Invar Effect in PbFe1/2Nb1/2O3 Ceramics

High-density lead ferroniobate PbFe1/2Nb1/2O3 (PFN) is prepared by conventional ceramic technology. Its structural properties are studied in a wide temperature range (293 <= T <= 973 K). The following chain of phase transitions is established in the vicinity of the transition to the polar phase: Rh (rhombohedral phase) (T < 363 K) -> Psc (pseudo-cubic phase) (363 < T < 387 K) -> C (cubic phase) (T > 387 K). The paraelectric range contains five ranges of constant unit-cell volume (invar effect): I (387 <= T <= 413 K), II (433 <= T <= 463 K), III (553 <= T <= 613 K), IV (743 <= T <= 773 K), and V (798 <= T <= 823 K). It is shown that the anomalous behavior of the PFN dielectric characteristics above the Curie temperature, which was revealed previously, is associated with the specific features of its real (defect) structure, which is caused by the crystal-chemical specificity of the main structure-forming agents: alpha-Fe2O3 and alpha(ht)-Nb2O5.