Lanthanum-gallium tantalate single crystal (La3Ta0.5Ga5.5O14, langatate, LGT) is a perspective piezoe-lectric material as an active component of pressure  sensors. An investigation of the growth conditions in-fluence (the  growth  atmosphere) on the electrophysical  сharacterization  of LGT, obtained in different at-mospheres (Ar, Ar + O2) was carried out. The frequency dependences of the relative dielectric constant 
(ε11/ε0) and of the admittance depend on the growth atmosphere. The langatate electrophysical сharacteri-zation  in alternating electric fields were analyzed by means of the impedance spectr oscopy method. The 
behavior of short circuit currents in specimens of polar cuts of LGT single  crystals with the same material
electrodes without preliminary polarization is described.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3628

Anfimov, N.S.

Bykov, A.S.

Kiselev, D.A.

Kozlova, A.P.

Kozlova, I.M

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 6 No 3, 03034(4pp) (2014) Том 6 № 3, 03034(4cc) (2014) 
 
 
The article was reported at the International Conference «The Advanced Technology, Equipment and Analytical Systems for Materials», Kursk, 13-14 May, 2014 
2077-6772/2014/6(3)03034(4) 03034-1  2014 Sumy State University 
Lanthanum-Gallium Tantalate Crystals and their Electrophysical Characterization 
 
A.P. Kozlova*, N.S. Kozlova, I.M. Anfimov, D.A. Kiselev, A.S. Bykov 
 
National University of Science and Technology "MISiS", 4, Leninskiy Pr., 119049 Moscow, Russia 
 
(Received 19 May 2014; published online 15 July 2014) 
 
Lanthanum-gallium tantalate single crystal (La3Ta0.5Ga5.5O14, langatate, LGT) is a perspective piezoe-
lectric material as an active component of pressure sensors. An investigation of the growth conditions in-
fluence (the growth atmosphere) on the electrophysical сharacterization of LGT, obtained in different at-
mospheres (Ar, Ar + O2) was carried out. The frequency dependences of the relative dielectric constant 
(ε11/ε0) and of the admittance depend on the growth atmosphere. The langatate electrophysical сharacteri-
zation in alternating electric fields were analyzed by means of the impedance spectroscopy method. The 
behavior of short circuit currents in specimens of polar cuts of LGT single crystals with the same material 
electrodes without preliminary polarization is described. 
 
Keywords: Langatate, Polar dielectric crystal, Impedance, Short circuit current. 
 
 PACS numbers: 82.45.Fk, 82.45.Gj, 82.45.Un 
 
 
                                                 
*
 anna_kozlova_2009@mail.ru 
1. INTRODUCTION 
 
Lanthanum-gallium tantalate single crystals 
La3Ta0.5Ga5.5O14 are of interest as piezoelectric materi-
al. A first characterization of La3Ga5,5Ta0,5O14 is dated 
1982 [1]. These crystals are widely used in the design 
of sensors based on the direct piezoelectric effect. The 
main requirements for pressure sensors are small size 
of the element and time stability of basic characteris-
tics. Particular requirements of materials for such 
pressure sensors are [2]: 
 absence of phase transitions up to ~ 1200 C; 
 absence of the pyroelectric effect; 
 absence of physical properties hysteresis; 
 high sensitivity, caused by high values of piezoe-
lectric coefficients; 
 high value of specific resistance. 
Lanthanum-gallium tantalate single crystal has all 
these necessary properties [2].  
The single crystals were grown by the Czochralski 
method in Ar or Ar + O2 atmospheres, but generally, the 
growth technology in argon-oxygen atmosphere is more 
frequently used. Some amount of oxygen in the growth 
chamber is necessary to prevent evaporation of gallium 
oxide and lack of gallium in the melt, which leads to the 
crystal non-stoichiometry. 
Polar cuts of langatate single crystals are used as 
pressure sensors active component in high temperature 
devices. The technology of high temperature sensors 
based on langatate single crystals includes the coating 
of their polar cuts surfaces with a thin conducting layer. 
One of the problems of such sensors is the surface aging 
and surface degradation during the sensor use 
especially at high temperatures. 
A peculiarity of langatate is the structural 
complexity of the unit cell and a tendency to form a 
disordered distribution of its matrix ions within the 
crystal lattice [2, 3]. Inhomogeneity of thus obtained 
crystals leads to heterogeneity of physical properties. 
The nature of the nonequilibrium crystals state, 
and consequently, the possibility of occurrence of pro-
cesses leading to crystals state changes may be caused 
by the crystal properties itself, by its post-growth 
treatment and by service conditions. It is well known 
that there are deviations from the equilibrium caused 
by external effects, for example, crystalline elements 
degradation under the effect of electric fields. Less 
evident are processes and their consequences 
determining the variation of crystals in the absence of 
external effects. This process includes the short circuit 
current (SCC) detected in polar crystals. This 
phenomenon was detected in lithium iodate [4].  
Thus, identifying the causes and nature of changes 
in the crystal state, which manifests itself in the 
anomalous behavior of some LGT properties and the 
SCC occurrence, is not only of practical interest, but is 
also important for a number of polar crystals fundamen-
tal studies. 
 
