Gd3+EPR study ofNH4Ce(SO4)2⋅4H2O andNH4Sm(SO4)2⋅4H2O single crystals: Structural phase transitions and the systematics of the zero-field-splitting parameter inGd3+-doped isostructuralNH4R(SO4)2⋅4H2O (R=La,Ce,Pr,Nd,Sm,Eu) single crystals

X-band (∼9.5-GHz) EPR measurements have been carried out on ${\mathrm{Gd}}^{3+}$-doped ${\mathrm{NH}}_4$Ce(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O (ACST) and ${\mathrm{NH}}_4$Sm(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O (ASST) single crystals in the temperature range 4.2–295 K. The features of the EPR spectra indicate the occurrences of first-order phase transitions at 34±0.5, 142±0.5, 164±0.5, and 247±0.5 K for ACST, and at 14±0.5, 165±0.5, 179±0.5, and 269±0.5 for ASST; the phase transitions detected below 77 K have not been detected by any other technique. The room-temperature values of the spin-Hamiltonian parameters for ${\mathrm{Gd}}^{3+}$ in ACST and ASST hosts have been evaluated; as well, the values of the intrinsic parameters b${\mathrm{¯}}_2$ for ${\mathrm{Gd}}^{3+}$ in ACST and ASST crystals have been estimated by the use of the superposition model of Newman. The present and previously reported results on ${\mathrm{Gd}}^{3+}$-doped isostructural single crystals of ${\mathrm{NH}}_4$R(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O (R=La,Ce,Pr,Nd,Sm,Eu) have been analyzed in order to unravel the systematics of the zero-field-splitting parameter ${\mathit{b}}_2^0$ and the intrinsic parameter b${\mathrm{¯}}_2$ of the ${\mathrm{Gd}}^{3+}$ ion in these hosts; it is found that both are linear functions of the radius of the host rare-earth ion.