Lattice Vibrational Characteristics Research Articles

Crystal structures, dielectric properties, and lattice vibrational characteristics of Zn1-xCaxWO4 (x = 0–0.25) composite ceramics

In this work, Zn1-xCaxWO4 (x=0–0.25, ZCWO) composite ceramics were prepared by using the conventional solid-state sintering method at 1070 ℃. The impact of Ca2+ substitution on the lattice vibrational characteristics and dielectric responses of the ZCWO composite ceramics was investigated in detail. The formation of composite ceramics was verified by the Rietveld refined X-ray diffraction data of the ZCWO samples. The lattice vibrational characteristics were also identified and analyzed by the Raman and infrared reflection spectra combined with the theoretical simulations. The vibrational modes analysis of the Raman spectra showed that the internal modes of the ZCWO ceramics correspond to the vibrations of the W-O octahedrons. From the Fourier transform infrared reflectance spectra, the existence of three new vibrational modes of Au (315 cm−1), Au (834 cm−1), and Eu (885 cm−1) at x = 0.15–0.25 was observed, which belong to the modes of the CaWO4 ceramic. In addition, the intrinsic dielectric properties fitted by the four-parameter semiquantum model were in direct agreement with the theoretical values obtained from the Clausius-Mossotti & damping equations and the measured values. Besides, the contribution of each lattice vibrational mode to the intrinsic properties of the ZCWO ceramics was thoroughly studied, which shows that external mode 1 contributes the most to the intrinsic properties. The structure-property relationships of the ZCWO ceramics were established using the Raman modes as a media. The optimum dielectric properties of the ZCWO ceramics were obtained when x = 0.25: εr =13.98 (10.48 GHz), Q × f = 32,188 GHz.

Phase Composition, Raman Spectrum and Microwave Dielectric Properties of glass-free low temperature co-fired ACaPr(MoO4)3 (A = Na, K) Ceramics

A novel low-temperature sintered ACaPr(MoO4)3 (A = Na, K) microwave dielectric material was prepared by the conventional solid-phase reaction method. The effects of temperatures on the crystal structure, lattice vibration characteristics, and microwave dielectric properties of the new ternary molybdate ACaPr(MoO4)3 (A = Na, K) ceramics were investigated. XRD and Rietveld refinement results show that ACaPr(MoO4)3 (A = Na, K) ceramics belong to a tetragonal scheelite structure with space group I41/a. The lattice vibration mode was analyzed by Raman spectroscopy. The ACaPr(MoO4)3 (A = Na, K) ceramics have high density(＞95%) and excellent microwave dielectric properties with εr = 10.78, Qf = 57726 GHz, τf = −44.6 ppm/°C at 775 °C and εr = 10.36, Qf = 50096 GHz, τf = −46.2 ppm/°C at 725 °C, respectively. In addition, the excellent chemical compatibility between ACaPr(MoO4)3 (A = Na, K) ceramics and Ag electrodes has developed a path for its application in LTCC technology.

Crystal structures, lattice vibrational characteristics, and dielectric response of Mg3(VO4)2 microwave dielectric ceramics sintered at different temperatures

Mg3(VO4)2 (MVO) microwave dielectric ceramics were fabricated by a conventional solid-state sintering method at different sintering temperatures (Ts) between 1000 °C and 1100 °C. The effects of Ts on the phase evolution, the crystal structures, the dielectric responses, and the lattice vibrational characteristics of MVO were investigated. XRD patterns after Rietveld refinement were applied to analyze the phase evolution and the crystal structures of the samples, which shows the orthorhombic structure with the space group of Cmca for the MVO ceramics. Micrograph from scanning electron microscopy shows a uniform and dense crystal structure with the highest bulk density for the MVO sample sintered at 1050 °C. Raman scattering spectroscopy and infrared reflectance spectroscopy were used to dissect the lattice vibrational characteristics of the MVO ceramics. There are two types of vibrational modes of the MVO samples, i.e., the external modes caused by the deformation of [VO4]3- and the rotational and translational vibrations of the lattices, the internal modes attributed to the asymmetric stretching vibrations of V-O bonds. The intrinsic dielectric properties were determined by fitting the data with a four-parameter semi-quantum (FPSQ) model. Modes 1, 2, and 4, associated with the external modes, have the greatest contribution coefficient of 65.58% to the dielectric constant, and Mode 1 has the greatest contribution coefficient of 51.40% to the dielectric loss. The structure-property relationships were semi-quantitatively mathematically established according to the Raman modes. The MVO sample sintered at 1050 °C has the best dielectric properties of εr = 9.59 and Q × f = 41,193 GHz (f = 13.69 GHz).

