Effects of pressure on the local atomic structure of CaWO4 and YLiF4: mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions

doi:10.1016/j.jssc.2003.10.017

Journal of Solid State Chemistry

Volume 177, Issues 4–5, April–May 2004, Pages 1087-1097

https://doi.org/10.1016/j.jssc.2003.10.017 Get rights and content

Abstract

The pressure response of the scheelite phase of CaWO₄ (YLiF₄) and the occurrence of the pressure-induced scheelite-to-wolframite (M-fergusonite) transition are reviewed and discussed. It is shown that the change of the axial parameters under compression is related to the different pressure dependences of the W–O (Li–F) and Ca–O (Y–F) interatomic bonds. Phase transition mechanisms for both compounds are proposed. Furthermore, a systematic study of the phase transition in 16 different scheelite ABX₄ compounds indicates that the transition pressure increases as the packing ratio of the anionic BX₄ units around the A cations increases.

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

References (62)

R.M. Hazen et al.
J. Phys. Chem. Solids
(1985)
D. Christofilos et al.
J. Phys. Chem. Solids
(1995)
A. LeBail et al.
Mater. Res. Bull.
(1988)
J.P. Bastide
J. Solid State Chem.
(1987)
L.E. Depero et al.
J. Solid State Chem.
(1997)
A. Jayaraman et al.
J. Phys. Chem. Solids
(1987)
Q.A. Wang et al.
Phase Transitions
(1995)
A. Sen et al.
J. Phys.: Condens. Matter
(2002)
A. Jayaraman et al.
J. Raman Spectrosc.
(1995)
G. Serghiou et al.
Phys. Rev. B
(1997)

G.A. Wolten et al.

Am. Miner.

(1967)

V.Ya. Markiv et al.

J. Alloys Compounds

(2002)

G.A. Wolten

Acta Crystallogr.

(1967)

R.W.G. Wyckoff, Crystal Structures, Vol. 3, 2nd Edition, Wiley, New York, 1964, pp....

D. Errandonea et al.

Phys. Stat. Sol. (b)

(2003)

D. Christofilos et al.

Phys. Stat. Sol. (b)

(1996)

A. Grzechnik et al.

Phys. Rev. B

(2002)

F.J. Manjón et al.

Phys. Rev. B

(2002)

A.W. Sleight

Acta Crystallogr. B

(1972)

P. Blanchfield et al.

J. Phys. C

(1982)

P. Blanchfield et al.

J. Phys. C

(1983)

E. Sarantopoulou et al.

Phys. Rev.

(1999)

Q.A. Wang, A. Bulou, J.Y. Gesland, cond-mat/0210491,...

D. Errandonea et al.

Phys. Stat. Sol. (b)

(2002)

A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR, 2000, pp....

W. Kraus et al.

J. Appl. Crystallogr.

(1996)

P. Blanchfield et al.

J. Phys. C

(1979)

O. Fukunaga et al.

Phys. Chem. Miner.

(1979)

J.W. Otto et al.

Phys. Rev. B

(1991)

J. Spitaler et al.

Phys. Rev. B

(2003)

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Effects of pressure on the local atomic structure of CaWO4 and YLiF4: mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions

Abstract

Introduction

Section snippets

Experimental background

Pressure effects on the local atomic structure

Concluding remarks

Acknowledgments

J. Phys. Chem. Solids

J. Phys. Chem. Solids

Mater. Res. Bull.

J. Solid State Chem.

J. Solid State Chem.

J. Phys. Chem. Solids

Phase Transitions

J. Phys.: Condens. Matter

J. Raman Spectrosc.

Phys. Rev. B

Am. Miner.

J. Alloys Compounds

Acta Crystallogr.

Phys. Stat. Sol. (b)

Phys. Stat. Sol. (b)

Phys. Rev. B

Phys. Rev. B

Acta Crystallogr. B

J. Phys. C

J. Phys. C

Phys. Rev.

Phys. Stat. Sol. (b)

J. Appl. Crystallogr.

J. Phys. C

Phys. Chem. Miner.

Phys. Rev. B

Phys. Rev. B

Effects of pressure on the local atomic structure of CaWO₄ and YLiF₄: mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions