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 (Sn0.85Pb0.15)P2S6. However, for (Sn0.975Ge0.025)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.
