Far-infrared Reflectivity Spectra Research Articles

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.

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.

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.

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.

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.

An infrared study of antiferromagnetic medium and high entropy rocksalt structure oxides

The infrared reflectance of a set of antiferromagnetic entropy-stabilized rocksalt structure oxides is measured in order to study the effect of a random array of cations, spin–phonon coupling, and anharmonicity on phonon mode behavior. Samples were prepared by conventional solid state reaction from quaternary or quinary equimolar mixtures of CoO, ZnO, CuO, NiO, and MgO. They were characterized by magnetometry, x-ray diffraction, and reflectance spectroscopy. It is found that the Néel temperature (TN) depends on the concentration of magnetic ions. The far-infrared reflectance spectra consist of one strong reststrahlen band and a weaker low frequency band, which is unexpected in a rocksalt structure. The resonance frequency of the main transverse optical (ωTO) mode varies between 355&amp;lt;ωTO&amp;lt;386 cm−1 and is controlled by both the average reduced mass of a cation–anion pair and the average effective charge. Unlike the parent compounds CoO and NiO, no splitting of ωTO due to spin–phonon coupling appeared in the reflectance spectra below TN. This is possibly due to the high scattering rate of the associated transverse optical mode, which is connected to static disorder.

Comparison of the results of optical and electrophysical measurements of free electron density in n-GaAs samples doped with tellurium

A theoretical model has been developed that allows one to determine free electron density in n-GaAs from the characteristic points on far-infrared reflection spectra. It was shown that, in this case, it is necessary to take into account the plasmon-phonon coupling (otherwise, the electron density is overestimated). The calculated dependence of electron density, Nopt, on the characteristic wave number, ν+, which is described by a second degree polynomial, has been obtained.Twenty-five tellurium-doped gallium arsenide samples were used to measure the electron density in two ways: according to traditional four-contact Hall method (Van der Pauw method) and using the optical method we developed (measurements were carried out at room temperature). Based on the experimental results, the dependence was constructed of the electron density values obtained from the Hall data, NHall, on the electron density obtained by the optical method, Nopt. It is shown that this dependence is described by linear function. It is established that the data of optical and electrophysical measurements coincide if the electron density is Neq = 1.07 ⋅ 1018 cm-3, for lower values of the Hall density NHall &lt; Nopt, and for large values NHall &gt; Nopt. A qualitative model is proposed to explain the results. It has been suggested that tellurium atoms bind to vacancies of arsenic into complexes, as a result of which the electron density decreases. On the surface of the crystal, the concentration of arsenic vacancies is lower and, therefore, the condition Nopt &gt; NHall should be satisfied. As the doping level increases, more and more tellurium atoms remain electrically active, so electron density in the volume begins to prevail over the surface one. However, with a further increase in the doping level, the ratio NHall/Nopt again decreases, tending to unity. This, probably, is due to the fact that the rate of decomposition of the complexes “tellurium atom + arsenic vacancy” decreases with increasing doping level.

Structure and Microwave Dielectric Properties of Gillespite-Type ACuSi4O10 (A = Ca, Sr, Ba) Ceramics and Quantitative Prediction of the Q × f Value via Machine Learning.

Structure and dielectric properties of gillespite-type ceramics ACuSi4O10 (A = Ca, Sr, Ba) were investigated by crystal structure refinement, far-infrared reflectivity spectroscopy, and microwave dielectric measurements. A series of (CaxSr1-x)CuSi4O10 (0 < x < 1) ceramics with relative permittivities of 5.70-5.82, Q × f values of 20391-48794 GHz (@ ∼ 13.5 GHz), and τf of -46.3 to -38.9 ppm/°C were synthesized. By Ca2+ substitution for Sr2+ at the A-site, the rigid double-layered copper silicate framework remains stable, resulting in the nearly unchanged relative permittivity, while the [(Ca,Sr)O8] dodecahedron undergoes shrinkage and distortion, which is correlated to the changes in the Q × f and τf values. The normalized bond valence sums indicate that almost all ions are rattling, weakening the bond strengths and enlarging the molecular dielectric polarizability. The fitting of far-infrared reflectivity spectra reveals that the local structure changes suppress the intermediate and low-frequency vibrational modes significantly and improves the contribution from electronic polarization to permittivity. Symmetry breaking of the [(Ca,Sr)O8] dodecahedron conforms to the elevated restoring forces acting on the ions and improves the τf value. The large span in Q × f value may have intricate correlations to local structure changes and defects. Machine learning methods were introduced to explore the decisive structural factors for the Q × f value. A Q × f value prediction model correlated with the A-O2 bond length and the variance of A-O bond lengths was established. The Q × f values of isostructural (BaySr1-y)CuSi4O10 ceramics were predicted and verified by experiments.

Crystal structure and optimised microwave dielectric properties of Ce2(Zr1-xTix)3(MoO4)9 solid solutions

Ce2(Zr1-xTix)3(MoO4)9 (CZ1-xTxM) (x = 0.02–0.10) solid solutions were prepared via solid-state reaction method. The effects of Ti4+ ions substituting for Zr4+ on sintering behavior, structure and microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics were investigated. X-ray diffraction (XRD) patterns combined with Rietveld refinements results confirmed that all compounds crystallized in a trigonal cell (R-3c). The Q·f values of the as-prepared samples were explained by the packing fraction and average grain size. Dielectric polarizability played a significant role in affecting the permittivity, and the τf values were associated with the bond valence of Zr-site. What's more, far-infrared reflectivity spectra were also used to explore the intrinsic dielectric properties of Ce2(Zr1-xTix)3(MoO4)9 (x = 0.08) ceramics. Typically, the desirable microwave dielectric properties of Ce2(Zr1-xTix)3(MoO4)9 ceramics were achieved in x = 0.08 when sintered at 775 °C for 6 h: εr = 11.28, Q·f = 84,200 GHz and τf = −7.86 ppm/°C.

