ISSN 10637826, Semiconductors, 2011, Vol. 45, No. 11, pp. 1387–1390. © Pleiades Publishing, Ltd., 2011. Original Russian Text © R.M. Sardarly, O.A. Samedov, A.P. Abdullayev, F.T. Salmanov, O.Z. Alekperov, E.K. Huseynov, N.A. Aliyeva, 2011, published in Fizika i Tekhnika Polu provodnikov, 2011, Vol. 45, No. 11, pp. 1441–1445.
NONELECTRONIC PROPERTIES OF SEMICONDUCTORS (ATOMIC STRUCTURE, DIFFUSION)
Superionic Conductivity and Switching Effect with Memory in TlInSe2 and TlInTe2 Crystals R. M. Sardarlya^, O. A. Samedova, A. P. Abdullayeva, F. T. Salmanova, O. Z. Alekperovb, E. K. Huseynovb, and N. A. Aliyevab aInstitute
of Radiation Problems, National Academy of Sciences of Azerbaijan, AZ1143 Baku, Azerbaijan ^email:
[email protected] bInstitute of Physics, National Academy of Sciences of Azerbaijan, AZ1143 Baku, Azerbaijan Submitted April 5, 2011; accepted for publication April 15, 2011
Abstract—The temperature dependences of the conductivity σ(T) and the switching and memory effects in onedimensional TlInSe2 and TlInTe2 single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic con ductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl+ ions over vacancies in the 3+
2– –
3+
2– –
thallium sublattice between ( In Te 2 ) and ( In Se 2 ) nanochains (nanorods). Stype switching and memory effects are revealed in TlInSe2 and TlInTe2 crystals, as well as voltage oscillations in the range of neg ative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electricfieldinduced transition of TlInSe2 and TlInTe2 crystals to the superionic state is found. DOI: 10.1134/S1063782611110224
1. INTRODUCTION Under normal conditions of ion charge transfer in solids, both crystalline and amorphous, their conduc tivity at room temperature does not exceed 10–10– 10–12 Ω–1 cm–1, whereas the conductivity of superi onic conductors is about 10–1 Ω–1 cm–1 [1–4]. Currently, superionic conductors are used in solid state fuel elements, gas and liquid sensors, miniature accumulators, and other devices. The existence of superionic conductivity depends in many respects on the structural features of the material, specifically: (i) the number of potentially mobile ions per unit cell should exceed the number of immobile ions; (ii) the energy of ion disordering over lattice posi tions and the energy lost during motion should be small; (iii) the network of channels in the crystal structure must be through for moving ions. These requirements are satisfied for only a few par ticular crystals, the structure of which excludes long range ordering in the spatial arrangement of one or several types of atoms but retains longrange order for other particles. These compounds are considered crys tals with intrinsic structural disordering. Crystals with structural disordering, which have mainly ion conductivity, can be in two qualitatively different states. At temperatures below critical they behave like conventional ionic crystals (dielectric phase), while at temperatures above critical they pass
to a peculiar superionic state (electrolytic phase). Thus, the case in point is materials having peculiar hybrid properties: conductivity of a liquid melt or a solution and mechanical strength and elasticity of solids. TlInSe2 and TlInTe2 are materials in which the fea tures of low (one)dimensional systems manifest themselves under certain conditions [5, 6], due to which these compounds are widely investigated. The temperature dependences of the specific heat, lattice parameters, and photoconductivity of TlInSe2 were studied in [7–9], where experiments were performed at temperatures T from 5 to 300 K, thermodynamic parameters of the crystal were calculated, and the presence of a phase transition to the temperature range T = 135–184 K was shown. Mamedov et al. [5] inves tigated the band structure of a TlInSe2 crystal by the method of linear combinations of atomic orbitals. The specific heat, Xray diffraction, and transport proper ties of a TlInTe2 crystal were investigated in [10–17]. Negative differential resistance was observed in crys tals of both types (TlInSe2 and TlInTe2) in [13–15]; this effect was believed to be purely thermal. Voltage oscillations were also observed in the range of negative differential resistance. Studies of the conductivity and dielectric proper ties of TlGaTe2 (a structural analog of TlInSe2 and TlInTe2) revealed its superionic conductivity at tem peratures above 300 K [18, 19]. In this paper, we report the results of an experimen tal study of the conductivity and switching effect in
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0.2
4.5 4.0 2.2
2.3
2.4 2.5 103/T, K−1
2.6
6 4 2 0
2
3
6 4
1.2
(b)
9.8
−1
−1.7 −2.1 −2.5 2.24
2.32
4
2.40 2.48 2.56 103/T, K−1
5
2.64
30 20
0
3
2
2.1. Electrical Properties The temperature dependences of conductivity, σ(T), in TlInSe2 and TlInTe2 are shown in Figs. 1 and 2, respectively. The measurements were per formed in an electric field oriented perpendicular to the tetragonal crystal axis σ⊥(T) and parallel to it σ||(T). One can see jumps in the dependences σ(T) for both crystals: for TlInSe2 a jump in σ||(T) is observed at = 391 K and a jump on σ⊥(T) at
9.4 9.2
4
2.4
2.6 2.8 103/T, K−1
5
3.0
6 7 103/T, K−1
Fig. 2. Temperature dependences of the conductivity σ(T) for the TlInTe2 crystal, measured (a) perpendicularly and (b) parallel to the tetragonal axis c of the crystal. The insets show the dependences ln(σT) on 1/T above the tempera ture jump in the dependence of ln(σT) on 1/T. ⊥
T cr = 388 K; for TlInTe2, the corresponding values are ||
2. RESULTS AND DISCUSSION The conductivity was measured by the fourprobe method in two directions: parallel (σ||(T)) and perpen dicular (σ⊥(T)) to tetragonal axis c of the crystal. The experimental samples were prepared in the form of rectangular plates 0.3–0.5 mm thick. Contacts with the samples were formed by depos ing a silver conducting paste on the plate surface. The permittivity and conductivity were measured by Е78, Е712, and Е720 digital immittance meters at fre quencies of 1–1000 kHz in a temperature range of 100–450 K. The measuring field amplitude did not exceed 1 V cm–1.
