Far-infrared Reflectivity Research Articles

Low sintering temperature, interface characteristic and microwave/terahertz dielectric properties of ternary-phase Nd2O3-P2O5 based ceramics for patch antenna application

In this work, LiF was chosen as a sintering aid to lower the sintering temperature required for monoclinic NdPO4 (NPO) ceramic. A comprehensive investigation was conducted to analyze the impact of LiF on the sintering characteristics, microstructures, phase compositions, crystalline structures, interface characteristics, and microwave dielectric properties of NPO ceramic. Structure-property relationships were discussed based on the density, crystallite size, and lattice strain. The complex impedance spectroscopy revealed the mechanism underlying the interface characteristics of LiF on NPO ceramic. Additionally, far-infrared reflectance spectra and THz time-domain spectra revealed the intrinsic dielectric properties of NdPO4–1 wt.%LiF (NPO1). Notably, 1 wt.% LiF effectively reduced the sintering temperature from 1300 °C to 850 °C, significantly improving densification and dielectric properties. Typical properties such as Q × f = 55,840 GHz (at 9.58 GHz), εr = 10.4, and τf = −43.1 ppm °C−1 were obtained for NPO1 ceramic after sintering at 850 °C. Furthermore, its compatibility with Ag was explored as a prerequisite for LTCC applications. Finally, the NPO1 ceramic was designed as a microstrip antenna with high gain (6.83 dB) and low return loss (−18.85 dB) at a centre frequency of 2.56 GHz, demonstrating its potential for Wi-Fi and Bluetooth applications. This work provides a theoretical foundation and practical guidance for developing orthophosphate ceramics.

Near-Zero Temperature Coefficient of Resonance Frequency in Rare-Earth Titanates via the Phase Transition Strategy.

This study presents an approach to achieve a near-zero temperature coefficient of resonance frequency (τf) in rare-earth titanate microwave dielectric ceramics (MWDCs) by inducing a phase transition. By Zr4+ substitution at the B site, a series of Sm2Ti1-xZrxO5 (0.02 ≤ x ≤ 0.55) ceramics are synthesized using the solid-state method to intentionally alter the radius ratio of the A/B sites, realizing in a controlled phase transition from orthorhombic (Pnma) to biphasic coexistence and ultimately to cubic (Fd3̅m) structure. The phase composition is rigorously identified through X-ray diffraction (XRD) Rietveld refinement, high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and Raman spectroscopy. A comprehensive analysis is conducted to elucidate the relationships between factors such as ionic polarizability, packing fraction, bond valence, complex chemical bonding, and far-infrared reflectivity spectra with microwave dielectric properties. The results demonstrate that these ceramics exhibit a broad range of permittivity (14.30-23.18), high-quality factors (14,828-22,300 GHz), opposite temperature coefficient of resonance frequency (-16.0 to + 22.4 ppm/°C), and nice thermal conductivity (1.81-2.76 W·m-1·K-1), particularly at x = 0.30 with a near-zero τf value of +1.6 ppm/°C. The findings not only provide insights into designing MWDCs with a near-zero τf but also offer a promising route for developing advanced microwave materials with improved performance and reliability.

Crystal structures, lattice vibrational characteristics, and dielectric responses of ZnWO4 microwave dielectric ceramics sintered at different temperatures

The study meticulously examined the impact of varying sintering temperatures on the phase transformations, crystallographic configurations, dielectric response mechanisms, and lattice vibrational characteristics of the ZnWO4 ceramics. Far-infrared reflectance spectra were fitted using a four-parameter semiquantum model to obtain the intrinsic dielectric properties. Moreover, the contribution to the dielectric properties for each vibrational mode was analyzed, indicating that the lower-frequency mode had the most contribution. The relationship linking the structural aspects to the properties of ZnWO4 ceramics was elucidated through an analysis of the Raman spectral modes across a range of sintering temperatures, overcoming the limitation of solely textual description without mathematical quantification. When sintered at 1070 °C, the ZnWO4 ceramic had the best dielectric properties: εr = 14.81, Q×f = 34,239 GHz (f = 9.69 GHz), with the highest relative density of 96.95 %.

