Phase Transitions During Ion-Beam Irradiation of the Perovskite-structure Oxides
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ABSTRACT oxide Several perovskite-structure compounds, including CaTiO 3 , SrTiO3, BaTiO 3 , KNbO 3, and KTaO3 were irradiated by 800 keV Kr* ions in order to investigate and compare their response to heavy-ion irradiation. The critical amorphization which Tc, above temperature amorphization could not be induced, was found to increase in the order SrTiO 3 --
0
B
0
CaTiO 3 -> BaTiO 3 -ý KNbO 3 -+ KTaO 3 . No
single physical parameter explains the observed sequence, although Tc correlates well with the melting temperatures. The phase solid state well-known transformations in these materials did not have a significant effect on the dose
Fig.1. The crystal structureof cubic perovskite. The smaller B-site cations
Domain for amorphization. required boundaries were observed in the pristine
occupy one fourth of the octahedral
sites defined by the cubic-closest
samples; however, after only a low dose, packed arrayof oxygen anions and Athe boundaries became poorly defined site cations(filled circles). and, with increasing dose, eventually Dislocation disappeared. Transition Space Group loops were observed to High T Temperature (K) ,ow T aggregate at the domain 1533 Pcmn Pm3m CaTiO 3 boundaries. INTRODUCTION The perovskitestructure oxides have the general formula ABO 3, where a variety of cations can occupy the A and B sites, with certain ionic radius and charge balance constraints. Generally, the tolerance factor (a crystalchemical measure of the
SrTiO 3 BaTiO 3
KNbO 3
I4/mcm
Pm3m
108
Amm2 P4mm R3m
P4mm
273
Pm3m Bmm2
393 230
Bmm2 P4mm
P4mm Pm3m
490 690
Pm3m
KTaO 3
Table 1. Space groups and transitiontemperaturesof the perovskite-structure oxides investigatedin this study. 401
Mat. Res. Soc. Symp. Proc. Vol. 481 @1998 Materials Research Society
deviation from the ideal cation radii for the perovskite structure [1]) must lie between 0.8 and 1.0, and the combined charge of the A- and B-site cations must equal +6. Thus there exists a large number of compounds with the perovskite structure. In this study we examine the effects of irradiation on CaTiO 3, SrTiO 3, BaTiO 3, KNbO 3, and KTaO 3. The ideal perovskite structure is cubic Pm3m (Z = 1 formula unit per unit cell). It can be viewed as a cubic close-packed array of oxygen anions with one fourth of the oxygens regularly replaced by an A-site cation (e.g., Ca, Sr, Ba, K; Fig. 1). The B-site cations fit into those octahedral sites that are defined by oxygen atoms only. This arrangement can be viewed as a simple-cubic array of B0 6 octahedra with the A site cations occupying the interstices. Many of the perovskite-structure oxides undergo one or more instantaneous displacive phase transitions at temperatures that are achievable in-situ in the transmission electron microscope (TEM). The cubic phase is stable at high temperatures but lowering the temperature causes the B06 octahedra to shift, decreasing the symmetry. Some of the materials (e.g., KNbO 3) undergo cubic-totetragonal-to-orthorhombic-to-rhombohedral phase transitions as the temperature is lo
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