Lattice vibrational characteristics and structure-property relationships of SrWO4- x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics

SrWO4-x wt.% LiF (SWL, x = 0.5–3.0) ceramics were obtained by a conventional solid-state reaction at 850 °C. The effects of the LiF additive on the phase evolution and dielectric response mechanism of the SWL ceramics were comprehensively studied. The densification temperature of the SWL ceramics was successfully reduced to 850 °C by adding LiF as the sintering additive. The X-ray diffraction data indicated that the SWL ceramics were composite ceramics. The lattice vibrational properties were investigated in depth by Raman scattering and Fourier transform infrared reflectance. The intrinsic dielectric property values fitted by the four-parameter semi-quantum model agreed well with the measured dielectric property values and the theoretical values obtained from the Clausius-Mossotti & damping equations. Moreover, structure-property relationships were obtained by studying the lattice vibrational modes of the SWL ceramics. The SWL ceramic achieved the optimum dielectric property at x = 2.0, with εr = 9.03 and Q × f = 47,830 GHz.

Effects of LiF additive on crystal structures, lattice vibrational characteristics and dielectric properties of CaWO4 microwave dielectric ceramics for LTCC applications

In this paper, CaWO4-x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics are prepared by conventional solid-state reaction at 850 °C. All the X-ray diffraction peaks are attributed to CaWO4, combined with the results of X-ray photoelectron spectroscopy and scanning electron microscope. They indicate that LiF forms liquid phase at high temperature, exists in amorphous phase, and the CaWO4-x wt.% LiF samples belong to the composite ceramics. With the increase of the LiF content, the liquid phase fills the pores of the ceramics and increases the compactness. Therefore, the quality factor increases from 59,055 GHz (x = 0.5) to 77,855 GHz (x = 3.0), and the dielectric constant decreases from 9.26 (x = 0.5) to 8.41 (x = 3.0). The lattice vibration characteristics of the CaWO4-x wt.% LiF samples are studied by Raman and infrared spectroscopy. The external modes associated with the vibration of the Ca–O bonds have the greatest influence on the dielectric response, as shown by the fitting results based on the four-parameter semi-quantum (FPSQ) model. In addition, the simulated intrinsic properties and measured values are in general agreement. When the x value increases, the dielectric constant decreases with the decrease of the bond length. The quality factor has a negative correlation with the FWHM value of the Bg mode and a positive correlation with the packing fraction. Finally, the CaWO4-3 wt.% LiF sample has good compatibility with the silver powders at 850 °C, and high application value in the LTCC field.

Lattice vibrational characteristics, crystal structures, and dielectric properties of LiMnPO4 microwave dielectric ceramics as a function of sintering temperature

LiMnPO4 (LMP) microwave dielectric ceramics were manufactured at different temperatures via a standard solid-state reaction method and the LMP ceramic sintered at 750 °C displayed dielectric properties of εr = 7.82 and Q × f = 29,189 (f = 12.7 GHz). The lattice vibrational characteristics of LMP ceramics were studied utilizing both infrared reflection and Raman scattering spectroscopy to clarify the basic principle of the dielectric response. The intrinsic properties that were fitted and simulated based on the infrared spectra agreed with the measured property values. The low-frequency vibrational modes contributed more to the dielectric properties than the high-frequency modes. Upon increasing the temperature, the permittivity was positively correlated with the bond length but showed the opposite trend of the Raman shift of mode 9 Ag(υ1). The Q × f value was positively correlated with the packing fraction but negatively correlated with the FWHM of mode 10 Ag(υ3). Thus, the structure–property relationships of LMP ceramics were established as a function of sintering temperature.

