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The crystal structures of NaK1−x(NH4)x(+)-C4H4O6.4H2O (RS1−xARSx, x = 0.90, 0.94 and 1.00) have been determined in the paraelectric phase (T = 294 K). The crystal data are: for ARS, orthorhombic, P21212, R = 0.038, wR = 0.032, no. of reflections = 3373, a = 12.206 (7), b = 14.451 (6), c = 6.250 (4) Å, V = 1102 (1) Å3, Z = 4, Dx = 1.573 g cm−3, λ(MoKα) = 0.70926 Å, F(000) = 552.00; for RS0.06ARS0.94, orthorhombic, P21212, R = 0.040, wR = 0.030, no. of reflections = 2446, a = 12.172 (4), b = 14.421 (4), c = 6.239 (2) Å, V = 1095.2 (6) Å3, Z = 4, Dx = 1.592 g cm−3, λ(Mo Kα1) = 0.70926 Å, F(000) = 553.92; for RS0.10ARS0.90, orthorhombic, P21212, R = 0.036, wR = 0.027, no. of reflections = 1898, a = 12.165 (9), b = 14.420 (7), c = 6.239 (4) Å, V = 1094 (1) Z = 4, Dx = 1.598 g cm−3, λ(Mo Kα1) = 0.70926 Å, F(000) = 555.20. In the crystals there are two crystallographically nonequivalent cation sites occupied by K/NH4 ions. The accommodation ratios of sites (1) [(2a) by the Wyckoff notation] and (2) [(2b)] are 0.94 (1) and 0.92 (1), respectively, in RS0.06ARS0.94, and 0.90 (1) and 0.93 (1), respectively, in RS0.10ARS0.90. As x increases, the location of site (2) displaces towards the origin. The hydrogen bonds around sites (1) and (2) are classified into two types: one is that the distances become longer as x increases, due to the difference in the ionic radii of K and NH4 ions, while the other is that the distances are equal. Comparing the crystal structure of ARS to that of RS, the following differences occur: the position of site (2), the interatomic distances between cations of sites (1) and (2) and O atoms, the form of the tartrate molecule and the position of some water molecules. These differences seem to be mainly caused by the replacement of NH4 ions by K.
