Crossover between Ferroelectric and Quantum Paraelectric in SrTiO 3 both by Isotopic Substitution and Hydrostatic Pressu
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Crossover between Ferroelectric and Quantum Paraelectric in SrTiO3 both by Isotopic Substitution and Hydrostatic Pressure Ruiping Wang1 and Mitsuru Itoh Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, 226-8503, Japan 1 Smart Structure Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1, Umezono, Tsukuba, 305-8568, Japan ABSTRACT Quantum paraelectric SrTiO3 undergoes a transition to ferroelectric by the substitution of 18O for 16O. The Tc vs. x in SrTi(16O1-x18Ox)3 follows Tc = 30.4(x - 0.33)1/2. Application of the hydrostatic pressure on the SrTiO3 and SrTi(16O0.0718O0.93)3 have the effects of decreasing ε’ and depressing Tc, respectively. Above the critical pressure pc, ferroelectricity of SrTi(16O0.0718O0.93)3 M disappears. Fitting the data to the Barrett’s formula, ε ′ = , elucidated that (T1 2) coth(T1 2T ) − T0 T0 and T1
for both compounds changes linearly with pressure.
INTRODUCTION Evolution of the ferroelectricity in the incipient ferroelectrics, SrTiO3 (STO) and KTaO3, by the chemical substitution for A- or B-site ions has been an important subject to elucidate the transition mechanism of the displacive-type ferroelectrics. Chemical substitution itself is a powerful method to change the overall lattice size. Isovalent substitution for quantum paraelectric SrTiO3 and KTaO3 yields a ferroelectric (FE) instability of these compounds to give a Curie temperature dependent on concentration as Tc(x) = A(x - xc)α. For Sr1-xAxTiO3 (A = Ca, Ba, Pb, Cd) [ 1-11] and KTa1-xNbxO3 [12-15], it was found that in the vicinity of xc, i.e., for low Tc, α ≈ 1 2 ; this is in full agreement with the theoretical prediction for low-temperature ferroelectrics [16-20]. However, the nature of the impurity-induced FE phase transition in perovskites remains unclear. The model of an off-center impurity ion moving in a multiwell potential is very popular. Interaction between the impurity-ion dipole moments via soft polar TO mode can give rise both D8.8.1
to the formation of a dipole glass phase and to the transition to the FE state [14]. This situation occurs in K1-xLixTaO3 [13, 14]. However, the applicability of this approach to other perovskites with isovalent impunities is questionable due to following reason. Kvyatkovskiĭ has shown that softening of kst for the impurity atom is small compared to that in the like compounds of the impurity ion [21], where kst denotes force constant. It is surprising that kst for Nb atom in KTa1-xNbxO3, whose ferroelectric origin has been considered due to the off-center impurity, is larger than that for KNbO3 [21]. In any case of chemical substitution in perovskite, we must always take the chemical and local structural changes into account for the detailed discussion of the induced properties. Compared to such a chemical substitution method, isotope exchange has a merit that both the chemical and structural changes can be avoided through the substitution. However, isotope exchange accompanie
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