2.  EXPERIMENTAL PROCEDURE 
 
In this study, langatate crystals produced by the 
“Fomos-Materials” company were used. The crystals 
were grown in iridium crucibles in an Ar atmosphere 
and in a mixture of argon and oxygen (Ar + (2 %)O2). 
The samples represent plates of polar cut polished on 
both sides. The samples represent plates of polar cut 
polished on both sides with DC-magnetron sputtered 
electrodes (gold with a titanium sublayer, pure gold 
and pure iridium). 
The electrical parameters measurement was per-
formed on a “Keithley” 6517A electrometer. The inves-
tigations of electrical parameters frequency dependen-
cies were performed in the frequency range from 5 to 
500 000 Hz on a “Tesla” BM 507 impedance meter. Im-
pedance measurement was carried out in the tempera-
ture range 20-450 C and SCC in the range 20-600 C. 
Specific admittance z and relative dielectric constant ε 
were calculated on the basis of measured impedance Z 
values and phase angle φ according to equations (1) 
and (2) 
 
 A.P. KOZLOVA, N.S. KOZLOVA, ET AL. J. NANO- ELECTRON. PHYS. 6, 03034 (2014) 
 
 
03034-2 
 
1
Z
d
l h Z
  

, (1) 
 
where z  specific admittance, Оhm – 1·m – 1, 
d  sample thickness, m; l  electrode length, m; 
h  electrode width, m; Z  impedance, Оhm; 
 
 
0
1 sin
2
d
l h f Z


 
  
   
, (2) 
 
where   relative dielectric constant, f  frequency, Hz; 
0  permittivity of vacuum (8,8610 – 12 F/m). 
The temperature-frequency dependences of admit-
tance and relative dielectric constant were investigat-
ed. According to these results the near-electrode 
processes contribution to electrophysical properties was 
determined. The measurements in the frequency range 
up to 500 kHz showed the admittance value increases 
on three orders for all investigated samples. 
The temperature dependence is absent up to 400 °C 
for the crystals grown in the Ar atmosphere, change of 
admittance starts only at 450 °C, while in crystals 
grown in the Ar + O2 atmosphere the temperature in-
fluence is absent up to 450 °C. 
The relative dielectric constant temperature-frequency 
dependences of crystals obtained in different atmosphere 
with identical iridium electrodes and obtained in identical 
atmosphere with different electrodes were investigated 
(Table 1). It allowed revealing the influence of the growth 
atmosphere and the conducting coating material on the 
relative dielectric constant value. Data with an uncertain-
ty limit are shown in Table 1. 
 
