Thermal emission and pyroelectric current in manganese chalcogenides

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Introduction

Electric energy storage devices are used in the aerospace industry. Rechargeable batteries and supercapacitors are mainly used [1–3]. To manufacture supercapacitors, it is necessary to use a material with a high dielectric constant, which depends on the electric polarization [4–8]. Semiconductors with migration and dipole polarization can be applied in capacitors [9–11]. Another aspect consists of alternative energy sources, the conversion of thermal energy into electrical energy [12–15].

Nonstoichiometric replacement of manganese ions with thulium forms electrically inhomogeneous states in the sample [16–17]. Hybridization of the wave functions of cations is accompanied by the participation of a chalcogen ion, which will lead to a charge gap. The excess charge on Tm3+ is compensated by the displacement of anions and free current carriers. Electric polarization with a p-n junction occurs at the interface between TmSe nanoregions. When phonons are absorbed by electrons in the region of the p-n junction, a thermionic current will arise.

As promising materials, we consider chalcogenides, a number of which are used for the manufacture of thermoelements [18–20]. We consider manganese chalcogenides substituted with 3d elements as current sources.

Pyroelectric current in thulium-substituted manganese selenide

A current in the absence of an external voltage can be caused by a temperature gradient (thermoEMF), a change in electrical polarization (pyroelectric current), the appearance of a potential difference when the sample is deformed (piezoelectric current), or thermionic emission (thermionic current).

Chalcogenides with variable valence have unique properties – transport, magnetic and thermoelectric [21–23]. A change in the valence of the ion changes the electronic structure and leads to lattice deformation [24]. Valence changes both with temperature and with increasing pressure [25]. Compensation for excess electrical charge will lead to local electrical polarization. Partial decompensation will cause the formation of an electric field in the sample and dipole polarization. A change in electric polarization with temperature will cause a pyroelectric current j_p = (dP/dT) (dT/dt). The current in zero electric field has the form:

$j=\sigma E_{in}\pm \frac{dP}{dt},$ (1)

where σ – sample conductivity; E_in – internal electric field.

The current in zero electric field was measured on a 6517B/E electrometer and is presented in Fig. 1, a for Tm_0,04Mn_0,95Se. When heated, the current changes sign at 110 K from negative to positive. The current passes through a maximum at 220 K, reaches a minimum at 285 K, and increases sharply above room temperature. Integrating the current will give polarization P ~ $\int jdT$, the relative value of which is shown in Fig. 1, b. Electric polarization in this sample disappears in the range of 220–240 K. 

 

Рис. 1. Температурная зависимость пиротока (a) и относительное изменение поляризации (б) для Tm_0,04Mn_0,95Se. Вставка: Температурная зависимость пиротока для Tm_0,04Mn_0,95Se

Fig. 1. Temperature dependence of the pyrocurrent (a) and relative change in polarization (b) for Tm_0.04Mn_0.95Se. Inset: Temperature dependence of the pyrocurrent for Tm_0.04Mn_0.95Se

 

Let us highlight two temperature ranges up to 240 K, where the current is caused by a change in polarization and above this temperature a thermionic current appears.

With increasing concentration, the current exhibits a maximum at 480 K in Tm_0,08Mn_0,9Se, which is associated with electrical polarization (Fig. 2). It is possible to form dipole polarization at low temperatures, which disappears when heated and migration polarization appears. The pyroelectric current reaches a value of 10 nA, current density j = 0.2 μA/cm².

 

Рис. 2. Температурная зависимость пиротока (a) и относительное изменение поляризации (б) для Tm_0,08Mn_0,9Se

Fig. 2. Temperature dependence of the pyrocurrent (a) and relative change in polarization (b) for Tm_0.08Mn_0.9Se

 

In the case of replacement of manganese ions with thulium in the concentration region where thulium ions flow through the lattice, the current changes sign above room temperature and has two small maxima at T = 110 and 280 K (Fig. 3).

