADP, which indicate that KDP is combined with a larger force and a smaller moving space.
The migration of a proton in KDP is therefore more
difficult, which contributes to a smaller conductivity
(< 3×10−7 Ω−1 cm−1 )[16] compared with ADP crystal.
(a)
=0
=0.3
=0.5
=0.7
) (
W
-1
(b)
=0
=0.3
=0.5
=0.7
Scm
-1
SK)
-3
-4
-5
-6
-7
-8
-9
-10
2.25 2.30 2.35 2.40 2.45 2.25 2.30 2.35 2.40 2.45
-direction
ln(
-direction
3
10 /
(K
-1
3
)
10 /
(K
phate, leading to a sharply increasing total current
and change of conductivity slope. However as mentioned in the literature,[7] the conductivity of a dielectric crystal like ADP is much larger than the influence
of the surface current and it is unnecessary to use a
guard ring to shunt it. The knee still exists even with
the use of a guard ring. We check all the samples after
the heating test in our experiments and find no liquid
film on the sample surface.
Table 1. Activation energy for conductivity in DADP crystals.
Composition 𝑥
0
0.3
0.5
0.7
Activation energy (eV)
𝑎-direction
𝑐-direction
2.159±0.0374
2.811±0.0297
2.100±0.0432
2.702±0.0440
1.960±0.0191
2.509±0.0588
1.813±0.0171
2.417±0.1088
Subhadra et al.[14] believed that the formation of a
knee is related to the high temperature phase transition of crystals, i.e., the crystal breakdown at the temperature around the knee. However, we did not find
any phase change from room temperature to melting
point (190∘C) by means of thermal-Raman (Fig. 5).
The raman spectra does not have any change until
the crystal is melted.
470 K
-1
)
450 K
Intensity (arb. units)
Fig. 3. Variation of electrical conductivity ln(𝜎𝑇 ) versus 1000/𝑇 for DADP crystals (at 10 kHz) (a) along 𝑎direction; (b) along 𝑐-direction.
b
a
430 K
410 K
390 K
370 K
350 K
330 K
320 K
310 K
300 K
210
P+5
O-2
H+1
420
630
840
1050 1260 1470 1680
Raman shift (cm
P+5
N-3
O-2
H+1
-1
)
Fig. 5. Thermal-Raman spectra of ADP crystal in the
temperature range of 300–470 K.
Fig. 4. The hydrogen bonds associated with the phosphate group in KDP and ADP crystals. The hydrogen
bonds associated with the ammonium group in ADP crystal.
The calculated activation energy values of these
crystals are listed in Table. 1. In the range of 0–0.7,
the activation energies decrease with the increasing
deuterium content. The results along the 𝑐-direction
are also larger than those along the 𝑎-direction. Such
an anisotropy is also consistent with the measured
conductivity values.
There are different interpretations about the reason to form the knee. Harris et al.[8] inferred that the
formation of a knee is due to the heating decomposition of ADP. After the high temperature test, surface
current was generated by the decomposition of phos-
The activation energy of the impurity is much more
smaller than that of the lattice ion, thus the impurity
conduction in ionic crystal plays an important role
at low temperature. The carriers number of intrinsic
conduction can be significantly enhanced with the increase of the thermal motion energy, hence the intrinsic conduction dominates the high temperature region.
We believe that the different conduction mechanism of
two parts makes the curve turn out a breaking point.
It is concluded that the high temperature
conductivity was proportional to the tunneling
frequency.[17,18] After the substitution of deuterium
to hydrogen, the energy gap between the doubleminimum potential energies well increases,[19] which
indicates that the tunneling rate of deuterium is higher
than that of hydrogen. This results in the increase of
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CHIN. PHYS. LETT. Vol. 32, No. 5 (2015) 057201
conductivity in deuterated crystals, which is consistent with our measurements.
In summary, a series of DADP crystals with
good crystal quality were grown by the traditional
temperature-reduction method. The ac conductivities were measured along the 𝑎 and 𝑐 directions in
the temperature range of 25∘C–170∘C. A knee exists
in the conductivity-temperature plot curve between
ln(𝜎𝑇 ) and 𝑇 −1 , separating the curve into two distinct regions. Conductivity in the higher temperature
part represents the intrinsic conductivity. The activation energy decreases with the increasing deuterium
content.
References
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[1] Reintjes J and Eckardt R C 1977 Appl. Phys. Lett. 30 91
[2] Roberts D A 1992 IEEE J. Quantum Electron. 28 2057
[19]
057201-4
Hu G H et al 2009 Chin. Phys. Lett. 26 097802
Hu G H et al 2009 Chin. Phys. Lett. 26 097803
Liu B A et al 2013 Chin. Phys. Lett. 30 067804
Schmidt V H and Uehling E A 1962 Phys. Rev. 126 447
Murphy E J 1964 J. Appl. Phys. 35 2609
Harris L B and Vella G J 1973 J. Chem. Phys. 58 4550
Glasser L 1975 Chem. Rev. 75 21
Assencia A A and Mahadevan C 2005 Bull. Mater. Sci. 28
415
Pollock J M and Sharan M 1969 J. Chem. Phys. 51 3604
Rath J K and Radhakrishna S 1987 J. Mater. Sci. Lett. 6
929
Abdel-Kader A 1991 J. Mater. Sci. -Mater. El. 2 7
Subhadra V, Syamaprasad U and Vallabhan C 1983 J. Appl.
Phys. 54 2593
Chandra S and Hashmi S A 1990 J. Mater. Sci. 25 2459
Liu B A et al 2012 Appl. Phys. A 109 159
O’Keeffe M and Perrino C T 1967 J. Phys. Chem. Solids
28 211
O’Keeffe M and Perrino C T 1967 J. Phys. Chem. Solids
28 1086
Kreuer K D et al 1995 Solid State Ionics 77 157