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Semiconductor-to-insulator transition of undoped- in quenched state
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Image of FIG. 1.
FIG. 1.

Defect structure of in equilibrium state at elevated temperatures when doped with fixed-valent acceptors, e.g., (a) and with variable-valent acceptors, e.g., (b). The oxygen activity for is denoted as and that for conductivity-transition as . Note that ( for type-transition) and fall in the same majority disorder regime when is fixed-valent but in different disorder regimes ( and , respectively) when variable-valent. The total amount of is denoted by “.” Reproduced from Ref. 1.

Image of FIG. 2.
FIG. 2.

Microstructures of undoped specimens made of custom-made powders with three different Ba/Ti ratios, (a) , (b) , and (c) . The mean grain sizes are , , and , respectively.

Image of FIG. 3.
FIG. 3.

Electrical conductivities vs oxygen activity of undoped made of powders from different sources and different Ba/Ti ratios, A-3N (◇), A-4N(▲), K-3N(▼), (●), (◼), and (★), in equilibrium state at (a) and in quenched state at [(b) and (c)]. For comparison purpose are shown the results of 1.8 mol % Al-doped (BT:Al) at (, from Ref. 7) and (◁, from Ref. 1), and of 1.0 mol % Mn-doped (BT:Mn) at (▷, from Ref. 1). Solid curves through the data in part (a) are as calculated and the nearby triangles to denote the ideal slope of ±1/4; solid lines in part (b) and (c), only for visual guidance. Note a V-shaped, type-transition on each equilibrium-conductivity isotherm and an L-shaped, conductivity-transition on each quenched-state conductivity isotherm.

Image of FIG. 4.
FIG. 4.

Color of as-quenched specimen vs quenching atmosphere oxygen activity corresponding to the quenched-state conductivity in Figs. 3(b) and 3(c). Note the change of color shade crossing the thick demarcating line in each column, representing a semiconductor (black shade) to insulator (brown shade) transition.


Generic image for table
Table I.

Best estimates for the type-transition activity , conductivity-transition activity , ionic conductivity , oxygen vacancy concentration in number/lattice molecule, and energy level above the valence band edge of the present specimens, together with those of BT:Al and BT:Mn for comparison.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Semiconductor-to-insulator transition of undoped-BaTiO3 in quenched state