Phase transition of pure zirconia under irradiation: A textbook example

https://doi.org/10.1016/j.nimb.2006.04.092Get rights and content

Abstract

One of the most important goals in ceramic and materials science is to be able to design materials with specific properties. Irradiation seems to be a powerful tool for the design of advanced ceramics because of its ability to modify over different scales the microstructure of solids. Nowadays, it is clearly proved that irradiation induces order–disorder phase transitions in metallic alloys and in some ceramics. In this paper, we show that a displacive phase transition can also be induced by irradiation. Based on many experimental facts, a microscopic model is proposed to explain the displacive phase transition observed in this material after irradiation. Defects, produced in the oxygen sublattice, induce important strain fields on a nanometric scale. This strain field can be handled as a secondary order parameter within the Landau theory approach, leading to a decrease of the phase transition temperature and thus quenching the high temperature tetragonal phase.

Introduction

At high temperature, most solids are in a thermodynamic equilibrium state. At low temperature this is not always true since their relaxation timescales can be very long. Therefore, a perturbation of the system will not always allow the system to reach the equilibrium ground state [1]. Sometimes, the system perturbation is so important that the phase transition sequence can be modified: several examples, like the occurrence of order–disorder phase transitions driven by irradiation in metallic alloys, are discussed by many authors [2], [3]. A phase transition induced by irradiation has also been observed in zirconia [4], [5], [6] where the monoclinic to tetragonal phase occurs. Some authors [7] have also observed the effect of grain size on radiation induced transformations in zirconia. Zirconia has then been the object of extensive investigations, and therefore it is a textbook example for describing the displacive phase transition out of irradiation within the Landau theory approach [8]. Under irradiation, it is quite surprising that displacive transitions associated to correlated movements of atoms can occur. In fact, displacement cascades and amorphous tracks, breaking the spatial coherence of the crystal, should forbid such a mechanism. Nevertheless, some authors [9] attemped to explain amorphisation of some oxides within the Landau theory framework. In this paper, the analysis of different experiments done on pure monoclinic zirconia out of and under irradiation allows us to build a simple microscopic model explaining the monoclinic to tetragonal phase transition observed in pure zirconia samples. To reach such a goal, we have studied the phase transition triggered by the temperature in this solid. The key parameters associated with this transition were described within the Landau framework of phase transitions. On the other hand, to understand the impact of different order parameters in this transition, the structural behavior of different nanocrystals of zirconia were studied. From this analysis, it seems clear that only a single order parameter controls the phase transition. All these facts explain the monoclinic to tetragonal phase transition observed in pure zirconia samples exposed to radiation damage, pointing out the key role of defects created by the radiation exposure and the way they couple to order parameters to induce the monoclinc to tetragonal phase transition.

Section snippets

The monoclinic to tetragonal phase transition of zirconia versus temperature

Zirconia undergoes two successive phase transitions on cooling. The former transition occurs at about 2573 K and it is second order or weakly first order [10], [11]. The cubic structure becomes tetragonal (P42/nmc) and it is characterized in particular by the onset of a shear strain along the fourfold axis; the coordination polyhedron for Zr in the ideal fluorite structure (ZrO8 unit) is only slightly modified. On the other hand, the second phase transition, occurring at about 1330 K on cooling

Links between the order parameters

To study the sensitivity of this phase transition to the order parameters, very pure nanocrystals of tetragonal zirconia have been characterized and their evolution has been followed as a function of temperature with the neutron diffraction technique. Moreover, many studies on nanocrystals have proved that the small grain size ensures that no Schottky nor Frenkel defects can exist in these materials even at high temperature. The nanocrystals of zirconia permit us to understand the impact of

Structural stability of monoclinic zirconia under irradiation

To study in detail the mechanism associated with the structural evolution of pure monoclinic zirconia under irradiation, ZrO2 samples were irradiated at room temperature using 400 keV Xe ions to maximize the creation of displacement cascades and then the defect concentrations. During the irradiation by low energy ions, displacement cascades occur. They produce point defects in the target. A balance between production and recombination of point defects occurs leading to a non-equilibrium defect

Conclusion

This work suggests a microscopic mechanism to explain the appearance of a displacive phase transition under irradiation in pure zirconia. The phase transition induced by irradiation can be considered as a two step mechanism. The radiation damage creates a non-equilibrium concentration of defects in this solid. The spectroscopic signature of these defects clearly shows that these defects are mainly O vacancies forming Fa color centers, as already observed in several other ionic compounds under

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