Search Results (475)

Heavy metal fluoride glasses (HMFGs), particularly fluorozirconate glass systems such as ZBLAN have attracted considerable attention due to their unique physical properties, including low phonon energies, wide transparency from the UV to the mid-IR, and high rare-earth ion doping solubility, making them promising materials for photonic applications. This review provides an overview of fluoride glass synthesis methods, structural characteristics, and physical properties of fluorozirconate glasses, with emphasis on glass processing conditions, thermal, mechanical, and optical properties. The structural characteristics are discussed in terms of zirconium–fluorine polyhedral networks and their compositional dependence, while physical properties are analyzed, including glass transition behavior, crystallization tendency, elastic moduli, and infrared transmission. Rare-earth doped Er³⁺, Ho³⁺, and Tm³⁺ ZBLAN glasses are also discussed, which exhibit efficient emissions in the near and mid-IR spectral regions. Although significant progress has been achieved, limitations related to thermal stability, mechanical strength, and incomplete understanding of structure–property relationships persist. Future research should therefore focus on compositional optimization and predictive structural modeling to enable the design of improved fluoride glasses for various applications. Full article

Yb³⁺-containing glasses and glass-ceramics are attractive photonic materials because Yb³⁺ can act simultaneously as a near-infrared absorber, an energy-transfer sensitizer, a luminescent center, and a composition-dependent modifier of glass structure and crystallization. This brief review focuses on crystallization from parent glasses to glass-ceramics and examines glass-network chemistry, local Yb³⁺ coordination, phase separation, viscosity, heating rate, treatment temperature, holding time control nucleation, crystal growth, phase selection, rare-earth partitioning, transparency, and optical performance. Representative oxyfluoride, phosphate, oxyapatite, borosilicate, and aluminosilicate systems are compared using thermal analysis, X-ray diffraction, electron microscopy, vibrational spectroscopy, and optical spectroscopy. The available data show that Yb₂O₃ or YbF₃ does not have a universal effect on crystallization: low concentrations can promote fluoride-rich clustering or lower the apparent crystallization barrier, whereas higher concentrations can increase packing density, stabilize the residual glass, change the competitive phase assemblage, or suppress crystallization. Crystallization-enhanced luminescence is most consistently obtained when Yb³⁺ and the activator partition into low-phonon-energy nanocrystals while crystal size and refractive-index mismatch remain sufficiently small to preserve transparency. This review also identifies major reporting gaps, including limited quantification of crystalline fraction, partition coefficients, luminescence lifetime, quantum efficiency, and long-term thermal stability. Practical design guidelines and unresolved questions are proposed to support the rational development of transparent Yb³⁺-containing glass-ceramics for lasers, sensing, optical amplification, and related photonic applications. Full article

Garnet-structured compounds have long been recognized as versatile host platforms for luminescent phosphors, owing to their robust chemical stability and highly tunable cationic sublattices. Conventional aluminate and gallate garnets, however, generally face intrinsic limitations, including severe concentration quenching at high dopant levels, relatively high phonon energy, and unsatisfactory efficiency for long-wavelength near-infrared (NIR) emission. In recent years, sodium-based germanate garnets, constructed by introducing aliovalent Na⁺ into dodecahedral sites combined with Ge⁴⁺ substitution in tetrahedral frameworks, have emerged as a promising branch of garnet phosphors. The aliovalent Na+ incorporation brings unique structural effects, including energy migration blocking, coordination environment distortion, and defect level modulation, which endow the materials with advantages in mitigating concentration quenching, boosting energy transfer efficiency, and enhancing thermal stability. This review systematically summarizes the crystal structure classification and luminescence fundamentals of sodium-based germanate garnet systems, and introduces mainstream synthesis techniques represented by the high-temperature solid-state method. Focusing on representative host systems, the luminescence characteristics and energy transfer mechanisms of both rare earth- and transition metal-doped systems are elaborated in detail, followed by a summary of four representative performance regulation strategies: cationic disorder engineering, crystal field engineering, defect engineering and dual-site cooperative regulation. The multifunctional applications of these materials in white light-emitting diodes, plant growth lighting, fluorescence temperature sensing, NIR imaging, and information encryption are also presented. Finally, existing challenges and future research perspectives are proposed to provide guidance for the development of high-performance garnet phosphors. Full article

