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The d33 piezoelectric coefficient in sodium oxonium trihydroxotrioxoiodate (2-) is 4.3 ± 0.5 × 10-12 CN-1 at 298 K. The p3 pyroelectric coefficient is 4.3 ± 0.3 × 10-6 Cm-2 K-1 at 298 K, increasing to 14.5 × 10-6 Cm-2 K-1 at 220 K and becoming zero at about 137 K; p3 changes sign at lower temperatures. The absolute sense of the two coefficients at 298 K, determined with respect to the atomic arrangement, is given by d33>0, p3 <0. The Na+ ions are embedded within layers of I(OH)3O32- ions: the H3O+ ions are attached to these anion layers by strong hydrogen bonds. Compressive stress equivalent to finger pressure on a 1 mm2 crystal face displaces the Na+ ions toward the OH- surfaces and the H3O+ ions toward the O- surfaces of the anion layer by about 10-4 Å. An increase in temperature of 100 K displaces the positive ions in the same sense by about 2 × 10-3 Å. A similar model for elbaite tourmaline predicts an average cation displacement of 5-10 × 10-4 Å along the polar axis, on cooling the crystal through 100 K, consistent with the recent results of G. Donnay [Acta Cryst. (1977), A33, 927-932].