A study of phase transitions in (NH4)3H(SO4)2 crystals through Raman scattering

Abstract

Phase transitions in triammonium hydrogen disulphate (TAHS), occurring below room temperature, are investigated by measuring polarized Raman scattering. A careful examination of changes in frequency, intensity, and width of thermosensitive bands near the transition temperatures is helpful in deciding about the type of phase transition. The value of the order parameter exponent β (0.45 ± 0.03) as estimated by us for the transition at 265 K is consistent with the value for a second‐order transition in the mean field theory approximation. The transitions at 265 and 140 K are found to be of second order, whereas the one at 46 K shows features of a first‐order transition. A coupling of ammonium and sulphate ion translational modes with a soft mode becomes evident on cooling the crystal to 265 K. The increase in ordering of the ammonium ion sublattice seems to be mainly responsible for the transitions at 265 and 140 K. It is determined that hydrogen bonding is strongest in the ferroelectric phase (T < 46 K). The rotation of the SO(2) bond around 46 K appears to be involved in bringing about the transition to the ferroelectric state.

Phase Transitions of Aqueous Atmospheric Particles

“…For this reason, the abscissa in Figure 7 does not terminate but is extended by an arrow. Comparison of Figures 3 and 7 shows that molality skews the appearance of phase diagrams to favor concentrated δT ) K f m solute (1) 66,131 H2SO4/H2O, 132 HCl/H2O, 133 NH4HSO4/H2O, 14,34,135 NaCl/H2O, 136 (NH4)3H(SO4)2/H2O, 134,135 NH4NO3/H2O, 137 and (NH4)2SO4/H2O. 138,139 The solute mole fraction (x) relates to molality (m) and weight percent (w) as follows: m ) 1000x/( electrolytes.…”

“…Abbreviations are as follows: AHS, NH 4 HSO 4 ; AN, NH 4 NO 3 ; AS, (NH 4 ) 2 SO 4 ; NAD, HNO 3 ·2H 2 O; NAM, HNO 3 ·H 2 O; NAT, HNO 3 ·3H 2 O; SAD, H 2 SO 4 ·2H 2 O; SAH, H 2 SO 4 ·6.5H 2 O; SAM, H 2 SO 4 ·H 2 O; SAT, H 2 SO 4 ·4H 2 O; SATr, H 2 SO 4 ·3H 2 O; and THD, (NH 4 ) 3 H(SO 4 ) 2 . Data sources are as follows: HNO 3 /H 2 O, , H 2 SO 4 /H 2 O, HCl/H 2 O, NH 4 HSO 4 /H 2 O, ,, NaCl/H 2 O, (NH 4 ) 3 H(SO 4 ) 2 /H 2 O, , NH 4 NO 3 /H 2 O, and (NH 4 ) 2 SO 4 /H 2 O. , The solute mole fraction ( x ) relates to molality ( m ) and weight percent ( w ) as follows: m = 1000 x /(18(1 − x )) and w = 100 M w x /(18 + ( M w − 18) x ) where M w is the molecular weight of the solute. …”

Raman linewidths calculated as a function of temperature in NaNO2

“…For the damping constant due to pseudospinphonon interactions over a narrow temperature range close to T C where the reorientation of the NO 2 ions occur, the vibrational relaxation (Γ vib ) can be neglected, or it can be taken as independent of temperature close to the critical temperature [66]. This has also been considered for lithium ammonium sulphate (LAS) [62] and (NH 4 ) 3 H(SO 4 ) 2 [63] crystals. By neglecting the vibrational effects close to T C for NaNO 2 , the activation energy U can be calculated due to reorientations of the NO 2 ions from the relation…”