Because antimony trisulfide (Sb₂S₃) is abundant on Earth, non-toxic, and naturally has a low lattice thermal conductivity, it has garnered increasing interest as a possible thermoelectric material. One factor contributing to its anisotropic transport behavior is the orthorhombic structure in which Sb₂S₃ crystallizes, which is made up of one-dimensional (Sb₄S₄)_n ribbons. For thermoelectric energy conversion, its comparatively broad band gap (~1.5–1.7 eV) leads to a high Seebeck coefficient, usually in the 200–600 μV/K range. However, due to its inherently low carrier mobility, pristine Sb₂S₃ exhibits poor electrical conductivity, thereby restricting its power factor. Recent research indicates that composite engineering, nanostructuring, and doping (e.g., with elements such as Ln, As, Se, Ni, Zn, and Fe) can enhance the dimensionless figure of merit (ZT) by increasing carrier concentration while suppressing phonon transport. ZT values in bulk Sb₂S₃ range from 0.1 to 0.2 to approximately 0.5 in optimized nanostructured or doped systems. Higher ZT values (>1) are expected to be possible with advanced band engineering and defect management. According to these results, Sb₂S₃ is a promising mid-temperature thermoelectric material that can be used for waste-heat recovery and possibly integrated into hybrid photovoltaic–thermoelectric systems. Full article

Discrete breathers (DBs) are nonlinear vibrational excitations localized on small groups of atoms in perfect crystal lattices. While theoretically proven, a systematic understanding of DB formation in binary crystals with the B2 structure remains limited. We employ molecular dynamics simulations using the LAMMPS package to investigate the nonlinear dynamics of three representative B2 crystals: ionic CsCl, and intermetallic LiPb and NiTi. We calculate the amplitude-frequency dependencies of delocalized nonlinear vibrational modes (DNVMs) and analyze DB existence conditions based on phonon spectrum features and anharmonicity type. Our analysis reveals that a significant atomic mass difference creates a phonon band gap, enabling gap DBs in CsCl and LiPb, whereas NiTi, with similar atomic masses, exhibits no gap. A simplified model assuming identical bond stiffnesses accurately predicts frequency ratios in CsCl and LiPb but fails for NiTi due to strong bond stiffness asymmetry. We demonstrate the successful excitation of long-lived gap DBs in LiPb by initializing atomic displacements based on the G1 DNVM pattern on heavy Pb atoms. These gap DBs remain stable for over 20 ps with negligible energy dissipation. In contrast, DBs with frequencies above the phonon spectrum (excited on light Li atoms) exhibit shorter lifetimes (~2 ps). The study establishes that both atomic mass ratio and interatomic bond stiffness asymmetry are critical parameters governing nonlinear dynamics in B2 crystals. The predicted long-lived gap DBs in LiPb provide a target for future experimental detection via inelastic neutron or X-ray scattering, offering new insights into energy localization and transport in biatomic alloys. Full article

Two-dimensional (2D) materials have emerged as a key platform for next-generation electronics due to their atomic thickness and tunable properties. Iron disulfide (FeS₂), known as pyrite, with a bandgap of ~0.95 eV, is suitable for solar energy applications. However, its performance is limited by defects in bulk crystals. Reducing FeS₂ to a single layer eliminates bulk defects and enables strain engineering of the bandgap. In this study, First-principles density functional theory (DFT) calculations are performed using the CASTEP code and the PBEsol functional to examine the structural, electronic, and optical properties of a distorted 1T′-phase FeS₂ monolayer. Full geometry optimization yields lattice parameters a′ = 17.594 Å, b′ = 3.20231 Å, c′ = 5.28091 Å, and Fe–S bond angles of ~75.8° and ~98.2°, confirming symmetry-breaking distortion. The monolayer is dynamically stable, showing no imaginary modes in the phonon dispersion, and remains structurally intact up to 1000 K in molecular dynamics simulations. The unstrained system has an indirect bandgap of 0.70 eV, with the valence band maximum at the Γ point (dominated by S-p states) and conduction band minimum near the X point (Fe-d states). Under mechanical strain (±4%), the bandgap decreases significantly: from 0.70 eV to 0.44 eV under +4% tensile strain along the y-axis, and to 0.53 eV under −4% compressive strain. Biaxial strain causes weaker modulation, reducing the gap to 0.66 eV (+4%) and 0.62 eV (−4%). Optical absorption exceeds 10⁴ cm⁻¹ for photon energies above the bandgap, with tensile strain causing redshifts and compressive strain inducing blueshifts. These findings demonstrate that 2D FeS₂ is mechanically robust, electronically tunable, and optically active, making it a promising candidate material for flexible strain sensors and photovoltaic devices. This work is intended to motivate and inform future synthesis efforts. Full article

