Ordered Structure and Thermal Expansion in Tungsten Bronze Pb₂K_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_0.5Nb₅O₁₅ displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group <i>Bb</i>2₁<i>m</i>. The transition to a paraelectric phase (<i>P</i>4/<i>mbm</i>) occurs at 500 °C. Compared with Pb₂KNb₅O₁₅ (PKN), the substitution of 0.5K⁺ with small 0.5Li⁺ into PKN causes the tilting of NbO₆ octahedra away from the <i>c</i> axis with Δθ ≈ 10° and raises the Curie temperature by 40 °C, and the negative thermal expansion coefficient along the polar <i>b</i> 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 <i>b</i> axis, which may be caused by more Pb²⁺ in the pentagonal caves