Origin and stability of the dipolar response in a family of tetragonal tungsten bronze relaxors

Abstract

A new family of relaxor dielectrics with the tetragonal tungsten bronze structure (nominal composition Ba 6 M 3+ Nb 9 O 30 , M 3+ = Ga, Sc or In) were studied using dielectric spectroscopy to probe the dynamic dipole response and correlate this with the crystal structure as determined from powder neutron diffraction. Independent analyses of real and imaginary parts of the complex dielectric function were used to determine characteristic temperature parameters, T VF , and T UDR , respectively. In each composition both these temperatures correlated with the temperature of maximum crystallographic strain, T c/a determined from diffraction data. The overall behaviour is consistent with dipole freezing and the data indicate that the dipole stability increases with increasing M 3+ cation size as a result of increased tetragonality of the unit cell. Crystallographic data suggests that these materials are uniaxial relaxors with the dipole moment predominantly restricted to the B1 cation site in the structure. Possible origins of the relaxor behaviour are discussed.

Relaxor nature in lead-free Sr5LaTi3Nb7O30 tetragonal tungsten bronze ceramics

“…13,18,24 As shown in Figure 1, in the temperature range from 128 K to 573 K, diffuse peaks of the complex dielectric permittivity, e ¼ e 0 -ie 00 , and the loss factor, tand ¼ e 0 /e 00 , move towards higher temperatures with increasing frequency. Dielectric dispersion of the main relaxation takes place in broad temperature and frequency regions both below and Bond lengths (Å ) Bond lengths (Å )…”

“…A symmetric loss peak with the Debye response can be fitted to a Gaussian distribution of relaxation times. 13 Due to the multi-dispersive nature of the dielectric relaxation, the frequency dependence of the dielectric loss data in Sr 5 LaTi 3 Nb 7 O 30 ( Figure 6(b)) indicates a high degree of asymmetry. This asymmetric dielectric loss data can be fitted to the empirical two-exponent model of Jonscher's universal dielectric response 34 (UDR) with the following relationship:…”

“…9 In Ba 6 M 3þ Nb 9 O 30 system, dipole freezing behavior is observed and the dipole stability increases with increasing M 3þ cation size as a result of increased tetragonality of the unit cell. 13 The relaxor and ferroelectric nature in the quaternary M 4 on the A1 (tetragonal) and the A2 (pentagonal) sites. 19,20 Larger DR increases the driving force of the transformation of an incommensurate tilted structure to a commensurate superstructure, which associated with the normal ferroelectric transition.…”

Manipulation of polar order in the “empty” tetragonal tungsten bronzes: Ba4-xSrxDy0.67□1.33Nb10O30, x = 0, 0.25, 0.5, 1, 2, 3

“…19,20 Higher occupancy of Sr on the A1-site leads to an increased t A1 tolerance factor for x = 2, such that t A1 (x = 2) > t A1 (x = 3) , Table 1. This multi-site occupancy by Sr generates two contributing factors which favour relaxor behaviour: 1) A-site disorder; and 2) reduced octahedral tilting due to the increase in A1-cation size (increased t A1 ).…”

“…where f is the frequency of the applied ac field, f 0 is the limiting response frequency, E a is the activation energy, k is Boltzmann's constant and T m is the maximum of the relative permittivity measured with ac field of frequency, f. [19][20][21] The polarisation freezes out at T f , the static freezing temperature, also known as the Vogel-Fulcher temperature. 19,20 Higher occupancy of Sr on the A1-site leads to an increased t A1 tolerance factor for x = 2, such that t A1 (x = 2) > t A1 (x = 3) , Table 1.…”