Phase Behavior of TmVO₄ under Hydrostatic Compression: An Experimental and Theoretical Study

Abstract

We present a structural and optical characterization of magnetoelastic zircon-type TmVO 4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO 4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO 4 is found to be E g = 3.7(2) eV. Under compression the band gap opens linearly, until it undergoes a huge collapse following the structural phase transition (ΔE g = 1.15 eV). Ab initio structural and free energy calculations support our findings. Moreover, calculations of the band structure and density of states reveal that for both zircon and scheelite TmVO 4 the band gap is entirely determined by the V 3d and O 2p states of the VO 4 3− ion. The behavior of the band gap can thus be understood entirely in terms of the structural modifications of the VO 4 units under compression. Additionally, we have calculated the evolution of the infrared and Raman phonons of both phases upon compression. The presence of soft modes is related to the dynamic instability of the low-pressure phase and to the phase transition.

PrVO4 under High Pressure: Effects on Structural, Optical, and Electrical Properties

“…Similar bulk moduli have been found in the isostructural CeVO 4 , TbVO 4 , and TmVO 4 vanadates. 21,47 For an easier comparison, EOSs of other REVO 4 species in zircon, monazite, and BaWO 4 -II are summarized in Table 2. Under compression in nonhydrostatic media, most zircon REVO 4 vanadates show larger bulk moduli, often above 130 GPa.…”

“…The underestimation of the band gap of zircon-phase orthovanadates by DFT calculations is not an unusual result. 45,47 The inclusion of the Hubbard U potential increases the magnitude of the gap, in comparison to that when only PBE is applied. However, since U is an empirical parameter, we must note that the band gap width is dependent on this parameter.…”

“…The axial compressibility of the zircon phase is strongly anisotropic (Figure ), as it is larger along a than along c , with k a = [3.1(1)] × 10 –3 GPa –1 and k c = [1.29(8)] × 10 –3 GPa –1 . The anisotropy of the zircon phase is not unusual for RE VO 4 systems, due to their polyhedral arrangement, and in the case of PrVO 4 it is in line with previous reports. ,, For zircon PrVO 4 the volume of the VO 4 units is almost 11 times lower than that of the PrO 8 polyhedra, implying that at first order the compressibility is dominated by the changes in PrO 8 molecular units. For this zircon phase, the PrO 8 units lie in zigzag chains along [100] and [010] and are stacked along [001], separated by the scarcely compressible VO 4 tetrahedra (Figure ).…”

“…Similar bulk moduli have been found in the isostructural CeVO4, TbVO4 and TmVO4 vanadates. 31,34 Under compression in non-hydrostatic media most zircon REVO4 vanadates have higher bulk moduli, often above 130 GPa. 11 Zircon PrVO4, together with TmVO4, is thus one of the softest vanadates.…”