Second-harmonic-generation of [(Se,Te)Cl3_{3}]+^{+} [GaCl4_{4}]–^{–} with aligned ionic tetrahedra

[SeCl$_{3}$][GaCl$_{4}$] (1) and [TeCl$_{3}$][GaCl$_{4}$] (2) are prepared via Lewis-acid–base reaction of SeCl$_{4}$ or TeCl$_{4}$ with GaCl$_{3}$ at 50 °C (1) and 140 °C (2) in quantitative yield. The ionic compounds contain pseudo-tetrahedral [SeCl$_{3}$]$^{+}$/[TeCl$_{3}$]$^{+}$ cations with a prominent stereochemically active electron lone pair at Se(IV)/Te(IV) as well as tetrahedral [GaCl$_{4}$]$^{–}$ anions. Both compounds crystallize in the polar chiral space group P1 with an unidirectional alignment of all tetrahedral building units. They can be considered as the first examples of a much larger group of ionic compounds [MX$_{3}$]$^{+}$[M′X$_{4}$]$^{–}$ (M, M′: metal or main-group element, X: halogen) showing nonlinear optical effects. Material characterization is performed by X-ray structure analysis based on single crystals and powder samples, thermogravimetry, optical spectroscopy, infrared and Raman spectroscopy. Second harmonic generation (SHG) is observed with intensities about 3-times stronger than for potassium dihydrogen phosphate (KDP) in the visible spectral regime with narrow-band-gap materials (2.8, 3.2 eV). Density functional theory calculations are employed to complement the experimental findings, interpret the Raman spectra, visualize the stereochemically active lone electron pair, and compute the SHG tensor

Order and Disorder in Mixed (Si, P)–N Networks Sr2SiP2N6:Eu2+ and Sr5Si2P6N16:Eu2+

In the field of nitride phosphors, which are crucial for phosphor-converted light-emitting diodes, mixed tetrahedral networks hold a significant position. With respect to the wide range of compositions, the largely unexplored (Si, P)–N networks are investigated as potential host structures. In this work, two highly condensed structures, namely Sr2SiP2N6 and Sr5Si2P6N16 are reported to address the challenges that arise from the similarities of the network-forming cations Si4+ and P5+ in terms of charge, ionic radius, and atomic scattering factor, a multistep workflow is employed to elucidate their structure. Using single-crystal X-ray diffraction, energy-dispersive X-ray spectroscopy (EDX), atomic-resolution scanning transmission electron microscopy (STEM)-EDX maps, and straightforward crystallographic calculations, it is found that Sr2SiP2N6 is the first ordered, and Sr5Si2P6N16 the first disordered, anionic tetrahedral (Si, P)–N network. After doping with Eu2+, Sr2SiP2N6:Eu2+ shows narrow cyan emission (λmax = 506 nm, fwhm = 60 nm/2311 cm−1), while for Sr5Si2P6N16:Eu2+ a broad emission with three maxima at 534, 662, and 745 nm upon irradiation with ultraviolet light is observed. An assignment of Sr sites as probable positions for Eu2+ and their relation to the emission bands of Sr5Si2P6N16:Eu2+ is discussed

Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)

A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd

18-Crown-6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects

The crown-ether coordination compounds ZnX$_{2}$(18-crown-6), EuX$_{2}$(18-crown-6) (X: Cl, Br, I), MnI$_{2}$(18-crown-6), Mn$_{3}$Cl$_{6}$(18-crown-6)2, Mn$_{3}$I$_{6}$(18-crown-6)$_{2}$, and Mn$_{2}$I$_{4}$(18-crown-6) are obtained by ionic-liquid-based synthesis. Whereas MX$_{2}$(18-crown-6) (M: Zn, Eu) show conventional structural motives, Mn$_{3}$Cl$_{6}$(18-crown-6)$_{2}$, Mn$_{3}$I$_{6}$(18-crown-6)$_{2}$, and Mn$_{2}$I$_{4}$(18-crown-6) exhibit unusual single MnX$_{4}$ tetrahedra coordinated to the crown-ether complex. Surprisingly, some compounds show outstanding photoluminescence. Thus, rare Zn$^{2+}$-based luminescence is observed and unexpectedly efficient for ZnI$_{2}$(18-crown-6) with a quantum yield of 54%. Unprecedented quantum yields are also observed for Mn$_{3}$I$_{6}$(18-crown-6)$_{2}$, EuBr$_{2}$(18-crown-6), and EuI$_{2}$(18-crown-6) with values of 98, 72, and 82%, respectively, which can be rationalized based on the specific structural features. Most remarkable, however, is Mn$_{2}$I$_{4}$(18-crown-6). Its specific structural features with finite sensitizer–activator couples result in an extremely strong emission with an outstanding quantum yield of 100%. Consistent with its structural features, moreover, anisotropic angle-dependent emission under polarized light and nonlinear optical (NLO) effects occur, including second-harmonic generation (SHG). The title compounds and their optical properties are characterized by single-crystal structure analysis, X-ray powder diffraction, chemical analysis, density functional theory (DFT) calculations, and advanced spectroscopic methods