Lattice vibrational characteristics and structure-property relationships of Ca(Mg1/2W1/2)O3 microwave dielectric ceramics with different sintering temperatures

Ca(Mg 1/2 W 1/2 )O 3 (CMW) microwave dielectric ceramics were synthesized by the solid state reaction method and were sintered at the temperatures in the range of 1450–1550 °C for 4 h. X-ray diffraction and Rietveld refinement results confirmed that the main phase of the samples belongs to the monoclinic crystal structure (P2 1 /c space group). The vibrational modes were determined using the Raman scattering and Fourier transform infrared reflection (FTIR) spectra, and the lattice vibrational characteristics were revealed in detail. The fitting curves of FTIR spectra were obtained by the four-parameter semi-quantum model that was used to calculate the intrinsic properties of samples, and the influence of the infrared mode on the intrinsic properties was elucidated. The structure-property relationships were constructed based on the Raman modes. The optimal dielectric properties of CMW ceramics are of ε r = 18.37, tanδ = 2.71 × 10 −4 and were obtained for the sintering temperature of 1500 °C.

Lattice wave solution and its dispersion relation of two coupled Frenkel-Kontorova chains

We obtain the lattice wave solution and the dispersion relation of the lattice vibration equation of the two identical coupled Frenkel-Kontorova (FK) chains, and we study the lattice vibration characteristics of the coupled FK chains. The results show that the dispersion relation of coupled FK chain contains an acoustic branch and an optical branch. The spectral range and frequency band gap are related to the coefficient of restoring force of each chain and the inter-chain coupling strength, and it is also related to the depth of the on-site potential for the low temperature case and finite temperature case. Moreover, it is found that there is no frequency band gap for weak inter-chain coupling. The frequency gap appears when the inter-chain coupling strength exceeds a critical value, and the frequency band gap will become bigger with the inter-chain interaction increasing. This is because the optical branch moves towards high frequency region with the inter-chain coupling increasing. We also find that the critical inter-chain coupling strength of frequency band gap is always twice the restoring force coefficient of FK chain, and it does not depend on temperature. In addition, we study the effect of temperature on the dispersion relationship of coupled FK chain with a fixed inter-chain coupling strength. These results provide a theoretical basis for analyzing the effects of inter-chain coupling and temperature on the vibrational characteristics and physical properties of lattice, and thus providing an important guide for the energy transport, thermal management and other practical applications.

Na-doped diopside microwave ceramics with dielectric properties influenced by the sintering, lattice vibration and chemical bond characteristics

The Na-doped diopside (CaMg1-xNa2xSi2O6, x = 0–0.18) microwave dielectric ceramics were fabricated via solid-state reaction method. Three phases were identified through the XRD data that was refined with the Rietveld method to obtain structural information. The role of Na+ substitution to promote the densification process could be corroborated by the SEM images and relative density. The P-V-L bond theory was used to explore the chemical bond characteristics and structure-performance relationships. Through discussion, the dielectric constant was determined by relative density rather than the bond ionicity. The Q × f value was dominated by the lattice energy, lattice vibration and was also related to the relative density and grain size. And the linear thermal expansion coefficient was closely correlated with the τf value. Excellent microwave dielectric properties, (εr = 7.573, Q × f = 35049 GHz, tan δ = 4.015 × 10−4, τf = −68.71 ppm/°C) and (εr = 7.646, Q × f = 37844 GHz, tan δ = 3.706 ×10−4, τf = −54.2 ppm/°C) were achieved for the CaMg0.97Na0.06Si2O6 ceramics sintered at 1150 °C/3 h and 1200 °C/3 h, respectively.

