High-pressure synthesis, structure, phase relation of polar LiNbO₃-type ZnTiO₃

Ferroelectricity, piezoelectricity, pyroelectricity, and second-order nonlinear optical behavior are technologically important. Because these properties are attributable to the noncentrosymmetric (NCS) structure[1], the search for materials exhibiting such characteristics must begin with a search of NCS materials. Among them, LiNbO3-type (LN-type) compounds with a chemical formula of ABX3 exhibit NSC structures with hexagonal polar space group R3c whose BX6 octahedra three-dimensionally share their corners the same as perovskite-type compounds[2]. In LN-type compounds, the A cation - B cation repulsion directs the spontaneous polarization along c-axis(Fig. 1). Therefore, we might find attractive functional properties by the selection of constituent ions based on their having a naturally occurring polar LN-type structure attributable to the cation-cation repulsion. With the ideas mentioned above, we have investigated the high-pressure synthesis and characterization of novel LN-type oxides such as ZnSnO3, PbNiO3, CdPbO3 as well as known MnMO3 (M = Ti, Sn). Recently, we have successfully synthesized a polar LN-type titanate ZnTiO3 (LN-ZTO) under high pressure and high temperature [3]. The first principles calculation indicates that LN-ZTO is a meta-stable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement reveals that LN-ZTO exhibits greater intra-distortion of the TiO6 in LN-ZTO than that of the SnO6 in LN-type ZnSnO3 (LN-ZSO). The estimated spontaneous polarization are greater than those of LN-ZSO, which is attributed to the great displacement of Ti along c-axis and the greater Born effective charge of Ti (+6.1) than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-ZTO is greater than that of LiNbO3, indicating that LN-ZTO, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-ZTO is 24 times greater than that of LN-ZSO. The findings indicate that the intra-octahedral distortion, spontaneous polarization, and SHG response are caused by the stabilization of the polar LN-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d0 ions such as Ti4+. We also discuss the relationship between the intra-distortion of BO6 and polarity in several LN-type oxides.