Mn2+EPR study of the phase transition in an ammonium sulfate single crystal: Existence of two inequivalent sublattices

Angular variations of X-band EPR spectra, for the magnetic field orientations in three mutually perpendicular planes, of a single crystal of ${\mathrm{Mn}}^{2+}$-doped (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{SO}}_4$, have been recorded from 113 to 398 K. Two ${\mathrm{Mn}}^{2+}$ centers, the principal axes of whose zero-field splitting tensors ($b_2^m$) are found to be oriented very close to each other, exhibit ferroelectric phase transitions of entirely different natures, although occurring at the same transition temperature ($T_c$=223 K). At $T_c$, the EPR lines corresponding to one center exhibit a jump (first-order transition), while those for the other a continuity (second-order transition). For both the centers each EPR hyperfine line splits into two below $T_c$. The ${\mathrm{Mn}}^{2+}$ spin-Hamiltonian parameters in (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{SO}}_4$ are evaluated at room temperature, using a rigorous least-squares-fitting procedure, combined with numerical diagonalization of the spin-Hamiltonian matrix. The present EPR data confirm the existence of two inequivalent sublattices in the (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{SO}}_4$ crystal. The unusual crossing of the EPR line positions, below $T_c$, for the ${\mathrm{Mn}}^{2+}$ center which undergoes a first-order phase transition at $T_c$, has been related to the reversal of the spontaneous polarization. The dynamic behavior of the two ${\mathrm{Mn}}^{2+}$ centers, in this ferroelastic crystal, has here been interpreted to be due to the deformation of the ${\mathrm{SO}}_4^{2\mathrm{-}}$ groups and the spontaneous strain produced by the acoustic mode. The critical exponent β has been determined, for both the centers, to be 0.49±0.03, from the line splitting below $T_c$. It has been verified that the scaling law and Rushbrooke inequality are well satisfied in (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{SO}}_4$. .AE