SnO2 Nanopowders Research Articles

Enhancing Tin Dioxide Anode Performance by Narrowing the Potential Range and Optimizing Electrolytes

Tin dioxide (SnO2) is a low-cost and high-capacity anode material for lithium-ion batteries, but the fast capacity fading significantly limits its practical applications. Current research efforts have focused on preparing sophisticated composite structures or optimizing functional binders, both of which increase material manufacturing costs. Herein, we utilize pristine and commercially available SnO2 nanopowders and enhance their cycling performance by simply narrowing the potential range and optimizing electrolytes. Specifically, a narrower potential range (0–1 V) mitigates the capacity fading associated with the conversion reaction, whereas an ether-based electrolyte further suppresses the volume expansion related to the alloy reaction. Consequently, this SnO2 anode delivers a promising battery performance, with a high capacity of ~650 mAhg−1 and stable cycling for 100 cycles. Our work provides an alternative approach to developing high-capacity and long-cycling metal oxide anode materials.

Highly dispersed spherical phosphors obtained by encapsulating silica hollow spheres with SnO2: Eu3+ layers for enhancing red light emission

Orange light (593 nm, 5D0→7F1) and red light (613 nm, 5D0→7F2) are the two most important emission lines belonging to Eu3+ ions in high and low symmetries, respectively, making them important indicators for judging the nanoenvironment around Eu3+ ions. Therefore, obtaining significant red light emission in Eu3+ doped SnO2 nanocrystals requires anchoring Eu3+ ions onto lattice sites with low symmetries, which can be achieved through maximizing surface doping. To achieve this goal, we coated very thin Eu3+ doped SnO2 nanocrystal (10–20 nm) coatings on extremely dispersed SiO2 hollow spheres (500 nm in diameter) to investigate doping dependent luminescent properties. A significant increase in red emission was observed in such a system, accompanied by a significant attenuation of orange light, and the intensity of red emission increases with increasing Eu3+ concentration. When the concentration of Eu3+ exceeds 3 at%, the intensity of red light emission sharply decreases due to the quenching effect. For the optimal doped hollow sphere sample (3 at%), the asymmetry ratio (red/orange light integration intensity ratio) under indirect excitation is 5.58, which further rises to 11.3 under direct excitation, while the asymmetry ratio of SnO2 nanopowder with the same doping concentration is less than 0.2 under both excitation modes, indicating that the current structural design plays a significant role in the modulation of Eu3+ ion luminescence. Our research not only demonstrates an effective method to enhance the red light emission of Eu3+ doped SnO2 nanocrystals, but also provides a method for manipulating the polychromatic luminescence of such systems.

Highly Tuning of Sunlight-Photocatalytic Properties of SnO2 Nanocatalysts: Function of Gd/Fe Dopants

Gd/Fe-SnO2 nanopowders as novel photocatalysts for the active removal of Rose Bengal dye and methyl parathion pesticide were synthesized with a low-cost coprecipitation route. The X-ray diffraction analysis of SnO2, Sn0.96Gd0.02Fe0.02O2 and Sn0.94Gd0.02Fe0.04O2 nanopowders proved the formation of a tetragonal phase of tin oxide with average crystallite sizes in the range of 13–18 nm. The Fourier transform infrared (FTIR) spectra of all samples displayed the characteristic absorption bands of SnO2. The nanopowder of the pure SnO2 sample, as seen in its transmission electron microscope (TEM) image, contains spherical-like particles of variable sizes. The TEM images of the Sn0.96Gd0.02Fe0.02O2 and Sn0.94Gd0.02Fe0.04O2 powders revealed the synthesis of fine spherical nanoparticles. Based on the TEM images, the average particle size of the pure, (Gd, 2 wt% Fe) and (Gd, 4 wt% Fe) codoped SnO2 nanopowders was estimated to be 14, 10 and 12 nm, respectively. After the addition of (Gd, 2 wt% Fe) and (Gd, 4 wt% Fe) to the SnO2 structure, the band gap energy of SnO2 was reduced from 3.4 eV to 2.88 and 2.82 eV, respectively. Significantly, the Sn0.96Gd0.02Fe0.02O2 nanocatalyst exhibited a high removal efficiency of 98 and 96% for Rose Bengal dye and methyl parathion pesticide after activation by sunlight for 35 and 48 min, respectively. Furthermore, this catalyst has shown perfect mineralization as well as high stability properties for the treatment of Rose Bengal dye and methyl parathion pesticide. These results suggest the suitability of the Sn0.96Gd0.02Fe0.02O2 nanocatalyst for the treatment of agriculture and industrial effluent under sunlight light energy.

