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On the basis of an analysis of data on a large number of compounds exhibiting displacive phase transitions, it is observed (with no known exceptions) that the transition temperature decreases with hydrostatic pressure for those transitions associated with soft zonecenter optic phonons and increases for those transitions associated with soft zone-boundary optic phonons. This difference in behavior is tentatively interpreted in terms of a reversal in the roles of the short-range and long-range forces in the lattice dynamics of the two cases.

We have measured the temperature dependence of the dielectric constants for singledomain SrTi0 3 crystals. Dielectric anisotropics and deviations from the simple Curie-Weiss law have been observed and are discussed in connection with the structural phase transitions at 110 and 65°K.The structural phase transition at T a~ 110°K in SrTi0 3 is a direct consequence of the condensation of the r 25 optical-phonon mode at the Brillouin-zone boundary. 1 " 4 The dielectric constant of this crystal was reported to obey a simple form, e = C(T-T c )~1 9 with paraelectric Curie temperature T C «35°K. At lower temperatures, an appreciable departure from the Curie-Weiss law was observed, which was ascribed to the quantum effect of ionic polarizability. 5 " 8 Hitherto, the dielectric properties were obscured by the presence of multiple domains when the crystal is cooled through T a . The nonferroelectric structural transitions were generally considered not to affect the dielectric behavior. Recently, it was demonstrated that a uniaxial compressive stress can produce a single-domain crystal at low temperatures. 9,10 We report here dielectric measurements on single-domain crystals produced by this technique.A tetragonal prism with surfaces of (110), (ITO), and (001) and of size 2x2x3 mm 3 was cut from a pure single crystal. Geometries of the crystal axes, electrodes for dielectric measurement, and the direction of compressive stress are shown in Fig. 1, Samples were always cooled from room temperature to liquid-helium temperature under an applied stress higher than 10 kg/ mm 2 and the stress was released during dielectric measurement in order to avoid the pressure dependence of the permittivity. This procedure (a) (b) (c) (d) FIG. 1. Geometries of crystal axes, electrodes, external uniaxial stress (thick arrows) and the direction of c axes in the sample (thin arrows), for measuring (a) e a , (b) € c , (c) € 110 , and (d) efo.Dielectric permittivity was measured by a threeterminal method using a GR model 1615A capacitance bridge at 10 kHz with a field level of 5 V/ cm. Temperature inside the sample cryostat was raised slowly, at the rate of 0.2°C/min.Components of the dielectric permittivity tensor for crystals subjected to the stress treatment were determined as functions of temperature, with results as shown in Fig. 2. In this figure we notice that the dielectric tensor becomes anisotropic below T a9 with e a larger than e c . Furthermore, the dielectric anisotropy in the plane perpendicular to the c axis was found to develop around T b ^65°K 12_14 ; that is, [110] and [1T0] axes I-i-i-i-i-|-i-i-i-i-|-i-i-i-i-|-i-i-i-r 0 50 100 150 200 TEMPERATURE (°K) FIG. 2. Temperature dependence of the dielectric constants in SrTi0 3 for various orientations (see Fig. 1). The insets show the details of the dielectric anisotropics near T a and T b9 respectively. 851

Shubnikov de Haas(SdH) oscillations have been measured for the semiconducting single domain SrTiO3 with Carrier concentration of 3.4×1018 cm-3 and 5.9×1018 cm-3. From the temperature dependence of the SdH amplitude, the cyclotron mass values are determined for the [001] and [110] magnetic field directions. We have found, from the angular dependence of the SdH oscillation, that there exist several branches of the frequencies among which the highest and lowest branches originate from the Fermi spheres at the Brillouin zone center, and the other ones from the extremal area which is interconnected through the magnetic breakdown tunneling paths. Energy band parameters based on the k p perturbation method are determined.

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The nonlinear optical coefficients of AlN have been determined as ‖d33‖= (23.2±35%) dQ11 and ‖d31‖ ≲0.04‖d33‖ for the c-plate film and for a fundamental beam at 1.064 μm. The result is discussed with reference to the bond-charge model.