The metastable perovskite phases of the compounds PbB1/23+Nb1/2O3 (B3+ = Er, Tb, Gd, Eu, Sm) were first obtained under high pressures at high temperatures, and their stability limits at atmospheric pressure were determined. They are characterized by a monoclinic crystal lattice distortion. For the PbB1/23+Nb1/2O3 compound series with the isomorphous substitution B3+, graphs of average lattice parameters of the perovskite and pyrochlore phases, the change of the specific unit cell volume at the perovskite-pyrochlore transition, the temperature of maximum dielectric permittivity, and the temperature of structure anomalies as functions of the B3+ cation radius were constructed and are discussed.