From this it can be seen that in forming either the 3 : 5 or 2: 1 structures from the molten state or the oxides, the aluminum must decrease its coordination. On the other hand, the coordination of this ion in YAIOJ is the same or greater than in the oxides or in the molten state. As a result, YAlOa is readily formed from these starting materials even though it is a metastable compound under equilibrium conditions. The aluminum oxygen octahedra are, of course, the most important structural element of these structures and hence the yttrium plays a minor role in metastability.One of the most important results found from this study and another one1* involving the tetravalent ceramic oxides F. A. Mumpton and Rustum Roy, "Experimental Study of the Zircon-Thorite Group, Bull. Geol. Soc. Am., 68,1772 (1957).is that very great difficulty is encountered in attaining equilibrium or even in obtaining indications of the direction of equilibrium in systems which contain ions with high Z / r values because of the high bond strength between the cation and the surrounding oxygen ions. Previously this has been observed in silicate systems, but more recently it has also been found in aluminate, zirconate, and titanate systems. Indeed, the surprising fact is that the problem is even more acute in the latter systems than in silicates. Highradial coupling coefficients are observed for compositions having up to 50 mole % sodium niobate additions to potassium niobate. The activity diminishes with additional sodium niobate content and disappears beyond about 98 mole yo additions. Dielectric constants are relatively low, varying from 450 to 125, depending on composition. A.-c. losses are high and d.-c. resistivities are -lo1* ohm-cm. The low dielectric constants and high coupling obtained in certain regions of the system make these materials promising for solid ultrasonic delay line use, especially where thin sectioned plates are required, as in high-frequency thickness ertensional or thickness shear mode transducers.

Compositions in the system KNbO3‐NaNbO2 have been hot‐pressed to yield ceramics with relative densities greater than 99%. Because these materials lack any degree of pyroplastic behavior, temperatures approaching those required for air sintering are needed. These hot‐pressed specimens exhibit a finer microstructure than that obtainable by conventional processing. Enhanced piezoelectric coefficients are observed, and in certain compositional regions radial coupling coefficients have been increased from 32 to approximately 48%. Dielectric constants increase with densification and ac losses remain relatively high. The fine structure and improved piezoelectric activity, especially near the equimolar composition, make these materials desirable for electromechanical transducers, particularly in the 10 to 20 Mc per second range. The rate equation of Murray, Live, and Williams for hot‐pressing ceramics was found to apply in this system.

In the preparation of barium titanate ceramics for dielectric and piezoelectric applications, careful attention must be given to many factors which determine the ultimate performance of the ceramic. In this study structural differences were observed with the aid of electron micrographs as the time and temperature of firing were varied. At lower temperatures and shorter firing intervals crystals remained small, grain-boundary areas were relatively large, and there were numerous voids where several crystals joined. As the time and/or temperature were increased, crystals became larger with an improved grainboundary condition that resulted in better structure. An optimum condition was soon reached beyond which rounding of edges at grain boundaries where several crystals joined became more pronounced. This caused an increase in the number of voids. Accompanying these changes, the 1-kc. dielectric constant, the piezoelectric constants (thickness mode), and coupling coefficients first improved, reached optimum values, and then declined slightly, but the resistance to depoling continually decreased as the crystal size increased. An experimental technique of rapid heating and cooling was used to prepare dense specimens of chemically pure barium titanate with relatively small average crystal sizes. Dielectric constants as high as 3000 with Q values of 80 were obtained for these high-density ceramics, accompanied by low effective piezoelectric constants and coupling coefficients.

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