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Influence of hydration on the dielectric properties of the high permittivity material Rb 2 Ti 2 O 5
Sofia de Sousa Coutinho, David Bérardan, Nita Dragoe, Stéphane Holé, Brigitte Leridon
To cite this version:
Sofia de Sousa Coutinho, David Bérardan, Nita Dragoe, Stéphane Holé, Brigitte Leridon. Influence
of hydration on the dielectric properties of the high permittivity material Rb 2 Ti 2 O 5. 2020 IEEE
3rd International Conference on Dielectrics (ICD), IEEE, 2021, pp.598-600. �hal-03055024�
Influence of hydration on the dielectric properties of the high permittivity material Rb 2 Ti 2 O 5
Sofia De Sousa Coutinho*1, David Bérardan2, Nita Dragoe2, Stéphane Holé1 and Brigitte Leridon1
1 LPEM, CNRS, PSL Research University, ESPCI-Paris, Sorbonne Université – 10 rue Vauquelin – 75005 Paris - France
2 ICMMO, CNRS, Université Paris-Saclay – rue du doyen Georges Poitou – 91405 Orsay cedex – France
*Email: sofia.de-sousa-coutinho@espci.fr
Abstract- Previous results have reported a huge equivalent relative permittivity of up to 109 for the solid electrolyte material Rb2Ti2O5 (RTO) together with a high ionic conductivity between 200 K and 300 K when placed under voltage between two metallic electrodes. More recently, space charge measurements have highlighted the strong water content dependence on the charge accumulation taking place at the anode interface inside the RTO material. In this paper, dielectric spectroscopy measurements were carried out in order to better understand the effect of hydration on the dielectric properties of RTO.
I. INTRODUCTION
In the literature, many perovskite-type oxides have been reported to exhibit high ionic conductivity in humid atmospheres [1, 2]. The water incorporation into these oxygen vacancy-dominated oxides leads to protons introduction as hydroxide defects (𝑂𝐻)𝑂∙ as follows:
𝐻2𝑂 + 𝑉𝑂∙∙+ 𝑂𝑜𝑥⇌ 2(𝑂𝐻)𝑂∙, (1) with 𝑉𝑂∙∙ the oxygen vacancies and 𝑂𝑜𝑥 the oxygen sites in the RTO matrix, according to the notation of Kröger and Vink [3]. Previous results have reported a huge equivalent relative permittivity of up to 109 for the solid electrolyte material Rb2Ti2O5 (RTO). This material exhibit also a high ionic conductivity between 200 K and 300 K [4]. Recently, the strong water content dependence on the charge accumulation taking place inside the RTO material has been highlighted.
Placed between two metallic electrodes and submitted to an applied voltage, the system presents an accumulation of negative charges at the anode interface which has been attributed to hydroxide ions [5, 6]. The accumulation of hydroxide ions seems to be at the origin of the high permittivity observed in RTO.
In order to improve the understanding of the influence of hydration on the dielectric properties of Rb2Ti2O5, dielectric spectroscopy measurements were carried out on both as- grown and 350°C-dehydrated RTO ceramic samples. In this paper, samples preparation and measurement setup are described in Section II. Experimental results are presented and discussed in Section III before conclusion.
II. SAMPLES AND EXPERIMENTAL SETUP
Crystallites of Rb2Ti2O5 were obtained using a standard solid-state reaction route. Starting powders of TiO2 (> 99.8%)
and RbNO3 (> 99.9%) were weighed in stoichiometric proportion, mixed and heated at 930°C for 3 hours in a platinum crucible under air and at atmospheric pressure.
Several temperature stages have to be followed in order to obtain RTO crystals [4]. RTO ceramic samples were obtained from as-grown crystals ground and dehydrated separately under O2 atmosphere. Two different RTO ceramic samples were measured: as-grown one and 350°C-dehydrated one.
The two RTO ceramic samples were prepared in glove box under N2 atmosphere by applying a pressure of 6 tons/cm2 in a 13-mm diameter die. Gold electrodes were then vacuum deposited and aluminum foils were stuck on both sides using silver lacquer to form the samples.
Dielectric spectroscopy measurements were performed using Solartron 1296 Dielectric interface and 1260 impedance/gain analyzer both controlled by SMaRT software. Real and imaginary parts of equivalent capacitance were measured as a function of frequency from 10−1 Hz to 106 Hz under 100 mV ac-voltage and the equivalent relative permittivity was inferred from the capacitance knowing the thickness and the area of each RTO ceramic samples. Sample measurements were performed in a glove box under N2
atmosphere at about atmospheric pressure and room temperature in order to maintain the sample dehydration state. A typical RTO ceramic sample structure is depicted in Fig.1.
