Ca Vacancies Research Articles

Toward Controllable Self‐Reduction of Mn4+ to Mn2+ by Lanthanide Ions for Luminescence Based Colorimetric Sensing of Temperature

AbstractLuminescence intensity ratio (LIR) based optical thermometers have attracted lots of attention. In case of color tunable luminescence with the variation of LIR, “RGB colorimetry” based on convenient temperature sensing using a smartphone or digital camera is available. However, narrow emission bands with primary colors are needed for avoiding color crosstalk and achieving high sensitivities. In this paper, Mn4+ and Mn2+ coactivated CaAl12O19 (CAO) phosphors are reported that show a narrow red emission of Mn4+ and green one of Mn2+ with the number of the two ions controlled by lanthanide ions through manipulation of self‐reduction of Mn4+ to Mn2+ in air. The self‐reduction in CAO via the addition of Ce4+ is mainly studied. First‐principles calculation shows that the self‐reduction is driven by Ca vacancies that are produced in aliovalent substitution of Ce4+ for Ca2+ and can reduce the formation energy of Mn2+. The comparative study of temperature sensing based on LIR and RG colorimetry is conducted. The relative sensitivity based on LIR is high as 4.5% K−1 at 373 K and that for RG colorimetry can reach 2% K−1 at 350 K. These findings provide a strategy for manipulation of manganese valance states and a phosphor for RG colorimetry temperature sensing.

Substitution of europium (III) in hydroxyapatite lattice for luminescence applications: Integrating experimental and theoretical insights

Europium(III) (Eu3+/Eu(III)) substitution in the hydroxyapatite (HAp) lattice imparts luminescence properties, enabling its application in diagnostics and bioimaging. Despite numerous experimental studies, there has been a lack of comprehensive discussion on the substitution behavior and local structural stability of Eu3+ within HAp. In this study, experimental and first-principles investigations were integrated to explore and understand the substitution of Eu3+ in HAp. Optimization of various HAp models revealed the dependency of Eu3+ substitution site on the charge compensation mechanism. Eu3+ prefers the Ca(2) site when OH is converted to O and the Ca(1) site in the presence of a Ca vacancy. Phonon calculations showed that temperature influences charge compensation and site preference, favoring OH to O compensation and Eu3+ in Ca(2) at higher temperatures and Ca vacancy and Eu3+ in Ca(1) at lower temperatures. Electronic structure simulations explained that the luminescence in Eu(III)-substituted HAp primarily arises from charge transfer between Eu3+ and O atoms, supported by experimental photoluminescence measurements. Correspondence between simulations and experimental photoluminescence data emphasizes the reliability of the computational approach in this study. The integration of insights from experimental and simulation results enhances the understanding of substitution of Eu3+ in the HAp lattice, providing valuable guidance for designing and optimizing HAp-based luminescent and optical materials.

Codoping Eu3+ with Sc3+ and La3+ in CaSnO3 for possible site selective doping: Is size selection in site selective doping really effective?

Local site of dopant in host lattice with multiple cationic sites is very important from light emission perspective in dictating its various photophysical features. The same is very important particularly in perovskites which is considered a very important invention for optoelectronic applications. Here in this work we have investigated the local site of dopant and time resolved photoluminescence in CaSnO3:Eu red emitting phosphor on co-doping La3+ and Sc3+ based on their size proximity to A and B-site respectively. EXAFS spectroscopy suggested increase in coordination number of Eu3+ from 6 to 8 and formation of Sn and Ca vacancies as charge compensating defects (CCDs) in La3+ and Sc3+ co-doped samples respectively. Co-doping with either La3+or Sc3+ is directing more fraction of Eu3+ to asymmetric 8-coordinated Ca2+ site compared to relatively symmetric 6-coordinated Sn4+ site. Sc3+ codoping direct more Eu3+ to Ca2+ site but with less distortions in the lattice owing to smaller size of Eu3+ than Ca2+ and higher size of Sc3+ than Sn4+. In case of La3+ co-doping, directing Eu3+ to Sn4+ site is not so effective owing to larger size of Eu3+ than Sn4+ as well as La3+ than Ca2+ and both cause significant distortions in the lattice. PL intensity also improved in the co-doped samples with Sc3+ co-doping emerging victorious over La3+ due to better charge compensation encountered in the former. Excitation dependent PL suggested Eu3+ at Ca2+ sites is preferentially excited under 250 nm and Eu3+ at Sn4+ symmetric site is preferentially excited under 300 nm.

Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials.

Thorium-doped vacuum ultraviolet (VUV) transparent crystals is a promising candidate for establishing a solid-state nuclear clock. Here, we report the research results on high-concentration doping of 232Th:CaF2 single crystals. The structures, defects, and VUV transmittance performances of highly doped Th:CaF2 crystals are investigated by theoretical and experimental methods. The defect configurations formed by Th and the charge compensation mechanism (Ca vacancy or interstitial F atoms) located at its first nearest neighbor position are mainly considered and studied. The preferred defect configuration is identified according to the doping concentration dependence of structural changes caused by the defects and the formation energies of the defects at different Ca or F chemical potentials. The cultivated Th:CaF2 crystals maintain considerable high VUV transmittance levels while accommodating high doping concentrations, showcasing an exceptional comprehensive performance. The transmittances of 1-mm-thick samples with doping concentrations of 1.91 × 1020 and 2.76 × 1020 cm-3 can reach ∼62% and 53% at 150 nm, respectively. The VUV transmittance exhibits a weak negative doping concentration dependence. The system factors that may cause distortion and additional deterioration of the VUV transmittance are discussed. Balancing and controlling the impacts of various factors will be of great significance for fully exploiting the advantages of Th:CaF2 and other Th-doped crystals for a solid-state nuclear optical clock.

Harnessing reaction media as charge compensator to improve red emission from CaWO4:Eu3+ a new strategy

This is probably the first report wherein we have correlated the optical properties of red emitting doped scheelite (CaWO4:Eu3+) synthesized by solid state (SS) reaction and sodium tungstate employed ethylene glycol (EG) method. The presence of sodium in EG sample was confirmed using neutron activation analysis whereas defect formation was probed using positron annihilation lifetime spectroscopy (PALS). The positron lifetimes suggested formation of calcium vacancies in CaWO4:Eu3+synthesized by SS method whereas Na from Na2WO4 in EG method enters the lattice and charge compensates for trivalent Eu3+ doping at divalent Ca2+ sites and plays pivotal role in removal of the calcium vacancies. Due to the removal of Ca vacancies, Eu doped sample prepared by EG method demonstrated higher internal quantum efficiency (IQE) and longer excited state lifetime with lower non-radiative channels and improved branching ratio compared to SS. The comparison with externally added sodium ion in SS samples with EG showed slightly higher IQE in former but at the cost of branching ratio and asymmetry factor. This work paved a new strategic pathway for research community working in the area of phosphor converted light emitting diodes to employ a alkali rich precursor to compensate for aliovalent doping and in lowering the non-radiative channels to improve the luminescence output of the phosphor.

Effects of seawater on the formation and mechanical properties of Friedel's salt associated with tricalcium aluminate

This work examines the micromechanical properties of AFm phases formed in seawater and deionized water. As the interlayer SO42− in the AFm phase is gradually replaced by Cl− in seawater, the basal spacing of the AFm crystal narrows, which promotes the packing density of nanocrystals and their indentation modulus and hardness, i.e., Cl-AFm (Friedel's salt) > SO4-Cl-AFm (Kuzel's salt) > SO4-AFm (monosulphate). It is found that Friedel's salt formed in seawater feature multiple structural and compositional defects. First, the anions (Cl−, OH−, SO42−) are bound in the interlayer of Friedel's salt, and the bound OH− contents have negative relations with the Cl− concentration in the seawater. Second, the incorporation of Mg ions in the Friedel's salt barely changes its micromechanical properties based on the experimental data and molecular dynamics simulations. Third, when portlandite is present in the seawater, some Ca vacancies would be formed in the Friedel's salt, which would decrease its Young's modulus significantly. This accounts for the decrease in the indentation modulus of Friedel's salt as observed in the experiments. These findings enable us to better understand and control the micromechanical properties of Cl-containing AFm phase formed in seawater-mixed cementitious materials.