Defect Engineering for SnO2 Improves NO2 Gas Sensitivity by Plasma Spraying.

Large emissions of nitrogen dioxide (NO2) pose a significant threat to human health, Monitoring its content and implementing timely measures are crucial. Utilizing oxide semiconductors, such as tin dioxide (SnO2), has proven to be an effective way to detect and analyze NO2. The design and preparation of sensing materials with high sensitivity and excellent selectivity is the key to improve the detection efficiency. SnO2 nanopowders with small and uniform particle size, large specific surface area, adjustable defect content, and no impurities were prepared by a new plasma spraying method. The SnO2 nanopowders exhibit outstanding performance in detecting NO2 at a low temperature of 100 °C, the response to 5 ppm of NO2 reaches 48, and the material demonstrates rapid response and recovery times, coupled with excellent selectivity. The exceptional gas-sensitive properties can be attributed to the superior morphology and structure of SnO2. It provides more reaction sites for gas sensitive reactions, fast electron transport, a large number of charge carriers, and improved adsorption of the material to the target gas. This study provides valuable insights into nanomaterial preparation and the enhancement of gas-sensitive properties for SnO2.

Enhanced degradation of ciprofloxacin in water using ternary photocatalysts TiO2/SnO2/g-C3N4 under UV, visible, and solar light.

In this study, we report on the synthesis of ternary photocatalysts comprising TiO2/SnO2/g-C3N4 for the degradation of ciprofloxacin (CIP) in water. SnO2 nanoparticles were synthesized via the sol-gel method, while g-C3N4 was obtained through melamine calcination. Commercial TiO2 and SnO2 nanopowders were also used. The heterojunctions were synthesized via the wet impregnation method. The photocatalysts were characterized via various techniques, including XRD, TEM, STEM, FTIR, N2 adsorption, UV-Vis DR, and hole tests. Photocatalytic degradation tests of CIP were carried out under UV, visible, and solar radiation. The P25/npA/g-C3N4 (90/10) material exhibited the best performance, achieving CIP degradation of over 97%. The synthesized materials demonstrated excellent initial adsorption of CIP, around 30%, which facilitated subsequent degradation. Notably, the CIP photocatalytic degradation tests performed under solar radiation showed a synergistic effect between the base materials and carbon nitride in highly energetic environments. These results highlight the effectiveness of ternary photocatalysts TiO2/SnO2/g-C3N4 for CIP degradation, particularly under solar radiation.

Effect of pH of the precursor solution on the photocatalytic and biomedical applications of enzyme coupled ZnO and SnO2 nanomaterials: A comparative study

The focus of this study is to investigate the effects of pH of the precursor solution and the addition of vermiwash on the photocatalytic and certain biomedical applications of zinc oxide (ZnO) and tin oxide (SnO2) nanopowders prepared via co-precipitation method. The pH of the starting solution was varied as 8, 9 and 10 through the addition of suitable amount of sodium hydroxide (NaOH) and vermiwash. These vermiwash-derived enzymes activated ZnO and SnO2 nanopowders (prepared at different pH values) were applied for biomedical applications viz. antibacterial, antioxidant and antidiabetic activity for the first time. The prepared samples were also characterized using various analytical techniques viz. XRD, SEM, TEM, EDS, FTIR, XPS, BET, and UV-vis spectra. The results of these analytical studies are correlated with their photocatalytic degradation abilities and also biological applications. The photocatalytic activity of the nanoparticle were analyzed using a visible annular type photo reactor by degrading methylene Blue (MB) - a standard cationic dye - under UV-visible irradiation. A remarkable dye degradation efficiency of 98% and 94% were achieved for the best samples of vermiwash activated ZnO and SnO2 (prepared from starting solution with pH 9), respectively. The lower band gap values (2.97 and 3.1 eV) of these samples as calculated from Tauc's plots and their enhanced surface area as observed from the BET analysis play crucial roles in improving their photocatalytic efficiency. Remarkable antibacterial, antioxidant and antidiabetic activities were also recorded for the samples prepared at pH 9.

