Quantum Limit of Ferroelectric Phase Transitions inKTa1−xNbxO3

Dielectric and elastic constants of $\mathrm{K}{\mathrm{Ta}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ are presented as a function of $x$ and of the temperature. For $x=8\mathrm{}\times {10}^{-3\mathrm{}}$, the system exhibits the characteristics of a ferroelectric at the displacive limit (i.e., a "quantum ferroelectric") as evidenced by three critical indices and the temperature independence of all responses near $T=0\mathrm{}$.