Ordered Structure and Thermal Expansion in Tungsten Bronze Pb₂K_0.5Li_0.5Nb₅O₁₅

Abstract

The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K(0.5)Li(0.5)Nb5O15, 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(0.5)Li(0.5)Nb5O15 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 Pb(2+) in the pentagonal caves.

Role of structural modulation in electrical properties of tungsten bronze (Ca0.28Ba0.72)2.5−0.5Na Nb5O15 ceramics

“…The abnormal dielectric peak moves to higher temperatures with increasing frequencies, which is similar to a diffused phase transition of relaxor characteristics. The low temperature dielectric relaxation is quite common in other TTB ferroelectrics, such as Sr 2 [18,17,29,30], many of them have been ascribed to the effects of local structural fluctuation rather than the phase transition of macro-structure [8]. These effects of local structural fluctuation may include the concerted rotation of oxygen octahedron in the ab plane and the delocalization of small cations in A-sites of the tungsten bronze structure.…”

Niobium tungsten oxides for high-rate lithium-ion energy storage

“…From this point, the unit cell volume actually decreases with increasing lithium intercalation and is the same volume at 1.0 Li + /TM as at 0.5 Li + /TM, a total increase of only 2.8% from the unlithiated host. These is phenomenologically related to negative thermal expansion (NTE) 35,36 or negative linear compressibility (NLC) 37 and may have shared origins related to tilting of the polyhedral units (vide infra). The small volume change has implications for the suppression of intergranular cracking and long-term cycle performance 38 .…”

Variation of electrical properties with structural vacancies in ferroelectric niobates (Sr0.53Ba0.47)2.5−0.5Na Nb5O15 ceramics

“…However, the presence of crystal nonequivalent A and B sites and an extra C site provide extra degrees of freedom for manipulation of the structure to offer huge compositional flexibility [3,4]. Generally, A1 with 12-fold coordinated sites in the crystal lattice structure and A2 (15-fold coordinated sites) sites can be filled by Sr 2+ , Ba 2+ , Ca 2+ , Pb 2+ , Na + and some rare earth cations, whereas B1 (9-fold coordinated sites) and B2 (6-fold coordinated sites) sites by either Nb 5+ or Ta 5+ , and the C sites by Li + and other small cations in the formula.…”