Tailoring Negative Thermal Expansion in Ferroelectric Sn2P2S6 by Lone-Pair Cations

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• 作者：Yangchun Rong ; Kun Lin ; Fangmin Guo ; Ronghui Kou ; Jun ChenYang Ren ; Xianran Xing
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：January 26, 2017
• 年：2017
• 卷：121
• 期：3
• 页码：1832-1837
• 全文大小：490K
• ISSN：1932-7455

文摘

The rare negative thermal expansion (NTE) in ferroelectrics has received significant attention in lead-titanate perovskite oxides. Recently, a notable NTE of −4.7(1) × 10^–5 K^–1 was reported in a lead-free and nonperovskite ferroelectric Sn₂P₂S₆. The stereochemically active lone-pair of Sn(II) was considered to be responsible for the NTE. Here, the role of the lone-pair is further explored via substitution of Ge(II)/Pb(II) for the cation Sn(II). Both high-energy as well as high-resolution synchrotron X-ray diffraction were employed to reveal the tailored NTE behavior and structure evolution, respectively. Due to the stereochemically inactive Pb(II) 6s² pair when bonding with anion sulfur, the Pb(II)-substitution depresses the ferroelectricity and reduces the NTE of Sn₂P₂S₆ to –1.9(2) × 10^–5/K in (Sn_0.85Pb_0.15)P₂S₆. However, for (Sn_0.975Ge_0.025)P₂S₆, the ferroelectricity is enhanced by the tiny amount of stereochemically active Ge(II) 4s² pair but the NTE is weakened to –3.9(1) × 10^–5/K. The Raman spectra helps reveal the disparate effects of Ge(II)/Pb(II)-substitution on the local/average spontaneous polarization and the NTE. This work clarifies a further understanding of the role of the lone-pair in the spontaneous volume ferroelectrostriction (SVFS) and the NTE among ferroelectrics.