Crystal growth, crystal structure, optical, electro-optic and electrostrictive properties of the complex silver thiocyanates Cs₃Sr[Ag₂(SCN)₇] and Cs₃Ba[Ag₂(SCN)₇]

Abstract

The crystal structure of Cs3Sr[Ag2(SCN)7] was determined from single crystal X-ray diffraction data (tetragonal, I[unk]2m, a = 11.083(2) Å, c = 20.117(3) Å, Z = 4, Dx = 2.98 Mgm−3, μ(MoKα) = 8.55 mm−1, T = 293 K, R = 0.040 for 1183 unique reflections). The main structural feature is the binuclear μ-thiocyanato(S)-bis[trithiocyanato(S)-argentate(I)]-complex [Ag2(SCN)7]5−, where the silver atoms are tetrahedral coordinated by the sulphur atoms of the linear SCN groups. Cs3Ba[Ag2(SCN)7] [a = 11.210(9) Å, c = 20.30(1) Å] is isotypic with the strontium compound. Large single crystals of optical quality of Cs3Sr[Ag2(SCN)7] and Cs3Ba[Ag2(SCN)7] were grown from stoichiometric aqueous solutions at 28°C. The tensor of linear electrostriction [dijk] and the unclamped electro-optic tensor [rijkσ] were determined for both compounds at room temperature and λ = 633 nm. The value of the electro-optic constants is only one third of the corresponding constants of KH2PO4(KDP). The maximum value of the electrostrictive coefficients is six times that of α-quartz. A comparison with the isotypic copper compound Cs3Sr[Cu2(SCN)7] shows that the exchange of Ag for Cu within the binuclear complex causes a strong increase in both effects.