High temperature phase transitions in barium sodium niobate: the wall roughening 1q–2qincommensurate transition and mean field tricritical behaviour in a disordered exclusion model

Abstract

Barium sodium niobate (Ba2NaNb5O15) is a tungsten bronze structure that exhibits a complicated sequence of six structural phase transitions, including three incommensurate (IC) phases. The phases are unusual in that all but the highest temperature P4/mbm structure are ferroelectric. Unlike the situation for most incommensurate insulators, in which ferroelectricity develops at low temperatures along the modulation direction, the polarization direction in barium sodium niobate is orthogonal to the modulation(s), permitting some unusual phenomena. In the present study we analyse the thermal and dielectric behaviour at the Curie temperature TC near 830 K as well as that at the Ccm 21–IC(1q) transition near 543 K, the IC(1q)–IC(2q) transition near 565 K and the IC(2q)–P4bm transition at 582 K. The entropy change at 565 K is related to the wall roughening model of Rice et al (1981 Phys. Rev. B 24 2751). Data near TC = 830 K indicate close proximity to a tricritical point, and discussions of critical exponents are presented, all of which are found to be mean field. Because of Na vacancies, transition temperature variation is found among specimens Ba2Na1−xNb5O15 (830 K< TC(x)< 865 K), and the system appears to be describable by the disordered exclusion model as a slightly first-order intrinsic system whose dynamics are suppressed by weak disorder. Near TC the specific heat C(T) is compared with the random bond prediction of Harris (1974 J. Phys. C: Solid State Phys. 7 1671): C(T) = C0(T)/[1+bx2C0(T)], where C0(T) is the intrinsic specific heat of the vacancy-free crystal varying as (TC−T)−1/2 and x is the sodium vacancy concentration. In agreement with Harris’s model, the shifts in TC(x) are to lower T with increasing x and scale as x; the broadening scales as x2; and the effective critical exponent remains unchanged at α = 1/2.

Dielectric anomalies in Nd3+ doped Ba2NaNb5O15 laser crystal

“…The present study indicates that the ferroelectric (P4bm)-paraelectric (P4/mbm) phase transition temperature is T 4 = 812 K (in pure BNN is 830 K [7]). This result is in very good agreement with the thermal expansion measurements (Fig.…”

“…The shoulders at T 2 and T 3 may be assigned to the orthorhombic (Pba2)-incommensurate-tetragonal (P4bm) phase transitions [7,8], which cannot be well detected in the ε r curve (Fig. 2a), but they have a strong effect on the imaginary part of the complex dielectric permittivity ε r (Fig.…”

Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze K3Li2Ta5OSmith1, Gardner2, Morrison3 et al. 2018Phys. Rev. Materials3020View full textAdd to dashboardCite

“…In TTBs it is not unprecedented to have orthorhombic distortion and expansions of the unit cells from the P 4/mbm (Z = 2) cell. Other examples [56][57][58][59][60][61][62] in the ab plane. From the analysis reported here, there is a high likelihood that at least one phase transition occurs in the region of 25-50 K. Variable temperature PXRD was used in an attempt to probe any possible changes in structures.…”

Elastic behavior associated with phase transitions in incommensurateBa2NaNb5O15

“…9 It undergoes a transition to a 1q incommensurate phase, losing the short range modulations. The 2q phase is explained by Scott et al 8 as a wall roughening of the 1q phase, with the 1q-2q phase transition resulting in a change of the effective point group from tetragonal ͑2q͒ to orthorhombic ͑1q͒. The two incommensurate phases may coexist, and a complicated memory effect in the incommensurate region has been observed.…”

“…The transition is slightly first order, but is strongly affected by the stoichiometry of the sample, with samples containing higher concentrations of Na vacancies exhibiting a ferroelectric transition with tricritical dynamics and a shift in the apparent Curie temperature. 8 The hightemperature 2q incommensurate phase consists of two orthogonal modulations, one of which is long range and the other short range. 9 It undergoes a transition to a 1q incommensurate phase, losing the short range modulations.…”