Terahertz permittivity of rutile TiO₂ single crystal measured by anisotropic far-infrared ellipsometry

Abstract

Rutile structured TiO 2 has very high dielectric permittivities in non-ferroelectric materials. To understand the reason why rutile TiO 2 has high ionic polarizabilities, it is essential to analyze accurate dielectric spectra of rutile TiO 2 in the THz region. In this study, the complex permittivity of rutile TiO 2 single crystal in the range 30700 cm ¹1 (0.9021 THz) was directly measured using the anisotropic far-infrared spectroscopic ellipsometer. The three E u modes and one A 2u mode, which are all infrared-active phonon modes in rutile TiO 2 , were observed. Moreover, the resonance frequencies of the phonon modes approximately were coincident with a calculation result of first-principles and the analysis data by a harmonic oscillator model. From the examination of the dielectric response, it was found that E u (1) mode, which indicates the oscillation of Ti ions against O ions in the same phase, was dominant contributor to the static permittivity for a-axis. On the other hand, the high permittivity for c-axis was due to "soft" A 2u mode.

Usage of P–V–L bond theory in studying the structural/property regulation of microwave dielectric ceramics: a review

“…Up to now, several theories have been developed to study the regulation between crystal structure and dielectric pro-perties. For example, THz time-domain spectroscopy and farinfrared reflectance spectroscopy can be used to analyse the intrinsic properties of the lattice vibration of material systems; [28][29][30][31] the Clausius-Mossotti equation predicts the macroscopic dielectric constant of an ionic crystal by using ionic polarizabilities; 32,33 bond-valence theory gives the actual valence state of ions in a crystal structure and provides the basis for the state of the ions. [34][35][36][37] These concepts help researchers gain better insights into the relationship between crystal structure and dielectric properties.…”

Temperature-Stable Linear Dielectric Response of Low-Temperature Sintered La-Doped Bi2SiO5 Ceramics

“…Figure a compares the temperature dependence of the dielectric permittivity of the sample with x = 0.03 [i.e., (Bi 0.97 La 0.03 ) 2 SiO 5 ] with conventional paraelectric materials, namely, rutile-type TiO 2 , CaZrO 3 , BaZrO 3 , and Ba 0.9 Ca 0.1 ZrO 3 . TiO 2 has a high dielectric permittivity of around 100 at room temperature due to the soft-mode phonon . However, the temperature stability of its permittivity is poor due to the temperature dependence of the soft-mode frequency; the permittivity drops rapidly to 90 at 200 °C.…”

“…10 TiO 2 has a high dielectric permittivity of around 100 at room temperature due to the soft-mode phonon. 7 However, the temperature stability of its permittivity is poor due to the temperature dependence of the soft-mode frequency; the permittivity drops rapidly to 90 at 200 °C. The perovskite-type zirconates of CaZrO 3 and BaZrO 3 possess excellent temperature stability with a near-zero temperature coefficient, but their permittivity is much lower than that of TiO 2 .…”

“…Modern electronic devices and systems, such as smartphones, portable personal computers, and electric vehicles, require a large number of multilayer ceramic capacitors (MLCCs) as essential passive components . The dielectric ceramics used in MLCCs are categorized mainly into two types: (1) Class I dielectrics, which are paraelectric materials with low dielectric permittivity that is not sensitive to temperature changes, and (2) Class II dielectrics, which are derived from ferroelectric materials and thereby have high dielectric permittivity but poor temperature stability. , The dielectric permittivity of Class I dielectrics is less dependent on DC bias voltage, while that of the usual Class II dielectrics decreases significantly with the bias voltage. , Class I dielectrics are thus widely used in MLCCs for filtering and resonant circuits that require superior temperature and bias stabilities and low dielectric (hysteresis) loss. − The discovery of novel paraelectric materials with high dielectric permittivity allows us to use Class I MLCCs for applications requiring high capacitance such as snubber capacitors . Thus, paraelectric ceramics with temperature-stable high dielectric permittivity are widely desired.…”

“…Currently available commercial Class I MLCCs are made from paraelectric materials, such as rutile-type TiO 2 , CaTiO 3 , and CaZrO 3 , whose dielectric responses stem from ionic polarization in M O 6 oxygen octahedra in their crystal lattice, where M is a d 0 transition metal cation. Among them, rutile-type TiO 2 and perovskite-type titanates (e.g., CaTiO 3 ) show a relatively high permittivity exceeding 100 originating from soft-mode phonons. , However, their permittivity decreases with temperature due to the strong temperature dependence of the soft-mode frequency. Meanwhile, temperature-insensitive permittivity can be achieved using perovskite-type zirconates (e.g., CaZrO 3 ) but their permittivity drops to below 50. , As a result, oxygen-octahedra-based paraelectric materials have faced a trade-off between dielectric permittivity and temperature stability for many decades.…”