THERMAL-STABILITY OF PB(ZN1 3NB2/3)O-3 (PZN) AND CONSIDERATION OF STABILIZATION CONDITIONS OF PEROVSKITE-TYPE COMPOUNDS/

Thermal stability of Pb(Zn1/3Nb2/3O3 (PZN) single crystal was examined in the saturated PbO vapor at 1150 degrees C for 2h and found that PZ N single crystal decomposes into pyrochlore type compound, PbO and ZnO , which indicating that PZN is unstable at high temperature. The reaso n why PZN single crystal can be grown in the PbO flux at temperature h igher than the decomposition into pyrochlore was explained by applying the Ostwald's step rule. The reason PZN can be stabilized by partial substitution of A or B site cations was considered on the basis of the decrease of steric hindrance of lone pair of Pb2+ in the perovskite l attice and decrease of mutual interactions between lone pair and Zn2+. The result of electrostatic potential calculation for each cation and anion in Pb(B'B'')O-3 showed that the site potential ratio, phi(B')/p hi(B'') (the valence of B' is lower than that of B'') is much smaller in A(2+)(B1/32+B2/35+)O-3 and A(2+)(B1/22+B1/26+)O-3 than in A(2+)(B1/ 23+B1/25+)O-3 and A(2+)(B2/33+B1/36+)O-3 perovskite type compounds. Th e result suggested that in A(2+)(B1/32+B2/35+)O-3 and A(2+)(B1/22+B1/2 6+)O-3 type perovskite type compounds, large electronegativity differe nce from oxygen is required for B' cation to decrease interaction with lone pair of Pb2+ in addition to the geometrical limit of the ion rad ius prescribed by the tolerance factor.