Microwave dielectric properties of Li2SrTa2(1−x)Nb2xO7 ceramics investigated by Raman spectroscopy

We investigated the crystal structure and microwave dielectric properties of the Li2SrTa2(1−x)Nb2xO7 (x=0−1.0) compounds using Raman spectroscopy. The Raman spectra clearly exhibit the orthorhombic structure (Cmcm space group) of these compounds. The Raman mode at 800 cm−1 corresponds to the B-O3 (B=Nb and Ta) bond and shifted towards the higher frequency with increase of Nb content. The increase in separation between the two modes at 589 and 596 cm−1 gives an indication of change in the difference between B-O1 and B-O2 bond lengths. The microwave dielectric properties of these compounds were found to be strongly dependent on the BO6 octahedra. The peak width of Ag (800 cm−1) mode and quality factor (Q×f) is inversely proportional to each other and the shift of this mode is correlated to dielectric constant (εr). Further analysis of the Raman modes at 800, 596 and 589 cm−1 (due to the BO6 octahedra) indicates the enhancement of octahedral distortion with increase in Nb content, which led to increase of temperature coefficient of resonant frequency (τƒ).