Broadband and infrared spectroscopy of Ag0.98Li0.02NbO3 ceramics

The dielectric properties of Ag0.98Li0.02NbO3 (ALN2) ceramics were investigated in a broad frequency range (20 Hz – 60 THz). The dielectric spectra of ALN2 ceramics are mainly impacted by electrical conductivity at higher temperatures (above 400 K) and low frequencies (below 100 Hz), ferroelectric domains below ferroelectric phase transition temperature Tc = 330 K and at low frequencies (below 1 MHz), and contribution of the soft ferroelectric mode, the frequency of which is below 50 cm–1. All phononic modes are slightly temperature dependent, thus confirming the influence of Ag, O and Li ions dynamics on the phase transitions. However, the most important contribution to the dynamics of phase transition is made by Nb ions. Ceramics exhibits a huge value of dielectric permittivity and relatively low losses in a microwave frequency range (ε΄ ≈ 250 and ε˝ ≈ 20 at 10 GHz and room temperature), indicating that it is suitable for various microwave dielectric applications.

Thermodynamics of ultrastrongly coupled light-matter systems

We study the thermodynamic properties of a system of two-level dipoles that are coupled ultrastrongly to a single cavity mode. By using exact numerical and approximate analytical methods, we evaluate the free energy of this system at arbitrary interaction strengths and discuss strong-coupling modifications of derivative quantities such as the specific heat or the electric susceptibility. From this analysis we identify the lowest-order cavity-induced corrections to those quantities in the collective ultrastrong coupling regime and show that for even stronger interactions the presence of a single cavity mode can strongly modify extensive thermodynamic quantities of a large ensemble of dipoles. In this non-perturbative coupling regime we also observe a significant shift of the ferroelectric phase transition temperature and a characteristic broadening and collapse of the black-body spectrum of the cavity mode. Apart from a purely fundamental interest, these general insights will be important for identifying potential applications of ultrastrong-coupling effects, for example, in the field of quantum chemistry or for realizing quantum thermal machines.

Crossover from Ferroelectric to Relaxor Behavior in Ba1−xCaxTiO3 (x = 0.17) System

The dielectric properties of Ba1−xCaxTiO3 (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba1−xCaxTiO3 (x = 0.17) ceramics is discussed in the paper.

Phase transition and dielectric properties of BaTiO3 ceramics containing 10 mol% BaGeO3

Dielectric properties and the cubic ? tetragonal phase transition temperature ofdense BaTiO3 ceramics containing 10 mol% BaGeO3, sintered between 840 and 1350 °C,have been investigated. The ceramic bodies were prepared from a nano-sized BaTi0.9/Ge0.1O3powder consisting of both BaTiO3 and BaGeO3 phases. The addition of BaGeO3 leads to areduction and broadening of the permittivity maximum, and to a small downshift of theparaelectric ? ferroelectric phase transition temperature, compared to a pure BaTiO3 ceramic.Lower sintering temperatures and thus small grain sizes of the ceramics cause an additionalreduction of the maximum permittivity down to 2800. Both DTA and dilatometricmeasurements reveal also a downshifting of the phase transition temperature, as well as adecrease of the latent heat.