Insights into the crystal structure and multifunctional optical properties of A2CdTeO6 (A=Ba, Sr, Ca) double perovskites

Abstract

Crystal structure and multifunctional optical properties of A-site substituted double perovskite compounds in A2CdTeO6 (A=Ba, Sr, Ca) system are reported in this work. These double perovskites are synthesized through high-temperature solid-state reaction route, and the crystal structure is determined using XRD, Raman and FTIR spectroscopy. The exact crystal structure is investigated by analyzing the observed number of vibrational modes in accordance with the group theoretical predictions. Even though the reported symmetry of Ba2CdTeO6 was cubic, the observed number of Raman modes corresponds to tetragonal symmetry with I4/m space group. Further vibrational analysis also points out the compounds Sr2CdTeO6, and Ca2CdTeO6 belongs to monoclinic symmetry with space group P21/n. The crystal symmetry of A2CdTeO6 (A = Ba, Sr, Ca) is further confirmed by Rietveld refinement of XRD patterns. From diffuse reflectance spectroscopy (DRS), the optical band gaps of Ba2CdTeO6, Sr2CdTeO6 and Ca2CdTeO6 were calculated as 3.3 eV, 3.8 eV and 3.9 eV respectively which correspond to wavelengths of absorption in the UV region indicating the possibility of these tellurates as suitable host lattices for novel phosphor materials. It is observed that the band gap is increased with decrease in Cd-O-Te bond angle which indicate the narrowing of conduction band with respect to octahedral tilting. Photoluminescence studies reveal that Eu3+ substituted Sr2CdTeO6 shows intense red emission under 394 nm excitation, and the dominance of electric dipole transition (5D0-7F2) over magnetic dipole transition (5D0-7F1) indicates the substitution of Eu3+ ions at the asymmetric A-site of the host. Concentration dependant PL properties show the optimum concentration of Eu3+ ions is about x = 0.14 and the critical distance for quenching effect of Eu3+ions is calculated as 9.75 Å, indicating the energy transfer mechanism involved is electric multipolar interaction. Obtained CIE color coordinates (0.643, 0.346) is close to the standard red color, and the low value of CCT (2572 K) ensure warmer red light. PL emission intensity of 0.14 Eu3+ doped Sr2CdTeO6 shows a decreasing trend with increase in temperature which indicates the potentiality of Eu3+ doped sample for temperature sensing applications. Relative sensitivity calculated for Sr2CdTeO6: 0.14 Eu3+ is 0.28 %K−1. Furthermore Judd-Ofelt intensity parameters Ωλ (λ = 2, 4) are calculated and the larger value of Ω2 compared to Ω4 confirms that Eu3+ ions occupy the asymmetric A- site of Sr2CdTeO6 host. Hence, Eu3+activated Sr2CdTeO6 can be treated as a suitable material for both photoluminescence and temperature sensing applications.