Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze K3Li2Ta5O

Abstract

We have synthesized ceramic specimens of the tetragonal tungsten bronze K 3 Li 2 Ta 5 O 15 (KLT) and characterized its phase transition via x-ray diffraction, dielectric permittivity, resonant ultrasonic spectroscopy, and heat capacity measurements. The space group of KLT is reported as both P 4/mbm and Cmmm with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used inside the closed crucibles for the solid state synthesis. The data show strong relaxor behavior, with the temperature at which the two dielectric relative permittivity peaks decreasing, with 104 T m1 69 K and 69 T m2 46 K as probe frequency f is reduced from 1 MHz to 316 Hz. F tests show that the data satisfies a Vogel-Fulcher model better than Arrhenius with an extrapolated freezing temperature for ε and ε of T f 1 = +15.8 and −11.8 K and T f 2 = −5.0 and −15.0 K for f → 0 (tending to dc). This difference between T f from real and imaginary values, albeit counterintuitive, is mandatory, according to the theory of Tagantsev. Therefore, by tuning frequency, the transition could be shifted to absolute zero, suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalyzed the conflicting literature for Pb 2 Nb 2 O 7 pyrochlore which suggests that this also has a relaxor-type quantum critical point since the freezing temperature from the Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at approximately 100 kHz shifts below 0 K for very low frequencies, this transition would not be seen with heat capacity data collected in the zero-frequency (dc) limit. Both of these materials show promise for possible new relaxor-type quantum critical points with nonperovskite based structures. DOI: 10.1103/PhysRevMaterials.2.084409 I. THE SEARCH FOR NEW QUANTUM CRITICAL POINT (QCP) FERROELECTRICS INCLUDING RELAXOR QCPSQuantum critical point (QCP) studies of ferroelectrics have been limited to a few crystal structures, emphasizing perovskite oxides. The drive to find new QCPs with ferroelectrics (FEs) are of interest due to the likelihood that they will exhibit novel electrical and thermal properties over wide ranges in temperature and tuning parameters, similar to what is seen in more widely studied magnetic counterparts. However, as the dynamical exponent is 1 for displacive FE rather than 3 for itinerant ferromagnets, the understanding and modeling of their properties are likely to be more complex, since real systems can exceed the upper and lower critical dimensionality [1]. For a QCP to occur, the transition should be driven by quantum fluctuations rather than classical fluctuations, and such quantum fluctuations tend to dominate in a region just above 0 K. Interest in ferroelectric quantum critical points has grown rapidly in the past several years, with emphasis upon perovskites [1,2] and several other materials, including hexaferrites [3][4][5][6] and organic or molecular crystals [7][8][9][10]. However, the QCPs studied thus ...

Ferroelectric Relaxor Quantum Crystals

“…Our fit to the lead pyrochlore relaxation, Figure 6, yields a freezing temperature of T f = −14 K (below absolute zero) [13]. Note that the frequency f 0 in the fit is unreliable; the data fitting is not sensitive to this value, and the fit simply shows that it is very large.…”

“…We show in the graphical abstract here that this is because the transition is that of a relaxor QCP. Hence, if measurements are made via resonant ultrasonic spectroscopy or dielectric studies in the 300 kHz regime, a phase transition is measured (up to T = 40 K), whereas if a dc technique is used, such as specific heat, no transition is observed in this temperature range [13]. The situation in lead pyrochlore is rather more confusing.…”