Relationship between structure and microwave dielectric properties of new low-K garnet-type Ca3Cr2Ge3O12 ceramics

In order to solve the problem of signal delay, a novel low-K Ca3Cr2Ge3O12 ceramic for 5G and millimeter-wave communication was prepared by a solid-state reaction method. XRD and Rietveld refinement results confirm that it belongs to the garnet structure with the Ia-3d space group. The shrinkage rate of the sample sintered at 1350 °C reached 11.9 %, and the relative density exceeded 94.3 %. Excellent microwave dielectric properties were obtained. The permittivity was 8.38, the quality factor was 23,107 GHz, and the temperature coefficient of resonant frequency was −43.57 ppm/°C. The relationship between the crystal structure and microwave dielectric properties of ceramics was further studied. The results show that lattice vibration, stacking fraction and bond strength have a great influence on the Q × f value of ceramics, and the bond valence parameter is the key factor affecting the temperature stability. In addition, the theoretical quality factor of the ceramic can reach ∼13,0000 GHz by far-infrared reflection spectroscopy, and the ceramic has great research potential. The development of Ca3Cr2Ge3O12 ceramics and the study of microwave dielectric properties have filled the research gap of related systems. The excellent microwave dielectric properties indicate that it can be used as an alternative material for the preparation of components in the field of communication.

Ionic doping engineered Ce2(Zr1−xBx)3(MoO4)9 (B[dbnd]Cd1/3Nb2/3, Cd1/3Ta2/3) ceramics: Structural, chemical bond, and microwave/terahertz dielectric performance

A series of Ce2(Zr1−xBx)3(MoO4)9 ceramics, where BCd1/3Nb2/3, Cd1/3Ta2/3 (abbreviated as CZ1−xNx, CZ1−xTx), were synthesized using the classical solid-state reaction method. This study focused on the intrinsic mechanisms affected by doping in terms of chemical bond parameters, polarizability, lattice parameters, and bond valence. X-ray diffraction (XRD) results showed that Cd/Nb and Cd/Ta were solidly dissolved into the matrix lattice, which crystallized in a pure trigonal structure (Space group of R-3c, No. 167). According to the Phillips-van Vechten-Levine (P–V-L) theory, the chemical bond features have been parameterized for exploring the relationships between crystal structure and property. Interestingly, the findings indicated that the dielectric parameters varied closely with the chemical bond features of Zr1–O4. Far-infrared reflectivity (FIR) spectrum and terahertz time-domain (THz-TD) were used to obtain information on lattice vibration. The results showed that the complex permittivity extrapolated from THz-TD was closer to the measured value (at microwave) than the former. The relaxation polarization mechanism may be a key intrinsic factor affecting dielectric loss. Additionally, the bond valence results showed a linear relationship between Zr1–O4 and τf. CZ1−xNx and CZ1−xTx exhibited Qf values that were 2.86 and 5.46 times greater than the matrix, respectively, with low permittivity (εr, roughly 10) suitable for high-frequency communication materials. Notably, CZ0.94T0.06 ceramic achieved excellent performance with Qf = 103,974 GHz, εr = 10.63, and τf = −17.68 ppm/°C at 725 °C.

Broad-range high-resolution optical spectroscopy of CH3NH3PbBr3 hybrid perovskite single crystals: Optical phonons, absorption edge, phase transitions

Vibrational and structural properties have an enormous influence on optoelectronic properties of hybrid halide perovskites CH3NH3PbX3 (X = I, Br, Cl), which are perspective materials for different photovoltaic applications. Here, we report results of the first high-resolution optical spectroscopy measurements of large CH3NH3PbBr3 single crystals in wide frequency (20–20 000 cm−1) and temperature (5–300 K) ranges. Fit of polarized far-infrared reflection spectra using Lorentz model of damped oscillators revealed parameters of three phonons at room temperature and 38 phonons at 10 K. An analysis of mid-infrared transmission spectra gave evidence of a strong anharmonicity in this hybrid perovskite. A splitting of some lines observed in the orthorhombic phase was presumably assigned to the tunneling of the CH3NH3+ cation molecule between several potential energy minima. The temperature behavior of the transmission spectra near the fundamental absorption edge was tentatively explained by the competition between two different exciton transitions in CH3NH3PbBr3. Positions of spectral lines and of the absorption edge measured at cooling and heating the sample demonstrate hysteresis loops in the vicinities of phase transitions around 237, 155, and 149 K, thus revealing the first-order nature of all three phase transitions in CH3NH3PbBr3.

