Sillenite Compounds Research Articles

Indium containing sillenite semiconductor: Synthesis, structural, spectroscopic, and thermogravimetric analysis

AbstractSillenite‐type ceramics are non‐centrosymmetric phases of ongoing research interest because of their structural defects and optoelectronic properties. We report a series of sillenite compounds with a general composition Bi12(Bi3+4/5−3xIn3+5x□1/5−2x)O19.2+3x□0.8−3x for x = 0.03–0.27 to understand how the crystal–physicochemical properties change with a successive filling of empty Bi3+ positions in the tetrahedral site by In3+ cation. Conventional solid‐state synthesis method is used to prepare the microcrystalline samples. Each sample is characterized by X‐ray diffraction, Raman, UV/Vis diffuse reflectance spectroscopy, and thermogravimetry (TG/DSC). X‐ray powder data Rietveld refinement reveals that phase‐pure samples can be obtained for x = 0.03–0.08 in the space group I23. Appearance of starting In2O3 as minor phases with the final products for 0.10 < x < 0.27 suggests for xmax = 0.08. The successive decrease of the lattice parameter indicates the incorporation of smaller In3+ cations in the structure. The effect of the lone electron pairs of Bi3+ and the structural cation vacancies lead to the modification of the interatomic bond lengths. At least one Raman active phonon mode shows hardening for decreasing cation vacancy concentration in the system. The bandgap energy increases with increasing indium content. An additional absorption band at lower energy for x = 0.03–0.08 complements the theoretical study, which completely disappears for x > 0.08. The stronger In–O bonds play pivotal roles in the thermal stability of the phases studied by TGA/DSC analysis.

First principles study of Bi12GeO20: Electronic, optical and thermodynamic characterizations

Bismuth germanium oxide (Bi12GeO20) is one of the attractive members of sillenite compounds having fascinating photorefractive characteristics. The electronic, optical and thermodynamic properties of Bi12GeO20 were investigated using density functional theory (DFT) calculations. The experimental and calculated X-ray diffraction patterns were obtained as well-consistent with each other. The lattice constant of the cubic crystalline structure of Bi12GeO20 compound was calculated as 10.304 Å. The electronic band structure and partial density of states plots were reported and contribution of constituent atoms (Bi, Ge, O) to the valence and conduction bands was presented. The band gap energy of the Bi12GeO20 was calculated as 3.20 eV. This wide direct band gap energy provides Bi12GeO20 significant potential in ultraviolet applications. The spectra of real and imaginary components of dielectric function, refractive index, extinction coefficient and absorption coefficient were drawn in the 0−10 eV energy range. Temperature-dependent heat capacity plot indicated the Dulong-Petit limit as 825 J/mol.K. The results of the present study would present worthwhile information to device application areas of Bi12GeO20 compound.

Structural and temperature-tuned optical characteristics of Bi12GeO20 sillenite crystals

Sillenite compounds exhibit unique photorefractive and electro-optic characteristics providing attractiveness to these materials in various optoelectronic applications. The present paper aims at investigating one of the members of this family. Structural and optical characteristics of Bi12GeO20 (BGO) were studied by means of x-ray diffraction, Raman spectroscopy and temperature-dependent transmittance measurements. Obtained transmission curves in the wavelength range of 350−1100 nm and at different applied temperatures between 10 and 300 K were employed to find out the absorption coefficient dependence on the photon energy. Tauc relation revealed the presence of an energy gap of 2.49 eV at room temperature. Extension of energy gap up to 2.57 eV due to decreased temperature down to 10 K was deduced by the analysis. In order to have reliable results, the energy gap value was corroborated by utilizing derivative spectral method and well consistency between both methods was indicated. Energy gap change with temperature was also discussed in the study using an empirical formula developed by Varshni. Energy gap at absolute zero and rate of band gap alteration with temperature were determined as 2.57 eV and −2.4 × 10−4 eV K − 1, respectively. Taking into account the previously reported studies on investigation of band gap characteristics of BGO, intrinsic BiGe3++VO+ defect could be responsible for the revealed energy value of 2.49 eV which is much lower than reported band gap energy of ~3.2 eV.