Wittichenite (Cu₃BiS₃) was synthesized by mechanical alloying (MA) followed by hot pressing (HP), and its phase evolution, thermal stability, charge transport behavior, and thermoelectric performance were systematically examined. X-ray diffraction analysis of the MA powders revealed broadened diffraction peaks, indicating reduced crystallinity and refined crystallite size. After HP consolidation, a well-defined single-phase orthorhombic wittichenite structure was obtained. These results demonstrate that the mechanically induced solid-state synthesis was effectively initiated during MA and subsequently completed through crystallization, defect relaxation, and densification during HP. The MA–HP processed specimens exhibited high relative densities of 94–98% of the theoretical value and a homogeneous microstructure without detectable compositional segregation or grain-boundary enrichment, confirming the formation of a structurally and chemically stable single-phase bulk material. Thermal analysis identified a reversible polymorphic phase transition from P2₁2₁2₁ to Pnma at low temperature, followed by structural relaxation and the onset of partial decomposition at higher temperatures, indicating that Cu₃BiS₃ retains structural integrity below 700 K, which defines the relevant operating window for thermoelectric evaluation. The samples exhibited p-type semiconducting behavior, with electrical conductivity increasing with temperature due to thermally activated hole transport and showing an additional enhancement across the structural transition region. The Seebeck coefficient remained positive over the entire temperature range and decreased gradually with increasing temperature, consistent with semiconductor transport characteristics. The thermal conductivity remained low at 0.30–0.38 W·m⁻¹·K⁻¹, with a negligible electronic contribution, confirming that heat transport is dominated by lattice phonon scattering. As a result of the combined increase in electrical conductivity and intrinsically low thermal conductivity, the dimensionless figure of merit (ZT) increased continuously with temperature and reached 0.17 at 673 K. These results demonstrate that the MA–HP route provides an effective and scalable strategy for producing phase-pure Cu₃BiS₃ with controlled microstructure and reproducible thermoelectric performance. Full article

Recent first principles simulation of the Raman spectral intensity of a bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF or ET) singly charged dimer suggested that the very important C=C stretching region of a κ-phase BEDT-TTF salt needs a reinterpretation. In fact, the possible presence of the out-of-phase coupled infrared active C=C anti-symmetric stretching b_1uν₂₇ did not receive the proper attention Here, the comparison of the calculated Raman spectra of ET⁺ cation with those of a doubly charged dimer, (ET)₂²⁺, shows that in a dimer, not only the C=C stretching modes but also anti-symmetric C-S stretching and ring breathing b_1u modes show Raman intensity comparable to that of the corresponding totally symmetric phonons. The calculations are validated through the comparison with the experimental Raman spectra of two charge-transfer salts, (ET)₂[Re₂Cl₈] and (ET)₂[Re₂Br₆CH₃COO]0.5(C₂H₃Cl₃), in which the ET molecules form almost isolated centrosymmetric (ET)₂²⁺ dimers. We also present the Raman spectra of a well-known ET salt, κ-(ET)₂Cu[N(CN)₂]Br (κ-CuBr), the experiment being performed on the rarely investigated (101) crystal face. The comparison with the Raman calculations on a singly charged ET dimer oriented as in the κ-CuBr salt allows us to clearly identify the out-of-phase coupled b_1u phonons, yielding a reliable interpretation of the most important (ET)₂⁺ spectral regions, i.e., the C=C and C-S stretching, and the ring breathing ones. The present results constitute a reliable basis for the interpretation of the Raman spectra of ET crystals characterized by the presence of isolated or almost isolated singly or doubly charged ET dimers. Full article

