Infrared bound states in the continuum: random forest method.

In this Letter, we consider optical bound states in the continuum (BICs) in the infrared range supported by an all-dielectric metasurface in the form of subwavelength dielectric grating. We apply the random forest machine learning method to predict the frequency of the BICs as dependent on the optical and geometric parameters of the metasurface. It is found that the machine learning approach outperforms the standard least square method at the size of the dataset of ≈4000 specimens. It is shown that the random forest approach can be applied for predicting the subband in the infrared spectrum into which the BIC falls. The important feature parameters that affect the BIC wavelength are identified.

Monoclinic SmAl3(BO3)4: synthesis, structural and spectroscopic properties.

Single crystals of SmAl3(BO3)4 were synthesized by the group growth on seeds method. The crystal structure was solved using a single-crystal experiment and the purity of the bulk material was proved by the Rietveld method. This borate crystallizes in the monoclinic C2/c space group with unit-cell parameters a = 7.2386 (3), b = 9.3412 (5), c = 11.1013 (4) Šand ß = 103.2240 (10)°. IR and Raman spectroscopic analyses confirmed the monoclinic structure of SmAl3(BO3)4. Under 532.1 nm excitation, luminescence spectra exhibit bands assignable to the transitions from 4G5/2 to 6H5/2, 6H7/2, 6H9/2 and 6H11/2. The similarity of the luminescence spectra of the trigonal and monoclinic polymorphs is explained by the minor role of Sm-O bond distortion and the primary role of rotational distortion of SmO6 octahedra. The smaller covalency of the Sm-O bond in alumoborates is deduced in comparison with galloborates. Calorimetric measurements did not reveal high-temperature structural phase transitions up to a temperature of 720 K.

Characterization of the iron oxide phases formed during the synthesis of core-shell FexOy@C nanoparticles modified with Ag.

Core-shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.

Synthesis of Samarium OxysulfateSm2O2SO4 in the High-Temperature Oxidation Reaction and Its Structural, Thermal and Luminescent Properties.

The oxidation process of samariumoxysulfide was studied in the temperature range of 500-1000 °C. Our DTA investigation allowed for establishing the main thermodynamic (∆Hºexp = -654.6 kJ/mol) and kinetic characteristics of the process (Ea = 244 kJ/mol, A = 2 × 1010). The enthalpy value of samarium oxysulfate (ΔHºf (Sm2O2SO4(monocl)) = -2294.0 kJ/mol) formation was calculated. The calculated process enthalpy value coincides with the value determined in the experiment. It was established that samarium oxysulfate crystallizes in the monoclinic symmetry class and its crystal structure belongs to space group C2/c with unit cell parameters a = 13.7442 (2), b = 4.20178 (4) and c = 8.16711 (8)Å, ß = 107.224 (1)°, V = 450.498 (9)Å3, Z = 4. The main elements of the crystalline structure are obtained and the cation coordination environment is analyzed in detail. Vibrational spectroscopy methods confirmed the structural model adequacy. The Sm2O2SO4luminescence spectra exhibit three main bands easily assignable to the transitions from 4G5/2 state to 6H5/2, 6H7/2, and 6H9/2 multiplets.

Negative thermal expansion and electronic structure variation of chalcopyrite type LiGaTe2.

The LiGaTe2 crystals up to 5 mm in size were grown by the modified Bridgman-Stockbarger technique and the cell parameter dependence on temperature in the range of 303-563 K was evaluated by the X-ray diffraction analysis. The thermal behavior of LiGaTe2 is evidently anisotropic and a negative thermal expansion is found along crystallographic direction c with coefficient -8.6 × 10-6. However, the normal thermal expansion in two a directions with coefficient 19.1 × 10-6 is dominant providing unit cell volume increase on heating. The atomic mechanism is proposed to describe this pronounced anisotropic expansion effect. The electronic structure of LiGaTe2 is measured by X-ray photoelectron spectroscopy and the band structure is obtained by DFT calculations. The pressure response from 0 to 5 GPa was calculated and a normal crystal compression is found. This work indicates that LiGaTe2 is promising as an IR NLO or window material for many practical applications because the thermal expansion coefficients of this telluride are not big. We believe that these results would be beneficial for the discovery and exploration of new IR optoelectronic polyfunctional metal tellurides.

Sequence of phase transitions in (NH4)3SiF7.

Single crystals of silicon double salt (NH4)3SiF7 = (NH4)2SiF6·NH4F = (NH4)3[SiF6]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G1) ↔ Pbam (Z = 4) (G2) ↔ P21/c (Z = 4) (G3) ↔ P1[combining macron] (Z = 4) (G4) ↔ P21/c (Z = 8) (G5). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N-HF hydrogen bonds can be a driving force of the observed phase transitions.

