Tellurium oxide (TeO₂) due to its glass-forming characteristics, highly non-linear optical properties, good chemical resistivity, low UV- IR absorption, etc. are widely used in a variety of application like acoustic optical devices, radiation detectors, gas sensors. Apart from these applications very little studies emphasize on the dielectric property and its application. Prime aspect for the interest of this material is that the dielectric constant of about 17-25 have been reported for α-TeO₂. Such a high dielectric constant may be considered as a potential characteristic sprouting the possibility of TeO₂ in the microelectronic industry. In the present study (In_0.5Nb_0.5) _x Te_1-xO₂ (x=0, 1, 3, 5 %) ceramics were prepared by the solid-state method by co-doping indium oxide and niobium pentoxide with TeO₂ and we studied the composition dependence of doping on the dielectric properties (e.g., permittivity, dielectric loss, temperature, and frequency stability) of TeO₂ ceramic. The crystal structure and vibrational modes were determined by XRD and Raman spectra respectively. SEM micrographs were used to view the morphology of the sample. XRD and Raman data shows the co-existence of the tetragonal phase of (In_0.5 Nb_0.5) _x Te_1-xO₂ with space group P4₁2₁2 along with highly symmetric cubic phase of In_0.5 Nb_0.5 Te₃O₈ with space group Ia-3 in the doped samples.

From the quantification of XRD data, it is evident that by increasing the doping concentration evolution of the cubic phase is enhanced. Also, decrease in intensity of major Raman vibration modes related to TeO4 unit and the existence of new modes of vibration indicates the new bonding bridges formed which in turn has an effect on the basic structure of TeO₂. Dielectric measurement of co-doped samples was carried out over frequency range 500 Hz to 1 MHz and temperature range 90 K to 420 K. Dielectric measurement shows a relevant enhancement of the dielectric constant for co-doped samples with ten times decline in loss (tan δ). As shown in the figure, it can be observed that with 1%, 3%, 5% doping there is an enhancement in ε value are 21.5, 22 and 27.5 respectively (at room temperature) with respect to un-doped TeO₂. (ε =21). Also, the temperature and frequency dependence of dielectric constant shows a stable response. The incorporation of the cubic phase does influence the dielectric constant of the co-doped TeO₂. By increasing the doping concentration, there is a proportional increase in the cubic phase and from XRD correspondingly the characteristic peak for TeO₂ tetragonal phase gradually shifts towards larger diffraction angles indicating the reduction of unit cell volume [1]. This observation is a clear association reciting to the relation between the lattice constant from the XRD peaks and the dielectric constant of the co-doped TeO₂. From the Clausius-Mossotti relation, it is can be interpreted that the dielectric constant enhancement through the cubic phase appearance accounts from the molar volume shrinkage [2].

Figure 1. Dielectric constant ε and tan δ of (In_0.5 Nb_0.5) _x Te_1-xO₂ the samples as a function of indium and niobium co-doping.

Reference:

[1] Tomida K, Kita K, Toriumi A (2006) Dielectric constant enhancement due to Si incorporation into HfO₂. Appl Phys Lett. 89:142902

[2] Shi F et al (2017) Crystal structure characteristics, dielectric properties and vibrational spectra of Nb-rich non-stoichiometric Ba[(Zn_1/3Nb_2/3)1−xNb_x] O₃ ceramics J. Mater. Sci., Mater. Electron. 2811455–63