Low loss and high dielectric constant materials are essentially desired for wide varieties of applications in electronics and communication technology. Herein the structure and dielectric properties of two titanosilicates, Ln2Ti2SiO9, for Ln = Pr3+ and Nd3+ are reported. Both materials are isostructural and have layered structure with layers of [LnTi2SiO9]3- and Ln3+ ions. Electrical properties of both have been analyzed by using the temperature and frequency dependent permittivity, loss, conductivity and modulus data. At room temperature, appreciable relative permittivity (35 for Nd2Ti2SiO9 and 22 for Pr2Ti2SiO9 over the frequency range of 100Hz to 5MHz) and low dielectric loss (like 0.007-0.03 at 100Hz and 10-4-10-3 at 1MHz). Analysis of dc-conductivity data of both compounds indicate that the conduction in both materials is due to the correlated barrier hopping (CBH) of polarons. The relaxation peak of modulus spectra indicates more non-Debye like relaxation in Pr2Ti2SiO9 than that in Nd2Ti2SiO9, and that is possibly due to increased correlation in the dynamics of hopping polarons in Pr2Ti2SiO9.

The present work reports detailed structural and electrical studies on A -site substituted La x Gd 1-x InO 3 (0.0 1.0) system with structurally different end members in search for tailored lead-free relaxors. The motivation is the possibility to manipulate the crystal structure to maneuver electrical properties in hexagonal GdInO 3 , a candidate for geometric ferroelectricity. The hybrid synthesis protocol involving gel -combustion led to stabilisation of metastable polymorphs in some nominal compositions due to kinetic stabilisation. Non -preference of La 3+ for 7- fold coordinated A -site in hexagonal structure manifested in wide-ranging orthorhombic phase field prevailing even in Gd 3+ -rich region, also supported by theoretical studies. P -E studies on Gd 3+ -rich compositions exhibited hysteresis loop which suddenly became narrow for the nominal composition La 0.4 Gd 0.6 InO 3 with very low remanent polarisation. This along with broad maximum exhibited in plot of dielectric constant vs temperature that shifts to higher T upon increasing frequency, for La 0.4 Gd 0.6 InO 3 , supports presence of relaxor-type behavior. Signature of hexagonal -type vibrations in otherwise bulk orthorhombic (by XRD) La 0.4 Gd 0.6 InO 3 and anomalous trend in La/Gd vibrational mode along with narrowing down of Raman modes have been cited as plausible structural reasons for the relaxor behaviour. This has significance in context of structure -electrical property relationship leading to development of potential lead-free relaxors.

Herein we reported the crystal structure and crystal chemistry of orthorhombic perovskite type Nd2CuTiO6 in between 2 K and 290 K as observed from the in situ temperature-dependent powder neutron diffraction (PND) studies. It is observed that the cations in octahedral sites are statistically occupied, and the ambient temperature orthorhombic structure is retained throughout the temperature range of the study. Absence of any long-range magnetic ordering down to 2 K is confirmed by both low-temperature PND and magnetization studies. The lattice shows strong anisotropic thermal expansion with increasing temperature, viz. almost no or feeble negative expansion along the a-axis while appreciably larger expansion along the other two axes (αb = 10.6 × 10−6 K−1 and αc = 9.8 × 10−6 K−1). A systematic change in the rotation of octahedral units with temperature was observed in the studied temperature range, while the expansion of unit cells is predominantly associated with the polyhedral units around the Nd3Ions. The temperature-dependent relative change in unit cell parameters as well as coefficients of axial thermal expansion show anomalous behavior at lower temperatures, and that seems to be related to the electronic contributions to lattice expansion.

Dielectric studies were carried out on Nd2CuTiO6 samples prepared by high temperature solid-state reaction followed by annealing under different oxygen partial pressures. X-ray diffraction studies revealed that the orthorhombic (Pnma) structure of Nd2CuTiO6 is retained in all the samples. The room temperature permittivity at 100Hz was found to be about 200 in the air-annealed sample, and it increased considerably to about 3000 upon annealing in argon atmosphere. The electrical conductivity in the argon-annealed compound was found to be an order of magnitude higher than that for the air-annealed sample. On annealing in oxygen, both permittivity and electrical conductivity of the sample were found to be significantly reduced. The variation in dielectric properties could be attributed to the induction or annihilation of mixed valence state of copper and oxygen vacancies, which was supported by X-ray photoelectron and diffuse reflectance spectroscopy data. The analysis of temperature and frequency dependent conductivity and dielectric data showed that the overlapping large polaron tunnelling (OLPT) mechanism is responsible for the observed dielectric relaxations and conduction. The observed systematic dependence of properties on the oxygen partial pressure of the annealing environment suggests a simple method to tune the electrical properties of Nd2CuTiO6.