9.6
9.0 2.2
10
6 7 103/T, K−1
TlInSe2 and TlInTe2 compounds in a wide tempera ture range.
a temperature
1.4
1.0 2.35 2.40 2.45 2.50 2.55 2.60 103/T, K−1
−1.3
Fig. 1. Temperature dependences of the conductivity σ(T) for the TlInSe2 crystal, measured (a) perpendicular and (b) parallel to tetragonal axis c of the crystal. The insets show the dependences ln(σT) on 1/T above the tempera ture jump in the dependence of ln(σT) on 1/T.
|| T cr
1.6
0 40
(b) σ, Ω−1 cm−1
lnσT [K Ω−1 cm−1]
σ, 10−4 Ω−1 cm−1
8
8
2
0 −0.9
10
lnσT [K Ω cm ]
0.4
5.0
lnσT [K Ω−1 cm−1]
0.6
6.0 5.5
(a)
1.8
12
−1
0.8
14
(a)
6.5
σ, 10−3 Ω−1 cm−1
lnσT [K Ω−1 cm−1]
σ, Ω−1 cm−1
1.0
⊥
T cr = 388 K and T cr = 333 K. As follows from the insets to Figs. 1a and 2a, for both crystals, the experimental points of the tempera ture dependence ln(σT) in the range of the sharp jump of conductivity are described well by a straight line, which is given by the following equation [1–4] for ionic conductivity: σT = σ 0 exp ( – ΔE/kT ),
(1)
where ΔE is the conductivity activation energy and k is the Boltzmann constant. The activation energies for the TlInSe2 crystal in the measurements with field ori entation perpendicular and parallel to the c axis turned out to be ΔE⊥ = 0.377 eV and ΔE|| = 0.450 eV, respec tively; for the TlInTe2 crystal the corresponding values are ΔE⊥ = 0.101 eV and ΔE|| = 0.228 eV. It is known that this temperature dependence of conductivity indicates the dominant character of ionic conductivity above the critical temperature [1–4, 19, 20]. The observed jumps in the conductivity compo nents σ⊥ and σ|| for the TlInSe2 and TlInTe2 crystals at ||
⊥
the critical temperatures T cr = 391 K, T cr = 388 K and ||
⊥
T cr = 388 K, T cr = 333 K, respectively (Figs. 1, 2) can be explained by the sharp change in the number of ions SEMICONDUCTORS
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SUPERIONIC CONDUCTIVITY AND SWITCHING EFFECT WITH MEMORY
in the states characterized by high ion mobility, i.e., by the phase transition to the superionic state. TlInSe2 and TlInTe2 semiconductor crystals belong
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10−1 5
(a)
6
3+
2– –
crystals is that their structure is formed by ( In Se 2 ) 3+
2– – Te 2 )
and ( In chains, elongated along tetragonal axis c of the crystal. The tetragonal axis is an optical axis. Monovalent Tl+ atoms are in the octahedral envi ronment of Se and Te atoms in the TlInSe2 and TlInTe2 crystals, respectively. Based on the crystal lochemical considerations, it can be suggested that this crystal structure is most favorable for mobile Tl+ cations. In this case, the favorable factor is the presence of extensive cavities, which are linked by shared faces (conductivity windows), as well as the fundamental possibility of deficit of monovalent thal lium ions, as a result of which the ionic conductivity may significantly rise. The linear character of the dependence ln(σT) on 1/T above the conductivity jump in the dependence of σ on 1/T indicates dominance of ionic conductivity, which is mainly due to the diffusion of Tl+ ions over vacancies in the thallium sublattice of the TlInSe2 and TlInTe2 crystals. This change occurs as a result of the phase transition, which is accompanied by disordering (“melting”) of the Tl sublattice in the TlInSe2 and TlInTe2 crystals. This conduction mechanism is typi cal of superionic conductors [1–4, 19–21]. 2.2. ElectricFieldInduced Transition of TlInSe2 and TlInTe2 Crystals to the Superionic State The transition to the high–conductivity state in superionic conductors occurs generally as a result of the firstorder phase transition and is explained by the stepwise disordering of one of the crystal sublattices (generally cationic) with the other sublattice either remaining invariable or transforming but retaining the crystal hardness [1–4, 19–21]. This effect was found experimentally in αAgSbS2 [20], where application of an external electric field caused a gradual increase in conductivity with a subsequent sharp increase (by a factor of 620) when the field reached the critical value. The measured dependences of the conductivity of TlInSe2 and TlInTe2 crystals on the electric field strength E at different temperatures are shown in Fig. 3. The measurements were performed along the tetragonal crystal axis and perpendicular to it. In rela tively weak fields, conductivity σ is almost indepen dent of the applied field E because of the dominance of the electronic component in σ in this range of field strengths. A further increase in E led to a linear increase in σ, which is explained by the increase in the ionic component of conductivity as a result of gradual SEMICONDUCTORS
