Characterization and strong piezoelectric response of an organometallic nonlinear optical crystal: CdHg(SCN)4(C2H6SO)2

Abstract

Bulk single crystals of CdHg(SCN)₄(C₂H₆SO)₂ (CMTD) have been grown by the temperature-lowering method. Refractive index measurements reveal that the refractive index n_x along the c axis is much smaller than the n_y and n_z indices in the ab-plane. The detailed powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate that the crystal is a phase matchable nonlinear optical material which exhibits a strong SHG effect, 1.2 times that of KTiOPO₄. The complete set of dielectric, elastic and piezoelectric constants of CMTD have been measured and the results show that the crystal possesses excellent piezoelectric properties of d₃₆ = 20.40 and d₁₄ = 17.80 pC N⁻¹ (much larger than those of α-SiO₂). The crystal exhibits a neutral layered structure consisting of the asymmetric HgS₄ tetrahedron and CdN₄O₂ octahedron which are connected by π-conjugated –SCN– ligands. The physical properties have been interpreted on the basis of the crystal structure combined with theoretical calculations. The strong piezoelectric response and high optical SHG efficiency make CMTD a promising candidate for both piezoelectric and nonlinear optical applications.