Terahertz response of Er3+/Yb3+ co-doped La2Zr2O7 synthesized using the sol-gel precipitation method

Oxide ceramics with pyrochlore structure such as La2Zr2O7 are promising materials for advanced multifunctional applications including extreme environment, such as space applications. They combine low thermal conductivity, suitable coefficient of thermal expansion to join supporting materials and have optical response in the UV–VIS region with up-converting and down-converting properties after doping, required for coating, sensors, and electro-optic applications. The effect of doping elements such as Er3+ and Yb3+, replacing isomorphically lanthanum in the La2Zr2O7 with pyrochlore cubic structure, on material photophysical properties was investigated by selective doping, in the molar ratio of Er3+/Yb3+ ranging from 0.5 to 5. The reflectance spectra of materials prepared by sol-gel co-precipitation method after sintering at 1400 °C, investigated in the UV–VIS–NIR range showed as expected well resolved absorptions due to the 4f–4f intra-configurational transitions. At the same time, dielectric properties of sintered samples were investigated by terahertz time domain spectroscopy. The refractive indices observed in the range of ~4.46–5.15, indicate high dielectric permittivity at THz frequencies applicable in energy storage materials or THz communication components. For all samples a broad absorption band was found in the THz region and the band intensities were clearly associated with the doping amount of Er and Yb cations into the La2Zr2O7 structure, at frequencies 0.75–1.02 THz. Significant correlation at the level of R2 = 0.911 was found between the absorption band corresponding to the 4I15/2 → 2H11/2 transition at 520.5 nm and the integrated intensities of THz bands. Dielectric ceramic materials such as titanium-, tantalum- and niobium- oxide based ceramics have permittivity in the range 20–30. The permittivity of studied La2Zr2O7 was ~20 and increased after doping by Er3+/Yb3+ to ~21–26.