A non-typical sequence of phase transitions in (NH4)3GeF7: optical and structural characterization.

Single crystals of germanium double salt (NH4)3GeF7 = (NH4)2GeF6·NH4F = (NH4)3[GeF6]F were grown and studied by the methods of polarization optics and X-ray diffraction. The birefringence Δn = (no - ne), the rotation angle of the optical indicatrix ϕ(T) and unit cell parameters were measured in the temperature range 100-400 K. Three structural phase transitions were found at the temperatures: T1↓ = 279.2 K (T1↑ = 279.4 K), T2↑ = 270 K (T2↓ = 268.9 K), T3↓ = 218 K (T3↑ = 227 K). An unusual sequence of symmetry transformations with temperature change was established: P4/mbm (Z = 2) (G1) ↔ Pbam (Z = 4) (G2) ↔ P21/c (Z = 4) (G3) ↔ Pa3[combining macron] (Z = 8) (G4). The crystal structures of different phases were determined. The experimental data were additionally interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account the critical and noncritical atomic displacements. Strengthening of the N-HF hydrogen bonds can be a driving force of the observed phase transitions.

Electronic structure of ß-RbSm(MoO4)2 and chemical bonding in molybdates.

Microcrystals of orthorhombic rubidium samarium molybdate, ß-RbSm(MoO4)2, have been fabricated by solid state synthesis at T = 450 °C, 70 h, and at T = 600 °C, 150 h. The crystal structure has been refined by the Rietveld method in space group Pbcn with cell parameters a = 5.0984(2), b = 18.9742(6) and c = 8.0449(3) Å (R(B) = 1.72%). Thermal properties of ß-RbSm(MoO4)2 were traced by DSC over the temperature range of T = 20-965 °C, and the earlier reported ß â†” α phase transition at T ∼ 860-910 °C was not verified. The electronic structure of ß-RbSm(MoO4)2 was studied by employing theoretical calculations and X-ray photoelectron spectroscopy. It has been established that the O 2p-like states contribute mainly to the upper part of the valence band and occupy the valence band maximum, whereas the Mo 4d-like states contribute mainly to the lower part of the valence band. Chemical bonding effects have been analysed from the element core level binding energy data. In addition, it was found that the luminescence spectrum of ß-RbSm(MoO4)2 is rather peculiar among the Sm(3+) containing materials. The optical refractive index dispersion in ß-RbSm(MoO4)2 was also predicted by the first-principles calculations.

Magnetic phase diagram of the olivine-type Mn2GeO4 single crystal estimated from magnetic, resonance and thermodynamic properties.

Mn2GeO4 single crystals with the olivine structure grown by the modified flux method have been investigated. Pronounced magnetic phase transitions at T1 = 47.7 K, T2 = 17 K and T3 = 5.5 K, with T2 being dependent on an applied magnetic field, have been found. Based on the data of magnetic, resonance and temperature measurements, the entire phase diagram of Mn2GeO4 has been built. Mn2GeO4 is shown to be a material with a complex magnetic structure consisting of two magnetic subsystems.

Electronic structure of α-SrB4O7: experiment and theory.

The investigation of valence band structure and electronic parameters of constituent element core levels of α-SrB(4)O(7) has been carried out with x-ray photoemission spectroscopy. Optical-quality crystal α-SrB(4)O(7) has been grown by the Czochralski method. Detailed photoemission spectra of the element core levels have been recorded from the powder sample under excitation by nonmonochromatic Al Kα radiation (1486.6 eV). The band structure of α-SrB(4)O(7) has been calculated by ab initio methods and compared to XPS measurements. It has been found that the band structure of α-SrB(4)O(7) is weakly dependent on the Sr-related states.

Powder diffraction crystal structure analysis using derivative difference minimization: example of the potassium salt of 1-(tetrazol-5-yl)-2-nitroguanidine.

The crystal structure of the potassium salt of 1-(tetrazol-5-yl)-2-nitroguanidine [K(C2H3N8O2)] was solved and refined from X-ray powder diffraction data by applying the derivative difference minimization (DDM) method. The compound is of interest as an energetic substance. The structure model was found from a Patterson search. The reflection intensities for the Patterson synthesis were derived from the powder profile by applying a newly developed DDM-based profile decomposition procedure. The use of the DDM method allowed successful location and unconstrained refinement of all the atomic positions, including those of three independent H atoms. The advantages of DDM in terms of the precision and reproducibility of the structural parameters are discussed in comparison to Rietveld refinement results. The failure to refine the H-atom positions by the Rietveld method was attributed to systematic errors associated with the background modelling, which are avoided by DDM.