The crystal structure of disordered - Springer Link

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U. Troitzsch Ж A. G. Christy Ж D. J. Ellis ... unit cell parameters a=4.8495(3) A˚ , b=5.4635(3) A˚ , ... at increased pressures (Troitzsch and Ellis 2004), the.
Phys Chem Minerals (2005) 32: 504–514 DOI 10.1007/s00269-005-0027-0

O R I GI N A L P A P E R

U. Troitzsch Æ A. G. Christy Æ D. J. Ellis

The crystal structure of disordered (Zr,Ti)O2 solid solution including srilankite: evolution towards tetragonal ZrO2 with increasing Zr

Received: 18 February 2005 / Accepted: 9 August 2005 / Published online: 25 October 2005  Springer-Verlag 2005

Abstract Crystal structure data are presented for seven synthetic samples of disordered zirconium-titanate solid solution (Zr,Ti)O2, ranging in composition from xTi=0.43 to 0.67, thus covering compounds such as ZrTiO4, Zr5Ti7O24, and ZrTi2O6 (srilankite). The compounds, synthesized at high temperatures and various pressures in their respective stability fields, are well crystallized and of homogeneous composition. The resulting structure data are less scattered compared to previous studies that were based on compounds synthesized metastably at low temperatures and room pressure. The compounds have the structure of scrutinyite (a-PbO2) with space group Pbcn, Z=4, unit cell parameters a=4.8495(3) A˚, b=5.4635(3) A˚, c=5.0462(3) A˚ at xTi=0.425 to a=4.7112(2) A˚, ˚ b=5.4944(1) A, c=4.9962(1) A˚ at xTi=0.666. The first structure refinement of pure, synthetic srilankite is presented, which is in good agreement with that of the natural counterpart. Structural trends observed in disordered zirconium-titanate solid solution along the binary join ZrO2–TiO2 are relatively smooth and continuous, except for rapid lengthening of an unshared octahedral edge which is anomalously short in scrutinyite-structure TiO2. The shortness of this edge may explain the observed instability of this structure with the relatively small Ti as the dominant cation. With increasing Zr content, the average cation position moves off-centre inside the octahedron, away from two shared edges, which permits the 12 closest cation–cation distances in the structure to become more equal. The shortening of the b dimension with increasing amount of the larger cation Zr decreases the distance between octahedral Zr and two additional oxygens in an adjacent chain of edge-sharing octahedra, implying that the Zr U. Troitzsch (&) Æ A. G. Christy Æ D. J. Ellis Department of Earth and Marine Sciences, Australian National University, Canberra, ACT, 0200 Australia E-mail: [email protected] Tel.: +61-2-61252071 Fax: +61-2-61255544

environment is evolving towards eightfold coordination. If the two additional oxygens are considered as part of the Zr coordination polyhedron, the bonding topology of tetragonal zirconia is obtained. The compositional evolution of the cell parameters, Zr atomic coordinates and Zr coordination environment is consistent with the idea that the structure is evolving towards that of tetragonal ZrO2. Group-theoretical relationships between scrutinyite, tetragonal zirconia, baddeleyite and fluorite structures show that the sequence of structures fluorite > tetragonal zirconia > scrutinyite > baddeleyite are all related by potentially diffusionless phase transitions driven by wavelike displacements of the oxygen substructure. The scrutinyite and tetragonal structures can act outside their stability fields as ‘‘transition states’’ between the structures on either side. Keywords Zirconium titanate Æ Srilankite Æ Crystal structure Æ Solid solution Æ Tetragonal zirconia

Introduction Zirconium titanate (Zr,Ti)O2, the intermediate compound of the ZrO2–TiO2 system (Coughanour et al. 1954; McHale and Roth 1986), is a solid solution that spans compositions xTi=0.42–0.67 [xTi = molar Ti/ (Ti+Zr)] depending on pressure and temperature of formation (Troitzsch and Ellis 2004) (Fig. 1). The mineral srilankite ZrTi2O6 (Willgallis et al. 1983), and synthetic compounds such as ZrTiO4 (Newnham 1967) and Zr5Ti7O24 (Bordet et al. 1986) are all specific compositions within this solid solution. The present study focuses on the structure of the high temperature, cationdisordered polymorph of (Zr,Ti)O2, which crystallizes in the orthorhombic scrutinyite (=a-PbO2) structure, space group Pbcn, with random distribution of Ti and Zr on the octahedral sites (shaded area in Fig. 1). Below T1,150C at atmospheric pressure, or slightly higher T at increased pressures (Troitzsch and Ellis 2004), the cations begin to order over a temperature interval of

505

disordered

1600 ZrO2 tetrag.

TiO2

T [C°]

1400 28 kbar 20 kbar

1200

15 kbar

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1000 ZrO2 monocl.