STUDY OF THE MICROSTRUCTURE AND PHASE FORMATION IN CERAMICS BASED ON Zn, Sn AND Ti OXIDES

Ceramics was obtained from the mixture of ZnO, SnO2 and TiO2 nanopowders by the method of solid-state sintering at 1123 K and 1443 K. The indicated nanopowders are used as components in arc dampers and for dispersion-strengthening in silver-based electrical contact materials. The phase formation and microstructure of the obtained material were studied by the methods of scanning electron microscopy, energy dispersive microanalysis and X-ray phase analysis. It is shown that at a sintering temperature of 1123 K a dispersed structure was formed, with the size of the Zn2TiO4 and ZnO phases being 0.5 – 1.0 µm. The solid phase reactions in the system ZnO/TiO2 at Тsint = 1123 K resulted in the formation of two-phase ceramics ZnO/Zn2TiO4 with the residual amounts of the phases of zinc and tin oxides. At Тsint = 1443 K there occurred the grain growth, Zn2SnO4 became the predominant phase and there remained a certain amount of the zinc and tin oxide phases.

Morphological modification and charge carrier separation in tin oxide nanomaterials towards improved photocatalytic dye degradation: Enzyme coupling – An effective way

This paper reports the preparation of enzyme-coupled tin oxide (SnO2) nanomaterial using vermiwash and its photocatalytic dye decomposing efficiency, for the first time. The vermiwash used in the precursor solution was varied as 10, 20, 100 and 200 mL. It is found that the vermiwash powered nanoproducts inherit enzymes of vermiwash like phosphatase, urease, amylase and protease which act as co-catalysts to improve the efficiency. The mechanism behind the improved photocatalytic activity of the enzyme-coupled nanomaterial has been illustrated. The vermiwash-activated SnO2 nanopowders were also characterized using XRD, SEM, TEM, BET, UV–vis absorption and FT-IR. These results are correlated with the photocatalytic degradation abilities of the samples.

Hydrothermal Synthesis and Properties of SnO2 Nanopowder

The tin oxide nanopowder is synthesized by hydrothermal method and the hydrothermal time effect on its phase structure, morphology and optical properties are studied. The results show that the samples are all tetragonal rutile structure with high purity, and possess flowers-shaped morphology. The crystallites size increases as the hydrothermal time does. The sample obtained through 12 h hydrothermal time has excellent morphology, uniform size and great dispersibility. Photoluminescence spectra shows photoluminesce peak appear near 500 nm, which is related to the oxygen vacancy. With the increase of hydrothermal time, the photoluminesce peak has a blue shift and the peak intensity decreases.

Role of (Cu, Al) codoping in tuning the optical, structural and magnetic properties of Co-doped SnO2 nanostructures: A comparative study

Tin oxide (SnO2) co-doped with Co/Cu and Co/Al nanoparticles were synthesized by using a co-precipitation technique to compare the effect of non-ferromagnetic codoping of Al and Cu ions on the structural-optical-magnetic properties of the Co-doped SnO2samples. The energy dispersive x-ray analysis measurements were performed to check the possible presence of any impurity elements in the nanocrystals. The samples were studied and characterised by several methods; X‐Ray Diffraction, UV–visible absorption spectroscopy, and magnetization measurement methods were executed. The optical bandgap of the SnO2 nanopowders (3.7 eV) was studied by diffuse reflectance spectroscopy (DRS) and found it was redshifted by doping. It was found that the hydrogenation process of the host samples should be used as supplementary necessary action that should be processed to create ferromagnetic properties in SnO2:Co:Cu sample. The optical measurements were used to measure the optical properties including the surface plasmon resonance (SPR) signal that generated by the accumulated liberated metallic nanograins (clusters) in form of intermetallic Sn0.5Co1.5 by the hydrogenation process and thus connect the SPR with the XRD results.

