Structural phase transition and its consequences for the optical behavior of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>LaV</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Nb</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>
Hemanshu Dua, Rishabh Shukla, R. S. Dhaka
Abstract
We present the structural, electronic, vibrational, and photoluminescence properties of polycrystalline ${\mathrm{LaV}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{4}$ $(x=0--0.2)$ samples at room temperature. The substitution of Nb at the V site shows the fascinating structural and optical behavior due to their isoelectronic character and larger ionic radii of ${\mathrm{Nb}}^{5+}$ as compared to the ${\mathrm{V}}^{5+}$. The Rietveld refinement of x-ray diffraction patterns demonstrate that the $x=0$ sample exists in a monoclinic $(P{2}_{1}/n)$ phase, whereas for the $x>0$, both monoclinic and scheelite-tetragonal $(I{4}_{1}/a)$ phases coexist in a certain proportion. Interestingly, a monotonous enhancement in the Raman spectral intensity with Nb substitution is correlated with the substitution induced increase in the scheelite-tetragonal phase. The x-ray absorption measurements reveal that the La ions exist in a trivalent oxidation state, while V and Nb cations possess $5+$ oxidation state in tetrahedral coordination. Moreover, the Fourier-transform infrared (FTIR) spectra indicate that the Nb substitution give origin to some additional IR modes owing to the deformation of the ${\mathrm{VO}}_{4}^{3\ensuremath{-}}$ tetrahedra and mixing of monoclinic and tetragonal phases. The photoluminescence measurements on these samples exhibit broadband spectra and their deconvolution designate the availability of more than one electron-hole pairs recombination center.