The structures of the room-temperature and the lowest-temperature ferroelectric phases of fully deuterated betaine (trimethylammonioacetate) calcium chloride dihydrate have been determined by means of elastic neutron diffraction. In previous structural studies the structural distortions of the incommensurate phase at 130 K and the fourfold phase at 90 K were analyzed in terms of symmetry modes. The same approach has been applied in the present case and, as a result, a quantitative comparison of the distortions at different temperatures can be performed. This shows that the Λ
3 (for q ≠ 0) and B
2u
(for q = 0) modes, corresponding to the order parameter, are predominant for the three phases. In addition, it has been found that the structure of this primary mode remains essentially unaltered in the three phases. There is only an increase in its global amplitude as the temperature decreases. Accordingly, it can be expected that the structures of the remaining intermediate modulated phases in the transition sequence are similar to that of the first incommensurate phase, except for the change of wavevector values and greater amplitude of the distortion. Crystal data: (CD3)3NCD2COOCaCl2.2D2O, Mr
= 279.0, λ = 0.8308 Å, Z = 4, F(000) = 552. At room temperature: Pnma, a = 10.95 (1), b = 10.15 (1), c = 10.82 (1) Å, V = 1203 (2) Å3, Dx
= 1.54 g cm−3, μ = 0.10 cm−1, final R = 0.048 for 748 unique observed reflections. At 35 K: Pn21
a, a = 10.905 (9), b = 10.00 (1), c = 10.80 (1) Å, V = 1178 (2) Å3, Dx
= 1.57 g cm−3, μ = 0.11 cm−1, final R = 0.042 for 1726 unique observed reflections.