 

Рис. 3. Температурная зависимость тока (a) и экспоненциальная зависимость тока от обратной температуры (б) для Tm_0,2Mn_0,8Se. Вставка: Температурная зависимость тока для Tm_0,2Mn_0,8Se

Fig. 3. Temperature dependence of the current (a) and exponential dependence of the current on the inverse temperature (b) for Tm_0.2Mn_0.8Se. Inset: Temperature dependence of the current for Tm_0.2Mn_0.8Se

 

At high temperatures, the current density is j = 0.05 mA/cm². In the free electron gas model j = env, the electron concentration increases exponentially with heating n = n₀exp(–∆E/T). The current is well described by the exponential dependence depicted in Fig. 3, b, with activation energy ∆E = 0.82 eV. The wave functions of electrons on thulium ions are interrupted and form an impurity subband. As a result of thermal fluctuations, electrons move from the impurity subband to the conduction band and an electric current appears in the electric field of defects. 

Thermionic current in manganese sulfide substituted with thulium

Let us consider how the chalcogen anion affects the current in a zero field and what causes it. Fig. 4 shows the temperature dependence of the current for Tm_xMn_1–xS solid solutions with x = 0.05; 0.15 in the temperature range 80–380 K. The maximum current value is achieved at temperatures of 320 K for x = 0.05 and 355 K for x = 0.15. These maxima are due to the emission of electrons from deep traps, and the temperatures of the thermionic current maxima correlate with the temperatures at which IR absorption disappears at the frequency ω1 = 3116 cm^–1 [26].

 

Рис. 4. Температурная зависимость термоэмиссионного тока для образцов Tm_xMn_1–xS с x = 0,05 (1); 0,15 (2)

Fig. 4. Temperature dependence of the thermionic current for samples Tm_xMn_1–xS with x = 0.05 (1); 0.15 (2)

 

For a composition with x = 0.15, the sign of the thermoEMF changes at T = 350 K, which is possibly caused by an increase in the thermionic current. It is possible that at this temperature the dissociation of vibronic states formed by the strong interaction of electrons and vibration modes of the octahedron occurs.

The replacement of manganese by ions of variable valence causes local deformation of the lattice, which is accompanied by a change in the magnitude of the ion charge. How will the current change in a zero external electric field when replacing manganese with a rare earth ion with constant valence, for example, holmium. In Fig. 5 the current for Но_0,1Mn_0,9S is given. The current magnitude decreases compared to the current in Tm_xMn_1–xS with x = 0.05; 0.15, there is an additional maximum at 205 K and a sharp peak at 348 K. Substitution with holmium enhances the electrical heterogeneity, the excess charge is compensated by holes and creates a charge density wave. As a result, a potential difference and an internal electric field Е_in arise on the surface of the sample. A change in electrical polarization induces a pyrocurrent j_p = dP/dt. The polarization obtained by integrating the current is shown in Fig. 5, b. When heated, P(T) has an anomaly at 200 K and disappears above T = 350 K.   

 

Рис. 5. Температурная зависимость тока (a) и относительное изменение поляризации (б) для Но_0,1Mn_0,9S

Fig. 5. Temperature dependence of the current (a) and relative change in polarization (b) for Ho_0.1Mn_0.9S

 

Polarization occurs due to the displacement of sulfur ions from octahedral positions; the angle between the bonds of magnetic ions and the ligand changes, affecting the magnitude of the exchange field. As a result, this will cause a change in magnetic characteristics.

Conclusion

Anomalies in the temperature dependence of the current in a zero external field make it possible to distinguish the emission current and the pyrocurrent caused by electric polarization. A pyroelectric current was found in selenide solid solutions below the percolation concentration, as well as upon substitution with holmium ions. In manganese sulfides replaced by thulium, deep traps are formed, which are a source of thermionic current.

Acknowledgements

The study was supported by a grant from the Russian Science Foundation N 23-22-10016, the Krasnoyarsk Regional Science Foundation.

Sobre autores

Maksim Sitnikov

Reshetnev Siberian State University of Science and Technology

Email: kineru@mail.ru

Cand. Sc., Associate Professor of the Department of Physics

Rússia, 31, Krasnoyarskii rabochii prospekt, Krasnoyarsk, 660037

Anton Kharkov

Reshetnev Siberian State University of Science and Technology

Autor responsável pela correspondência
Email: khark.anton@mail.ru

Cand. Sc., Associate Professor of the Department of Physics

Rússia, 31, Krasnoyarskii rabochii prospekt, Krasnoyarsk, 660037

Sergey Aplesnin

Reshetnev Siberian State University of Science and Technology

Email: aplesnin@sibsau.ru

Dr. Sc., Professor of the Department of Physics

Rússia, 31, Krasnoyarskii rabochii prospekt, Krasnoyarsk, 660037

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