Structural Effects Behind the Low Temperature Nonconventional Relaxor Behavior of the Sr₂NaNb₅O₁₅ Bronze

Abstract

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.

Relaxor-to-Ferroelectric Crossover and Disruption of Polar Order in “Empty” Tetragonal Tungsten Bronzes

“…19,37,[64][65][66][67][68][69][70][71] However, the relaxor-like response in these is often subtle or convoluted with the main permittivity peak associated with T C and has not been demonstratively linked changes in the structural or P-E properties and so the origins of such behavior has not been convincingly assigned. The pronounced separation of the features presented here for Ba 4 La 0.67 !…”

“…19 The origins of these superstructures are principally variations in the oxygen positions due to octahedral tilting out of the ab plane. [20][21][22][23][24][25][26][27][28][29][30] .…”

Complex impedance properties of LiSr2Nb5O15 ceramic

“…The distribution of metal cations in different atomic sites of TB structure plays a vital role in tailoring the physical and electrical properties of the materials for device applications. Among many ferroelectric oxides, significant work has been focused on the study of TB type ferroelectric niobates and tantalates [7][8][9][10][11][12] [16][17][18]. However, several works have been reported on electrical (impedance) properties of TB type compounds [19][20][21][22].…”

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

“…The TTB structure, with the general formula (A1) 2 (A2) 4 (C) 4 (B1) 2 (B2) 8 O 30 , consists of a complex array of distorted BO 6 octahedron linked by corners, 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 offering huge compositional flexibility [4][5][6]. In the formula, A1, A2, C and B are 12-, 15-, 9-and 6-fold coordinated sites in the crystal lattice structure, respectively.…”