Temperature-induced crystallinity and vibrational properties in samarium orthovanadate

The Samarium orthovanadates particles were prepared using the sol-gel method, and the effect of sintering temperature on the crystallinity was studied in detail through x-ray diffraction (XRD) and the Rietveld refinement of the obtained XRD data. Density functional theory (DFT) based calculations were used to describe the dynamic properties in Samarium orthovanadate. The data obtained from the DFT calculations were made instrumental in confirming the vibrational properties, and both the Raman and infrared modes obtained through the experiments. Temperature-dependent Raman spectroscopy was used to provide a deeper insight into all the eigenmodes and their behavior with varying temperature. A temperature-dependent (experiment) and pressure-dependent (DFT) Raman spectra were produced for the investigation of eigenmodes in the samples. A correlation between the two methods was seen, and the alignment between the two data sets was found to be ineluctable. The Grüneisen parameter of individual modes were calculated with the aid of the theoretically obtained variation in the Raman shift with respect to the bulk modulus. Ultraviolet-visible spectroscopy of the samples was explored to study possible correlation with temperature, and theoretical bandgaps were obtained through standard DFT and hybrid exchange-correlation functional calculations. Photoluminescence spectroscopy was performed in the hopes of unveiling possible applications of the material.

Figure 1

FESEM images of SV1 (top left), SV2 (top right) and SV3 (bottom left) and graph showing the variation in grain size (bottom right).

Figure 2

(a) XRD pattern obtained for the samples. The first inset in (a) shows the variation in lattice parameters with temperature and the second inset in (a) shows variation of (200) peak with temperature. (b) The refinement data showing the experimental, calculated and difference profiles, and ticks indicating the positions of the Bragg reflections in the SV1 sample.

Figure 3

(a) Tetragonal zircon unit-cell of $\mathrm{SmV}{\mathrm{O}}_4$, (b) the Vanadium tetrahedron surrounded by O atoms, and (c) Eight-coordinated Samarium atom.

Figure 4

Geometrical parameters of $\mathrm{SmV}{\mathrm{O}}_4$ expressed as $\alpha$ (O-V-O bond angles) and $\beta$ (O-Sm-O bond angles).

Figure 5

Variation in the two types of Sm-O and V-O bond lengths and the bond angles with temperature.

Figure 6

(a) A free energy versus temperature plot, (b) A plot for thermal expansion and heat capacity ($C_p$) with temperature, (c) Phonon lifetime for $\sim 0$ K structure, and (d) Phonon lifetime for SV1.

Figure 7

(a) Raman spectra of the samples with sintering temperature. The first inset of (a) shows the Raman shift of the most intense $A_{1g}$ mode, the second inset of (a) shows the variation in the FWHM of the $A_{1g}$ mode, and the third inset of (a) shows the shift in the same $A_{1g}$ mode. (b) Temperature-dependent Raman spectra of SV3. (c) Variation in peak parameters of the most intense $A_{1g}$ with varying temperatures. (d) Variation in the $B_{2g}$ in the temperature-dependent Raman spectra showing an opposing trend by blue shifting with increasing temperature. (e) Fitting showing all ten component modes of the SV3 Raman spectrum. (f) The experimental FTIR spectra for $\mathrm{SmV}{\mathrm{O}}_4$ samples projected along with the calculated IR spectra under DFT theory at room temperature (for SV1 at 300 K).

Figure 8

Variation in phonon frequencies: (left) with temperature (experimental) and (right) with the bulk modulus (B) (theoretical).

Figure 9

The UV-Vis spectra of the samples, with the arrows showing an approximate value of the band gap. Atom-resolved band-structure for $\mathrm{SmV}{\mathrm{O}}_4$ with HSE06 level of theory. Here, the red color represents the O states, the green color represents the V states, and Fermi level ($E_F$) is set to 0 eV.

Figure 10

(a) The photoluminescence spectra of the $\mathrm{SmV}{\mathrm{O}}_4$ samples. (b) Partial energy level diagram of along with radiative and nonradiative (NR) channels and energy transfer (ET) cross-relaxation (CR) channels.