Tetragonal-cubic phase boundary in nanocrystalline ZrO2–Y2O3 solid solutions synthesized by gel-combustion

https://doi.org/10.1016/j.jallcom.2011.01.213Get rights and content

Abstract

By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO2–Y2O3 solid solutions, the presence at room temperature of three different phases depending on Y2O3 content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7–10 nm, were synthesized by a nitrate–citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO2–Y2O3 solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined.

Research highlights

► Gel-combustion synthesis yields compositionally homogeneous, single-phased ZrO2–Y2O3 nanopowders, that exhibit the presence at room temperature of three different phases depending on Y2O3 content, namely two tetragonal forms (t′ and t″) and the cubic phase. ► Phase identification can be achieved by synchrotron XPD (SXPD) and Raman spectroscopy since the tetragonal forms and the cubic phase can be distinguished by these techniques. ► The crystallographic features of ZrO2–Y2O3 nanopowders were determined by SXPD. They are similar to those reported by Yashima and coworkers for compositionally homogeneous materials containing larger (micro)crystals. However, the lattice parameters are slightly different and the axial ratios c/a of our t′ samples are smaller than those reported by these authors. ► Compositional t′/t″ and t″/cubic phase boundaries are located at (9 ± 1) and (10.5 ± 0.5) mol% Y2O3, respectively. ► For the whole series of nanocrystalline ZrO2–Y2O3 solid solutions studied in the present work, no evidences of the presence of a mixture of phases – as reported by Yashima and coworkers for microcrystalline solid solutions – were detected.

Introduction

Zirconia-based ceramics are intensely investigated because of their excellent electric and mechanical properties. In particular, TZP (‘tetragonal zirconia polycrystals’) ceramics exhibit high ionic conductivity at intermediate temperatures and high fracture toughness [1], [2]. These ceramics are used in many applications, mainly in electrochemical devices such as oxygen sensors, oxygen pumps, and solid-oxide fuel cells [3].

Pure, undoped zirconia presents three stable phases that depend on temperature: monoclinic (stable below 1473 K), tetragonal (stable between 1473 and 2553 K) and cubic (stable from 2533 K up to the melting point at 2988 K) [1], [2]. Unfortunately, only the high temperature tetragonal and cubic phases have adequate properties for technological applications. The cubic polymorph can be fully stabilized at room temperature by introducing dopants such as Y2O3, CaO, MgO, and CeO2. In contrast, the tetragonal phase cannot be stabilized in all these systems, being the monoclinic phase the thermodynamically stable polymorph at room temperature. However, it has been found that the tetragonal phase can be retained in nanocrystalline powders and fine-grained ceramics [2].

For compositionally homogeneous ZrO2-based materials, the existence of three tetragonal forms, known as t, t′ and t″, all belonging to the P42/nmc space group have been reported [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The t-form is the stable one, limited to the solubility limit predicted by the equilibrium phase diagram. The dopant oxides exhibit a wider solubility range in the t′-form, which is unstable against the mixture of the t-form and cubic phase. The t″-form – that can be retained for high dopant contents – has a c/a ratio of unity, but the oxygen atoms are displaced along the c-axis from their ideal sites in the cubic phase (8c sites of the space group). Finally, the cubic phase having a fluorite-type structure (Fm3¯m space group) is fully stabilized for even higher dopant concentration.

Yashima et al. [4], [5], [6], [7], [8], [9] investigated the conditions for retention of metastable tetragonal forms, at room temperature, in compositionally homogeneous ZrO2-based solid solutions for several systems composed of coarse (micro) crystals. In the case of ZrO2–Y2O3 solid solutions, these authors have carefully characterized compositionally homogeneous powders synthesized by arc melting and rapid quenching [4], [6], [7] or by solid-state reaction [5]. Yashima and coworkers established that the t′/t″ and t″/cubic compositional boundaries are located around 9 and 11 mol% Y2O3, respectively. They also established the presence of a mixture of t′ and t″ for a solid solution with composition of ZrO2-14 mol% YO1.5 (ZrO2-7.5 mol% Y2O3) [5].