Structural features of glass-free Al2Mo3O12 ceramic with excellent microwave dielectric properties

The Al2Mo3O12 ceramic with monoclinic type is prepared at the sintering temperature as low as 750 °C, which exhibits excellent microwave dielectric properties of εr = 4.99, Q × f = 49,579 GHz (f = 14.8 GHz), and τf = −26 ppm/°C. The relationship between structure and characteristics is analyzed through the P–V–L chemical bond theory in depth, and the dielectric properties are investigated in combination with far-infrared spectroscopy. Results show that it is Mo–O bond that is responsible mainly for bond ionicity and dielectric polarizability, and structural stability as well. Based on the far-infrared reflectivity spectra and complicated dielectric function analysis, the calculated and measured results are in relative agreement using the classical damped oscillation mode, indicating that the mode at 80.23 cm−1 is dominant for the contribution to dielectric properties. This study gives an insight into the structural characteristic and provides a guide to further improve the microwave dielectric performance of Al2Mo3O12 ceramic.

Effect of adding Sn4+ on crystal structure, sintering behaviors, and microwave dielectric properties of La2Zr3(MoO4)9 ceramics

La2(Zr1-xSnx)3(MoO4)9 (0.02 ≤ x ≤ 0.10) (LZMS) materials were successfully synthesized to scientifically investigate the effects of Sn4+ addition on phase composition, microstructures, lattice vibration, and dielectric response at microwave frequency. XRD patterns indicate that all samples belong to the trigonal crystal (S.G. R-3c). The crystallite size and lattice strain were analyzed using the Willimson-Hall equation. According to the apparent density and SEM, the LZMS ceramics achieved a dense and homogeneous microstructure when sintered at 700 °C. Analysis of the microwave dielectric properties revealed that the ceramics with x = 0.06 sintered at 700 °C achieved excellent performances (Qf = 75,508 GHz, εr = 10.18, τf = −22.21 ppm/°C). The far-infrared reflectance spectrum shows that the theoretical permittivity of the ceramic was mainly from the absorption of phonon vibrations below 400 cm-1. Consequently, the La2(Zr0.94Sn0.06)3(MoO4)9 ceramic has excellent microwave dielectric properties and can be used as a material for 5G communication devices.

Structure, infrared spectrum, and microwave dielectric properties of NiO–TiO2–Nb2O5 ceramics

This study reports the crystal structure and intrinsic dielectric properties of NiO–TiO2–Nb2O5 ceramics. The NiO–TiO2–Nb2O5 system is crystallized as a rutile structure based on X-ray diffraction, Rietveld refinement, and TEM analysis. The sintering temperature has a certain positive effect on the dielectric constant and Q × f value, which is attributed to the reduction of porosity and the growth of crystals. Because of the particularity of the rutile structure, ionic polarizability cannot be used to evaluate the micro-polarization mechanism. Thus, using the far-infrared reflectance spectrum and complex dielectric function analysis, the high-frequency micro-polarization mechanism of the structure has been fully explained, which is expected to be applied to other high-εr dielectric ceramic systems.

Crystal structure, far-infrared spectroscopy, and microwave dielectric properties of two novel vanadates CaRECeV3O12 (RE = Nd, Sm)

Two novel low-εr CaRECeV3O12 (RE = Nd, Sm) microwave dielectric ceramics were prepared to utilize the solid-state method. According to XRD and Raman spectroscopy data, the CaRECeV3O12 (RE = Nd, Sm) ceramics had a tetragonal zircon-type with an I41/amd space group. Dense CaNdCeV3O12 and CaSmCeV3O12 ceramics sintered at 1225 °C and 1200 °C showed εr ∼11.85 ± 0.1 and 11.33 ± 0.1, Q × f ∼40,100 ± 500 GHz and 43,290 ± 500 GHz, τf ∼ −27.2 ± 0.5 ppm/°C and −22.4 ± 0.5 ppm/°C, along with αL of 9.4 ppm/°C and 8.5 ppm/°C, respectively. The measured εr and corrected εr(corr) values were larger than εr(C-M) (9.41 and 9.47) in CaNdCeV3O12 and CaSmCeV3O12 due to the cations being, on average, under-bonded and the overall structures being in a slightly expanded state. The slightly higher |<d>| value of CaNdCeV3O12 led to its higher εr(corr) value, even though it had a lower αD/Vm value. The sign and magnitude of τf values for CaNdCeV3O12 and CaSmCeV3O12 were mainly determined by their positive temperature coefficients of dielectric polarizability ταm of +36.2 ppm/°C and +32.1 ppm/°C, respectively. The inherent dielectric constant and dielectric loss were examined using far-infrared reflectivity spectra, Raman spectra, and the packing fraction.

