Effects of pressure on the local atomic structure of CaWO4 and YLiF4: mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions
Introduction
Many ABX4 compounds, like calcium tungstate (CaWO4) and yttrium lithium fluoride (YLiF4), crystallize in the tetragonal scheelite structure (SG: I41/a, No. 88, Z=4) [1], [2] under ambient conditions. The strong interest in the structural stability of scheelite compounds under compression is evident in the numerous experimental studies on the pressure effects on their phase behavior [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In particular, it has been demonstrated recently that CaWO4 transforms under compression from the scheelite structure to the monoclinic wolframite structure (SG: P2/c, No. 13, Z=2) [1], [2] at 11±1 GPa [3], [4]. On the other hand, YLiF4 transforms under compression from the scheelite structure to the monoclinic M-fergusonite structure (SG: C2/c, No. 15, Z=4) [1], [2] also at 11±1 GPa [5], [6]. In both these compounds, the reversibility to the initial scheelite structure after a decrease in the pressure has been shown.
From the cationic point of view, the scheelite structure consists of two intercalated diamond lattices: one for A cations and another for B cations (see Fig. 1), where the A–A distances are equal to B–B distances. In the scheelite structure, A cations, calcium (Ca) and yttrium (Y), are coordinated by eight X anions, oxygen (O) or fluorine (F), thus forming AX8 polyhedral units. On the other hand, B cations, tungsten (W) and lithium (Li), are coordinated by four X anions forming relatively isolated BX4 tetrahedral units [7]. In the cation coordination notation for ABX4 compounds ([cation A coordination–cation B coordination]), scheelites have cation coordination [8–4]. Fig. 1 shows a detail of the scheelite structure with the AX8 and BX4 polyhedra.
The study of the pressure effects on the local atomic structure can be a powerful tool for understanding the transformation mechanisms of the pressure-driven transitions. While a systematic analysis of the effects of pressure on the local atomic structure of YLiF4 has already been performed [5], the same analysis in CaWO4 has not been performed yet. In this work, we report and discuss the pressure response of the local structure of W (Li) ions in CaWO4 (YLiF4) in the light of the recently reported high-pressure X-ray diffraction data [3], [5] and other high-pressure techniques. The aim of discussing the effects of pressure in the local structure of both the compounds is to understand more precisely the occurrence of the scheelite-to-monoclinic transitions, and particularly, the scheelite-to-wolframite and scheelite-to-fergusonite transitions. From the characterization of the similarities and differences of the pressure response of the local structure of CaWO4 and YLiF4, possible transformation mechanisms for both transitions are identified.
Section snippets
Experimental background
The lattice parameters and bond distances presented here for CaWO4 were obtained from the energy-dispersive X-ray powder diffraction (EDXD) patterns measured at the X-17C beamline at the National Synchrotron Light Source (NSLS) using a diamond-anvil-cell (DAC) at a diffraction angle 2θ=13°. As CaWO4 is soft (bulk modulus, B0=77 [3]), this material was used as its own quasi-hydrostatic pressure medium. A detailed description of these experiments was given in Ref. [3]. There, we reported the
Pressure effects on the local atomic structure
In order to know the microscopic mechanisms governing the scheelite-to-monoclinic phase transitions in CaWO4 and YLiF4, we analyzed the pressure dependence of the lattice parameters and bond distances in these two compounds. Fig. 3 shows the pressure dependence of the lattice parameters for the scheelite phase of CaWO4 and YLiF4. Both these compounds show a clearly anisotropic character, the compressibility of the c-axis being larger in CaWO4, and the compressibility of the a-axis being larger
Concluding remarks
We report the pressure dependence of the lattice parameters and bond distances of the scheelite phase of CaWO4 and compare them to those previously reported for YLiF4. The comparison of the thermal expansion coefficients and the pressure coefficients found for the lattice parameters, bond distances, and Raman modes in both the compounds has allowed us to understand why these two scheelites do not show the same high-pressure phase transitions. A mechanism for each of the two
Acknowledgments
The authors gratefully acknowledge the contribution of A. Vegas and A. Segura, who reviewed this paper and made valuable comments. They also thank Dr. J. Hu of beamline X-17C at NSLS for valuable technical advice and assistance. This work was supported by the NSF, the DOE, and the W.M. Keck Foundation. F.J.M. acknowledges financial support from the European Union under Contract No. HPMF-CT-1999-00074. D.E. also acknowledges the financial support from the MCYT of Spain through the “Ramón y
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