Table 1– The values (ε11 / ε0) of dielectric constant of 
langatate samples at different temperatures in the fre-
quency range 5 Hz-500 kHz 
 
 Relative dielectric 
 constant 
Atmosphere,  
Electrode material 
(ε11 / ε0) at 
Т  20-400 С 
(ε11 / ε0)  at 
Т  450 С 
Ar, electrode - Ir 34  2 43  3 
Ar + (2 %)O2, electrode – Ir 43  3 49  3 
Ar + (2 %)O2, electrode – 
Au+Ti 
35  2 39  3 
 
The classical solid electrolyte is connected in paral-
lel resistor and capacitor. We suppose that the near-
electrode processes on polar cuts of LGT have an elec-
trochemical nature. 
To analyze the results of measurements of the spe-
cific impedance the impedance hodographs were con-
structed. The shape of the hodograph can be seen in the 
equivalent circuit of the investigated process, and thus 
about the process. Several equivalent circuits for the 
pattern “electrode-solid electrolyte-electrode” were ana-
lyzed: the equivalent circuit of Ershler-Randles 
(Fig. 1a), and Frumkin-Melik-Gaykazyan (Fig. 1b). 
According to the Ershler-Randles circuit the impedance 
of such a reaction can be represented as the impedance 
of the circuit consisting of series-connected resistors RF, 
characterizing the charge transfer, and the Warburg 
impedance Zw, which characterizes the diffusion pro-
cess. The resistor Re is the resistance of the electrolyte, 
Cd is part of the double layer capacity due to indiffer-
ent ions. Equivalent Frumkin-Melik-Gaykazyan circuit 
describes the electrochemical processes taking place 
into account of the electrochemical reactions products 
adsorption in “electrode-solid electrolyte-electrode” sys-
tems. R' characterizes own speed elementary act of 
adsorption-desorption and capacity С' – additional 
double layer capacity associated with adsorption of 
surface – active particles. Based on the experimental 
results of impedance spectroscopy for all samples of 
LGT crystals complex impedance Z components for 
both equivalent circuits have been identified by analyt-
ical method. For both circuits of LGT sample grown in 
Ar + O2 mixture with Au/Ti electrodes [5] impedance 
components demonstrate that the main contribution to 
the value of the electrical resistance is the resistance of 
near-electrode processes. The near-electrode processes 
are of electrochemical nature.  
 
 
a 
 
b 
 
Fig. 1 – The equivalent circuit of Ershler-Randles (a), and 
Frumkin-Melik-Gaykazyan (b) 
 
We have shown that the short circuit current in 
LGT is detected always even if the crystal is between 
symmetric (identical electrodes). The short circuit cur-
rent is recorded without preliminary polarization of the 
specimen. The diagram of the experiment shows time 
dependences of the temperature and current (Fig. 2). 
SCC measurement process consists of three stages. The 
first stage t1 is SCC measurement after shortening the 
circuit – the current decreases with time (t) in accord-
ance with the law 0 0( / )
aI I t t    to a stationary value 
which is retained for any period of time within about 1 
hour. During the second stage t2 the sample is kept for 
30 minutes at room temperature after the current de-
cay process. This step is necessary for accurate SCC 
measurements at the beginning of the heating process. 
After that, the linear heating with rate of 2 C/min to 
the required temperature and the simultaneous regis-
tration of SCC parameters were carried out (stage t3). 
The resulting dependence characterizes the near-
electrode processes kinetics and activation of active 
defects in the bulk of the crystal. The duration of this 
stage is about 8 hours. 
 
 LANTHANUM-GALLIUM TANTALATE CRYSTALS… J. NANO- ELECTRON. PHYS. 6, 03034 (2014) 
 
 
03034-3 
 
 
Fig. 2 – Scheme of the experiment: the plot of temperature 
and current change from time 
 
Charge state of each surface was determined by the 
piezoresponse technique. In the experiment, the crystal 
is always placed with the same orientation due to sur-
face charge sign with respect to the electrode holder. 
However, for testing, the sample was also placed on the 
opposite side with respect to the holder. The current 
direction changing is due to the processes within the 
sample. The behavior of the current schedule, depend-
ing on the setting of the sample shown in Figure 3. 
 
