SOLID-SOLUTIONS OF A JAHN-TELLER COMPOUND IN AN UNDISTORTED HOST .4. NEUTRON AND X-RAY SINGLE-CRYSTAL STRUCTURES OF 2 CR/ZN TUTTON SALT SOLID-SOLUTIONS AND THE OBSERVATION OF DISORDER BY LOW-TEMPERATURE NEUTRON-DIFFRACTION

Solid solutions of the ammonium Cr/Zn Tutton salt with two different compositions, (NH4)2[Cr0.10Zn0.90(H2O)6](SO4)(2), 1, and (NH4)(2)[Cr0.22Zn0.78(H2O)(6)](SO4)(2), 2, have been studied by single-crystal X-ray diffraction at room temperature and by single crystal time-of-flight neutron diffraction at 11-16 K for 1 and at 11-17 K for 2. Composition was also characterized by atomic absorption, AA, analysis. The solid solutions crystallize in the monoclinic space group P2(1)/c, Z = 2, with the following cell dimensions, where in each case the room temperature X-ray value is given first followed by the low-temperature neutron value. For 1: a = 6.249(2), 6.286(1) Angstrom; b = 12.508(3), 12.367(2) Angstrom; c = 9.237(2), 9.147(2) Angstrom; beta = 106.82(2), 106.87(1)degrees; V = 691.1(3), 680.4(2) Angstrom(3). For 2: a = 6.252(1), 6.260(1) Angstrom; b = 12.503(1), 12.242(2) Angstrom; c = 9.247(1), 9.093(1) Angstrom; beta = 106.77(1), 106.89(2)degrees; V = 692.1(1), 666.8(2) Angstrom(3). The structure of 1 was refined to residuals of R = 0.0226 and 0.0670 and quality of fit = 1.099 and 1.165 for X-ray and neutron data, respectively; the corresponding residuals for the X-ray and neutron structures of 2 are R = 0.0215 and 0.0682, and quality-of-fit = 1.057 and 0.994. The anisotropic displacement parameters of the oxygen and hydrogen atoms from the neutron diffraction studies are examined for their implications as to the presence or absence of disorder in the ligands about the composite Cr/Zn metal center. The results for composition 2, x(Cr) = 0.22, clearly indicate the presence of disorder for the aqua ligand affected by Jahn-Teller distortion in the pure chromium complex. For composition 1, x(Cr) = 0.10, the observation of disorder is not clear. The structures of 1 and 2 are isotypic at a given temperature, but the room-temperature and low-temperature structures differ subtly in the hydrogen bonding pattern.