Ordered structure and thermal expansion in tungsten bronze Pb ₂K_0.5Li_0.5Nb₅O₁₅

The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb₂K_0.5Li _0.5Nb₅O₁₅, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb₂K_0.5Li_0.5Nb ₅O₁₅ displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2₁m. The transition to a paraelectric phase (P4/mbm) occurs at 500°C. Compared with Pb₂KNb₅O ₁₅ (PKN), the substitution of 0.5K⁺ with small 0.5Li ⁺ into PKN causes the tilting of NbO₆ octahedra away from the c axis with Δθ ≈ 10°and raises the Curie temperature by 40°C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500°C. We present that, by introduction of Li⁺, the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb²⁺ in the pentagonal caves. © 2014 American Chemical Society.