Optical second-harmonic characterization of ferroelectricity in double perovskites Ca$_{\mathrm{2-x}}$Mn$_{\mathrm{x}}$Ti$_{2}$O$_{6}$

Perovskite-type ferroelectric oxides such as BaTiO$_{3}$ are used widely as actuators and memory storage devices. Recently ferroelectricity was demonstrated in the double perovskite CaMnTi$_{2}$O$_{6}$, which represents a fundamental new class of ferroelectrics in which dipoles from Mn$^{2+}$ at the A-site and Ti$^{4+}$ at the B site are cooperatively coupled [1]. However, synthesis of CaMnTi$_{2}$O$_{6}$ from CaTiO$_{3}$-MnTiO$_{3}$ required pressure as high as 7GPa. We are developing spark plasma sintering (SPS) methods to synthesize Ca$_{\mathrm{2-x}}$Mn$_{\mathrm{x}}$Ti$_{2}$O$_{6}$ at pressures as low as 50 MPa, and using Second Harmonic Generation (SHG) microscopy to characterize the strength of ferroelectricity. Preliminary SHG results show that ferroelectric CaMnTi$_{2}$O$_{6}$ can be synthesized at low pressure with stronger ferroelectricity achieved with higher x and synthesis pressure. We will present comparative SHG results for SPS-synthesized and high-pressure-synthesized CaMnTi$_{2}$O$_{6}$ and relate them to the underlying origins of ferroelectricity.\\[4pt] [1] A. Aimi \textit{et al}., Chem. Mat. 26, 2601 (2014).