We measured the electro-optical properties of Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> single crystals for electric fields parallel to the crystallographic <i>x</i>-axis. A direct interferometric technique is used and unclamped as well as clamped values are determined. The measured coefficients are large, with a maximum of 174 ± 10pm/V for the unclamped coefficient <i>r<sup>T</sup> </i> <sub>111</sub> at λ = 633nm. The corresponding reduced half-wave voltage is <i>V</i> <sub>π</sub> = 124 V.
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