Formation and thermal stability of hydrate-like compounds of Ba ₂(In_1-xM_x^III) ₂O5·nH₂O (M^III = Ga, Sc, Lu, and Y)

The thermal stability of the hydrate-like Ba₂(In _1-xM_x^III)₂O ₅·nH₂O, in which M^III denotes the trivalent elements Ga, Sc, Lu, and Y, was studied. When brownmillerite and its related type Ba₂(In_1-xM_x^III) ₂O₅ were cooled from 873 K to room temperature in a thermogravimetry-differential thermal analysis (TG-DTA) apparatus under a wet atmosphere, the exothermal peak in DTA accompanying an abrupt mass increase attributable to the formation of hydrate-like Ba₂(In _1-xM_x^III)₂O ₅·nH₂O was observed; the mass increase occurred in the temperature range of 573-673 K. For M^III = Sc, Lu, and Y, the value of n equaled 1, and the structure was the same as that of Ba ₂In₂O₅·H₂O. For M ^III = Ga, the hydrate-like compound of Ba₂(In _1-xGa_x)₂O₅·0.6H₂O (n = 0.6) appeared. The phase transformation temperature from that of Ba ₂(In_1-xM_x^III)₂O ₅ into that of Ba₂(In_1-xM_x ^III)₂O₅·nH₂O for M ^III = Sc, Lu, and Y was ∼50 K higher than that for the unsubstituted Ba₂In₂O₅·H₂O, while it was 10-20 K lower for M^III = Ga than for Ba ₂In₂O₅·H₂O. The increase in thermal stability by ∼50 K of Ba₂(In_1-xM _x^III)₂O₅·H₂O with M^III = Sc, Lu, and Y was discussed in terms of the higher affinity for the hydroxyl group of the basic Sc, Lu, and Y than that of amphoteric In and Ga.