 
 
Fig. 3 – SCC behavior at different positions of the sample 
relative to the electrodes 
 
SCC dependences of LGT samples with different elec-
trodes (pure gold, gold with a titanium sublayer) are 
shown in Figure 4. A level of short circuit is 10 – 14÷10 – 6 A 
current in LGT with gold-on-titanium electrodes and  
10 – 15÷10 – 9 A in LGT with gold electrodes.  
As follows from these graphs, the temperature de-
pendences of SCC are different for samples with differ-
ent electrodes, however, there are main general pat-
terns: 
 nonmonotonic SCC dependences; 
 SCC inversions; 
 with a certain temperature current linearly in-
creases by several orders. 
Extrema are observed at 270 C, 300 C, 350 C, 
380÷430 C, 480 C, 530 C, 580 C for the sample with 
gold electrodes, at temperatures 160 C, 260 C, 300 C, 
360 C, 460 C, 520 C for the sample with gold-on-
titanium electrodes. Tendency to current increase is 
observed from 600 C for both samples. Magnitude and 
direction of the stationary current value depend on the 
 
a 
 
b 
 
Fig. 4 – SCC dependences of LGT samples with gold elec-
trodes (a) and with gold-on-titanium electrodes (b) 
 
nature of the electrode. This shows that the detected 
short circuit current is determined by near-electrode 
processes. 
The samples surfaces before and after short-circuit 
current temperature measurement were investigated 
by XRD on a “Bruker” D8 Discover diffractometer with 
Cu K  radiation. Analysis of results showed that both 
identical and different phases appeared on the opposite 
surfaces of samples after thermal testing. These results 
confirmed our assumption that short circuit currents 
have electrochemical nature.  
 
3. CONCLUSION 
 
The dependences of the LGT crystal electrical proper-
ties from its growth atmospheres and electrode material 
in alternating electric fields are investigated. Analysis of 
impedance spectroscopy results found that electrochemi-
cal processes on LGT crystal polar surfaces determinated 
the appearance of short circuit currents. Short circuit cur-
rents are recorded on LGT crystals polar cut with sym-
metrical electrodes without preliminary polarization of 
the specimen. The inversion of SCC taken place during 
heating, is associated with different phase changes on the 
polar surfaces of the crystal which are caused by the elec-
trochemical processes.  
 
ACKNOWLEDGEMENT 
 
Investigations were carried out on equipment of the 
Interdepartmental Educational-Testing Laboratory of 
Semiconductor and Dielectric Materials “Single Crys-
tals and Stock on their Base” (NUST "MISIS"). 
 
 A.P. KOZLOVA, N.S. KOZLOVA, ET AL. J. NANO- ELECTRON. PHYS. 6, 03034 (2014) 
 
 
03034-4 
REFERENCES 
 
1. B.V. Mill, А.V. Butashin, G.G. Hodzhabagyan, 
E.A. Belokoneva, N.V. Belov, Doklady AN USSR 264 No 6, 
1385 (1982) [in Russian].  
2. K. Shimamura, H. Takeda, T. Kohno, J. Crystal Growth 
163, 388 (1996). 
3. O.A. Buzanov, E.V. Zabelina. N.S. Kozlova, Crystall. Rep. 
52 No 4, 691 (2007).  
4. A.A. Blistanov, N.S. Kozlova, V.V. Geras’kin, Izvestiya 
Vysshih Uchebnyh Zavedeniy. Cvetnaya Metallurgiya 
No 4, 66 (1996) [in Russian]. 
5. I.M. Anfimov, O.A. Buzanov, А.P. Kozlova, N.S. Kozlova, 
Izvestiya Vysshih Uchebnyh Zavedeniy. Materialy El-
ektronnoy Tehniki No 2, 21 (2011) [in Russian]. 
 


