Thermal stability, dynamics of dipoles and elastic anelastic relaxations accompanying the relaxor dielectric behaviour of tetragonal tungsten bronze oxides: from modelling to the development of novel ferroelectrics and further design of multiferroics

摘要

Polar dielectrics, particularly ferroelectrics (FE), are used in a wide range of electronic devices including capacitors, electro-optic switches, non-volatile memory chips and a number of piezoelectric transducers and sensors. The materials in use today are often Pb-containing e.g. Pb(Zr,Ti)O_3 (PZT) or have limited operating temperatures (e.g. BaTiO_3, TC = 130 °C) and so new materials are required for future development. The recent renaissance in the search for novel polar dielectrics and also multiferroics and magnetoelectrics [1,2] has largely focussed on perovskite materials; we have directed our attention to the related tetragonal tungsten bronze (TTB) structure (A1)_2(A2)_4C_4(B1)_2(B2)_8O_(30), which offers similar compositional flexibility [3]. TTBs comprise non-equivalent B06 corner sharing octahedra creating three distinct channels which can be occupied by cations: Al (15 coordinated), A2 (12 coordinated) and C (9 coordinated). This gives an extra degree of freedom to the flexibility of the structure, enhancing the separation of magnetic and ferroelectric ordering to distinct sublattices and makes possible various compositional substitutions. Of particular interest are niobium based TTBs due to their enhanced ferroelectric properties over other analogues such as tantalum or tungsten. In TTB oxides, large cations such as Ba~(2+), Sr~(2+), Ca~(2+), La~(3+), Nd~(3+), Bi~(3+) occupy the larger A-sites, while smaller cations of transitional metals such as Fe~(3+), Ga~(3+), Sc~(3+), In~(3+), Ti~(4+) prefer the B-sites. Despite the extensive past and recent work on TTBs our understanding of how to manipulate this structure is poor compared to other systems. A combination of dielectric spectroscopy (DS) and powder neutron diffraction (PND) as a function of temperature was used as to probe the thermal stability and dipole dynamics in a family of tetragonal tungsten bronze (TTB) relaxor dielectrics with the nominal formula Ba_(6-x-y)Sr_xCa_yM ~(3+)Nb_9O_(30) (where M~(3+) = Ga~(3+), Sc~(3+), In~(3+)). Electrical measurements indicate frequency dependence of both permittivity and dielectric loss characteristic of relaxor behaviour in all compositions. The dynamics of the dipole response has been investigated via Vogel-Fulcher [4-6] and Universal Dielectric Response (UDR) [7,8] analyses of dielectric data and correlated with crystallographic studies. For B-site substitutions the dipole freezing temperature, as determined by dielectric spectroscopy, increased with increasing M cation size [9]; dipole freezing was directly correlated to the structural anisotropy (maximal crystallographic strain) [3]. For A-site substitutions the thermal stability of dipoles is determined by local strain gradients which can be quantified by the statistical cation size variance [10]. All the materials studied are relaxor dielectrics and analysis of the frequency response indicates that the freezing of the dipolar response in these materials is glass-like rather than one of nucleation and growth [11,12]. The ferroic properties of interest are coupled with strain, which will be important in the context of stability, switching dynamics and thin film properties. Coupling of strain with the ferroelectric order parameter give rise to changes in elastic properties and these have been investigated for a ceramic sample of Ba_6GaNb_9O_(30) (BGNO) by resonant ultrasound spectroscopy (RUS). Room temperature values of the shear and bulk moduli for BGNO are rather higher than for TTB's with related composition which are orthorhombic at room temperature, consistent with suppression of the ferroelectric transition. Instead, a broad, rounded minimum in the shear modulus measured at ~1 MHz is accompanied by a broad rounded maximum in acoustic loss near 115 K, and signifies relaxor freezing behaviour. Elastic softening with falling temperature from room temperature, ahead of the freezing interval, is attributed to the development of dynamical polar nanoregions (PNRs),