KTaO₃ (KTO) is a quantum paraelectric perovskite oxide which belongs to the cubic space group Pm$\overline{3}$m in a large temperature range. Polar optical modes with a T_1u symmetry of KTO are infrared-active and Raman-inactive according to the centrosymmetric exclusion principle of the selection rule. In general, the soft modes responsible for ferroelectric instability are infrared-active and Raman-inactive in the paraelectric phase. Therefore, there are still not enough studies on Raman-inactive soft modes and related phonon polaritons. In the present study, Raman-inactive polar modes and related polaritons of KTO crystals are studied by Terahertz Time-Domain spectroscopy (THz-TDS) and FTIR. The real and imaginary parts of a dielectric constant along the [100] axis are uniquely determined by transmission and reflection THz-TDS without any fitting in the low-frequency range between 6 and 225 cm⁻¹, which covers the two lowest-frequency polar modes. The reflectivity is determined by reflection FTIR in the range between 50 and 1200 cm⁻¹, and the complex dielectric constant is also estimated by the fitting in the range between 6 and 1200 cm⁻¹. The phonon–polariton dispersion relations of the real and imaginary parts of the polariton wavevector are also studied in the range between 6 and 1200 cm⁻¹. The crossover from photon-like to phonon-like polaritons and related polariton decay are observed, while no anomaly related to polariton scattering and coupling to other elementary excitations is observed in the polariton dispersion. Full article

Low-frequency traffic noise below 500 Hz is difficult to mitigate because its long wavelengths require impractically large conventional resonators. Here, we report an active-learning-guided inverse-design approach for scalable phononic-crystal-based acoustic metamaterial resonators that simultaneously suppress low-frequency noise transmission and harvest acoustic energy. The approach combines Gaussian process regression surrogate modeling with genetic algorithm optimization to efficiently explore high-dimensional cavity geometries. By iteratively retraining the surrogate with FEM-validated designs, the active-learning process guides the search toward high-performance structures while reducing costly FEM evaluations compared with conventional GA optimization. After geometric scaling, the 2.5D prototype derived from the nine-point optimized cavity achieved a pressure amplification factor of approximately 20 near 490 Hz, while the revolved 3D cavity exhibited amplification exceeding 30 and a transmission loss of approximately 14 dB near the target frequency. Integrated with a mass-loaded five-PZT stack, the device generated 5.5 V_pp and 0.25 mW under 100 dB SPL, corresponding to a normalized power density of 0.58 μW Pa⁻² cm⁻³. These results demonstrate a route toward multifunctional piezoelectric acoustic devices for noise mitigation, localized energy harvesting, and self-powered sensing. Full article

A microscopic model has been elaborated for the description of charge transfer-induced spin transitions in crystals containing tetranuclear Fe₂Co₂ clusters as structural units. The model takes into account the energy spectrum of each Fe₂Co₂ cluster, formed by the states arising from its initial configuration, two low-spin Fe^II and two low-spin Co^III, final configuration two low-spin Fe^III, and two high-spin Co^II, as well as the states that originate from four intermediate configurations of the type of low-spin Fe^II, low-spin Co^III, low-spin Fe^III, and high-spin Co^II. Two different types of cooperative interactions are accounted for in the model, namely, the electron–deformational coupling arising as a result of the observed elongation of the cobalt-nitrogen bonds under the low-spin Co^III$\to$ high-spin Co^II transition and the interaction via the field of phonons that originates from the coupling of the Co-ions with the full symmetric displacements of the nearest ligand surrounding, which are modulated by crystalline vibrations. The role of cooperative interactions is discussed in detail. Different types of spin transitions are predicted, including the gradual and abrupt ones as well as those manifesting hysteretic behavior. Within the framework of the developed approach, a qualitative and quantitative explanation of the experimental data on the {[(Tp*)Fe(CN)₃]₂[Co(bpy^Me)₂]₂}(OTf)₂·2DMF·H₂O compound recently reported oniere is given. Full article

