Transitions between P21, , and P6322 modifications of SrAl2O4 by in situ high-temperature X-ray and neutron diffraction

Tunable luminescence and electrical properties of cerium doped strontium aluminate (SrAl2O4:Ce3+) phosphors for white LED applications

In new research growth long after glow material is a potential candidate due to its physical properties, chemical stability and wide application in modern solid-state lightning (LED), display devices, dosimetry and sensors. A cerium doped strontium aluminate phosphor (SrAl₂O₄:Ce³⁺) was synthesized by conventional solid-state reaction method. The crystal structure and morphology of phosphors, while doping rare earth metal and lithium metal ion was investigated by using X-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy. Fourier transformed infrared spectrum results of the synthesized phosphor composition conforms the characteristic vibration bands of synthesized phosphor. Surface composition analysis of the prepared samples was examined using X-ray photoelectron spectroscopy. Photoluminescence emission band observed at ∼420 nm, ∼490 nm and ∼610 nm region under the excitation wavelength of 256 nm. Wight light emission was confirmed using the Commission Internationale de L'Eclairage (CIE) chromatic coordinate graph. The correlated colour temperature (CCT) value of 0.5% Ce³⁺ doped SAO of phosphors was calculated is in the range of 1543 K, which is indicated the synthesized phosphors performance as warm white light source. The obtained phosphor has a high dielectric constant and low loss tangent which is useful to optoelectronic devices.

Structural Diversity and Incompatibility Induced Complex Phase Formation Behavior in the Stuffed Tridymites Ca1-xSrxGa2O4

Stuffed tridymites AM₂O₄ composed of a condensed MO₄-tetrahedra-based framework have been widely investigated due to their structural diversity and rich physical properties. Herein, the strategy of stuffing mixed Ca²⁺ and Sr²⁺ cations into the [Ga₂O₄]^2- framework in (Ca_1-xSr_x)Ga₂O₄ (CSGO, 0 ≤ x ≤ 1) is utilized to manipulate the phase formation behavior with different structure types at particular annealing temperatures. Five derivatives, including α- and β-CaGa₂O₄, β- and γ-SrGa₂O₄, and new CSGO-type structures, were observed. The distinctive feature of the CSGO-structure is the coexistence of UUDDUD- and UDUDUD-type six-membered rings, where U (up) and D (down) denote the orientations of GaO₄-tetrahedra with respect to the plane grids, in a ratio of 2:1. Single-phase α-Ca_1-xSr_xGa₂O₄ (x < 0.2) and γ-Ca_1-xSr_xGa₂O₄ (x > 0.67) could be obtained at low temperatures. Biphasic regions, including α-Ca_1-xSr_xGa₂O₄/CSGO (0.2 ≤ x ≤ 0.67), γ-Ca_1-xSr_xGa₂O₄/CSGO (0.67 < x ≤ 0.8), and β-Ca_1-xSr_xGa₂O₄/CSGO (0.8 < x < 1), were observed at the intermediate temperature region and evolve irreversibly into the CSGO single-phase region upon elevating the temperature. Moreover, the structure-property relationship of the new CSGO-phase was further studied by doping coordination-sensitive Bi³⁺ activators to advance the development and applications of stuffed tridymites.

Unveiling the role of the hexagonal polymorph on SrAl2O4-based phosphors

In persistent luminescence materials, the SrO–Al₂O₃ system has been mainly studied due to its chemical stability, higher photoluminescence response and longest green-afterglow times. Specifically, the research has focused on SrAl₂O₄ doped with europium and dysprosium. SrAl₂O₄ has two polymorphs: monoclinic polymorph (space group P2₁) and hexagonal polymorph (space group P6₃22). Besides, the coexistence of these two phases, monoclinic and hexagonal, appears in almost all the results. However, it is not clear how this coexistence influences optical response. Some authors have reported that only the monoclinic structure exhibits luminescence properties, while another suggests that the hexagonal SrAl₂O₄ polymorph has a higher emission efficiency than the monoclinic polymorph. Here we report a systematic evaluation of the effects of the stabilization of the hexagonal SrAl₂O₄ polymorph. We show that an interrelationship between the hexagonal polymorph and phosphorescent properties is the linchpin for the development of good luminescence properties. Remarkably, the stabilization of the hexagonal SrAl₂O₄ polymorph on the monoclinic–hexagonal polymorphic coexistence appears to be related to the preservation of the nanometric nature of the SrAl₂O₄-based system. Our results will help to understand the role of the hexagonal polymorph in the polymorphic coexistence on SrAl₂O₄-based systems and may facilitate the development of luminescent nanometric particles for the design and preparation of new light emitting materials.

In persistent luminescence materials, the SrO–Al₂O₃ system has been mainly studied due to its chemical stability, higher photoluminescence response and longest green-afterglow times.

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research.

A Fluctuating State in the Framework Compounds (Ba,Sr)Al2O4

The structural fluctuation in hexagonal Ba_1−xSr_xAl₂O₄ with a corner-sharing AlO₄ tetrahedral network was characterized at various temperatures using transmission electron microscopy experiments. For x ≤ 0.05, soft modes of q ~ (1/2, 1/2, 0) and equivalent wave vectors condense at a transition temperature (T_C) and form a superstructure with a cell volume of 2a × 2b × c. However, T_C is largely suppressed by Sr-substitution, and disappears for x ≥ 0.1. Furthermore, the q ~ (1/2, 1/2, 0) soft mode deviates from the commensurate value as temperature decreases and survives in nanoscaled regions below ~200 K. These results strongly suggest the presence of a new quantum criticality induced by the soft mode. Two distinct soft modes were observed as honeycomb-type diffuse scatterings in the high-temperature region up to 800 K. This intrinsic structural instability is a unique characteristic of the framework compound and is responsible for this unusually fluctuating state.

Average and local structure, debye temperature, and structural rigidity in some oxide compounds related to phosphor hosts

The average and local structure of the oxides Ba2SiO4, BaAl2O4, SrAl2O4, and Y2SiO5 are examined to evaluate crystal rigidity in light of recent studies suggesting that highly connected and rigid structures yield the best phosphor hosts. Simultaneous momentum-space refinements of synchrotron X-ray and neutron scattering yield accurate average crystal structures, with reliable atomic displacement parameters. The Debye temperature ΘD, which has proven to be a useful proxy for structural rigidity, is extracted from the experimental atomic displacement parameters and compared with predictions from density functional theory calculations and experimental low-temperature heat capacity measurements. The role of static disorder on the measured displacement parameters, and the resulting Debye temperatures, are also analyzed using pair distribution function of total neutron scattering, as refined over varying distance ranges of the pair distribution function. The interplay between optimal bonding in the structure, structural rigidity, and correlated motion in these structures is examined, and the different contributions are delineated.