Structure and stability ofZrSiO4under hydrostatic pressure

Abstract

We present the results of a combined experimental and theoretical investigation aimed to determine structural and equation-of-state parameters and phase stability thermodynamic boundaries of ZrSiO 4 polymorphs. Experimental unit-cell data have been obtained for a powdered sample in a diamond-anvil cell using energydispersive synchrotron x-ray diffraction with emphasis on the pressure range 0 -15 GPa. Static total-energy calculations have been performed within the density functional theory at local density and generalized gradient approximation levels using a plane-wave pseudopotential scheme. Our quantum-mechanical simulations explore the pressure response of the two observed tetragonal structures ͑zircon-and scheelite-type reidite͒ as well as of other potential post-scheelite polymorphs up to about 60 GPa. We find very good agreement between our experimental and calculated pressure-volume values for the low-pressure phase of ZrSiO 4 . A microscopic analysis of the bulk compressibility of zircon and reidite in terms of polyhedral and atomic contributions is proposed to clarify some of the discrepancies found in recent theoretical and experimental studies. Our results show the relevant role played by the oxygen atoms in the description of this property. The zircon-reidite equilibrium phase transition pressure is computed around 5 GPa. No other post-scheelite phase is found stable above this pressure though a decomposition into ZrO 2 ͑cottunite͒ and SiO 2 ͑stishovite͒ is predicted at about 6 GPa. These two transition pressure values are well below the experimental ranges detected in the laboratory in concordance with the large hysteresis associated with these transformations.

Zircon to scheelite phase transition induced by pressure and magnetism inTbCrO4

“…29 This reconstructive mechanism explains why the scheelite phases are metastable at ambient pressure and do not return to the zircon phase on release of pressure. 30,31 These results agree with our structural refinements, where it can be observed that the Cr-O and Tb-O distances do not change very much in both zircon and scheelite polymorphs, but there are remarkable changes in bond angles. These important changes appear to support the reconstructive model to explain this structural zircon-scheelite phase transition.…”

High-pressure structural phase transitions inCuWO4

“…Indeed, there are many examples in the literature of ternary oxides (e.g. ZrSiO 4 , CaWO 4 ) [27,30] where bulk modulus differences of up to 20% are found depending on hydrostaticity despite being highly uncompressible materials. Usually, larger bulk moduli are systematically obtained with the less hydrostatic media [31] a fact that agrees with our findings for CuWO 4 .…”

On the mechanism of the zircon-reidite pressure induced transformation

“…Details of the computational parameters and the comparison with the results provided by the generalized gradient approximation (GGA) have been previously discussed [1]. In general, presure (p)-volume (V ) points and all our energetic profiles have been computed at zero temperature and neglecting zero point vibrational contributions.…”

“…According to the transformation matrices shown in Table I Table I, all these values can be compared with the zero pressure parameters of the corresponding conventional cells collected in Table I of Ref. [1].…”

“…We have previously reported a combined experimental-theoretical work on the pressure behavior of zircon, reidite and post-scheelite phases of ZrSiO 4 with special emphasis on the equations of state and the relative thermodynamic stability of these structures [1]. Here, we focus on the characterization of the zircon-reidite phase transition mechanism, i. e., the microscopic description of the structural, energetic and bonding changes that take place across the transition pathway connecting both structures.…”