Crystal structure, dielectric properties, and far-infrared reflectivity spectrum of a novel phosphate KBaPO4 ceramic at microwave frequency

A novel orthophosphate ceramic KBaPO4 was successfully synthesized via a solid-state reaction route and surveyed systematically in microwave properties. X-ray diffraction patterns confirmed that single phases were obtained of all ceramics sintered at 900–1100 °C. The dense ceramics with homogeneous microstructure was obtained when sintered at 1050 °C and revealed by density and microstructural studies. According to the chemical bond theory, P–O made a more significant contribution to the microwave properties of KBaPO4 ceramics. Infrared reflection spectrum results showed that the permittivity was related to the absorption of phonon oscillation. The optimum properties (εr = 7.58, Q×f = 31,577 GHz, τf = −13.3 ppm/°C) of KBaPO4 ceramics were obtained when sintered at 1050 °C.

Relationship between distortion of crystal structure and dielectric properties of complex perovskite oxide Ba(Co,Zn)1/3Nb2/3O3 thin films

The oxygen octahedral can be distorted by epitaxial strain due to the lattice mismatch. The epitaxial strain (εyy) linearly decreases from − 0.244% to − 0.445% with the growth temperature. Thin films grow along c axis on the SrTiO3 substrate and exhibit the epitaxial relationship of Ba(Co,Zn)1/3Nb2/3O3 (001) // SrTiO3 (001). The superlattice reflections arising from in-phase tilting of the oxygen octahedra are clearly visible along [010] and [111] zone axis. The IR modes at 330 cm−1 and 390 cm−1 related to in-phase tilting are observed in far-infrared reflectivity spectroscopy. The calculated Q×f values from far-infrared reflectivity spectra of films grown at 550 °C to 700 °C increases from 51,000 GHz to 91,000 GHz mainly due to the enhancement of crystalline quality. The intrinsic quality factor (Q) is mainly contributed by O-(Co, Nb)-O and O-(Zn, Nb)-O bonding modes, while in-phase tilting in BCZN films may result in enhanced dielectric constants.

Optical spectroscopy and unusual temperature shift of f – f zero-phonon luminescence lines: KTaO3:Er

We present a study of KTaO3:Er (≈0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO3. We provide information about energy level structures controlling f – f optical transitions in Er3+, on formation of minor cubic octahedral and major orthorhombic Er3+ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.

Revealing plasmon-phonon interaction in nanocrystalline MgFe2O4 spinels by far-infrared reflection spectroscopy

Room-temperature far-infrared reflectivity spectra of nanocrystalline, partially inverse MgFe2O4 were investigated. MgFe2O4 samples were prepared by sol-gel method and sintered at three different temperatures (400, 600 and 800 °C). Raman spectroscopy was employed to estimate the degree of inversion in the sintered samples. The degree of inversion was found to increase from 0.52 to 0.74 as the sintering temperature increased from 400 °C to 800 °C. The reflectivity spectra, besides the four infrared modes characteristic for spinels (ν1, ν2, ν3, ν4), revealed the presence of free carriers. Plasmon-longitudinal optical (LO) phonon interaction was analyzed using factorized coupled and decoupled plasmon-phonon models, combined with the Bruggeman effective medium approximation. From these models it was obtained that the ν1 and ν3 phonon modes are more strongly coupled with plasmons than the ν2 mode. A potential mechanism of plasmon-phonon interaction in inverse MgFe2O4 spinel has been discussed.

Assessing thermodynamical properties of Al1−xGaxSb alloys and optical modes for Al1−xGaxSb/GaAs epifilms and (AlSb)m/(GaSb)n superlattices

A generalized Green's function (GF) theory is adopted in the framework of a realistic rigid-ion-model (RIM) to assess the composition, x-dependent lattice dynamics, and thermodynamical characteristics of ideal random Al1−xGaxSb alloys. For simulating phonons, the alloy parameters are achieved by interpolating the values of the RIM force constants between AlSb and GaSb without requiring any additional interactions. The outcomes of phonon dispersions ωj(q→), Debye temperature ΘD(T), and specific heat Cv(T) compare favorably well with the existing experimental and theoretical data. An established methodology of multilayer optics is also employed for modeling the far-infrared reflectance and transmission spectra of ultrathin GaSb/GaAs, AlSb/GaAs, Al1−xGaxSb/GaAs epilayers, and (AlSb)m/(GaSb)n/GaAs superlattices at near normal (θi = 0) incidence and oblique (θi ≠ 0) incidence. An accurate appraisal of the x-dependent longitudinal-optical [ωLO(Γ)] and transverse-optical [ωTO(Γ)] phonon splitting by Berreman's effect, along with the calculated GF results of localized vibrational mode (GaSb:Al) and gap mode (AlSb:Ga), is carefully integrated into the modified-random-iso-displacement model to validate the two-phonon mode behavior in Al1−xGaxSb ternary alloys.

