The results of the search for dopants to optimize the ferroelectric phase transition temperature and pyroelectric and piezoelectric properties of Sn(2)P(2)S(6) crystals are reported. Among all dopants only germanium causes a pronounced shift of the phase transition toward higher temperatures. The highest Curie temperature achieved by Ge doping is as high as 88 degrees C. The major advantage of the Ge-doped crystals as compared to pure Sn(2)P(2)S(6) is that the hydrostatic piezoelectric and pyroelectric response is kept high in a wider temperature range with much lower temperature dependence. The effects of the doping on pyroelectric and piezoelectric properties, characterizing the sensitivity of the material to thermal and hydroacoustic excitation, are discussed.

direction in the incommensurate phase, showing the prime importance of such movements to the incommensurate phase formation.

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Abstract. The memory effect in the incommensurate phase of the semiconducting ferroelectric SnzPzSes is studied by dielectric measurements. The mechanism of the memory effect is associated with the pinning of the incommensumte modulauon by a charged, most probably dipolar, defect density wave. It is suggested that a defect density wave is formed by a recharging Of certain ions in the crystal lattice. The spatial distribution of the defects is defined by the population of trapping levels in the band gap.Comparison of the shape and temperature behaviour of the memory effect anomaly with theory and additional &periments. such as measurement of the thermostimulated current and determination of the activation energy af the c h q e c-ers by various techniques, showed that the memory effect for the most p m Originates in the modulated population of the charge carriers an trap levels in the band gap

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We have established the magnetic-field independence up to 20 T of a capacitance thermometer based on the incommensurate crystal ͑Pb 0.45 Sn 0.55 ͒ 2 P 2 Se 6 in the temperature range from 25 mK to 0.5 K. The sensitivity of the thermometer, d ln C/d ln T, is 5ϫ10 Ϫ3 at low temperatures and is about an order of magnitude larger than for any other previously reported capacitance thermometer. The most sensitive temperature range of the thermometer can be tuned by varying the measuring ac frequency and voltage. The reproducibility of the thermometer upon cycling below 1 K is better than 1 mK.

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