Na1/2Bi1/2VO3 and K1/2Bi1/2VO3: New Lead-Free Tetragonal Perovskites with Moderate c/a Ratios

New lead-free tetragonal perovskites Na1/2Bi1/2VO3 and K1/2Bi1/2VO3 were synthesized under high pressure (6 GPa) and high temperature (1473 K), based on the design of materials optimizing the lone pair effect of the A-site ion and utilizing the Jahn–Teller effect in the B-site V4+. The magnitudes of the c/a ratio and spontaneous polarization, PS, were 1.085 and 108 μC/cm2, respectively (73 μC/cm2 by the point charge model), for Na1/2Bi1/2VO3 and 1.054 and 92 μC/cm2, respectively (56 μC/cm2 by the point charge model), for K1/2Bi1/2VO3, which are comparable to the well-known lead-based ferroelectric PbTiO3. This approach can guide the design of new lead-free ferroelectric and piezoelectric materials.