Modification of crystal structure of ammonium Rochelle salt [NaNH₄(+)-C₄H₄O₆.4H₂O] owing to the replacement of NH₄ ions with K ions

The crystal structures of NaK_1−x(NH₄)_x(+)-C₄H₄O₆.4H₂O (RS_1−xARS_x, 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) ų, Z = 4, D_x = 1.573 g cm⁻³, λ(MoKα) = 0.70926 Å, F(000) = 552.00; for RS_0.06ARS_0.94, 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) ų, Z = 4, D_x = 1.592 g cm⁻³, λ(Mo Kα₁) = 0.70926 Å, F(000) = 553.92; for RS_0.10ARS_0.90, 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, D_x = 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 RS_0.06ARS_0.94, and 0.90 (1) and 0.93 (1), respectively, in RS_0.10ARS_0.90. 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.

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