The relative linear birefringence of ${\mathrm{Cs}}_2$${\mathrm{ZnI}}_4$ was measured along all orthorhombic Pnam axes with use of the rotating-analyzer method. These measurements showed three distinct phase transitions at 120, 109, and 94 K and another possible phase transition near 101 K. The transition at 94 K showed definite hysteresis (≊1 K), which indicates that it was a first-order transition. No hysteresis was observed at either of the transitions at 109 and 120 K. The shapes of these transitions, however, indicate that the one at 120 K was second order, and the one at 109 K was first order in nature. Rough Brewster’s-angle measurements were made which yielded a crude estimate of the average index of refraction of between 1.77 and 1.80. X-ray-diffraction measurements were also made to confirm our identification of the crystals grown and to identify the crystal orientation. These measurements yielded accurate values for the axial lengths of the room-temperature structure that are in very good agreement with previous measurements.

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