Crystal structure, lattice vibrational characteristics, and dielectric properties of Ba(Mg1/2Mo1/2)O3 ceramics sintered at different temperatures

Double perovskite structured Ba(Mg1/2Mo1/2)O3 (BMM) ceramics were prepared by traditional solid sintering methods at different sintering temperatures (Ts=1250–1325 °C). X-ray diffraction (XRD) data proved a 1:1 ordered face-centered cubic lattice (Fm-3 m), and the ceramic sintered at 1300 °C obtained maximum relative density. The lattice vibration characteristics were identified by Raman spectroscopy and far-infrared reflection spectroscopy. Based on the far-infrared data, a four-parameter semi-quantum model (FPSQ) was applied to calculate the intrinsic properties of BMM ceramics, and the obtained results approximated the measured values. The Raman shift of the BMM F2g(B) mode (attributed to the centrosymmetric vibration of Ba atom) was shown to be negatively correlated with the corrected permittivity and Ba-O bond length. With increasing sintering temperature (Ts), the relative density is positively correlated with the quality factor (Q × f), and the change of Ba-O bond length leads to distortion of the oxygen octahedron, which is positively correlated with the dielectric constant. The sintered BMM ceramics obtained their best dielectric properties at 1300 °C (εr= 4.987, Q × f = 23,069 GHz, f = 11.16 GHz).

Temperature-Dependent Phonon Shifts in van der Waals Crystals.

In recent years, there has been an explosive increase in the research on van der Waals (vdW) crystals because of their great potential applications in many optoelectronic devices. It is necessary to determine their temperature-dependent lattice vibration characteristics because their thermal and electrical transport are closely related to the anharmonic phonon effect, which will affect the performance of the devices. We review the temperature-dependent Raman spectroscopy of vdW crystals, systematically introduce the thermal behavior of optical phonons, and summarize their shift with temperature. Upon analyzing the theoretical models and summarizing the reported experimental data, it is found that the phonon shifts of vdW crystals have a "quasi-linear" relationship with temperature, which is widely described with first-order temperature (FOT) coefficients obtained through a linear fit. Thus, subsequently, the phonon shifts of monolayer materials, different-thickness crystals, suspended and supported samples, in-plane and out-of-plane modes in the same vdW materials, as well as heterostructures and alloys are discussed through comparative analysis of FOT coefficients.

Sintering behavior, infrared spectra and microwave dielectric characteristics analysis for Ce2Zr3(MoO4)9 ceramics achieved by reaction-sintering method

Ce2Zr3(MoO4)9 (CZM) ceramics were prepared successfully through the reaction-sintering (RS) method and a single phase was detected in X-ray diffraction patterns. The microstructures of ceramics were observed by a scanning electron microscopy. Based on the results of Rietveld refinement, the influence of chemical bonds parameters on microwave dielectric properties were analyzed by complex chemical bond theory. The lattice vibrational characteristics were investigated by far-infrared reflection spectroscopy. The promising dielectric properties of CZM ceramics were εr= 10.1, Q×f= 69,705 GHz and τf= −6.34 ppm/°C, sintered at 850 °C.

Lattice vibrational characteristics, crystal structure and dielectric properties of Ba2MgWO6 microwave dielectric ceramic

Ba2MgWO6 (BMW) microwave dielectric ceramic was prepared by the traditional solid-state sintering. According to the XRD pattern, the sample belongs to a double perovskite structure (Fm-3m). The SEM image shows that the ceramic is dense with high crystallinity, whose average particle size is of 3.22 μm. The lattice vibrational characteristics of BMW ceramic were identified and analyzed through Raman and FTIR spectroscopy, whose vibrational modes were determined. The intrinsic properties (εr = 16.281, tanδ = 1.795 × 10−4) calculated from the FPSQ model based on the FTIR spectroscopy are close to the measured values (εr = 16.95, tanδ = 2.44 × 10−4), and the dielectric response mainly comes from the modes of F1u(1) at 145 cm−1 and F1u(2) at 284 cm−1.