Lanthanum-Gallium Tantalate Crystals and their Electrophysical Characterization

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 6 No 3, 03034(4pp) (2014) Том 6 № 3, 03034(4cc) (2014)   The article was reported at the International Conference «The Advanced Technology, Equipment and Analytical Systems for Materials», Kursk, 13-14 May, 2014 2077-6772/2014/6(3)03034(4) 03034-1  2014 Sumy State University Lanthanum-Gallium Tantalate Crystals and their Electrophysical Characterization  A.P. Kozlova*, N.S. Kozlova, I.M. Anfimov, D.A. Kiselev, A.S. Bykov  National University of Science and Technology "MISiS", 4, Leninskiy Pr., 119049 Moscow, Russia  (Received 19 May 2014; published online 15 July 2014)  Lanthanum-gallium tantalate single crystal (La3Ta0.5Ga5.5O14, langatate, LGT) is a perspective piezoe-lectric material as an active component of pressure sensors. An investigation of the growth conditions in-fluence (the growth atmosphere) on the electrophysical сharacterization of LGT, obtained in different at-mospheres (Ar, Ar + O2) was carried out. The frequency dependences of the relative dielectric constant (ε11/ε0) and of the admittance depend on the growth atmosphere. The langatate electrophysical сharacteri-zation in alternating electric fields were analyzed by means of the impedance spectroscopy method. The behavior of short circuit currents in specimens of polar cuts of LGT single crystals with the same material electrodes without preliminary polarization is described.  Keywords: Langatate, Polar dielectric crystal, Impedance, Short circuit current.   PACS numbers: 82.45.Fk, 82.45.Gj, 82.45.Un                                                    * anna_kozlova_2009@mail.ru 1. INTRODUCTION  Lanthanum-gallium tantalate single crystals La3Ta0.5Ga5.5O14 are of interest as piezoelectric materi-al. A first characterization of La3Ga5,5Ta0,5O14 is dated 1982 [1]. These crystals are widely used in the design of sensors based on the direct piezoelectric effect. The main requirements for pressure sensors are small size of the element and time stability of basic characteris-tics. Particular requirements of materials for such pressure sensors are [2]:  absence of phase transitions up to ~ 1200 C;  absence of the pyroelectric effect;  absence of physical properties hysteresis;  high sensitivity, caused by high values of piezoe-lectric coefficients;  high value of specific resistance. Lanthanum-gallium tantalate single crystal has all these necessary properties [2].  The single crystals were grown by the Czochralski method in Ar or Ar + O2 atmospheres, but generally, the growth technology in argon-oxygen atmosphere is more frequently used. Some amount of oxygen in the growth chamber is necessary to prevent evaporation of gallium oxide and lack of gallium in the melt, which leads to the crystal non-stoichiometry. Polar cuts of langatate single crystals are used as pressure sensors active component in high temperature devices. The technology of high temperature sensors based on langatate single crystals includes the coating of their polar cuts surfaces with a thin conducting layer. One of the problems of such sensors is the surface aging and surface degradation during the sensor use especially at high temperatures. A peculiarity of langatate is the structural complexity of the unit cell and a tendency to form a disordered distribution of its matrix ions within the crystal lattice [2, 3]. Inhomogeneity of thus obtained crystals leads to heterogeneity of physical properties. The nature of the nonequilibrium crystals state, and consequently, the possibility of occurrence of pro-cesses leading to crystals state changes may be caused by the crystal properties itself, by its post-growth treatment and by service conditions. It is well known that there are deviations from the equilibrium caused by external effects, for example, crystalline elements degradation under the effect of electric fields. Less evident are processes and their consequences determining the variation of crystals in the absence of external effects. This process includes the short circuit current (SCC) detected in polar crystals. This phenomenon was detected in lithium iodate [4].  Thus, identifying the causes and nature of changes in the crystal state, which manifests itself in the anomalous behavior of some LGT properties and the SCC occurrence, is not only of practical interest, but is also important for a number of polar crystals fundamen-tal studies.  2.  