III. EXPERIMENTAL RESULTS AND DISCUSSION
Dielectric spectroscopy measurements are reported in Fig.2. The real and imaginary parts of the equivalent relative permittivity are plotted as a function of frequency for both the as-grown and the 350°C-dehydrated RTO ceramic samples.
A typical dielectric behavior is found for the as-grown sample, as reported in [4], with a maximum value of equivalent relative permittivity of 108 at low frequency. For the 350°C-dehydrated sample, the real and imaginary parts of the equivalent relative permittivity is found to follow a similar behavior but shift towards low frequencies by a factor of about 104. However, a high value of equivalent relative permittivity is still reached at lower frequency. It can be expected from the upward slope of the curve that a maximum value of the equivalent relative permittivity will be reached at frequencies of about 104 -105 Hz for this sample.
After measuring the 350°C-dehydrated RTO ceramic sample into glove box, the sample is taken out and
permittivity measurements were performed in air for 9 days.
Results are shown in Fig.3.
Fig. 1. Typical structure of a RTO ceramic sample. Gold electrodes are vacuum deposited on both sides of the RTO ceramic before aluminum foils
were pasted using silver lacquer. Samples are typically 1 to 2 mm thick.
Fig. 2. Comparison of the real (blue dots) and the imaginary (red dots) parts of the equivalent relative permittivity as a function of frequency under 100
mV ac-voltage for the as-grown and for the 350°C-dehydrated RTO ceramic samples.
Fig. 3. Comparison of the real (blue dots) and the imaginary (red dots) parts of the equivalent relative permittivity as a function of frequency under 100
mV ac-voltage for the as-grown sample and for the 350°C-dehydrated sample after being exposed to air for 9 days.
The real and imaginary parts of the equivalent relative permittivity are displayed as a function of frequency for the as-grown sample and for the 350°C-dehydrated sample after being exposed to air for 9 days. Both curves show the same dielectric behavior in respect with frequency. It clearly appears that the rehydration of the 350°C-dehydrated sample allows to recover a similar dielectric behavior with that obtained for an as-grown sample.
From these two measurements, it comes that water has a high influence on the dielectric properties of Rb2Ti2O5
material. In Fig. 2, the shift towards low frequency of the equivalent relative permittivity of the 350°C-dehydrated sample compared to the as-grown one point towards the role of water in the implementation of ionic conduction in RTO material. In absence of sufficient water inside the material, the ionic conductivity seems to be harder to initiate leading to a shift towards low frequency of the equivalent relative permittivity. From Fig. 3, it comes that the water incorporation through the 350°C-dehydrated RTO sample leads to an increase of the equivalent relative permittivity over air exposure time. The same as-grown dielectric behavior is found after 9 days of time exposure to air.
CONCLUSION
In the literature, the effect of water incorporation in oxides properties has always been intensively investigated in perovskite-type oxide materials. Using dielectric spectroscopy measurements, the water content dependence of the dielectric properties of Rb2Ti2O5 material has been investigated. Results show a shift towards low frequency of the equivalent relative permittivity for a 350°C-dehydrated sample compared with an as-grown one. The rehydration of the 350°C-dehydrated sample leads to recover the same dielectric properties as for the as-grown sample. One can propose that the ionic mechanism and thus the resulting accumulation of charges inside RTO material is water- mediated and controlled. Further measurements are ongoing in order to clarify the adsorption and/or dissociation mechanism of water inside the material.
REFERENCES
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[2] J-H. Yu, J-S. Lee, and J. Maier. “Water incorporation in oxides: A moving boundary problem,” Solid State Ionics, vol. 181, pp.154-162, February 2010.
[3] F. A. Kröger and H. J. Vink, “Relations between the concentrations of imperfections in solids,” Journal of Physics and Chemistry of Solids, vol. 5, pp.208-233, May 1958.
[4] R. Federicci, S. Hole, A. F. Popa, L. Brohan, B. Baptiste, S. Mercone, and B. Leridon, “Rb2ti2o5: Superionic conductor with colossal dielectric constant,” Physical Review Materials, vol. 1, 2017.
[5] S. De Sousa Coutinho, R. Federicci, S. Holé, and B. Leridon, “High ionic conductivity in the solid electrolyte material rb2ti2o5,” 2018 IEEE 2nd International Conference on Dielectrics (ICD) IEEE, July 2018.
[6] S. De Sousa Coutinho, R. Federicci, S. Holé, and B. Leridon, “Virtual cathode induced in rb2ti2o5 solid electrolyte,” Solid State Ionics, Vol.
333, pp.72-75, May 2019.
