Ordering behaviors of divalent and pentavalent cations in lead magnesium niobates modified by lanthanum, nickel, zinc, and cadmium

The ordering behaviors of the divalent and pentavalent cations in the well-known relaxor ferroelectric lead magnesium niobates, the lead of which are partly replaced by lanthanum and the magnesium of which are partly replaced by isovalent dopants having different ionic radii such as nickel, zinc, and cadmium, Pb1−xLax{(Mg1−yDy)(1+x)/3Nb(2−x)/3}O3 where D=Ni, Zn, and Cd, x=0∼0.2 and y=0∼0.2, were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). It was revealed that the B′-site cations (Mg2+–Ni2+, Mg2+–Zn2+, and Mg2+–Cd2+) are disordered while these B′-site cations and the B′′-site cations (Nb5+) are 1:1 ordered within the ordered nano-domains dispersed in the Nb5+-rich disordered matrix. For a given La3+content, the cadmium modified material always has the highest degree of nonstoichiometric 1:1 ordering, whereas the nickel modified material always has the lowest degree of nonstoichiometric 1:1 ordering. The results were discussed on the basis of the larger ionic size difference between B′- and B′′-site cations in A(<mt4><rm>B&prime;</rm><inf><fen>1<cp style=&0x0022;/&0x0022;></fen>3</inf>^{<rm> </rm><hsp sp=&0x0022;0.5&0x0022;><rm>2</rm>+}</mt4>)O3-type perovskite structure. {rNb5+=0.64 Å<rNi2+=0.69 Å<rMg2+=0.72 Å<rZn2+=0.74 Å<rCd2+=0.95 Å}.