Project supported by the National Natural Science Foundation of China (Grant No. 51571006).
Project supported by the National Natural Science Foundation of China (Grant No. 51571006).
† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 51571006).
The ceramics La0.85Cr0.15TiOx and La0.7Cr0.3TiOx are prepared by conventional solid-state reaction method. The dielectric properties of Cr-doped LaTiOx as a function of frequency (0.1 kHz ≤ f ≤ 1 MHz) and temperature (77 K ≤ T ≤ 360 K) are studied. The blocks are then annealed in a flowing O2 or Ar/H2 to convert their oxygen content and the tests mentioned above are performed. The highly oxygenated samples exhibit extremely high low-frequency dielectric constants at room temperature (∼106). The results show that the oxygen stoichiometry could significantly influence the dielectric properties of Cr-doped LaTiOx.
Perovskite oxides have been extensively studied in recent years due to their various exotic properties. This is mainly because of their superior properties in dielectric, ferroelectric, piezoelectric, superconductivity, etc.[1–4] Lanthanum titanium oxide (LTO), as an important titanate material, has attracted a great deal of researchers’ attention recently.[5] Many researchers have studied LTO materials from different perspectives. For example, Schmehl et al. reported the transport properties of LaTiOx films and heterostructures.[6] Schmitz et al. discussed the effect of the experimentally observed Jahn-Teller distortion of the oxygen octahedronin LaTiO3 on the magnetic exchange.[7] Lunkenheimer studied the dielectric properties and dynamical conductivity of LaTiOx from dc to optical frequencies.[8]
Perovskite has the flexibility of changing the effective negative charges by performing doping substitution on the cationic sites. This class of compound can go through a wide range of structural phase transition, which can strongly affect its physical and chemical properties. Among various types of titanate perovskites, LTO is considered to be a very promising material.[9,10] Cu doping in LTO can not only improve its dielectric properties, but also enhance the Fenton degradation of rhodamine B.[11,12] The Sr doping lowers the conduction band and reduces the band gap of La1−xSrxTiO3.[13] As can be seen, the enhancement in total conductivity of the LTO perovskite can be achieved only at lower doping concentration. The opposite effect is observed for higher doping concentration.[14]
The Cr is an important doping element commonly used to improve the properties of the material.[15–17] It has been verified by first-principles calculations that Cr doping improves the conductivity of LiFePO4.[18] The Cr-doped SrTiO3 single crystals exhibit current-driven insulator–conductor transition characteristics which can be used for fabricating the next-generation non-volatile memory devices.[19] In Li3V2−xCrx(PO4)3/C, the initial specific capacity decreases with the decrease in Cr content at a lower current rate. Under the condition of the moderate Cr doping, both the cycle performance and rate performance have been improved.[20]
Annealing in different atmospheres may change the oxygen content of the material, thereby changing the properties of the material. After deposition annealing, the permittivity and the dielectric Q (1/tanδ) of single-phase thin film of Ba0.5Sr0.5TiO3 showed significant changes.[21] Titanium oxide films have better electrical properties after being annealed in oxygen.[22] Polycrystalline La0.67Ba0.33MnO3 bulk samples, which were obtained under the annealing conditions at 700 °C in the flowing 95%Ar:5%H2(Ar/H2) mixed gas or a different time, have significantly different insulator–metal transition temperature and AC magnetic susceptibility.[23] Oxygen content can significantly affect the dielectric properties of TiO2 crystals.[24]
In this study, the Cr-doped LaTiOx is synthesized using the solid-state method and the samples are annealed in O2 or Ar/H2 to change the oxygen content. The effects of Cr-doped ratio and oxygen content on the dielectric properties of Cr-doped LaTiOx are systematically investigated.
The ceramics La0.85Cr0.15TiOx and La0.7Cr0.3TiOx were prepared by solid-state reaction. The starting powders La2O3, Cr2O5, and TiO2 (with purity > 99.99%) were mixed as required and fired at 1450 °C for 600 min in the air with intermediate grinding in an agate mortar. The grinding and sintering process was repeated three times. Then the mixture was reground, pressed into pellets of 8 mm in diameter and sintered at 1520 °C in the air for another 600 min followed by surface cooling. In order to explore the influence of oxygen content, the samples were annealed at 700 °C in flowing O2 or Ar/H2 mixed gas for 60 min and then cooled down.
The structural analysis of the pellet was checked by x-ray diffraction (XRD) using Cu-Kα radiation with 2θ ranging from 20° to 80°. The chemical compositions and chemical states of the as-prepared pellets were characterized by x-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Scientific). The silver paste was coated on both sides of the disk as an electrode to test the dielectric properties. The dielectric properties of bulks were measured by a precise impedance analyzer 6500B (Wayne Kerr corp.). The temperature was automatically monitored and controlled by LakeShore temperature controller (Model: 331) with temperature ranging from 77 K to 360 K. Data measurement frequency was in a range from 0.1 kHz to 1 MHz.[25–32]
The XRD spectra of Cr-doped LaTiOx composites at room temperature are shown in Fig.