EXPERIMENTAL PROCEDURE  In this study, langatate crystals produced by the “Fomos-Materials” company were used. The crystals were grown in iridium crucibles in an Ar atmosphere and in a mixture of argon and oxygen (Ar + (2 %)O2). The samples represent plates of polar cut polished on both sides. The samples represent plates of polar cut polished on both sides with DC-magnetron sputtered electrodes (gold with a titanium sublayer, pure gold and pure iridium). The electrical parameters measurement was per-formed on a “Keithley” 6517A electrometer. The inves-tigations of electrical parameters frequency dependen-cies were performed in the frequency range from 5 to 500 000 Hz on a “Tesla” BM 507 impedance meter. Im-pedance measurement was carried out in the tempera-ture range 20-450 C and SCC in the range 20-600 C. Specific admittance z and relative dielectric constant ε were calculated on the basis of measured impedance Z values and phase angle φ according to equations (1) and (2)   A.P. KOZLOVA, N.S. KOZLOVA, ET AL. J. NANO- ELECTRON. PHYS. 6, 03034 (2014)   03034-2  1Zdl h Z  , (1)  where z  specific admittance, Оhm – 1·m – 1, d  sample thickness, m; l  electrode length, m; h  electrode width, m; Z  impedance, Оhm;   01 sin2dl h f Z      , (2)  where   relative dielectric constant, f  frequency, Hz; 0  permittivity of vacuum (8,8610 – 12 F/m). The temperature-frequency dependences of admit-tance and relative dielectric constant were investigat-ed. According to these results the near-electrode processes contribution to electrophysical properties was determined. The measurements in the frequency range up to 500 kHz showed the admittance value increases on three orders for all investigated samples. The temperature dependence is absent up to 400 °C for the crystals grown in the Ar atmosphere, change of admittance starts only at 450 °C, while in crystals grown in the Ar + O2 atmosphere the temperature in-fluence is absent up to 450 °C. The relative dielectric constant temperature-frequency dependences of crystals obtained in different atmosphere with identical iridium electrodes and obtained in identical atmosphere with different electrodes were investigated (Table 1). It allowed revealing the influence of the growth atmosphere and the conducting coating material on the relative dielectric constant value. Data with an uncertain-ty limit are shown in Table 1.  Table 1– The values (ε11 / ε0) of dielectric constant of langatate samples at different temperatures in the fre-quency range 5 Hz-500 kHz   Relative dielectric  constant Atmosphere,  Electrode material (ε11 / ε0) at Т  20-400 С (ε11 / ε0)  at Т  450 С Ar, electrode - Ir 34  2 43  3 Ar + (2 %)O2, electrode – Ir 43  3 49  3 Ar + (2 %)O2, electrode – Au+Ti 35  2 39  3  The classical solid electrolyte is connected in paral-lel resistor and capacitor. We suppose that the near-electrode processes on polar cuts of LGT have an elec-trochemical nature. To analyze the results of measurements of the spe-cific impedance the impedance hodographs were con-structed. The shape of the hodograph can be seen in the equivalent circuit of the investigated process, and thus about the process. Several equivalent circuits for the pattern “electrode-solid electrolyte-electrode” were ana-lyzed: the equivalent circuit of Ershler-Randles (Fig. 1a), and Frumkin-Melik-Gaykazyan (Fig. 1b). According to the Ershler-Randles circuit the impedance of such a reaction can be represented as the impedance of the circuit consisting of series-connected resistors RF, characterizing the charge transfer, and the Warburg impedance Zw, which characterizes the diffusion pro-cess. The resistor Re is the resistance of the electrolyte, Cd is part of the double layer capacity due to indiffer-ent ions. Equivalent Frumkin-Melik-Gaykazyan circuit describes the electrochemical processes taking place into account of the electrochemical reactions products adsorption in “electrode-solid electrolyte-electrode” sys-tems. R' characterizes own speed elementary act of adsorption-desorption and capacity С' – additional double layer capacity associated with adsorption of surface – active particles. Based on the experimental results of impedance spectroscopy for all samples of LGT crystals complex impedance Z components for both equivalent circuits have been identified by analyt-ical method. For both circuits of LGT sample grown in Ar + O2 mixture with Au/Ti electrodes [5] impedance components demonstrate that the main contribution to the value of the electrical resistance is the resistance of near-electrode processes. The near-electrode processes are of electrochemical nature.    