posted on 2014-09-02, 00:00authored byKun Lin, Yangchun Rong, Hui Wu, Qingzhen Huang, Li You, Yang Ren, Longlong Fan, Jun Chen, Xianran Xing
The crystal structure and thermal
expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric,
Pb2K0.5Li0.5Nb5O15, 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 Pb2K0.5Li0.5Nb5O15 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 Pb2KNb5O15 (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.