800 0

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TiO2 [mol.%]

Fig. 1 ZrO2–TiO2 phase diagram at room pressure showing the composition range of disordered (Zr,Ti)O2 investigated in this study (shaded area). Dashed lines depict stability field of (Zr,Ti)O2 at higher pressures. Phase diagram modified from Troitzsch and Ellis (2004, 2005)

about 400C that is followed by a phase transition to a fully ordered commensurate superstructure (Park et al. 1996). The ordered phase has a smaller molar volume, and is slightly more Ti-rich compared to the disordered polymorph (Troitzsch and Ellis 2005). Neither phase approaches end-member TiO2 in composition. Despite the apparent composition gap between srilankite and scrutinyite-structure TiO2, this metastable ‘‘TiO2-II’’ phase is well known as a quench product from the highpressure baddeleyite-structure polymorph and as a compression product of anatase (Haines and Le`ger 1993; Gerward and Olsen 1997; Arlt et al. 2000) and has recently been synthesized hydrothermally at 9–9.5 GPa as nearly millimetric euhedral crystals (Dyuzheva et al. 2004). The mineral srilankite is a rare accessory phase in metamorphic and igneous rocks, reported from only a few locations in the world (eg., Willgallis et al. 1983; Merkle 1991; Kostrovitskiy et al. 1995; Wang et al. 1999; Bingen et al. 2001). Its conditions of formation, and thus its role in phase equilibrium studies, are still uncertain. Earlier work suggested a low-temperature, hydrothermal origin of srilankite based on hydrothermal synthesis of the compound at temperatures well below 900C (Willgallis et al. 1987; Buhl and Willgallis 1989). Moreover, natural srilankite is of similar composition to the ordered, low-T (Zr,Ti)O2 polymorph shown in the phase diagram favoured at the time (McHale and Roth 1986), which also supported the hypothesis of low-T formation of srilankite. In contrast to this, recent experimental evidence points to a possible high-pressure, high-temperature formation of the mineral, at least under oxidizing conditions. Troitzsch and Ellis (2004)

pointed out that srilankite has the disordered structure of the high-temperature polymorph, and thus should be stable at temperatures much higher than the previously supposed upper temperature limit of 900C. Their highpressure experimental study demonstrated that the Ticontent of the (Zr,Ti)O2 solid solution is dependent on pressure and temperature, and they were able to synthesize srilankite at 28 kbar and 1,440C in equilibrium with rutile. A recent report of srilankite crystallized from a gabbroic melt similarly implies the formation of this mineral at T>900C (Morishita et al. 2004). It is likely that the earlier studies of Willgallis et al. (1987) resulted in the metastable formation of srilankite outside the stability field of the disordered form, a common problem in low-temperature experiments on (Zr,Ti)O2. The present study reports the structural changes in quenched synthetic samples from across the entire solid solution range that is currently accessible with low and highpressure experiments. Zirconium titanate has received much attention in the past because of its outstanding electrical properties. It has been in use as temperature-stable dielectric material for ceramic capacitors for a long time (Wolfram and Go¨bel 1981; Azough et al. 1996; Wang et al. 1997), and more recently has become important as stable oscillator at microwave frequencies, used in satellite communication such as in cell phones and global positioning systems. ZrTiO4 is valued in industry for its high dielectric constant, low Q-factor and small temperature coefficient (Azough et al. 1996), a sought-after combination of properties found in few materials. The current market is focussed on the use of the high-temperature, disordered form of ZrTiO4 that can be manufactured easily by sintering at high temperatures (>1,250C) from the oxides. Additives such as Sn can be added to improve the dielectric properties even further. Since the physical properties of a compound intimately depend on its composition and crystal structure, the availability of accurate structure data is of fundamental importance for any fine tuning of physical properties that may be required for a specific electronic device. Motivation of the present study is to further our understanding of how the (Zr,Ti)O2 structure reacts to changes in composition. The structure of synthetic ZrTiO4 was first described by Newnham (1967). Unit cell dimensions of natural srilankite were documented by Willgallis et al. (1983), those of synthetic srilankite were given in Willgallis et al. (1987) and Buhl and Willgallis (1989). Willgallis and Hartl (1983) presented a structure refinement of natural srilankite. The structure of synthetic disordered (Zr,Ti)O2 solid solution was documented in earlier studies. Compositions between xTi=0.42 and about 0.55 were synthesized from the oxides at room pressure within the stability field of the disordered phase (Noguchi and Mizuno 1968), but unit cell data were only presented visually. The crystallization of more extreme compositions (0.34