Optical properties and visible light-induced photocatalytic activity of bismuth sillenites (Bi12XO20, X = Si, Ge, Ti)

Abstract Three bismuth sillenite compounds with the general composition, Bi 12 XO 20 (X = Si, Ge, Ti), were prepared by a conventional solid state synthesis route and their local crystal structure, light absorption and visible-light induced photocatalytic activities were systematically investigated. The prepared powder samples show comparable particle size distribution (~ 0.5–5 µm) and BET surface area (~ 0.8 m 2 /g). The optical band gap energies were estimated from diffuse reflectance spectra to be in the range 2.6–2.9 eV. The local crystal structure variation and its influence on the electronic band structure was elucidated in the three sillenite compounds by Raman analysis and density functional theory (DFT). The visible-light induced photocatalytic activity was evaluated by monitoring the degradation of rhodamine B dye solution under visible-light irradiation (> 420 nm). We found that the Bi 12 TiO 20 powder shows the highest photocatalytic activity, which is attributed to the higher visible light absorption and the smaller band gap resulting from its distinct local crystal structure (longer Bi-O distance).

Low temperature sintered of Bi2O3 deficient sillenite Bi12SiO20 ceramics

Bismuth oxide based Bi12SiO20 dielectric ceramics were prepared via a solid state reaction process. The bismuth oxide of the ceramics was vaporized during the calcination and sintering process, and became non-stoichiometric Bi2O3 deficient sillenite compounds. Different amounts of Bi2O3 were added to the sintered pellets in order to compensate for the loss of Bi2O3. The sillenite dielectric ceramics with different Bi2O3 additives were analyzed. Bismuth oxide Bi2O3 as a minor phase was detected from the Bi12SiO20 powders calcined at 750°C, and there was approximately 0.89wt% Bi2O3 vaporized after calcination. There was approximately 2wt% Bi2O3 loss when the pellet was sintered. The relative densities of the sintered pellets decreased with increasing temperature, this was attributed to the vaporization of the Bi2O3 during sintering. The 780°C sintered Bi12SiO20 pellet without Bi2O3 addition had the highest relative density, at roughly 98.7%. Dielectric constants of the sintered pellets with Bi2O3 additions lower than 4wt% were ranged between 38.6 and 39.6, Q׃ values of the sintered Bi12SiO20 pellets varied depending on the sintering conditions and Bi2O3 additions. The sample without Bi2O3 addition sintered at 820°C for 10h, had a dielectric constant εr=38.95, Q׃ value=14,910 (at 7.73GHz) and temperature coefficient of resonant frequency tƒ=−25.3ppm/°C.

First-principles study of the BiMO4 antisite defect in the Bi12MO20 (M=Si, Ge, Ti) sillenite compounds

Structural, electronic and optical properties of the antisite BiMO4 defect in Bi12MO20 sillenites (BMO, M=Si, Ge, Ti) were investigated using density functional theory. The defect is studied in neutral, positively and negatively charged states. It is demonstrated that within the neutral defect the Bi 6s2 lone pair is broken and the valence state of the Bi is 4+ (6s1). Within the charged defects, the Bi 6s orbital is found to be either full (Bi3+: 6s2) or empty (Bi5+: 6s0). All three charged states introduce energy bands within the BMO gap. By analyzing possible transitions between them we deduced a simple model of functioning of the BiMO4 defect that is able to explain the photochromic and photorefractive effect in sillenites and that reproduces almost all known experimental facts.

First-principles calculations of novel sillenite compounds Bi24M2O40 (M=Se or Te)

The electronic structures of novel sillenite compounds Bi24M2O40 (M=Se or Te) were studied by first-principles calculations. The PDOS and the band decomposed electron density contour maps projected on symmetric tetrahedral MO4 show that the novel band at the Fermi level is resulted from the antibonding interaction of cationic ns states and anionic O 2p states. Further interaction between this antibonding state and the cationic np states forms asymmetric electron densities around cation under distorted coordination. The novel band at Fermi level and the asymmetric electron densities of these two kinds of sillenite compounds may be advantageous for photocatalytic applications.

Growth of Sillenite-Structure Single Crystals

The main processes for preparing bulk single crystals and films of photorefractive and piezoelectric Bi 12 M x O 20 ± δ (M = Group II-VIII elements) sillenite compounds are considered. Experimental data are sum- marized on the crystal growth of Bi 12 M x O 20 ± δ from the melt and under hydrothermal conditions, and the key morphological features of sillenites are analyzed. Various types of macroscopic growth defects in sillenite-type crystals are described, and their origin is discussed. The compositions of second-phase inclusions in undoped and doped (Group I-VIII elements) Bi 12 SiO 20 , Bi 12 GeO 20 , and Bi 12 TiO 20 single crystals are presented, and the main physicochemical properties of various Bi 12 M x O 20 ± δ crystals are summarized.

