（Pb $_y$ Sn $_{1-y}$）$_2$ P $_2$ S $_6$混晶的铁电性及随机场BEG模型
Ferroelectricity in (Pb$_y$Sn$_{1-y}$)$_2$P$_2$S$_6$ mixed crystals and random field BEG model
摘要：
对于Sn $_2$ P $_2$ S $_6$铁电体，在压缩到250 K的转变温度下，观察到二级相变线，直至达到三临界点。通过DFT研究Sn $_2$ P $_2$ S $_6$晶体的局域势，得到的温度-压力相图与MC计算得到的相图相吻合。除三临界点外，还发现了无序相和四极相的可能性。对于铅代锡混晶，在铁电转变温度低于250 K附近的“温度水线”时，超声、超声波和低频介电性能也显示出异相特性。硒代硫混晶也出现了类似温度下的三临界。这种行为符合Blume-Emery-Griffiths（BEG）模型，该模型适用于所研究的具有三阱局域势的铁电系统的序参量（自发极化）涨落。
英文摘要：
For Sn$_2$P$_2$S$_6$ ferroelectrics the second order phase transitions line is observed until reaching the tricritical point at transition temperature lowering to 250 K by compression. Observed temperature-pressure phase diagram agrees with simulated diagram by MC calculations based on early founded by DFT study local potential for Sn$_2$P$_2$S$_6$ crystals. In addition to the tricritical point, the possibility of disordered and quadrupolar phases occurrence was shown. For mixed crystals with tin by lead substitution, the investigated ultrasound, hypersound and low frequency dielectric properties also reveal appearance of heterophase peculiarities at decreasing of ferroelectric transition temperature below so named "temperature waterline" near 250~K. The tricriticality at similar temperature level also appears in mixed crystals at sulfur by selenium substitution. Such behavior agree with Blume-Emery-Griffiths (BEG) model, that is appropriated for investigated ferroelectric system with three-well local potential for the order parameter (spontaneous polarization) fluctuations.
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