Magnetic-Field-Tunable Intensity Transfer from Optically Active Phonons to Crystal-Field Excitations in the Reflection Spectra of the PrFe3(BO3)4 Antiferromagnet

We analyze the field-dependent intensities of the coupled electron-phonon modes observed in the low-temperature far-infrared (terahertz) reflection spectra of PrFe3(BO3)4 and develop a theory based on the Green’s function approach. An excellent agreement between the experimental and theoretical data is achieved. The developed theory of the intensity transfer from phonons to quasi-electronic excitations can be applied to the electron-phonon modes in other compounds, in particular, in magnetodielectric materials, where it can be used to analyze the magnetodielectric response.

SnO2 doping effects on the phase constitution and microwave dielectric properties of a Co0.5Ti0.5NbO4 system

The effects of Sn4+ substitution on the structure–property relationship of Co0.5(Ti1−xSnx)0.5NbO4 ceramics were studied systemically using the dielectric theory of complex crystals, and the intrinsic dielectric properties were examined. A rutile Co0.5(Ti1−xSnx)0.5NbO4 solid solution was formed, and the properties were examined via X-ray diffraction and transmission electron microscopy. According to Rietveld refinement analysis, Sn4+ increased the axis length and unit-cell volume. The dielectric theory of complex crystals showed that the introduction of Sn4+ leads to a consistent variation between the average bond covalency value and dielectric constant, indicating a decline in dielectric polarizability. However, an evaluation of the lattice energy of chemical bonds revealed a decrease in the structural stability, with a concomitant increase in the dielectric loss. Moreover, far-infrared reflectivity analysis, complex dielectric function analysis, and Terahertz time-domain spectroscopy of the intrinsic dielectric properties of Co0.5(Ti1−xSnx)0.5NbO4 (x = 0.2) ceramic revealed similar results between the theoretical fitting and experimental test.

Relationship between the structure and microwave dielectric properties of garnet ceramics Ca3B2GeV2O12 (B = Mg, Mn)

Two garnet-structured ceramics, Ca3B2GeV2O12 (B = Mg, Mn), were synthesized using a traditional solid-state reaction method, and their sintering behavior, structures, vibrational spectra, and magnetic and microwave dielectric properties were characterized. Low permittivity (εr) of 9.75 (Ca3Mg2GeV2O12) and 11.06 (Ca3Mn2GeV2O12), quality factor (Q × f) of 54,000 GHz (Ca3Mg2GeV2O12) and 30,240 GHz (Ca3Mn2GeV2O12), and temperature coefficient of resonant frequency (τf) of −61.2 ppm/°C (Ca3Mg2GeV2O12) and −66.4 ppm/°C (Ca3Mn2GeV2O12) were obtained. The εr value was correlated with the sum of the ionic polarization per unit volume. The Q × f value was associated with the part with a fully filled 3d orbit, un-paired d electrons of B-site ion, and magnetic loss. The τf was correlated to the bond valance at the A- and C-sites. Moreover, the fitting analysis of the far-infrared reflectance spectra of Ca3Mg2GeV2O12 revealed that the active vibration modes in the low wave number range (100 - 400 cm-1) contributed mainly to the intrinsic microwave dielectric polarization.

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).

Investigation of the structural characteristics, dielectric properties, and infrared reflectivity spectra of AlON transparent ceramics

Al(8+x)/3O4−xNx (x = 0.299–0.575) transparent ceramics were fabricated by presintering and subsequent hot isostatic pressing (HIP) technology. The XRD and SEAD analyses verified that a complete solid solution with a cubic spinel structure was achieved in the entire composition range. The SEM characterization confirmed that the x = 0.299−0.510 samples are highly dense with rare residual pores, which was further demonstrated by HRTEM observations. In combination with phonon modes extracted by far-infrared reflectivity spectra, the intrinsic dielectric properties of AlON ceramics were estimated, which were consistent with the experimental results, revealing that the contribution to dielectric properties is dominated by the absorption of structural phonons. Moreover, as the nitrogen content increased, the measured permittivity (εr) of the samples was found to gradually increase from 9.41 to 9.68 at 7 GHz, while the quality factor (Q × f) values fluctuated from 43,750 to 76,923 GHz. Accordingly, the reported AlON transparent ceramics with exceptional dielectric performances are promising candidates for high-frequency 5 G applications.