Excellent antimicrobial and anti-biofilm activities of Fe–SnO2 nanoparticles as promising antiseptics and disinfectants

In recent years, nosocomial infections and antibiotic resistance are major public health problems. Nanotechnology should open new avenues to treat and prevent diseases. In this research work, iron-doped tin dioxide (Fe-SnO2) nanoparticles have been synthesized by the chemical co-precipitation method. X-ray diffraction analysis confirms the formation of nanocrystalline SnO2 single phase with Fe favoring grain growth, the crystallite size increases from 13 up to 29 nm. FT-infrared spectroscopy analysis shows the presence of peaks characteristic of the SnO2 phase. Ultraviolet-visible spectroscopy analysis indicates Fe doping induces red-shift of SnO2 bandgap. Morphological observations by scanning electron microscopy reveal randomly arranged irregular-sized compact grains with sponge-like microstructure. Energy dispersive X-ray spectroscopy analysis (EDX) confirms the purity of Fe-doped SnO2. Bacteria produce siderophores capable of capturing iron, a necessary nutrient. For this reason, SnO2 has been doped particularly with Fe to facilitate the entry of nanoparticles into the bacterial cell and improve its antibacterial activity. Fe-SnO2 nanoparticles exhibit significant antimicrobial and anti-biofilm activities against all tested microorganisms; it is found that biofilm eradication of the tested strains is higher than 90%. The fabricated Fe–SnO2 nanopowders have potential as antiseptics and disinfectants that will eliminate, kill, or inactivate microorganisms usually found on inert surfaces such as soil, walls, medical-surgical equipment, to prevent nosocomial infections in hospitals.

Electrochemical Properties of Flexible Anode with SnO2 Nanopowder for Sodium-Ion Batteries

Electrochemical Properties of Flexible Anode with SnO2 Nanopowder for Sodium-Ion Batteries

Nanocrystalline Zn2SnO4/SnO2: Crystal structure and humidity influence on complex impedance

Nanocrystalline Zn2SnO4/SnO2 powder was obtained by a solid state reaction of ZnO and SnO2 nanopowders mixed in the molar ratio 1:1. The phase composition of the obtained powder was studied by XRD and Raman spectroscopy, morphology and texture were characterized by FESEM, TEM, BET and Hg porosimetry, while XPS and FTIR spectroscopy were used to determine the surface chemistry. The influence of humidity on complex impedance was monitored on bulk samples in the relative humidity (RH) range 30–90% in a climatic chamber in the frequency range 42 Hz- 1 MHz at working temperatures of 25 and 50 °C. Change in RH had a significant influence on impedance reduction, especially noticeable in the lower frequency range, indicating potential application of this nanocomposite as a humidity sensing material. Increase in RH led to an increase in AC conductivity that changed with frequency according to the Jonscher power law. The frequency exponent decreased with increase in RH and sample temperature indicating that the correlated hopping barrier model is the dominant conduction mechanism. Complex impedance was analyzed using an equivalent circuit consisting of a parallel resistance and constant phase element, showing the dominant influence of grain boundaries at both working temperatures (25 and 50 °C). The resistance decreased, while the capacitance and relaxation frequency increased with increase in RH. At high humidity an added Wartburg element enabled modeling of the charge diffusion process.

Development of Nano-SnO2 and SnO2:V2O5 Thin Films for Selective Gas Sensor Devices

Pure and doped SnO2 with V2O5 nanopowders were synthesized via sol–gel method using different V2O5 ratios. Novel thin films of SnO2:V2O5 were thermally vacuum deposited from the nanopowders and utilized for gas sensor devices to detect volatile organic compounds hazardous gases. The morphological and crystalline structure, textural properties, functional groups, optical properties and thermal behavior were investigated by FESEM, XRD, HRTEM, surface area BET, FTIR and UV–Visible spectroscopy, respectively, for both the nanopowders, and thin films. From XRD patterns, the average calculated crystallite sizes decreased from 7.8 nm to 4.5 nm as the V2O5 concentration was varied from 0 to 10%. FESEM and HRTEM show that all the synthesized nanomaterials composed of mesoporous networks of aggregated nanoparticles that almost spherical. Thus, V2O5 doped SnO2 nanopowders synthesized by sol–gel method exhibited the structural and textural features required to be used as an active area for gas sensor devices. The effect of various doping weight amounts (1, 5 and 10 wt%) of V2O5 as the dopant element enhanced the gas response time and sensitivity. The electrical behavior of the sensors was determined by measuring the resistance of two deposited platinum electrodes for sensor’s devices for different kinds of gases (LPG, H2, NH3 and acetone) at different temperatures.

