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

Unlike potassium trichlorocuprate ${\mathrm{KCuCl}}_{3}$, the isostructural ammonium trichlorocuprate ${\mathrm{NH}}_{4}{\mathrm{CuCl}}_{3}$ 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 ${\mathrm{NH}}_{4}^{+}$ by the isomeric ${\mathrm{K}}^{+}$ ion results in a smearing of these steps. With the view of probing this change, we carried out on the mixed system ${({\mathrm{NH}}_{4})}_{x}{\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{CuCl}}_{3}$, with $x=0.0, 0.3, 0.6, 0.9, \mathrm{and} 1.0:$ (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 ${\mathrm{NH}}_{4}^{+}$ ion concentration, with the spin gap $\mathrm{\ensuremath{\Delta}}$ 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 ${\mathrm{Cu}}^{2+}\ensuremath{-}{\mathrm{Cu}}^{2+}$. This, in addition to the variation of $x$ with $\mathrm{\ensuremath{\Delta}}$, points to the need for additional theoretical modeling.