Pressure-Induced Phase Transformations of Zircon-Type LaVO₄ Nanorods

Abstract

A combination of synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy has been used to study high-pressure behavior of the zircon-type LaVO4 nanorods. In situ high-pressure XRD results identified an irreversible zircon-to-monazite phase transition at ∼5 GPa and a reversible transition to an undetermined second high-pressure phase (phase III) at ∼12.9 GPa. Through Le Bail refinements of the XRD patterns with zircon-type structure, we show that the zircon-type LaVO4 nanorods possess the smallest bulk modulus among zircon-type rare-earth orthovanadates. Furthermore, negative pressure coefficients of external translational T­(Eg) and internal υ2(B2g) bending modes have been observed in Raman measurements. The Raman spectra of phase III with distinctive features have been fully recorded for the first time, and a related structure associated with a coordination increase for V is suggested in terms of the postmonazite phase in LaVO4 nanorods. Finally, analysis of the transmission electron microscopy both before and after compression indicates that a large number of nanorods can be recovered in the quenched samples, allowing us to verify the orientation relationship for zircon-to-monazite phase transformation.

Phase Stability of Lanthanum Orthovanadate at High Pressure

“…Neither minority phases nor impurities were detected in the crystals. The lattice parameters are a = 7.021(2) Å, b = 7.242(2) Å, c = 6.704(2) Å, and β = 104.875 (5)º, which are in good agreement with the literature [4,9,18]. Single-crystal XRD (SXRD) measurements were also used to determine the orientation of the crystals.…”

“…Previous PXRD experiments on LaVO 4 [5,9] found that the PXRD patterns can be assigned to the monazite structure up to 11.6 GPa. However, a discontinuity in the unit-cell volume was detected at 8.6 GPa [9].…”

ScVO4under non-hydrostatic compression: a new metastable polymorph

“…In the case of RVO4 compounds with RE elements with the larger cationic radii (La-Nd), previous studies have shown that under compression they transform from the zircon to the monazite-type monoclinic structure (space group P21/n). This transition involves an increase in coordination number around the R +3 cation [29][30][31][32][33][34]. The HP phases of all RE orthovanadates are known to have band gaps smaller than the ambient-pressure zircon-type phases.…”

Pressure‐induced Fermi resonance between fundamental modes in 7,7,8,8‐tetracyanoquinodimethane

“…It is generally accepted that the frequency of Raman modes shifts to higher wavenumber region under external compression. As a result, the frequency changes in response of the Raman mode with pressure are a good indicator of a possible phase transition . Figure shows the Raman frequency–pressure relationship of TCNQ.…”