a  b  Fig. 1 – The equivalent circuit of Ershler-Randles (a), and Frumkin-Melik-Gaykazyan (b)  We have shown that the short circuit current in LGT is detected always even if the crystal is between symmetric (identical electrodes). The short circuit cur-rent is recorded without preliminary polarization of the specimen. The diagram of the experiment shows time dependences of the temperature and current (Fig. 2). SCC measurement process consists of three stages. The first stage t1 is SCC measurement after shortening the circuit – the current decreases with time (t) in accord-ance with the law 0 0( / )aI I t t    to a stationary value which is retained for any period of time within about 1 hour. During the second stage t2 the sample is kept for 30 minutes at room temperature after the current de-cay process. This step is necessary for accurate SCC measurements at the beginning of the heating process. After that, the linear heating with rate of 2 C/min to the required temperature and the simultaneous regis-tration of SCC parameters were carried out (stage t3). The resulting dependence characterizes the near-electrode processes kinetics and activation of active defects in the bulk of the crystal. The duration of this stage is about 8 hours.   LANTHANUM-GALLIUM TANTALATE CRYSTALS… J. NANO- ELECTRON. PHYS. 6, 03034 (2014)   03034-3   Fig. 2 – Scheme of the experiment: the plot of temperature and current change from time  Charge state of each surface was determined by the piezoresponse technique. In the experiment, the crystal is always placed with the same orientation due to sur-face charge sign with respect to the electrode holder. However, for testing, the sample was also placed on the opposite side with respect to the holder. The current direction changing is due to the processes within the sample. The behavior of the current schedule, depend-ing on the setting of the sample shown in Figure 3.    Fig. 3 – SCC behavior at different positions of the sample relative to the electrodes  SCC dependences of LGT samples with different elec-trodes (pure gold, gold with a titanium sublayer) are shown in Figure 4. A level of short circuit is 10 – 14÷10 – 6 A current in LGT with gold-on-titanium electrodes and  10 – 15÷10 – 9 A in LGT with gold electrodes.  As follows from these graphs, the temperature de-pendences of SCC are different for samples with differ-ent electrodes, however, there are main general pat-terns:  nonmonotonic SCC dependences;  SCC inversions;  with a certain temperature current linearly in-creases by several orders. Extrema are observed at 270 C, 300 C, 350 C, 380÷430 C, 480 C, 530 C, 580 C for the sample with gold electrodes, at temperatures 160 C, 260 C, 300 C, 360 C, 460 C, 520 C for the sample with gold-on-titanium electrodes. Tendency to current increase is observed from 600 C for both samples. Magnitude and direction of the stationary current value depend on the  a  b  Fig. 4 – SCC dependences of LGT samples with gold elec-trodes (a) and with gold-on-titanium electrodes (b)  nature of the electrode. This shows that the detected short circuit current is determined by near-electrode processes. The samples surfaces before and after short-circuit current temperature measurement were investigated by XRD on a “Bruker” D8 Discover diffractometer with Cu K  radiation. Analysis of results showed that both identical and different phases appeared on the opposite surfaces of samples after thermal testing. These results confirmed our assumption that short circuit currents have electrochemical nature.   3. CONCLUSION  The dependences of the LGT crystal electrical proper-ties from its growth atmospheres and electrode material in alternating electric fields are investigated. Analysis of impedance spectroscopy results found that electrochemi-cal processes on LGT crystal polar surfaces determinated the appearance of short circuit currents. Short circuit cur-rents are recorded on LGT crystals polar cut with sym-metrical electrodes without preliminary polarization of the specimen. The inversion of SCC taken place during heating, is associated with different phase changes on the polar surfaces of the crystal which are caused by the elec-trochemical processes.   ACKNOWLEDGEMENT  Investigations were carried out on equipment of the Interdepartmental Educational-Testing Laboratory of Semiconductor and Dielectric Materials “Single Crys-tals and Stock on their Base” (NUST "MISIS").   A.P. KOZLOVA, N.S. KOZLOVA, ET AL. J. NANO- ELECTRON. PHYS. 6, 03034 (2014)   03034-4 REFERENCES  1. B.V. Mill, А.V. Butashin, G.G. Hodzhabagyan, E.A. Belokoneva, N.V. Belov, Doklady AN USSR 264 No 6, 1385 (1982) [in Russian].  2. K. Shimamura, H. Takeda, T. Kohno, J. Crystal Growth 163, 388 (1996). 3. O.A. Buzanov, E.V. Zabelina. N.S. Kozlova, Crystall. Rep. 52 No 4, 691 (2007).  4. A.A. Blistanov, N.S. Kozlova, V.V. Geras’kin, Izvestiya Vysshih Uchebnyh Zavedeniy. Cvetnaya Metallurgiya No 4, 66 (1996) [in Russian]. 5. I.M. Anfimov, O.A. Buzanov, А.P. Kozlova, N.S. Kozlova, Izvestiya Vysshih Uchebnyh Zavedeniy. Materialy El-ektronnoy Tehniki No 2, 21 (2011) [in Russian].  

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Lanthanum-Gallium Tantalate Crystals and their Electrophysical Characterization

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