New Generation of LTCC Materials

To reduce the complexity of LTCC systems and so accelerate the development of LTCC tapes with new functionalities it is necessary to reduce the number of phases within a particular tape. This can best be done by using glass-free single-phase ceramic systems. Such a material system consisting of low- and high-permittivity LTCC materials was developed based on Bi eulytite (permittivity; K'=16) and sillenite (K'=40) compounds and the δ-Bi 2 O 3 solid solution with Nb 2 O 5 (K'=90). The Ge and Si analogues of the sillenites and eulytites, and the 0.75Bi 2 O 3 .0.25Nb 2 O 5 solid solution meet the main requirements for LTCC with respect to their sintering behavior (T s =850-900°C), their mutual chemical compatibility, their compatibility with a silver electrode as well as their dielectric properties.

Sillenites materials as novel photocatalysts for methyl orange decomposition

Sillenite compounds with a general formula Bi 12MO 20− x (M=Pb, Ni, Al, Ti, Fe) based on sillenite Bi 12O 20Si type are effective for the methyl orange decomposition and display higher activity than the TiO 2 catalysts.

Analysis of electro-optic crystal-based Fabry-Perot etalons for high-speed spatial light modulators.

The use of electro-optic (EO) crystal-based Fabry-Perot modulators (FPMs) as high-speed spatial light modulators is proposed. The FPMs operate with an extremely low drive voltage and a high extinction ratio. It is revealed by analysis of both the linear EO effect and the inverse piezoelectric effect of various EO crystals that three kinds of crystal configuration are suitable as FPMs. One of these is applicable to isotropic crystals, point groups 23 and (-)43m, and the others are better suited for uniaxial EO crystals, point groups (-)42m and 3m. Typical EO crystals suitable as FPMs are ferroelectric crystals such as LiNbO3, LiTaO3, and LiIO3 and sillenite compounds such as Bi12SiO20 and Bi12GeO20 as well as compound semiconductors such as GaAs and GaP.

A Stoichiometric Model for Sillenites

A critical review of crystallographic studies on sillenite compounds has allowed us to develop a general structural formula for the stoichiometric sillenites: Bi12(Bi4/5-nxMn+5x)O19.2+nx. We defined the stoichiometric sillenites as those with an oxygen sublattice that is fully occupied by either oxygen ions or lone electron pairs of Bi3+ ions. Nonstoichiometric groups of sillenites were identified as being either substoichiometric, with a deficient oxygen sublattice, or superstoichiometric, with an excess of oxygen. The compositions of the sillenites reported in the literature agree well with the proposed stoichiometric model. Two sillenites, Al and Zn, that were found to deviate from this model were experimentally investigated using X-ray diffraction, scanning electron microscopy, and wavelength-dispersive spectroscopy. The composition was found to be different from that reported previously and to correspond to the proposed stoichiometric model.

Processing and Dielectric Properties of Sillenite Compounds Bi12MO20−δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2)

Six sillenite compounds Bi12MO20‐δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) were synthesized, and the resulting single‐phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B‐site ion to a value of −20 ppm/K for the Bi12SiO20 and Bi12(B1/2P1/2)O20 compounds. The Q×f value is a maximum for Bi12SiO20 and Bi12GeO20 with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature‐stable permittivity, the Bi12SiO20 compound is a suitable material for applications in low‐temperature cofiring ceramic (LTCC) technology.

Heat transfer and the external morphology of Czochralski-grown sillenite compounds

Experimental results demonstrate changes in the external morphology of sillenite compounds grown by the Czochralski method. To explain these observations, a numerical model of heat transfer which accounts for radiative cooling and melt convection is applied to this system. Delicate shifts in the balance of heat transfer mechanisms during crystal growth and their resulting effects on the temperature distribution at the melt free surface correlate well the changes in external crystal morphology and provide a mechanistic explanation for the experimental results.

Vibrational spectra of bismuth oxide and the sillenite-structure bismuth oxide derivatives

Bismuth oxide is monoclinic, space group P2 1/c. The sillenite compounds are impurity stabilized derivatives with general formula MO x·6Bi 2O 3, body-centered cubic with space group 123. Far i.r. and Raman spectra have been measured on polycrystalline Bi 2O 3 and on sillenite structures in which the stabilizing oxide is SiO 2, GeO 2, TiO 2, Ga 2O 3, ZnO, or PbO. About half the predicted 24 i.r. bands and Raman bands are observed for the sillenite structures. Low frequency Raman bands are very sharp, but high frequency bands are greatly broadened. This is interpreted in terms of an orientational disorder of the lone-pair non-bonding orbital of Bi 3+ in the bismuth—oxygen framework. Although an isolated regular tetrahedron, MO 4, exists in the structure, internal modes from the tetrahedron are not identifiable, indicating a strong coupling with the framework. A cluster of relatively narrow bands in the i.r. spectra in the range of 400–650 cm −1 can be correlated with the various BiO bonds in the highly distorted polyhedra that make up the bismuth—oxygen framework.