GD3+ EPR STUDY OF NH4CE(SO4)2.4H2O AND NH4SM(SO4)2.4H2O SINGLE-CRYSTALS - STRUCTURAL PHASE-TRANSITIONS AND THE SYSTEMATICS OF THE ZERO-FIELD-SPLITTING PARAMETER IN GD3+-DOPED ISOSTRUCTURAL NH4R(SO4)2.4H2O (R = LA,CE,PR,ND,SM,EU) SINGLE-CRYSTALS

X-band (approximately 9.5-GHz) EPR measurements have been carried out on Gd3+-doped NH4Ce(SO4)2.4H2O (ACST) and NH4Sm(SO4)2.4H2O (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 Gd3+ in ACST and ASST hosts have been evaluated; as well, the values of the intrinsic parameters b2 for Gd3+ in ACST and ASST crystals have been estimated by the use of the superposition model of Newman. The present and previously reported results on Gd3+-doped isostructural single crystals of NH4R(SO4)2.4H2O (R = La,Ce,Pr,Nd,Sm,Eu) have been analyzed in order to unravel the systematics of the zero-field-splitting parameter b2(0) and the intrinsic parameter b2BAR of the Gd3+ ion in these hosts; it is found that both are linear functions of the radius of the host rare-earth ion.