Elastic properties of single crystal Bi₁₂SiO₂₀ as a function of pressure and temperature and acoustic attenuation effects in Bi₁₂MO₂₀ (M = Si, Ge and Ti)

A comprehensive study of sillenite Bi₁₂SiO₂₀ single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c₁₁ and c₁₂ are found to increase continuously with pressure while c₄₄ increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi₁₂SiO₂₀ crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c₄₄ and the piezoelectric stress coefficient e₁₂₃ are almost unaffected by this dissipation

High-pressure high-temperature behavior of polymer derived amorphous B-C-N

Dense diamond-like BCN compounds are of interest due to their extreme hardness and predicted excellent thermal and chemical stability, which are superior to those of diamond and c-BN. Here, we report on the high-pressure high-temperature (HP-HT) behavior of amorphous BC₂N and BC₄N -as potential precursors for HP-HT synthesis of diamond-like BCN. Prepared via hydroboration reaction of piperazine borane and pyridine borane, respectively, amorphous BC₂N and BC₄N are characterized by well-mixed B-N, C-C and C-N bonds, confirmed by XPS analysis. These BCN compositions were subjected to pressures between 5-12 GPa and temperatures up to 1700 °C using multi-anvil apparatus and toroid-type press. In- and ex-situ X-ray diffraction reveals the decomposition of BC₄N to graphite and h-BN between 5 and 12 GPa above 500 °C, in contrast to BC₂N which remains amorphous up to 1600 °C

High-Pressure Synthesis of β-Ir4B5 and Determination of the Compressibility of Various Iridium Borides

"A new iridium boride, beta-Ir4B5, was synthesized under high-pressure/high-temperature conditions of 10.5 GPa and 1500 degrees C in a multianvil press with a Walker-type module. The new modification beta-Ir4B5 crystallizes in a new structure type in the orthorhombic space group Pnma (no. 62) with the lattice parameters a = 10.772(2) angstrom, b = 2.844(1) angstrom, and c = 6.052(2) angstrom with R1 = 0.0286, wR2 = 0.0642 (all data), and Z = 2. The structure was determined by single-crystal X-ray and neutron powder diffraction on samples enriched in B-11. The compound is built up by an alternating stacking of boron and iridium layers with the sequence ABA'B'. Additionally, microcalorimetry, hardness, and compressibility measurements of the binary iridium borides alpha-Ir4B5, beta-Ir4B5, Ir5B4, hexagonal Ir4B3-x and orthorhombic Ir4B3-x were carried out and theoretical investigations based on density function theory (DFT) were employed to complement a comprehensive evaluation of structure-property relations. The incorporation of boron into the structures does not enhance the compressibility but leads to a significant reduction of the bulk moduli and elastic constants in comparison to elemental iridium.

Second harmonic generation measurements at high pressures on powder samples

This article reviews the most recent results concerning second harmonic generation (SHG) experiments of non-phase matchable and phase matchable powder samples at high pressures and explains the pressure dependence of the intensity of the SHG signal by correlating it to the ratio between the average coherence length and the average particle size. The examples discussed here include pressure-induced structural changes in quartz, ZnO, ice VII and KIO3. It is shown that the second harmonic generation technique is a unique tool for the detection of pressure-induced structural phase transitions. It is laboratory based and allows fast measurements. It is complementary to X-ray diffraction and provides additional information about the presence of an inversion center for unknown or controversially discussed structures at high pressure