Single-phase Cu₄Bi₄Se₉ was successfully synthesized through a simple and rapid process combining mechanical alloying (MA) and hot pressing (HP). The phase formation behavior, microstructural evolution, charge transport characteristics, and thermoelectric properties were systematically investigated. X-ray diffraction analysis as a function of MA time confirmed that all powders crystallized into a single orthorhombic phase with space group Pnma. No decompositions or secondary phases were observed after HP sintering, indicating high phase stability. Thermogravimetric and differential scanning calorimetric analyses revealed distinct endothermic peaks at 714–717 K for all samples, corresponding to the onset of the decomposition of Cu₄Bi₄Se₉. Microstructural observations showed that the relative density decreased with increasing HP temperature (>573 K), accompanied by grain growth and pore formation, reflecting the competition between Cu–Se interdiffusion and pore coarsening during high-temperature sintering. Hall effect measurements indicated p-type conduction for all samples, with carrier concentrations on the order of 10¹⁷ cm⁻³ and carrier mobilities of approximately 10² cm² V⁻¹ s⁻¹. With increasing temperature, the electrical conductivity increased monotonically, while the Seebeck coefficient gradually decreased, resulting in a maximum power factor of 0.12 mW m⁻¹ K⁻² at 573 K. The total thermal conductivity remained extremely low, ranging from 0.33 to 0.48 W m⁻¹ K⁻¹, with the electronic contribution accounting for less than 10%, indicating that lattice thermal transport is dominant. The suppressed lattice thermal conductivity is attributed to the combined effects of Cu atomic rattling, asymmetric bonding induced by Bi 6s² lone-pair electrons, and strong anharmonic phonon scattering arising from the complex crystal structure. Consequently, Cu₄Bi₄Se₉ achieved a peak dimensionless figure of merit ZT of 0.19 in the temperature range of 573–623 K, demonstrating that the MA–HP process enables stable phase formation and competitive thermoelectric performance without post-annealing. Full article

This study investigates the 2.0 μm luminescence efficiency of Tm³⁺-doped crystals under 793 nm excitation. An analytical model decomposing laser slope efficiency into the quantum defect, fluorescence quantum efficiency (${\eta }_q$), and a mode matching factor was established, highlighting ${\eta }_q$ optimization as key. Using a high-precision spectral system, the comparative study of Tm:YAG and Tm:YAP crystals revealed unprecedented ${\eta }_q$ values of 184.8% and 190.6%, respectively. This breakthrough, corroborated by double-exponential decay kinetics, verifies the cross-relaxation-dominated quantum cutting mechanism. Superior performance of Tm:YAP crystal is attributed to its lower phonon energy, effectively suppressing non-radiative losses, providing a foundation for high-performance 2.0 μm lasers. Full article

Capacitive micromechanical ultrasonic transducers (CMUTs), as microelectromechanical systems (MEMS) devices, have broad application prospects in ultrasonic imaging and sensing. This study investigates the influence of surface acoustic waves (SAWs) using periodically arranged CMUTs as the fundamental unit cells. We first utilize finite element analysis (FEA) to calculate and analyze the band structure and bandgap characteristics of phononic crystals under infinite periodic conditions. Subsequently, for finite periodic structures in practical applications, acoustic transmission spectra were further simulated using FEA to verify the bandgap characteristics of the structure for SAWs. Accordingly, this paper leverages a deep learning framework based on a multilayer perceptron (MLP) architecture to achieve the inverse design and optimization of CMUT geometric parameters, tailored to specific target bandgap requirements. The results demonstrate that this approach can efficiently and accurately determine the optimal structural configurations, offering a robust and novel technical paradigm for the precise control of SAWs using CMUT-based periodic arrays. Full article

This study investigates the phenomenon of supratransmission in three-dimensional crystals with a B2 (CsCl) structure, employing classical molecular dynamics with β-Fermi–Pasta–Ulam–Tsingou potentials up to fourth-nearest neighbors. We analyze energy transfer from a harmonically driven surface into the crystal bulk across various frequency regimes relative to the phonon spectrum. While low-amplitude excitation results in energy transmission only within the phononic bands, high-amplitude driving triggers supratransmission in the phononic gap and above the optical band. Our results demonstrate that in these nonlinear regimes, energy is transported not by linear phonon waves but by discrete breathers (DBs) emitted quasi-periodically from the surface. A key finding is the distinct sublattice selectivity of these excitations: gap DBs propagate primarily along the heavy atom sublattice, whereas above-spectrum DBs travel along the light atom sublattice. We quantify the velocities and oscillation periods of these localized modes, revealing their critical role in bypassing linear spectral restrictions. These findings provide new insights into nonlinear energy transport in binary alloys and suggest potential applications for controlling heat flow and signal processing in crystals. Full article