In the last few years, we have investigated the presence of metastable tetragonal forms in compositionally homogeneous ZrO2-based solid solutions composed of small nanocrystals, instead of microcrystalline materials as studied by Yashima and coworkers. The studied nanocrystalline solid solutions were synthesized by gel-combustion routes [10], [11], [12], [13], [14]. Interestingly, we have demonstrated that similar tetragonal forms can be retained in nanocrystalline ZrO2–Y2O3 [10], ZrO2–CeO2 [11], [12], ZrO2–CaO [13] and ZrO2–Sc2O3 [14] systems, but the lattice parameters, axial ratio and oxygen displacement may change with the crystallite size and/or synthesis route.

In a previous work [10], we investigated nanocrystalline ZrO2–Y2O3 solid solutions by powder diffraction using a conventional X-ray source (XPD). From these data, the lattice parameters and the position of oxide anions in the tetragonal cell could not be precisely determined. Moreover, the t″/cubic compositional boundary was not established. In order to more precisely determine the compositional dependence of the crystal structure of the nanocrystalline solid solutions and establish the t″/cubic boundary, a new study of compositionally homogeneous ZrO2-4 to 12 mol% Y2O3 nanopowders using synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy was performed.

As we will see, the techniques used here, SXPD and Raman spectroscopy, allowed us a precise analysis of the phase diagram and crystal structure of our nanocrystalline ZrO2–Y2O3 solid solutions. In particular, the measurement of the weak 1 1 2 X-ray reflection corresponding to the tetragonal phase – a forbidden reflection in the cubic fluorite structure – provided an accurate determination of the oxygen positions in the unit cell. Besides, in contrast to our previous work [10], both techniques made possible to distinguish between the tetragonal t″-form and the cubic phase and, therefore, determine the t″/cubic compositional boundary, a result that completes the whole phase diagram of nanocrystalline ZrO2–Y2O3 solid solutions.

Section snippets

Synthesis of nanocrystalline ZrO2–Y2O3 solid solutions

Nanocrystalline ZrO2-4, 6, 7, 8, 9, 10, 11 and 12 mol% Y2O3 nanopowders were synthesized by a pH-controlled nitrate–citrate gel-combustion process previously developed by the authors [10], [15], [16]. ZrOCl2·8H2O (Alpha Aesar, USA, 99.9%) and Y2O3 (GFS company, UK, 99.99%) were used as raw compounds.

A final calcination procedure at 600 °C for 2 h was performed in order to remove the organic residues.

Synchrotron X-ray powder diffraction (SXPD)

SXPD experiments were carried out using the D12A-XRD1 beamline of the Brazilian Synchrotron Light

Compositional boundary between the tetragonal t′-form (tetragonal unit cell) and the tetragonal t″-form (cubic unit cell)

Fig. 1 displays the whole diffraction pattern corresponding to the ZrO2-7 mol% Y2O3 solid solution. This pattern was well fitted by Rietveld procedure by assuming the presence of only one phase, namely the tetragonal t′-form. All the other SXPD patterns were also indexed and well fitted by assuming, in all cases, a single-phased material with one of the tetragonal forms or the cubic phase, depending on composition. No evidence of a mixture of phases was detected, as confirmed by a careful study

Conclusions

We have analyzed the crystal structure of nanocrystalline and compositionally homogeneous ZrO2–Y2O3 solid solutions synthesized by a gel-combustion process and identified the presence at room temperature of the metastable forms of the tetragonal phase, t′ and t″, and the stable cubic phase in these powdered materials.

The t′/t″ compositional boundary determined in this work for nanocrystalline ZrO2–Y2O3 solid solutions using Rietveld refinements of SXPD data was (9 ± 1) mol% Y2O3, in good

Acknowledgements

The present work was performed within the frame of the scientific collaboration agreements CAPES-MinCyT and CNPq-CONICET between Brazil and Argentina. It was also partially supported by the Brazilian Synchrotron Light Laboratory (LNLS, Brazil, proposal D12A-XRD1-1857), CNPq (Brazil, PROSUL programs 490289/2005-3 and 490580/2008-4), Agencia Nacional de Promoción Científica y Tecnológica (Argentina, PICT 2005 No. 38309, PICT 2007 No. 01152 and PAE-PICT 2007 No. 02288), CONICET (Argentina, PIP No.

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