Experimental and theoretical study of structural properties and phase transitions in YAsO${}_4$ and YCrO${}_4$

We have performed experimental and theoretical studies of the structural stability of YAsO${}_4$ and YCrO${}_4$ at high pressures. X-ray-diffraction experiments together with ab initio total-energy and lattice-dynamics calculations have allowed us to completely characterize a pressure-induced structural phase transition from the zircon to the scheelite structure in both compounds. Furthermore, total-energy calculations have been performed to check the relative stabilities of different candidate structures at different pressures and allow us to propose for YAsO${}_4$ the zircon → scheelite → SrUO${}_4$-type sequence of structures. In this sequence, sixfold arsenic coordination is attained for the SrUO${}_4$-type structure above 32 GPa. The whole sequence of transitions is discussed in comparison with YVO${}_4$, YPO${}_4$, YNbO${}_4$, YMoO${}_4$, and YTaO${}_4$. Also a comparative discussion of lattice-dynamics properties is presented. The band gap for YAsO${}_4$ and YCrO${}_4$ and the band structure for YAsO${}_4$ are also reported. Finally, the room-temperature equation of state of different compounds is also obtained.

Figure 1

Schematic view of the zircon (left), scheelite (center), and SrUO${}_4$-type (right) structures. Large circles: trivalent metal (e.g., Y), middle-size circles: $X$ atom (e.g., As, Cr, P, V). Small circles: oxygen atoms. The different polyhedral are shown as well as the unit cell.

Figure 2

Selection of diffraction patterns measured in YAsO${}_4$ at different pressures. The pattern denoted with (r) was taken after pressure release. The reflections of the zircon and scheelite structures are shown by ticks for the patterns collected at ambient pressure. The crosses ($+$) and asterisks (*) indicate the appearance of peaks of the high-pressure phase and vestiges of the peaks of the low-pressure phase.

Figure 3

Pressure evolution of the unit-cell parameters of the zircon-type and scheelite-type phases of YAsO${}_4$ and YCrO${}_4$. To facilitate the comparison, for the HP phase we plotted $c$/2 instead of $c$. Symbols: experiments. Solid lines: linear fit. The vertical dotted lines indicate the onset of the phase transition and the pressure range of phase coexistence. Data for pressures < 8 GPa (4.2 GPa) for the scheelite phase in YAsO${}_4$ (YCrO${}_4$) were obtained on decompression.

Figure 4

Pressure-volume relation in YAsO${}_4$ and YCrO${}_4$. Symbols: experiments. Lines: EOS fit. Data for pressures < 8 GPa (4.2 GPa) for the scheelite phase in YAsO${}_4$ (YCrO${}_4$) were obtained on decompression.

Figure 5

Theoretical energy as a function of volume curves for the structures zircon (filled circles), scheelite (filled squares), SrUO${}_4$ type (filled diamonds), monazite (filled triangles), iwashiroite (empty circles), BaWO${}_4$-II type (empty squares), LaTaO${}_4$ type (empty diamonds), BaMnF${}_4$ type (crosses), and CuTeO${}_4$ type (empty triangles). Energy and volume are written per formula unit.

Figure 6

Theoretical (a) volume as a function of pressure and (b) enthalpy difference as a function of pressure curves for the most stable phases of YAsO${}_4$ found in the calculations. Vertical dashed lines mark the theoretical transition pressures. In (b) at each pressure the enthalpy is measured with respect to the enthalpy of the zircon structure. Volume and enthalpy are written per formula unit.

Figure 7

Calculated pressure evolution of Raman modes for YAsO${}_4$.

Figure 8

Calculated pressure evolution of IR modes for YAsO${}_4$.

Figure 9

Band structure of zircon-type YAsO${}_4$.

Figure 10

Atomic partial densities of states of zircon-type YAsO${}_4$.

Figure 11

Structural sequence observed in different zircon-type oxides and related oxides. Data for YVO${}_4$ and YPO${}_4$ taken from Refs. 41 and 10. The higher coordination phase boxes represent either the SrUO-type structure or any of the high-coordination phases found in related oxides, e.g., orthorhombic Cmca (Ref. 5).

Figure 12

Schematic representation of Raman-active frequencies of YAsO${}_4$, YVO${}_4$ (Ref. 41), YCrO${}_4$, and YPO${}_4$ (Ref. 58). In the abscissa the compounds are ordered in decreasing order according to the atomic number of the $X$ atom.