Study of structural, optical and magnetic properties of hydrogenated Ni and (Ga, Zn) co-doped SnO2 nanocomposites

Tin oxide (SnO2) co-doped with 3%Ni - 5%Ga and 3%Ni - 5%Zn nanocomposites were manufactured by a co-precipitation method. The samples were characterized by several methods. Energy dispersive x‐ray fluorescence, X‐Ray Diffraction, UV–visible absorption spectroscopy, and magnetization measurements were performed to study the elemental analysis, crystal structure, optical band gap and magnetic properties, respectively. The present research is planned to investigate the conditions necessary to fabricate the dilute magnetic semiconductor, SnO2 of nanocomposites. It was found that the hydrogenation of the host samples is a necessary action that should be processed to create ferromagnetic properties. The effect of doping of non-magnetic Ga3+ and Zn2+ ions on the generated magnetic properties in the hydrogenated Ni-doped SnO2 nanopowder was investigated and reported. The optical bandgap of the SnO2 nanopowders was studied by diffuse reflectance spectroscopy (DRS) and found it was redshifted with Ni, Ga, and Zn doping. The higher magnetic energy observed with Ni and Ga codoped SnO2 was reported and discussed.

Eosin-Y sensitized tin oxide (SnO2): Fabrication and its analysis

In the present study, Tin oxide (SnO2) nanoparticles were synthesized at various molarities of precursor solutions, by precipitation method. Doctor Blade method was employed to prepare the films. The nanoparticles were characterized for structural, morphological and optical properties. The XRD pattern of all the SnO2 nanopowder synthesis at various molarities shows the tetragonal rutile crystal structure and the calculated particle size is in the range of 24.36–58.77 nm. The optical properties were investigated using UV–vis spectroscopy. The prepared films are sensitized by using low cost, ethanol based Eosin-Y dye solution. Eosin-Y Sensitized Tin Oxide based solar cells have been studied for its photo response. A nanocrystaline SnO2 synthesis at 0.3 M shows good photo conversion efficiency up to 1.6%, which is higher than that photoanode prepared at other molarities.

Highly Efficient Formaldehyde Sensors Based on Au-Decorated SnO2 Nanosheets with High-Energy {001} Facets

Introduction Formaldehyde (HCHO) have caused health problems and thus associated with various diseases, so there is an urgent need to develop an effective way for HCHO detection. Metal oxide-based gas sensors (e.g. SnO2) have been fully proved as active materials for monitoring HCHO. Recently, crystal facet engineering opens up a new way for enhanced response for nano SnO2 with exposed high-energy facts, for instance {221} [1], {001} [2], and {332} [3]. Moreover, SnO2 decorated with noble metals is an effective strategy for response improvement. Obviously, comprehensive optimization of gas sensing can be achieved via synergistic effects [4] of small size, high-energy facets and noble metal catalysis, however the related research has not been reported yet. Here, Au decorated SnO2 nanosheets with dominant {001} facets were synthesized, characterized, and HCHO sensing performance. Finally, the reasons of enhanced HCHO sensing were also discussed. Synthesis of SnO2 and Au-SnO2 Pristine SnO2 nanosheets (P-SnO2) was synthesized by a hydrothermal process: 2.0 mmol SnCl2·2H2O and 6.0 mmol NaOH were dissolved in 20 mL distilled water under stirring, transferred into a 25 mL Teflon-lined stainless steel autoclave, and maintained at 180 °C for 12 h. Precipitates were centrifuged, washed by deionized water and ethanol, dried at 80 °C for 12 h. Au decorated SO2 nanosheets (Au-SnO2) was obtained by a impregnation method: 0.5 mmol SnO2 products were dispersed into 10 mL ethanol by ultrasonication. Afterwards, 0.5 mL HAuCl4·4H2O solution (2 mmol/L) was added, ultrasonicated for 15 min, heated at 80 °C for 6 h, and calcined at 400 °C for 30 min. S ensor F abrication and Measurement The fabrication of side-heated SnO2 gas sensors is similar to the procedure [2]. Sensors were measured on a WS-30A static gas sensing system (Weisheng Electronics Co. Ltd., China). The response was defined as the ratio of sensor resistance in air (Ra) to that in a target gas (Rg): Sr = Ra/Rg. Results and Conclusions As shown in Fig. 1 (a), Au-SnO2 are featured in nanosheets with lengths of 30-50 nm and thicknesses of only 5-8 nm. The HRTEM indicates the decoration of Au particles on SnO2 in Fig. 1 (b). Temperature dependent HCHO response shows that Au-SnO2 holds the highest response value and the lowest optimum operating temperature (OOT) at 130 °C in all three SnO based HCHO gas sensors, as can be seen in Fig. 1 (c). We believe that the excellent results of Au-SnO2 can be attributed to synergistic effects of nanosheet caused structural sensitization, as well as Au catalysis induced chemical and electronic sensitization. Fig. 1 TEM (a) and HRTEM (b) images of Au-SnO2. Temperature dependent HCHO response of P-SnO2, Au-SnO2 and C-SnO2 (commercial SnO2 nanopowders).

