Evidence for anomalous relaxation of the ${\mathrm{CuCl}}_4$(${\mathrm{NH}}_3$${)}_2^{2\mathrm{-}}$ center in ${\mathrm{NH}}_4$Cl below ${\mathit{T}}_{\mathit{c}}$

The temperature dependence of the (a) ${\mathit{e}}_{\mathit{u}}$(π+σ,Cl)→${\mathit{a}}_{1\mathit{g}}^{\mathrm{*}}$(∼3${\mathit{z}}^2$-${\mathit{r}}^2$) charge-transfer transition, ${\nu }_{\mathrm{CT}}$; (b) ${\mathit{e}}_{\mathit{g}}^{\mathrm{*}}$→${\mathit{a}}_{1\mathit{g}}^{\mathrm{*}}$ crystal-field transition, ${\nu }_{\mathrm{CF}}$; (c) Cu-${\mathrm{NH}}_3$ and Cu-Cl ${\mathit{A}}_{1\mathit{g}}$ stretching mode frequencies (termed as ${\nu }_1$ and ${\nu }_2$, respectively) corresponding to the ${\mathrm{CuCl}}_4$(${\mathrm{NH}}_3$ ${)}_2^{2\mathrm{-}}$ center in ${\mathrm{NH}}_4$Cl have been measured in the 100–300-K range. Below ${\mathit{T}}_{\mathit{c}}$, ${\nu }_{\mathrm{CF}}$ and especially ${\nu }_{\mathrm{CT}}$ undergo a red shift ascribed to the increase of the ${\mathrm{Cu}}^{2+}$-${\mathrm{Cl}}^{\mathrm{-}}$ equatorial distance, ${\mathit{R}}_{\mathrm{eq}}$, of the center although the lattice parameter a decreases. This anomalous relaxation is strongly supported by Raman data, giving variations Δ${\nu }_1$=7±1 ${\mathrm{cm}}^{\mathrm{-}1}$ and Δ${\nu }_2$=-11±3 ${\mathrm{cm}}^{\mathrm{-}1}$ in the 243–213-K range. Based on these data, the variations undergone by ${\mathit{R}}_{\mathrm{eq}}$ and the axial ${\mathrm{Cu}}^{2+}$-${\mathrm{NH}}_3$ distance