Perovskite lead indium niobate(Pb$(In_{1/2}Nb_{1/2})O_3$, PIN) free of pyrochlore phase has been synthesized using the B site precursor method. $In_2O_3$ and $Nb_2O_5$ were prereacted at 1100$^\circ$C for 5 hr to obtain $InNbO_4$. $InNbO_4$ was calcined with PbO at 850$^\circ$C for 2 hr, and subsequently heated up to 1000$^\circ$C and the process was concluded after holding PIN powder at 1000$^\circ$C for 1 hr.
It has been reported that the single crystal PIN undergoes compositional B site ordering by suitable heat treatments below 1010$^\circ$C. Also, if the compositional B site In/Nb ordering proceeds, the dielectric constants maximum temperatures ($T_a$) shift to a higher temperature. In the light of this feature, the degree of long range order parameter S could be determined.
The clue of compositional B site In/Nb order-disorder transition of the heat-treated ceramic PIN specimen was not detected by X-ray diffractometry and dielectric constansts measurements. The former result is elucidated such as: the calculated X-ray diffraction intensity ratio of the superlattice line of fully ordered PIN and the fundamental line was 0.038 for (1/2 1/2 1/2) superlattice and (110)fundamental. It is too low to detect superlattice line even if fully ordered. The latter is thought as grain boundaries in ceramic PIN will modify the symmetry for micro regions close to the boundary surface, and resulting symmetry lowering may be responsible for the dielectric dispersion. Thus, any shift of Ta was not observed, especially, on the cooling stage of dielectric measurements.
Disordered PIN shows all the characteristic features of the diffuse phase transition. The relation between the reciprocal dielectric constant and temperature obeys power law, the exponent n lying between 1.6 and 1.65. And standard deviation of Curie temperature was fairly larger than that of other perovskite compounds. These facts reflect compositional fluctuation lied in disordered PIN.
To investigate whether spontaneous polarization far above $T_c$ will exist, as proposed by Burns and Cross respectively, pyroelectric current measurements of the specimen field cooled far above $T_c$ and of the normally poled (below $T_c$) specimen were carried out. The main difference between two models for the diffuse phase transition is that local spontaneous polarization exists far above $T_c$ is reversible(Cross') and irreversible(Burns'). Normally poled specimen shows that normal pyroelectric behavior such as pyroelectric coefficient maximum temperature was approximately coincident with the dielectric constant maximum temperature. However, the field-cooled specimen shows spontaneous polarization was induced by applied field far above $T_c$. This will be an indirect evidence for dynamical disorder of reversible spontaneous polarization.
Among the three proposed models of the diffuse phase transition, Cross' superparaelectric model complies well with the experimental results. The differential Scanning calorimetry measurement shows no anomaly associated with $T_a$. This fact suggests another evidence of diffuse phase transition in PIN.
