H.D. Bist

The ferroelectric phase transition in ammonium sulphate

Abstract On the basis of the temperature dependence of typical internal modes associated with SO 2− 4 and NH + 4 ions it is inferred that the ferroelectric phase transition near 223°K in ammonium sulphate in primarily due to sudden distortions in the SO 2− 4 ions.

Vibrational studies and phase transitions in Co(ClO4)2·6H2O and Mn(ClO4)2·6H2O

Abstract A vibrational assignment of cobalt and manganese perchlorate hexadrytes is proposed. Three phase transitions in each salt (at 333,240 and 166 K in the CO salt and at 341,248 and 161 K in the Mn salt) have been observed and have been attributed to the ordering of aquo-octahedral complexes occupying two distinguishable sites.

Vibrational spectra and phase transitions in Ni(CH3COO)24X2O (X = H, D)

Abstract The temperature dependence of the IR (4000—200 cm −1 , between 300 and 90 K) and Raman (4000—40 cm − , between 300 and 130 K) spectra of the title compounds are reported. The bands observed at 90 or 130 K have been correlated with those at 300 K. Analysis of the vibrational spectra indicates one type of acetate and two types of water in the unit cell, a result which is consistent with the room temperature crystallographic data. The temperature evolution of the thermosensitive CH 3 modes in the Raman and IR spectra suggests two reorientational phase transitions at 264 ± 3 and 178 ± 3 K.

Raman scattering study of the phase sequence in A2BX4 halides

Abstract Raman spectra of Cs 2 ZnI 4 single crystal have been measured in different scattering orientations covering the successive phase transitions down to 68 K. Two second order anomalies at around ice temperature (270 K) and at ∼ 92 K are observed. The anomaly around ice temperature exhibits the features of a normal-incommensurate transition evidenced by softening of a low-lying lattice phonon whose frequency decreases as the transition temperature is approached from below whereas the latter one appears to arise from usual structural distortions.

Spectroscopic correlation for OH stretching frequency shifts of water in crystal hydrates

Abstract The shift of the uncoupled OH stretching frequency of water (from its free state position) in crystal hydrates has been plotted against a parameter r 0 obtained by subtracting the H-bond (OH...Y) length from the sum of the free state ionic radii of O and Y. From an analysis of this curve it is inferred that MO w coordination in hydrates reduces the ionic size of water oxygen and its effect on the OH stretching frequency is opposite to that of H-bonding.

Vibrational studies and phase transition in mercury(II) perchlorate hexahydrate

The infrared spectra of polycrystalline mercury(II) perchlorate hexahydrate in the temperature range 100–300 K are reported for the region 4000–200 cm−1. A complete vibrational assignment is proposed using the infrared spectra of the deuterated analog and room temperature Raman spectra of the hydrate. The observed spectral changes in the thermosensitive IR bands are attributed to a transformation of the normal hydrate [Hg(OH2)6]2+ (ClO−4)2 at room temperature to a pseudohydrate [Hg(OH−)3 (H3O+)3]2+ (ClO−4)2 below 243 K.

Hypochromic effect in the bending mode of water at low temperature in Zn(ClO4)2⋅6H2O

The medium intensity infrared (IR) active bending mode of water near 164 cm−1 in zinc perchlorate hexahydrate disappears almost completely at low temperatures.

Host lattice effects in the EPR spectra of Gd3+ ion in some hydrated lanthanide sulfates

Electron paramagentic resonance (EPR) studies of Gd3+ in single crystals of Ln2(SO4)3⋅8H2O (where Ln=Pr, Nd, Sm, Dy, and Er as well as Y) have been carried out at 9.4 GHz and 297 K. The spin‐Hamiltonian parameters have been evaluated for each host. An empirical, linear relationship is found to exist between the magnitude of the Gd3+ spin‐Hamiltonian parameter b02 and the ionic radius of the lanthanide host. A similar relationship is found between the ionic radius of the lanthanide host and the total zero field splittings. A nonlinear relationship is observed between the magnitude of the spin‐Hamiltonian parameter b22 and the ionic radius of the host lanthanide. In the sulfates the b02, the b22, and the total zero field splitting show a variation opposite to that predicted on the basis of a point charge model. The EPR results on the sulfates have been compared with our results in LnCl3⋅6H2O.

Systematics of the librational modes of water in the IR spectra of MSO4 · H2O (M = Mn, Co, Ni, Zn)

Abstract In the isomorphous hydrates MSO 4 · H 2 O (M = Mn, Co, Ni, Zn), where M-O W distance is the only variable, the frequencies of the wagging, rocking and bending modes of water and of metal-oxygen stretching have been shown to correlate smoothly with the distance r (M-O W ). Quantitative relations have been found for ν W and ν(M-O W ) which vary linearly, while ν R and ν C 2 vary non-linearly with r (M-O W ).

Hydrate to pseudohydrate transformation in Cd(ClO4)2 ·6H2O: An I.R. study

Abstract The modes of water in Cd(ClO 4 ) 2 ·6H 2 O exhibit a sharp discontinuity at 248 ± 1 K. The appearance of new bands attributable to the OH − and H 3 O + ions below 248 K indicates the transformation of the normal hydrate to a pseudohydrate. A strong Fermi resonance is observed between v 1 and 2 v 4 bands of H 3 O + .