Closing the spin gap of (NH4)xK1−xCuCl3 through chemical substitution

Unlike potassium trichlorocuprate KCuCl3, the isostructural ammonium trichlorocuprate NH4CuCl3 exhibits the rather unusual effect of orientation-independent steps in the magnetization (M) as a function of the applied magnetic field (H). However, replacement of NH4+ by the isomeric K+ ion results in a smearing of these steps. With the view of probing this change, we carried out on the mixed system (NH4)(x)K1-xCuCl3, with x = 0.0, 0.3, 0.6, 0.9, and 1.0: (1) High-field pulsed measurements of M versus H at 1.5 K, (2) susceptibility measurements from 1.8 to 295 K and (3) zero-field specific heat measurements from 1.8 to 295 K. The magnetization steps were found to be highly sensitive to the NH4+ ion concentration, with the spin gap Delta generally decreasing with increasing x. Analysis of the susceptibility and specific heat data with several theoretical schemes also supports this general trend. This includes data analysis using the Bleaney-Bowers, Ising chain, and the mean-field model. These results yielded intradimer spin-exchange constants of the order of 20-40 K, somewhat smaller than expected for Cu2+ - Cu2+. This, in addition to the variation of x with Delta, points to the need for additional theoretical modeling.