From relaxor to ferroelectric behavior in K1-xLixTaO3 Z. Trybuła, Sz. Łoś, M. Trybuła
J. Dec, S. Miga
Institute of Molecular Physics Polish Academy of Science Poznań, Poland
[email protected]
Institute of Materials Science University of Silesia Chorzów, Poland
Abstract— The results of low-temperature linear and nonlinear susceptibilities, polarization measurements of the dielectric properties of the KTaO3 and K1-xLixTaO3, x=0.02 and 0.08 solid solution are presented. The coexistence of the relaxorlike and ferroelectric behavior and different mechanisms leading to either of them are discussed. The observed ferroelectric phase transition in K1-xLixTaO3, x=0.08 is of the first-order type with temperature hysteresis. Clusters of Li+ ions produce a relaxorlike behavior and random electric field. The ferroelectric phase transition is due to the off-center motions of Ta ions in the octahedral environment of oxygen ions. This process leads to the formation of a reversible spontaneous polarization.
Keywords—relaxors; doped incipient ferroelectrics; phase transitions; potassium tantalate; linear and nonlinear electric susceptibilities
I. INTRODUCTION The lithium-doped potassium tantalate K1-xLixTaO3 (KLT-x%Li for short), the perovskite solid solution is a complex polar system. The potassium tantalate KTaO3 (KT for short), remains in its centrosymmetric cubic structure down to the lowest temperatures (0.003K) and does not undergo any ferroelectric phase transition, because of quantum fluctuations [1-3]. KT has the polar soft TO1 transverse optic mode that is stabilized by the zero-point quantum fluctuations of atoms [4]. Potassium tantalate KT has a highly polarizable lattice. Their properties are very sensitive to even small amount of dopants as Li+, defects, electric and elastic applied fields [2, 5-13]. At low concentration of the Li+ ions in KLT-x%Li crystal, x0.022 a long-range order ferroelectric state sets in even in the absence of external electric field [5, 6, 12, 13]. Recently, the coexistence of the relaxor-like and ferroelectric behavior in KLT-3.4%Li has been reported by us [12]. Additional, evidence of polar nanoregions (PNRs) in nominally pure KT crystals [10], and a strain-induced ferroelectric order in epitaxial thin film of KT have been reported [8]. The fundamental question, concerns to the nature of PNRs and the origin of ferroelectricity in the solid solution of KLT-x%Li, is still remains open.
In this paper we report results of the low temperature dielectric linear and nonlinear behavior in pure KT crystal, as well as in doped by Li+ ions solid solution KLT-x%Li: x=2% and 8%. We will discuss the mechanism of polarization in KLT-x%Li solid solution. II. EXPERIMENTAL PROCEDURE Crystals of KT, KLT-2%Li, and KLT-8%Li was grown in an oxygen atmosphere by a slow-cooling technique from a high-temperature solution of K2CO3, Ta2O5 (KT crystal) and additional LiCO3 (KLT-x%Li crystals) [14]. The Li+ ions concentration x was determined as an empirical relation between x and the transition temperature [15]: TC=535 x2/3
(1)
Samples were prepared as a plate of dimensions of crystals: KT (2.70x3.25x0.85) mm3, KLT-2%Li (2.08x2.40x0.56)mm3, and KLT-8%Li (3.00x4.00x0.90)mm3. Electrodes of Cu-Au were evaporated onto the principal {100} faces. The thin copper interface is destined to improve the adhesion of gold. The silver wire of 0.03 mm in diameter were glued to the electrodes using the silver paste. The sample with electrodes and wires was placed in a leaky Teflon container, then silver wires were glued to the top of the Teflon container, to be sample mechanically free. The samples were measured in the temperature range of 4.2 K < T < 300 K using helium flow cryostat. The temperature of samples were measured using a Lake Shore Cernox CX1030 BC thermometer, and stabilized by an Oxford temperature controller ITC 503. The linear 1' , and nonlinear 2' , 3' , electric susceptibilities of the KLT-4.3%Li sample were measured at frequencies from 111 to 1000 Hz in the temperature range of 4.2 T 300 K using a self-constructed susceptometer [16]. The higher order nonlinear 2' , 3' electric susceptibilities in addition to the linear one 1' present in the series expansion of the polarization PE in respect of the applied electric field, E:
PE ε 0 χ1' E χ 2' E 2 χ 3' E 3 ,
978-1-5090-1871-0/16/$31.00 ©2016 IEEE
(2)
3
1 (10 )
2
-1
2 (10 mV )
b
4
-2 -2 2
Fig. 1 shows the temperature dependence of the linear electric susceptibility 1' , of the KTaO3 (KT) crystal, KLT-2%Li and KLT-8%Li solid solution for the frequency 111, 317 and 1000 Hz of the ac probing field in the ZFH after ZFC run. KT crystal does not exhibit dielectric relaxation. The characteristic relaxations related to relaxor behavior, occur in the temperature dependence of KLT-2%Li and KLT-8%Li crystals. This relaxation is related to the reorientation by 90and 180of Li+ ions between equivalent sites at orthogonal axes [7, 12, 17-20]. The broad peaks of 1' depend on frequency. The arrow shows a jump in the value of the 1' for all frequencies of KLT-8%Licrystals at TC=114 K. this is evidence of ferroelectric phase transitions. At temperatures below 114 K, relaxation is still observed. This is proof of the coexistence of the relaxor-like and ferroelectric behavior in KLT-8%Li solid solution. Recently, similar results have been reported by us in KLT-3%Li [12] and KLT-4.3%Li [13].
a
2 0 1.5
c
1.0
-8
III. RESULTS AND DISCUSSION
1000 Hz 316 Hz 111 Hz
4
0
3 (10 m V )
Equation (3) can provide useful information on the basic physics of electric polarization. Using a self-constructed susceptometer we are able to detect a dynamical nonlinear susceptibility related to ac dielectric nonlinearity. The sample is exposed to an ac probing field with sufficiently large amplitude much lower than the basis field strength used in conventional method [16]. A weak ac electric field with an amplitude of the value of Eac=15kVm-1 for the zero dc field cooling (ZFC) and zero dc field heating (ZFH) runs was applied. The remnant polarization P was detected by integrating the polarization current density j measured on ZFC and ZFH runs.
-3
(3)
0.5 0.0 10
12
10
11
10
10
d
5
' E 1' 2' E 3' E 2
Fig. 2 shows the temperature dependences of the linear, 1' , (Fig. 2(a)) and the nonlinear, 2' , (Fig. 2(b)), 3' , (Fig.2(c)), electric susceptibility and the scaled non-linear susceptibility, a3 , (Fig. 2(d)),, of the KLT-2%Li, for the frequencies of the ac probing electric field: 111, 317, and 1000 Hz at ZFH after ZFC regime.
-a3 (m VC )
where 0 stands for electric permittivity of the free space. The electric-field dependence of the electric susceptibility is represented by the equation:
0
20
40
60
80
100
T (K)
Fig. 2. Temperature dependences of (a) 1' , (b) 2' , (c) 3' electric
6
4
Tc= 114 K
KT
3
(10 )
1000 Hz 317 Hz 111 Hz
KLT-2%Li
5
(ZFH)
susceptibilities, and (d) a3 scaled nonlinear susceptibility, of KLT-2%Li for the frequency 111, 317 and 1000 Hz of the ac probing field in the ZFH after ZFC.
KLT-8%Li
3
' 1
The so-called scaled non-linear susceptibility, a3 , (Figure 2(d)) was given by Pirc and Blinc [21]:
2 1 0
a3 0
50
100
150
200
T(K)
Fig. 1. Temperature dependences of the linear electric susceptibility of KTaO3 (KT), K1-xLixTaO3: KLT-2%Li and KLT-8%Li solid solutions for the frequency 111, 317 and 1000 Hz of the ac probing field in the ZFH after ZFC.
3'
1' 3 0
4
(4)
The scaled non-linear susceptibility, a3 , is a suitable quantity because by measuring a3 (T ) one can discriminate between dipolar glass or relaxation behaviors and the
4 E = 0 V/cm
a
3
1000 Hz 316 Hz 111 Hz
2 1
'
2
Tc= 114K
0 0.0
-1
2 (10 m V )
' 1
3
1 (10 )
E = 700 V/cm
3
(10 )
3
1
'
0.8 0.6 0.4 0.0
5
10 15 10 14 10 13 10 12 10 11 10
d Tc= 114K
0
50
100
a
A)
4
heating
2 0
cooling
P (10 Cm )
-4
-2
TC= 109 K
2
' 1
b
1.2
cooling
-3
(ZFC)
3
(10 )
250
susceptibilities, and (d) a3 scaled nonlinear susceptibility, of KLT-8%Li for the frequency 111, 317 and 1000 Hz of the ac probing field in the ZFH after ZFC.