Lattice vibrational characteristics and structures-properties relationships of non-stoichiometric Nd[Mg0.5Sn0.5(1+x)]O3 ceramics

Non-stoichiometric Nd[Mg0.5Sn0.5(1+x)]O3 (− 0.04 ≤ x ≤ + 0.04)—type samples were prepared using a traditional two-step solid-state sintering process. Their crystal structures, phase compositions, dielectric properties and lattice vibrational characteristics were identified in detail. The double perovskite Nd(Mg0.5Sn0.5)O3 with a monoclinic structure was distinguished as the main crystalline phase, and pure phase was obtained when x < 0. The sample [Nd(Mg0.5Sn0.49)O3] at x = − 0.02 shows the excellent dielectric properties: er = 18.74, Q × f = 47,979 GHz. The lattice vibrational characteristics were clarified with Raman and infrared active mode spectroscopy. The intrinsic properties were fitted by four-parameter semi-quantum model, which show that the low-frequency infrared modes yield a more significant contribution to permittivity and dielectric loss. Raman shifts of modes F2g(A) and F2g(B) correlate with dielectric constants negatively, which implies that the bond length has a positive correlation with permittivity. Full widths at half maximum of vibrational modes A1g(O) correlate with dielectric loss values positively and correlate with the degree of order (S) negatively.

Lattice vibrational characteristics and dielectric properties of pure phase CaTiO3 ceramic

CaTiO3 microwave dielectric ceramic was fabricated utilizing traditional two-step sintering process. XRD pattern analysis after Rietveld refinement indicated a pure phase CaTiO3 sample. SEM image illustrated well-crystallized sample with uniform grain sizes and clear grain boundaries. The lattice vibrational characteristics were analyzed by Raman and IR spectroscopy, and the intrinsic properties were calculated in conjunction with the semi-quantum four-parameter (FPSQ) model, which turned out that the low-frequency vibrational modes contribute the most to the dielectric properties. Besides, the real and imaginary parts of the dielectric function were drawn from the FPSQ model. The intrinsic property results fitted from the FPSQ model agree well with the measured values.

The growth and characteristics of In2Se3/(Bi1−xInx)2Se3 superlattices with asymmetric graded interfaces by molecular beam epitaxy

The growth of In2Se3/Bi2Se3 superlattices (SLs) by molecular beam epitaxy at an elevated temperature is explored. The crystalline phase structure of In2Se3 layers in the as-grown SLs is determined to be α-In2Se3. The diffusion of In from In2Se3 to Bi2Se3 is significantly promoted, while Bi diffusion into In2Se3 layers is insignificant as manifested by the in situ lattice evolution analysis, so that the achieved SL structure is of graded (Bi1−xInx)2Se3 solid-solution layers periodically separated by α-In2Se3 layers. The lattice vibration characteristics due to phonon confinement in the achieved SLs are also exhibited.

Lattice vibration characteristics in layered InSe films and the electronic behavior of field-effect transistors

Understanding how temperature affects the structural and electronic properties for two-dimensional (2D) semiconductors could promote the application and development of nanoelectronic devices. Here, the temperature dependence of lattice structure for indium selenide (InSe) nanosheets and the corresponding electronic properties of 3 nm indium-deposited InSe field-effect transistors (FETs) are systematically demonstrated. Analyses of Raman spectra suggest that the difference of phonon frequency (Δω) for the A mode is found to be 3.14 cm−1, which is larger than that of the E mode due to the stronger electron-phonon coupling for the A mode. The device performance based on indium-deposited InSe is systematically explained using Kelvin probe force microscopy (KPFM) and the predicted energy band structure. Furthermore, FETs based on temperature and variable thickness InSe flakes are designed as applicable devices. Our findings are of fundamental importance to explain the underlying physics in intrinsic InSe transistors and improve further applications.

