Structure and thermal expansion of the tungsten bronze Pb₂KNb₅O₁₅

Abstract

The structure and thermal expansion behavior of the tetragonal tungsten bronze oxide Pb2KNb5O15 were investigated by neutron powder diffraction and high-temperature X-ray diffraction. Below the Curie temperature, T(C) (orthorhombic phase, T(C) ≈ 460 °C), the cell parameters a and c increase with temperature, while b decreases. The thermal expansion coefficients are α(a) = 1.29 × 10(-5) °C(-1), α(b) = -1.56 × 10(-5) °C(-1), and α(c) = 1.62 × 10(-5) °C(-1). Temperature-dependent second harmonic generation (SHG), dielectric, and polarization-electrical field (P-E) hysteresis loop measurements were performed to study the symmetry and electric properties. We show that the distortion and cooperative rotation of NbO6 octahedrons are directly responsible for the negative thermal expansion coefficient along the polar b axis. It is suggested that Pb-O covalency, especially in the large and asymmetric pentagonal prisms, may be related to orthorhombic distortion and abnormal spontaneous polarization along the b axis. This study shows that tungsten bronze families are possible candidates for exploring negative thermal expansion materials.

Photo-Hall, photorefractive and photomagnetoelectric effects in tungsten bronzes and related tetragonal ferroelectrics

“…The family of tetragonal tungsten bronze (TTB) materials, evidenced since 1949 and intensively studied both as single crystals and as ceramics [1][2][3][4][5][6][7], exhibits exceptional ferroelectric behaviors [8,9], together with non-linear optical properties. These features make them interesting materials for electro-optics and optoelectronics [1,[10][11][12][13][14][15][16] as well as promising candidates for computer memory applications based on resistive switching phenomena [17,18]. An additional advantage of these materials is the large variety of atoms present in their cell, which permit many chemical substitutions giving rise, for instance, to the possibility to give magnetic properties to these structures.…”

Structural modulation and electrical properties in ferroelectric niobates (Ca0.28Ba0.72)2.5−0.5K Nb5O15 (0.0≤x≤0.6) ceramics

“…For CBN compounds, the bigger Ba 2+ ions (1.61 Å) occupy the 15-fold coordinated oxygen octahedral A2 sites, the relative smaller Ca 2+ ions (1.34 Å) mainly occupy the 12-fold coordinated oxygen octahedral A1 sites [20,21]. Then the TTB structure with only five A-sites are occupied out of six and it results in a socalled 'unfilled' structure that is responsible for charge disorder and relaxor behavior [12,14,16].…”

Lead potassium niobate thin films grown by Pulsed Laser Deposition

“…In the recent years, some studies have shown the increasing interest of materials with tetragonal tungsten bronze (TTB) structure by demonstrating their potential technological applications [1][2][3][4][5].…”

“…It has an orthorhombic structure at room temperature with cell parameters: a=17.754 (2) Å, b = 18.014(2) Å and c = 3.915(1) Å [8]. PKN ceramic presents a behavior of a classical ferroelectric and undergoes a ferroelectric-paraelectric phase transition at Tc=450 °C [9], changing the point symmetry from P4/mbm (N° 127, paraelectric) to Cm2m (N° 38, ferroelectric). At room temperature PKN is ferroelectric and ferroelastic as refined by Sciau et al [7].…”

“…The ferroelectric polarization in PKN is oriented along the b axis of the orthorhombic unit cell. Above Curie temperature, PKN is paraelectric and paraelastic with a tetragonal symmetry (space group P4/mbm N° 127) [9]. PKN was previously studied for its piezoelectric, ferroelectricferroelastic coupling, optical properties, and surface-acousticwave (SAW) properties [6,[9][10][11][12][13].…”