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The new piezoelectric solid solution aluminium gallium orthophosphate single crystals have been successfully developed for the first time in our laboratory by hydrothermal method. It has been demonstrated that these crystals are homogeneous, and isomorphous with AIPOl and GaP04. The solid solution crystals A11-.GaXPOJ are easy to grown and have better optical quality than those of Alp04 crystals under the same conditions. A phase transition temperature T,-p is 587 f 3 "C for x = 0.12. Cell parameters, and elastic, piezoelectric, and dielectric constants are given.

In this paper, our work which was performed on order-disorder ferroelectrics by the Green's function method is reviewed. First.the Bose Green's function method is modified by introducing thrre undetermined constants to make the method self-consistent, If the constants are not introduced, the method would give a contradictory result.Then we apply this method to the transverse king model with four-spin coupling to study the properties of phase transitions of order-disorder ferroelectrics. A new condition to determine the transition temperature T, and the discontinuity at the transition temperature (F)< has been found. We have also determined the critical values of the model parameters, beyond which the transition is first order. Finally, a comparison of our results with the experimental data of PbHPO, and PbDPO, indicates that our method is more suitable for describing PbHPO,-type crystals, and can give a quantitative explanation.

A new approach employing only two differently oriented crystal plates to determine all the dielectric, piezoelectric, and elastic constants of crystals in class 32 has been put forward. Meanwhile, a method to improve the determination accuracy of the clamped dielectric constants of crystals in class 32 also has been suggested. For example, all the constants of a-quartz have been measured by the approach. The experimental results of o;-quartz have shown that the approach is very efFicient.

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