Lattice vibrational characteristics, crystal structures and dielectric properties of non-stoichiometric Nd(1+x)(Mg1/2Sn1/2)O3 ceramics

Non-stoichiometric Nd(1+x)(Mg1/2Sn1/2)O3 (−0.04 = x ≤ 0.04, NMS) ceramics were fabricated via a conventional solid-state reaction method. Crystal structures and morphologies were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The main crystalline phase is monoclinic Nd(Mg1/2Sn1/2)O3 with a double perovskite structure (P21/n space group) for the NMS system proved by XRD. The sample at x = 0.01 has the best crystallinity and evenly distributed crystal grains observed by SEM. The optimum performances (εr = 19.87, Q×f = 41840 GHz, f = 12.05 GHz) are obtained at x = 0.01. Lattice vibrational modes of the Raman spectra were assigned and illustrated, in detail. The dielectric properties obtained by fitting infrared reflectance spectra with the help of four-parameter semi-quantum model are consistent with the calculated values by microscopic polarization and damping coefficients. The reverse translational vibration of the NdMgO6, the F5u(5) mode, provides the greatest contribution to the dielectric response. The relationships between crystal structures and dielectric properties were mainly established using lattice vibrational modes as a media.

Intrinsic properties and lattice vibrational characteristics of NiWO4 ceramic

NiWO4 ceramic was prepared by solid-state synthesis, which was confirmed to be monoclinic wolframite structure (P12/c) by X-ray diffraction pattern. The vibrational modes were identified and illustrated at length by Raman scattering spectroscopy and far-infrared reflection spectroscopy to study the lattice vibrational characteristics. The tested properties are consistent well with the theoretical values obtained by Clausius-Mossotti & damping equations and four-parameter semi-quantum model, and the infrared vibrational mode Bu(263 cm−1) has the greatest contribution to the dielectric properties.

Stress mechanism of Y1-xGdxBCO thin film with Gd substitution prepared by F-free metal organic deposition method

The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction. Recently, the second-generation YBa2Cu3O7- (YBCO) high temperature superconducting materials have attracted much attention and become a hot research point. The YBCO coated conductors are widely used in transmission cables, motors, generators and magnetic energy storage systems due to their high critical current densities and high irreversible fields. To obtain high critical current, it is necessary to increase the thickness of YBCO film. However, as the thickness increases, the cracking of the film appears and the a-axis grains form, which causes the critical current density to decrease drastically, hence the critical current declines, i.e., the so called thickness effect appears. In order to overcome the thickness effect, a great many of efforts have been devoted to it. It is realized gradually that the growth orientation of the c-axis can be controlled by the stress of film, which can be achieved through the substitution of Y by Gd and Sm each with a larger ionic radius. However, the systematical study of the evolution of the stress mechanism with the substitution ratio is still lacking due to the extreme complexity of the stress manipulation. Therefore, a series of Y1-xGdxBCO thin films with different substitution ratios is deposited on lanthanum aluminate substrates by the fluorine-free metal organic deposition method in order to reveal the evolution of the stress mechanism with Gd substitution. The growth orientations, microstructures and lattice vibration characteristics of the films are analyzed by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The results show that the lattice constant of the film increases and the orientation of the c-axis changes with the Gd substitution ratio for x increasing to a value less than 0.5, and the blue shift of the O(2)/O(3) mode of the Raman spectrum decreases with increasing x. For x=0.5, the blue shift of the O(2)/O(3) mode vanishes, indicating the free standing film with optimal c-axis orientation. However, with the further increase of Gd content, the film structure is deteriorated, and the performance is degraded as well. The red shift of the O(2)/O(3) mode occurs and the frequency decreases with increasing x. Our results indicate that the stress mechanism can be manipulated by controlling the content of various ionic radii in Y1-xGdxBCO films. The free standing film with optimal c-axis orientation can be obtained through adopting an appropriate substitution ratio, i.e., the ratio of m Y:Gd equaling 1:1. These results suggest that manipulation of the stress mechanism is a promising method to overcome the thickness effect effectively.