Morphosynthesis of SnO2 nanocrystal networks as high-capacity anodes for lithium ion batteries

Tin dioxide (SnO2) is of interest as an alternative anode material for lithium ion batteries. However, the major concern is the significant volume expansion during lithiation process, resulting in poor capacity retention and rate capability. Here, a facile morphosynthesis route is adapted for the fabrication of hierarchical SnO2 nanocrystal networks with interwoven fibers, by immersing sacrificial eggshell membrane in a Sn-based sol followed by a calcination process in air. With both structural and compositional advantages, the synthesized SnO2 networks electrode manifests superior reversible capacities of 938, 802, 736, 636, and 506 mAh g−1 even at high current densities of 0.2, 0.5, 1, 2, and 5 A g−1, respectively. Besides, its initial Coulombic efficiency (72.6%) is also much enhanced compared with that of bare SnO2 nanoparticles (59.6%) and commercial SnO2 nanopowders (46.8%). The finding highlights the probable potential application of the SnO2 networks for next-generation lithium ion batteries.

Electron beam evaporated tin oxide thin films for thermistor applications

Tin oxide (SnO2) has been extensively used as a gas sensor and transparent conductive oxide owing to its excellent electrical and thermal properties. In this study, the thermal response of SnO2 based thin films has been investigated for negative-temperature-coefficient (NTC) thermistor applications. SnO2 nanopowder has been synthesized through co-precipitation method and subsequently used for depositing thin films via electron beam deposition technique. Eventually, the phase identification of the prepared thin films has been performed with the X-ray diffraction and Raman spectroscopic technique. Excitation and emission studies revealed the presence of defects in the films. Ultimately, temperature dependence properties have been studied in the range of 28–200 °C. The results confirmed the rutile structure of nanocrystalline thin films with grain size less than 8 nm. Further, the resistance of the samples turned out to be strongly dependent on the temperature showing typical NTC characteristics. High-temperature coefficient of resistance (TCR) equivalent to – 38.59 × 10−3 (°C)−1 has been obtained in the temperature range 28–44 °C, which makes the sensor competent for real-time human body temperature monitoring.

Tuning the CrIV/CrIII Valence States in Purple Cr-Doped SnO2 Nanopowders: The Key Role of CrIV Centers and Defects.

A low content of chromium (≤5 mol %) has been incorporated into a SnO2 cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O2 flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr4+ and Cr3+ ions as well as ferromagnetic Cr4+-Cr3+ and antiferromagnetic Cr3+-Cr3+ interactions. The strong purple hue is related to the isolated Cr4+ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10Dq value around 2.4 eV, whereas the 10Dq value is around 2 eV for isotropic Cr3+ ions, partially substituted for Sn4+ ions in cassiterite. Just after the coprecipitation process, only Cr3+ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr4+ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr3+ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature (T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr4+ species and limiting the Cr4+-Cr3+ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr4+ rate and the Cr4+-Cr3+ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.