Electron spin resonance of a high-T_c Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below T_c) may indicate that it arises from pair formation much above T_c.

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu²⁺ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

A new structural phase transition is induced in LiKSO₄ crystals by the application of a uniaxial pressure at 77 K. EPR spectra resulting from a MoO₄^3- probe undergo reversible changes as the pressure is increased to about 51 MPa. From a quantitative analysis of the new spectrum above 60 MPa, a space-group change is inferred. These results are corroborated by the evidence of a similar phase transition at 64 K and ambient pressure.

A new phase transition is observed in the KLiSO₄ crystals below 77 K using MoO₄^3- as a paramagnetic probe. The EPR spectra of this molecular ion, which substitutes for the host SO₄^2- ion, show triclinic symmetry at 77 K due to the joint action of a trigonal crystal field and a trigonal Jahn-Teller effect of E(X)e type. Above 77 K, motional averaging in the a-b plane occurs; thereby an axial spectrum is observed. Below 64 K, the two sulphate sites are no longer equivalent resulting in the appearance of two distinct EPR spectra of the molybdate ion. The possible space groups of the crystal in the vicinity of the phase transition are discussed. The authors' results indicate that one needs to distinguish carefully between the local dynamics and the structural changes in this crystal.

A solid state example that can clearly illustrate the rigorous application of IR and Raman selection rules in a pure Td symmetry.

ESR and ENDOR results on irradiated K₂SO₄:BF₄^− crystals are presented. Based on the ESR spectra and their symmetry at the X and Ka bands, a new paramagnetic radical trapped at the sulphate sites is identified. The ENDOR spectra are consistent with the involvement of a fluorine nucleus. The spectra are assigned to the symmetric FSO₂ radical.

Experimental results on KBr:ClO^−_4 single crystals, before and after x irradiation at 77 K, are reported. Infrared and polarized Raman spectra show clearly that the perchlorate molecules are isolated in the KBr matrix and possess T_d  symmetry. EPR spectra, at 9.5 and 36 GHz, of irradiated crystals at 4.2 K indicate the formation of ClO^2− ₄ in trigonal symmetry. The g and ³⁵Cl hyperfinetensors are found to be strictly axial and collinear. Temperature variation of the EPRspectra presented no motional averaging while uniaxial stress applied to the crystals yielded negative results. The structure of the ClO^2−₄ molecule is discussed in terms of a trigonal Jahn–Teller effect and a probable dissociation into (ClO^−_3 – O^−) complex.

Electron paramagnetic resonance of CrO²Â ^− ₄doped K₂SO crystals has been studied after xâ€�irradiation at 77 K. The resulting species, a CrO₄ ³Â ^− center, has the principal g values g _x=1.8743, g _y =1.8507, and g _z =1.9386. The EPR data indicate a small distortion of CrO₄ ³Â ^−tetrahedra. A temperature variation study of the spectra in the range 1.6–77 K shows that they broaden rapidly above 10 K and no motionally averaged spectrum is observed. The experimental results are discussed in terms of several effects including a possible Jahn–Teller effect.

E.P.R. of MnO^2- ₄ centres in KBr single crystals has been investigated at 9·5 and 35 GHz in the temperature range 1·6-77 K. At 4·2 K, the spectrum exhibits a more nearly cubic anisotropy. A temperature variation study indicates that the spectra disappear reversibly above about 8 K. Uniaxial stress measurements at X-band have shown the importance of random strain.

For the first time, experimental results have been obtained on a transition-metal tetroxy anion stabilized without any associated defects in a cubic crystal: the (ReO₄)^2- ion in KC1. Our EPR study, including experiments under uniaxial stress and static electric fields applied to the crystal, shows that a strong and essentially intramolecular Jahn-Teller effect occurs but not exclusively of the E⊗e type. Appropriate linear combinations of the T₂ vibrations involving a pseudo-Jahn-Teller effect and spin-orbit coupling must play an important part.