The influence of the oxygen stoichiometry on magnetization and relaxation of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R=Y,Tm) single crystals was studied for 0≤δ≤0.55. The field dependences of the shielding currents ${\mathit{j}}_{\mathit{s}}$(H) and flux creep rates S(H) were analyzed. Three different kinds of ${\mathit{j}}_{\mathit{s}}$(H) and S(H) behavior can be seen in highly oxygenated samples (δ0.1, when the fishtail position shifts below the matching field. (iii) The low-temperature region is characterized by a monotonic decrease of ${\mathit{j}}_{\mathit{s}}$(B). This region becomes narrower with larger δ and disappears for δ>0.3. Thus in highly deoxygenated samples, the fishtail feature is observed for practically all temperatures. The possible connection of the low-temperature region with the pinning of the small vortex bundles or with the slipping of vortices along the twin planes is considered.

This paper presents an overview of the data available in the literature and the results obtained by the authors in the study of the influence of the chemical composition on the structure and properties of crystals in the potassium titanyl phosphate family. The structure and properties of phosphates in which univalent cations are substituted for potassium cations, as well as of crystals with isovalent or heterovalent substitutions for tetravalent titanium cations in the structure of potassium titanyl phosphate crystals, are considered.