After briefly reviewing the theory and instrumentation, results from a variety of experiments carried out by the authors on the photoacoustic spectroscopy of solids and surfaces by employing an indigenous spectrometer are discussed in the light of the recent literature. Some of the important findings discussed are, phase angle spectroscopy, anomalous behaviour of monolayers, unusual frequency dependence in small cell volumes, spectra of a variety of solids including amorphous arsenic chalcogenides, photoacoustic detection of phase transitions and determination of surface areas and surface acidities of oxides. Recent developments such as piezoelectric photoacoustic spectroscopy, depth profiling and subsurface imaging are also presented.

Magnetic susceptibility studies of Mn_1-xM_xO (M = Zn, x≤0.1; M = Mg, x≤0.12;M = Fe, x≤0.4) in the range 77 to 300 K are reported. The methods of preparation of Mn_1-xM_xO systems preclude the presence of trivalent ions. The M_1-xFe_xO system shows anomalous behaviour aroundx = 0.2–0.3. The results are discussed in terms of competition between the nearest neighbour and the next-near-neighbour interactions, dilution effects and cooperative effects of FeO₆ octahedra.

The metal-insulator transition in 3d transition metal oxides such as the ternary perovskite oxides as well as the binary oxides both as a function of composition and temperature are reviewed. The description of metallisation in terms of a change in sign of the temperature coefficient of resistance as well as conductivity at 0 K are considered in terms of modern theories based on localisation. It is pointed out that there may be grounds to believe that in the high temperature limit a resistivity less than ≈10⁻¹ ohm cm may be associated with metallic properties despite a negative temperature coefficient of resistance. The correlation between magnetic and electrical properties have also been examined.

Quasi two-dimensional copper oxides derived from La_1·8Sr_0·2CuO₄ by substitution of La, Sr or Cu show highT_c superconductivity.

The prominent “1/3” effect observed in the Hall effect plateaus of two-dimensional electron gas (2DEG) systems has been postulated to indicating 1/3 fractional charge quasiparticle excitations arising from electron-electron interactions. Tunneling shot-noise experiments on 2DEF exhibiting fractional quantum Hall effect (FQHE) shows evidence for tunnelling of particles with eand e/3 charges for a constant band mass. A “1/3” effect in the hydrogen molecule is seen in as much as its internuclear distance,d_H-H = D⁻ + D⁺, with ¦D⁺/D⁻¦ = 1/3. This is examined in terms of electron-electron interactions involving electron-and hole quasiparticles, (e^-)and(h⁺), equivalent to those observed in FQHE shot -noise experiments. The(e/m) ratio of the (e⁻) and(h⁺) quasiparticles is kept at 1: −3. Instead of a 2DEG, these particles are treated as being in flat Bohr orbits. A treatment in the language of charge-flux tube composites for the hydrogen atom as well as the hydrogen molecule is attempted. Such treatment gives important insights into changes in chemical potential and bond energy on crossing a phase boundary during the atom-bond transition as well as on models for FQHE itself.