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

Abstract

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 ${\mathrm{SO}}_{4}^{2\mathrm{\ensuremath{-}}}$ groups and the spontaneous strain produced by the acoustic mode. The critical exponent \ensuremath{\beta} has been determined, for both the centers, to be 0.49\ifmmode\pm\else\textpm\fi{}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