Ultra-violet absorption spectra of ions in crystals are sensitive to their environment of neighbouring ions. This fact is utilized to study melting processes in nitrates and nitrites, with particular reference to changes in the average shell of nearest neighbours around any ion, on passing from crystal to melt. For the nitrates of sodium, potassium, rubidium, caesium, thallium and silver, for ammonium nitrate and for the nitrites of sodium and potassium this shell appears to contract on melting, which is attributed to the formation of association complexes in the melt. For the two crystal hydratesHNO3 . H20andHNO3 . 3H20, corresponding changes are observed, suggesting that the structure and absorption spectra correspond vwith the ions (OH3)+ and NO- in the crystals, likewise giving rise to tighter association complexes on melting. Pure nitric acid on the other hand behaves in a manner more closely similar to isopropyl nitrate. INTROD-ECTION X-ray and neutron-diffraction investigations of structural changes show that longrange order disappears on melting, but that nearest-neighbour packing in many melts is roughly the same as in the crystals. With particular reference to ionic crystals, radial distribution curves have been obtained from X-ray diffraction measurements for molten alkali halides (Agron, Bredig, Danford & Levy i960), carbonates and sulphates (Zarzycki i96i), and nitrates (Furukawa i96i). These can be interpreted on the basis that the cations in a series of salts of the alkali metals assume average positions with respect to any anion; for polyatomic anions these positions are close to the corners of the anions. However, radial distribution curves represent a statistical average that may blur and even obliterate fine details of changes in local environment which may be particularly important for elucidating mechanisms of melting. To describe the structure of melts more accurately, methods with higher resolving power are needed to give additional information about the nearest neighbours to any ion. Increased sensitiveness can sometimes be achieved by evaluating differences of properties between liquid and solid, in relation to differences of structure; this is usually known with fair accuracy for the crystals. In ionic crystals, electrostatic charges on the units of structure give rise to a characteristic strong mode of interaction between near neighbours. This affects mechanisms of melting for this class of crystals, in various distinctive ways. One consideration is that in such crystals, which normally have structures with quite high co-ordination number, electrostatic forces acting on any ion due to its near neighbours tend to be partly neutralized, owing to the effects of crystal symmetry. But on melting, the positional disorder introduced upsets this symmetry. Any strong tendency to alternative packings of lower symmetry can then be more

I. Changes in ultra-violet absorption spectra on melting

Physical applications of photoacoustic spectroscopy

Enhancement of the photoacoustic signal from condensed materials by several folds is achieved by the introduction of a liquid with high vapor pressure in the photoacoustic cell. The enhancement is especially marked for low absorption coefficients and high chopping frequencies. Typically the enhancement is two to nine times in the presence of diethyl ether at 293 K. A linear relationship is observed between the enhancement and the vapor pressure of the liquid.

A Novel Technique For Enhancing Photo-Acoustic Signals From Solids

Photoacoustic spectroscopy (PAS) is a non-destructive testing (NDT) tool and used here for analyzing the physical properties such as thermal diffusivity, thermal diffusion length, thermal conductivity and thermal effusivity of L-Threonine and L-Prolinium tartrate. These two crystals belong to a group of recently developed NLO materials, in our laboratory. The single crystals of the two compounds were grown by using submerged seed solution growth method. Characterization of the crystals was made by using single crystal X-ray diffraction. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoacoustic studies on two new organic NLO materials: L‐threonine and L‐prolinium tartrate

The successive phase transitions in a polycrystalline KNO3 sample have been studied using photacoustic and calorimetric techniques. The measured temperature dependence of the photoacoustic amplitude carries signatures of the transitions undergone by the sample. The results indicate that the intermediate ferroelectric phase in KNO3 is only a metastable one. This raises doubts regarding its applicability in fabricating ferroelectric memory devices.

Stability of the ferroelectric phase in polycrystalline KNO3 investigated by photoacoustic and calorimetric techniques

The use of the photoacoustic effect in the investigation of first- and second-order phase transitions has been examined. Changes in the amplitude of the photoacoustic signal across the phase transition are compared with changes in thermal properties such as specific heat and thermal diffusivity. The systems studied include NaNO2, TlNO3, CsNO3, NH4NO3, BaTiO3, CoO, Cu2HgI4, VO2 and V3O5. The current photoacoustic studies are discussed in the light of the theoretical models available.

/pdf/photoacoustic-investigation-of-phase-transitions-in-solids-3h23976i2w.pdf

P Ganguly

Papers

Photoacoustic investigation of phase transitions in solids