Proton, Deuterium, and Sodium-23 Magnetic Resonance of Ferroelectric Lecontite NaNH4SO4·2H2O

Abstract

Proton, deuterium, and sodium-23 nuclear magnetic resonances of deuterated and ordinary lecontite have been studied from 77° to 300°K and particularly in the vicinity of the ferroelectric phase transitions at 92° and 101°K. Proton–proton vector directions were determined for the two kinds of water molecules in the unit cell and are observed not to alter as the phase transitions are traversed. Above 230°K the deuterium magnetic resonance of one type of water molecule is observed in the 180° flip state of motion. Below 101°K axial and equatorial deuterons of ND4+ groups rotating about the threefold axis are observed with no detectable change at the transition temperatures. At 300°K one kind of 23Na site is observed; below 101°K, 23Na satellite line splitting occurs indicating that there are two kinds of 23Na site in the unit cell. This splitting is attributed to an abrupt lattice distortion which occurs as the crystal is cooled below 101°K. Proton spin–lattice relaxation time data show two minima, one at 170°K and one at 120°K, which are, respectively, assigned to rotation about the twofold and threefold axes of the NH4+ tetrahedron. Rotating frame relaxation times show minima at 80°, 110°, and 210°K, respectively, due to the threefold and twofold axis rotations of the NH4+ ion and a 180°, flip motion of one kind of H2O molecule. Near 101°K the activation energy for the threefold axis rotation of the NH4+ ion changes from 2.0 to 3.1 kcal mole−1. It is proposed on the basis of the magnetic resonance data that the transition at 101°K in lecontite is of the displacive type with a relatively large lattice distortion.

A study of the lattice region (0–400 cm−1) of the Raman spectrum of a single crystal of sodium ammonium sulphate dihydrate, NaNH4SO4·2H2O over the temperature range 293–87 K

“…The appearance of two ammonium librational frequencies in the spectrum below T, tends to confirm our conclusion of the formation of pairs of non-equivalent ions below the phase transition. This conclusion supports that from the magnetic resonance measurements [2] where the presence of two non-equivalent sodium ions were detected below the phase transition.…”

“…The nature and mechanism of these phase transitions have been investigated by magnetic resonance studies [2] of crystals of SASD and the deuterated compound SASD-d,. It was found that the proton-proton vectors in SASD and the deuterondeuteron vectors in SASD-d, were invariant indicating that the water molecules are not directly involved in either of the two phase transitions.…”

Re-refinement of sodium ammonium sulfate dihydrate at 170 K

“…Arzt & Glazer (1994) redetermined the crystal structure at room temperature based on serial detector data. Properties of the well-known ferroelectric SASD have been widely studied (Arzt & Glazer, 1994;Fawcett et al, 1975;Genin & O'Reilly, 1969;Hilczer et al, 1991Hilczer et al, , 1992Hilczer et al, , 1993Kanesaka & Ozaki, 1994;Kassem & Hedewy, 1988;Kloprogge et al, 2006;Lipinski et al, 2003;Lipinski & Kuriata, 2005;Ono et al, 1993;Osaka, 1978;Osaka & Makita, 1970;Ribeiro et al, 2006). Kloprogge et al (2006) also reported a Rietveld refinement of the structure of SASD at room temperature, thereby confirming the results of the previous single-crystal X-ray analyses.…”

Low temperature synthesis and characterisation of lecontite, (NH4)Na(SO4)·2H20

“…[7]). The vibrational spectroscopy has been hardly described and mainly focussed on for example the optical absorption spectra of Ni 2+ doped lecontite [8] The paramagnetic resonance of Cr 3+ ions in lecontite has also been studied [9], while Genin and O'Reilly have described the proton, deuterium and sodium-23 magnetic resonance spectra [10].…”