4
0.8 heating 0.4
TC= 114 K 1
(ZFH) 0.0
0 80
200
Fig. 5. Temperature dependences of (a) 1' , (b) 2' , (c) 3' electric
-2
3
150
T(K)
I (10
Results of ferroelectric behavior of KLT-8%Li are presented on Figs. 4-6.
0.2
-11
ferroelectric state more easily than on the basis of the linear, 1' , or the non-linear 3' electric susceptibilities alone. The value of a3 (T ) between 40 and 60 K is frequency dependent – characteristic to the relaxor behavior. The secondorder electric susceptibility 2' is proportional to the polarization [22] and reduces to zero if the polarization vanishes. The 3' , has a broad positive value within the relaxation range. This behavior is characteristic of relaxors [23]. The electric field-revealed precursor ferroelectric properties of KLT-2%Li is presented on Fig.3. Below TC=41 K temperature dependence of electric susceptibility 1' , in the ZFH after FC with Edc=700 V/cm (open circles on Fig. 3) slightly decreases from the value of ZFH after ZFC regime. The 1' decreases after application of a dc field. Recently, similar behavior in a solid solution KLT-1%Li was reported by us [11].
c
16
-3
Fig. 3. Temperature dependences of the linear electric susceptibility 1' , of KLT-2%Li for the frequency 1000 Hz of the ac probing field: open circles – ZFH after FC with Edc=700 V/cm; full circles ZFH after ZFC.
-1.2
-2
150
2
100
-7
T (K)
'
50
3 (10 m V )
0
-0.8
-a3 (m VC )
0
b
-0.4
-3
TC = 41 K
0
50
100
150
200
250
T (K)
100
120
140
160
T(K)
Fig. 4. Temperature dependences of the linear electric susceptibility 1' , of KLT-8%Li for the frequency 1000 Hz of the ac probing field: open circles – ZFC, full circles - ZFH after ZFC.
Fig. 6. Temperature dependences of (a)the polarization current I, (b) the remnant polarization P , measured on a KLT-8%Li on cooling and heating runs.
Apart from the relaxor properties, we have detected the first-order ferroelectric phase transition at TC 109 K and
TC 114 K
with characteristic hysteresis of the phase
transition TC 5 K (Figs. 4, 5). At TC 114 K the linear, 1' , (Fig. 5(a)) the nonlinear, 2' , (Fig. 5(b)), and scaled nonlinear, a3 , (Fig. 5(d)), susceptibilities exhibit small jumps at all frequencies. The temperature dependence of the remnant polarization P (Fig. 6) obtained by integrating the polarization current I measured in the ZFH after ZFC regime provides a supplementary confirmation of the appearance of a first-order phase transition. The observed transition is a discontinuous, sharp phase transition of the first-order type with temperature hysteresis. In addition, our studies [12, 13] and presented in this paper, show that the dielectric relaxation, typical for relaxor materials, appears also below the phase transition temperature. It indicates the coexistence of the relaxor-like and ferroelectric behavior, and different mechanisms leading to either of them. The phase transition cannot be ascribed to a sudden "freezing" of the off-center Li+ ions. Clusters of Li+ dipoles [24], produce a random electric field. This field affects the ordering process of the remainder of the system: the off-center motions of Ta ions in the octahedral environment of O ions. The motions of the Ta ions are correlated and form the polar nanoregions as precursors of ferroelectric domains [10-13, 25, 26]. Substitution of Li+ ions to KT leads to an appearance of dipole moments and local, "static" polar clusters. The local strain field around the Li+ ion induces a random electric field and the effective local restoring forces decrease [27], thus the depolarization field is low enough. In this way, below TC the spontaneous polarization, due to displacements of Ta ions along one of the eight equivalent off-center sites appears. ACKNOWLEDGMENT
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
The authors are grateful to Dr. V.V. Laguta for providing single crystalline samples of KT and KLT-x%Li.
[21]
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