内田 寛

Lead-free ferroelectric Bi4Ti3O12 (BIT) thin films were modified by the cosubstitution of Sr2+ for Bi3+ and of Nb5+ for Ti4+ by spin-coating and decomposition of chemical solutions of metal-alkoxide materials (the nominal compositions of Bi4-xSrxTi3-xNbxO12 (BSTN) are $x=0.5$, 1.0, 1.5). Single-phase thin films with a BIT-type structure were crystallized above 550°C. At room temperature, the ferroelectric and dielectric properties were found to be $P_{\text{r } }=10$ μC/cm2, $E_{\text{c } }=100$ kV/cm, $\varepsilon_{\text{r } }=300$, and $\tan\delta<5$% for $x=0.5$ after annealing at 650°C. The films with $x=1.0$ and 1.5 did not exhibit ferroelectric hysteresis behavior because of the decrease in Curie temperature ($T_{\text{c } }$) below room temperature. The $x$ dependence of $T_{\text{c } }$ was studied by considering the soft mode behavior in Raman scattering spectra and the $T_{\text{c } }$ values were 400, $-25$, and $-220$°C for $x=0.5$, 1.0, and 1.5, respectively. The cosubstitution by Sr2+ and Nb5+ is effective for reducing ferroelectric interaction between electrical dipoles leading to a large shift in $T_{\text{c } }$. Because of its high solubility of Sr2+ and Nb5+ and efficiency for shifting $T_{\text{c } }$, the BSTN system may find a novel application as a dielectric material rather than as a ferroelectric material.