Physical and Dielectric Properties of Polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$

TL;DR

Nb5+ substitution at the V5+ site in LaVO4 drives structural, vibrational, and dielectric changes in $LaV_{0.5}Nb_{0.5}O_4$. The authors synthesize the material by solid-state methods at two sintering temperatures (1000°C and 1250°C) and characterize phase composition, microstructure, vibrational modes, and electronic/dielectric properties using XRD/Rietveld, SEM/TEM, FTIR, Raman, XPS, UV-Vis DRS, and impedance measurements. A dominant shift from mixed monoclinic P2_1/n and tetragonal I4_1/a phases to ~96% tetragonal I4_1/a at 1250°C is observed, accompanied by band-gap narrowing from $E_g oughly 3.2$ eV to $E_g oughly 2.7$ eV and improved dielectric performance (higher $epsilon_r$ and lower loss). The work establishes sintering-temperature as a practical lever to tailor the structural, optical, and dielectric properties of LaV0.5Nb0.5O4 for potential luminescent and dielectric applications.

Abstract

We report a detailed investigation of the structural, electronic, vibrational, and dielectric properties of polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$ samples, prepared at two sintering temperatures (1000\degree C and 1250\degree C). The introduction of Nb$^{5+}$ at the V$^{5+}$ site leads to notable structural and vibrational changes, which can be attributed to their isoelectronic nature and the comparatively larger ionic radius of Nb$^{5+}$. The Rietveld refinement of the X-ray diffraction patterns confirms a coexistence of monoclinic ($P$2$_{1}$/$n$) and scheelite-type tetragonal ($I$4$_{1}$/$a$) phases; for example, with a fraction of 4\% and 96\% for the sample annealed at 1250\degree C. The particle morphology has altered from spherical (1000\degree C) to irregular-shaped (1250\degree C) as a result of increase in annealing temperature. The Raman spectroscopy, Fourier Transform Infrared spectroscopy and X-ray Photoemission Spectroscopy have been used to understand the vibrational and electronic properties. An optical band gap of 2.7~eV for the sample sintered at 1250\degree C is calculated using Ultraviolet-vis diffuse reflectance spectroscopy measurements. The dielectric studies shows the higher dielectric permittivity ($ε$$_{r}$) and lower dielectric loss for the sample annealed at 1250\degree C.

Figures (8)

• Figure 1: The room temperature XRD patterns with Rietveld refinement of (a) LVNO-1000 and (b) LVNO-1250 samples. The open red circles, black solid line, and blue solid line exhibits the experimental, calculated, and the difference between experimental and calculated pattern, respectively. The vertical green and purple markers show the Bragg positions corresponding to the $P$2$_{1}$/$n$ and $I$4$_{1}$/$a$ space groups.
• Figure 2: The FE-SEM images of the LVNO-1000 sample at (a) 1$\mu$m, (b) 500 nm and (c) probed region of scan (at 5$\mu$m) with the corresponding elemental mappings of all elements; the FE-SEM images of the LVNO-1250 sample at (d) 1$\mu$m, (e) 500 nm and (f) probed region of scan (at 5$\mu$m) with the corresponding elemental mappings of all elements; the HR-TEM images of (g) sintered at 1000° C and (h) sintered at 1250° C with the marked $d$-spacings across the selected region; the SAED patterns for the LVNO-1000 (i) and LVNO-1250 (j), respectively.
• Figure 3: (a) The FTIR spectra, (b) the UV-vis DRS plots and (c) the Kubelka-Munk plots obtained from the DRS data for the LVNO-1000 and LVNO-1250 samples, respectively.
• Figure 4: The room temperature Raman spectra of LVNO sample, (a, b) sintered at 1000° C and (c, d) for sample sintered at 1250$\degree$C in the low and middle wavenumber regions, respectively; fitted using the Voigt peak function and solid thick black line presents the total fit of the measured spectra.
• Figure 5: (a) The room temperature XPS survey spectrum; the core level spectra of (b, f) Nb 3$d$; (c, g) V 2$p$; (d, h) La 3$d$ and (e, i) O 1$s$ elements of the LVNO sample sintered at 1000° C (b, c, d, e) and 1250° C (f, g, h, i), respectively.