The x-ray photoelectron spectrum (XPS) analysis is also performed to explore the surface chemical compositions of the Cr-doped LaTiOx samples. The O 1s core level spectra of the prepared samples are displayed in Figs.
Figures
The Cr 2p spectra of the samples are exhibited in Fig.
The high-resolution XPS spectra of Ti2p are shown in Fig.
Finally, the different annealing atmospheres can affect oxygen concentration of the ceramic. However, the valence of oxygen ion is not affected. As shown in Fig.
The dielectric constant is a characteristic of material insulation, which indicates the ability of a substance to hold the charge. Figure
![]() | Fig. 6. (color online) Temperature-dependent ε′ and tanδ values of ((a), (c)) La0.85Cr0.15TiOx and ((b), (d)) La0.7Cr0.3TiOx in temperature range from 77 K to 360 K for different frequencies. |
Figures
It can be concluded that the dielectric constant ε′ increases as the concentration of doped chromium increases. The dielectric loss is also significantly affected by the Cr doping ratio. This may be because when chromium ions replace lanthanum ions, the conductivity and the polarizability change, affecting the dielectric constant of the sample.[43]
The Cr-doped LaTiOx is oxidized when annealed in oxygen, which may affect the crystal structures of some types of perovskites, and influence the physical and chemical properties.[21] The bulks are annealed in oxygen and tested as mentioned above. Figures
In conclusion, the dielectric properties of Cr-doped LaTiOx can be improved after being annealed in oxygen. It may be due to the fact that the number of activated carriers increases and the structure changes with rising oxygen content.[45–47] The high value of the dielectric constant may be attributed to the close-packed structure of oxygen ions. Oxygen ions with negative charges, because of two excess electrons, are more loosely bound to the nucleus than in a neutral atom or positive ion, so that the ion is readily distorted by an electric field. Also because of the close-packed structure, the number of polarized ions per unit volume is high, so when an electric field was applied, a high polarization increases, which results in a high dielectric constant.[24] In order to demonstrate the effect of oxygen content on the material property, the oxygen content of the material is reduced for further investigation.
The samples are reduced when annealed in Ar/H2 mixed gas. The oxygen content of the material became lower, which may affect the physical and chemical properties.[21] In order to study the effect of oxygen-deficient on the dielectric property, the samples are annealed in Ar/H2 and tested as done above. Figures
A comparison of Fig.
A series of distinct phases with variable oxygen stoichiometry exists in the Cr-doped LaTiOx system. Starting with fully oxidized compounds, the oxygen content is then reduced and the perovskite-related structure changes. The structural and physical properties vary drastically as oxygen quantity becomes stoichiometric.[47] In the Ar/H2-annealed samples, due to the low oxygen content, vacancies are formed in the samples. Charges trapped by the vacancies cannot participate in the long-distance conduction, but form a space charge. Many oxygen vacancies exist in the sample with low oxygen content, which can capture carriers. At lower voltage, most of the charges injected from the electrode fill the vacancies and cannot participate in the electrical conductivity resulting in a decrease in dielectric constant. For samples with high oxygen content, the oxygen vacancies are less and the oxygen ions are tightly linked, most of the charges are involved in the conductance. Due to the close-packed structure of oxygen ions, the number of polarizable ions is high. When an electric field is applied, a high polarization is established, resulting in a high dielectric constant. Space charges limit the current, thus dielectric loss varies with oxygen content. The current density decreases as the oxygen content decreases, resulting in a decrease in dielectric loss. This also shows that under the experimental conditions, the dielectric loss is determined mainly by the macroscopic charge transfer.[24,48]
The oxygen ions (O2 −) act as a bridge between Ti3+ and Ti4+ and play an essential role in the Ti3+–O2–Ti4+ superexchange interaction. Naturally, the extent to which the number of oxygen ions changes makes the crystal lattice structure collapse down, so the dielectric property of the material changes significantly.
In summary, doped Cr ion concentration can significantly influence the dielectric property of the LTO. The doping ratio increases from 15% to 30%, the dielectric constant is increased by almost an order of magnitude. Annealing in flowing O2 or Ar/H2 can change the oxygen content and influence the dielectric properties of the samples. The sample with high oxygen content shows higher dielectric constant, which means that O2 annealing is an effective method to improve the dielectric characteristics of Cr-doped LTO.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] | |
[34] | |
[35] | |
[36] | |
[37] | |
[38] | |
[39] | |
[40] | |
[41] | |
[42] | |
[43] | |
[44] | |
[45] | |
[46] | |
[47] | |
[48] |