High-temperature structural phase transition and infrared dielectric features of La2CoMnO6

In this paper, we present the temperature-dependent far-infrared reflectivity spectra of partially ordered magnetodielectric La2CoMnO6 from room temperature to 673 K. We demonstrate a clear first-order structural phase transition (SPT) from a monoclinic structure with P21/n symmetry to a rhombohedral phase with R3¯ symmetry from the behavior of the polar phonon modes at TC ∼ 590 K. The temperature-dependent transversal and longitudinal phonon branches, dielectric strengths, and damping of the strongest dielectric modes support the significant contribution of the phonon modes to the SPT and reveal an important lattice anharmonicity, especially for the low-frequency modes. Also, these investigations showed that structural ordering does not inhibit the SPT and provided valuable information regarding the polar phonons and their effects on the intrinsic dielectric constant in double perovskites and related compounds.

Structural and optical properties of ZnO–Al2O3 nanopowders prepared by chemical methods

The nanopowders of (ZnO)1–x(Al2O3)x, where x ranges from 0 to 0.7, were obtained by two chemical methods: the co-precipitation/calcination and hydrothermal synthesis. The first assessment of structural and optical properties of the obtained nanopowders was undertaken by the SEM, XRD, Raman and far-infrared spectroscopy, which was followed by the photoluminescence spectroscopy at room temperature. The obtained far-infrared reflectivity spectra were analyzed using the fitting procedure. The dielectric function of ZnO–Al2O3 nanopowders was modeled by the Maxwell-Garnet formula under the assumption that the nanopowders are a mixture of homogenous spherical inclusions in air. The combined plasmon-LO phonon modes (CPPM) were observed in the far-infrared reflection spectra. The photoluminescence spectra contain emissions related to the presence of ZnO, ZnAl2O4, and AlOOH phases in the nanomaterial, which is in agreement with the results obtained by other experiments.

Probing NaCl at High Pressure through Optical Studies and Ab Initio Calculations

The optical constants of sodium chloride in a wide pressure range were determined from the analysis of the reflectance and transmittance spectra of a minute quantity of NaCl powder placed in diamond anvil cells. The so-called “reststrahlen band” dominates the far-infrared reflectance spectra shifting from 150 cm–1 to 500 cm–1 at 100 GPa. For the 0–17.5 GPa pressure range, measurements allow accurate determination of both transverse and longitudinal mode frequencies. Higher pressure measurements reveal the B1 → B2 structural transition around 30 GPa and provide frequencies for the transverse and longitudinal modes. This spectroscopic signature on a sample smaller than 100 μm using light of a wavelength close to this dimension was observed, thanks to the high brilliance synchrotron source. In addition, ab initio calculations performed for the 0–200 GPa range predict the TO and LO frequencies. They are validated by the excellent agreement with the experiment.

Low-temperature infrared dielectric function of hyperbolic α -quartz

We report the infrared dielectric properties of $\alpha$-quartz in the temperature range from $1.5\ \mathrm{K}$ to $200\ \mathrm{K}$. Using an infrared free-electron laser, far-infrared reflectivity spectra of a single crystal $y$-cut were acquired along both principal axes, under two different incidence angles, in S- and P-polarization. These experimental data have been fitted globally for each temperature with a multioscillator model, allowing to extract frequencies and damping rates of the ordinary and extraordinary, transverse and longitudinal optic phonon modes, and hence the temperature-dependent dispersion of the infrared dielectric function. The results are in line with previous high-temperature studies, allowing for a parametrized description of all temperature-dependent phonon parameters and the resulting dielectric function from $1.5\ \mathrm{K}$ up to the $\alpha$-$\beta$-phase transition temperature, $T_C = 846\ \mathrm{K}$. Using these data, we predict remarkably high quality factors for polaritons in $\alpha$-quartz's hyperbolic spectral region at low temperatures.

NdNb1-x(Mg1/4W3/4)xO4 (0.02≤x≤0.06) solid solution characterized by infrared spectrum and complex chemical theory

The dependence of the bond characters and crystal structure on microwave dielectric properties of NdNb1-x (Mg1/4W3/4)xO4 (x = 0.02–0.06) solid solution ceramics is investigated. X-ray diffraction and Rietveld refinement analysis both indicate a formation of single monoclinic phase in the whole range. Based on complex chemical bond theory, theoretical dielectric constant of the complex crystal is slightly lower than observed one, which is consistent with the analysis of infrared reflectivity spectra. The Q × f (quality factor) values are associated with the total lattice energy and average grain size. Increment of Nb-site bond energy generates a more stable structure, which makes τf (temperature coefficient of resonant frequency) continuously move towards zero. Besides, far-infrared reflectivity spectra illustrates that low-frequency modes play primary role of intrinsic dielectric properties. Excellent microwave dielectric properties and an optimal combination are: εr = 19.59, Q × f = 50339 GHz, τf = −39.62 ppm/°C at x = 0.04.