A structure and phase analysis investigation of the '1:1' ordered A2InNbO6 perovskites (A=Ca2+, Sr2+, Ba2+)

The structures of the Ba{sub 2}InNbO{sub 6}, Sr{sub 2}InNbO{sub 6} and Ca{sub 2}InNbO{sub 6} ''1:1'' complex perovskites have been refined from neutron powder diffraction data. Both the A=Ca and Sr compounds occur at room temperature in P12{sub 1}/n1 (a=a{sub p}+b{sub p}, b=-a{sub p}+b{sub p}, c=2c{sub p}) perovskite-related superstructures while the A=Ba compound occurs in the Fm3-bar m, a=2a{sub p}, elpasolite structure type. In the case of the A=Ca compound, an extensive Ca{sub 2}[(Ca{sub 2x/3}In{sub 1-x}Nb{sub x/3})Nb]O{sub 6} 'solid solution' field spanning compositions between Ca{sub 4}Nb{sub 2}O{sub 9} and Ca{sub 2}InNbO{sub 6} in the CaO-InO{sub 3/2}-NbO{sub 5/2} ternary phase diagram is shown to exist. Under the conditions of synthesis used, the 'solid solution' field stops just short of the ideal 1:1 Ca{sub 2}InNbO{sub 6} composition.