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10−3 σ, Ω−1 cm−1
18
into the tetragonal system with the space group D 4h (TlSe structural type). A characteristic feature of these
4
3
10−5 2
10−7
10−9
1
0
400
800
1200 E, V/cm
101 (b)
10−1 σ, Ω−1 cm−1
to compounds of the A3B3C 2 group, which crystallize
1
2
10−3
10−5
0
20
40
60
80 E, V/cm
Fig. 3. Dependences of the conductivity σ on the electric field strength E for (a) the TlInSe2 crystal ((1, 2, 3) mea surements in the direction parallel to the c axis of the crys tal at temperatures of 168, 260, and 300 K, respectively, and (4, 5) measurements in the direction perpendicular to the c axis at temperatures of 177 and 240 K, respectively) and (b) the TlInTe2 crystal (measurements in the direc tions (1) parallel and (2) perpendicular to tetragonal axis c of the crystal, T = 300 K).
disordering of the cationic Tl sublattice in the electric field; in this range, the ionic conductivity begins to dominate over the electronic component and, when reaching the critical temperature, the conductivity sharply rises by a factor of 1000. The values of the critical transition field and the jump of conductivity during the phase transition increase with a decrease in temperature (Fig. 3). Volt age oscillations were observed in the range of negative
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differential resistance in both compounds; these were also revealed in [13–15]. Watzke et al. [15] explained the observed oscillations in these crystals under the assumption that the conductivity is due to two related effects: hops of carriers between different levels and fluctuations of these levels (crisis of induced intermit tency). In our opinion, the discovered effect of Stype switching in the TlInSe2 and TlInTe2 crystals, as well as the voltage oscillations in the range of negative dif ferential resistance, are related to the transition of these crystals to the superionic state, which is accom panied by melting of the Tl sublattice. The measured dependences of the TlInSe2 and TlInTe2 conductivities on the electric field strength E indicate that, at a certain value of critical field (at a temperature T = 300 K in the parallel direction, Ecr = 454.5 and 51.8 V/cm for TlInSe2 and TlInTe2, respec tively), the Tl+ ion sublattice may undergo stepwise disordering, which is accompanied by a stepwise change in conductivity. Thus, having applied an electric field to a crystal, one can implement disordering (melting) of the cat ionic sublattice, which leads to a stepwise increase in the occupancy of interstitial sites throughout the crys tal volume. The analysis also showed the presence of memory effect, which manifests itself as the conserva tion of a lowresistance state for a long time after switching off the field. For the TlInSe2 and TlInTe2 crystals, this time exceeds 48 h.
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7. 8. 9. 10. 11. 12. 13.
3. CONCLUSIONS The results obtained indicate that the electronic component of conductivity dominates in TlInSe2 and ||
⊥
TlInTe2 crystals at temperatures of T cr = 391 K, T cr = ||
14. 15.
⊥
388 K and T cr = 388 K, T cr = 333 K, respectively. A further increase in temperature (above Tcr) leads to a stepwise increase in conductivity, which is related to the increase in the ionic component due to Tl+ sublat tice disordering. In this temperature range the ionic conductivity of the crystal dominates over the elec tronic component. The study of the dependences of the TlInSe2 and TlInTe2 conductivities on electric field strength E showed that stepwise disordering of the Tl+ ion sublattice may occur at a certain critical electric field Ecr, which is accompanied by a stepwise change in conductivity. Note that the abovedescribed effect of field induced stepwise disordering in principle makes it possible to implement the superionic state in TlInSe2 and TlInTe2 crystals in a temperature range conve nient for practical applications (for example, the varis tor effect).
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18.
19.
20. 21.
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Translated by Yu. Sin’kov
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