Geochemistry of scheelite from Jiangligou skarn W-(Cu-Mo) deposit in the West Qinling orogenic belt, Northwest China: Implication on the multistage ore-forming processes

The Jiangligou skarn W-(Cu-Mo) deposit is a typical scheelite-dominated skarn deposit located in the West Qinling orogenic belt, northwestern China, hosting around 42, 000 tons of WO3 at an average grade of 1% with byproducts of Cu and Mo. Four generations of scheelite (Sch Ⅰ, Sch Ⅱ, SchIII, and SchIV) were recognized according to the mineral assemblages, inner textural, and geochemistry features. REE patterns of Sch Ⅰ have a close relationship with the garnet and diopside-hedenbergite and show significant LREE-enrichment and HREE-depletion, with δEu varying from negative (Sch Ⅰb) to positive (Sch Ⅰa). Sch Ⅱ grains show complex inner textures, zoning, syn-crystallization fractures, and filling, thus can be divided into four sub-generations with characteristic “flat” REE patterns. The early generation of scheelite gradually shifts from MREE-enriched with negative δEu (Sch Ⅱa and Sch Ⅱb) to MREE-depleted with positive δEu (Sch Ⅱc) or to lower REE with the same REE patterns (Sch Ⅱd). Sch III grains are commonly found in the sulfides overprinted ores, however, they have distinct REE patterns: Sch IIIa has relatively enriched MREE with negative δEu, while Sch IIIb is MREE depleted with positive δEu and negative δCe. The Sch IV has the largest (La/Lu)N with HREE down to the detection limit. Texture evidence implies that this may be due to later hydrothermal alteration which significantly removed the HREE. The Ca vacancy and Nb-REE coupled substitution dominantly account for the REE pattern of Sch Ⅰ, III, IV and Sch Ⅱ respectively. Revised batch crystallization modelling calculation on Sch Ⅱ and Sch III proved that the variation of δEu, MREE and Ce are controlled by the relatively low Eu2+/Eu3+ and Ce3+/Ce4+ ratios under relatively reduced conditions, and high Eu2+/Eu3+ and low Ce3+/Ce4+ ratios under relative oxidized conditions during scheelite precipitation. The extra HREE depletion of Sch Ⅱ from the measured data is interpreted as the low concentration of HREE in the ore-forming fluid caused by the precipitation of previous skarn minerals. This is also the main control in the depletion of HREE in Sch Ⅰ. Besides, the oxide condition change is also reflected by the Mo content change of the scheelite. Varied REE patterns of the four scheelite generations imply that at least four large volumes of flux of ore-forming fluids account for the whole scheelite mineralization.

Ca‐vacancy effect on the stability of substitutional divalent cations in calcium‐deficient hydroxyapatite

AbstractFirst‐principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca‐deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp.

The fracture mechanical behavior simulation of calcium-deficient hydroxyapatite crystals by molecular dynamics and first-principles calculation

Natural hydroxyapatite provides certain strength and stiffness to biological bones, and most of the studies on the strength of bone tissues have been carried out on hydroxyapatite (HAP). However, the Ca/P ratio of hydroxyapatite in bones is actually about 1.50, and the natural hydroxyapatite belongs to calcium-deficient hydroxyapatite (CDHA) with Ca vacancy defects. Therefore, this work focused on the effect of Ca vacancy defects on CDHA crystals through investigating the generation and expansion of microcracks under uniaxial tensile loading by combining molecular dynamics and first principles method. A series of crystal models with different Ca vacancy ratios are constructed and find that Ca vacancies degrade the mechanical properties of hydroxyapatite. Meanwhile the fracture behavior of crystals is detailed and find that the cracks arise at vacancies and extend along the direction of vacancies. Also, first-principles calculation is performed to reveal the mechanism of crack formation in MD simulations. It is found that the decrease of Ca–O bonding of CDHA causes the decrease of the stability of the crystal structure by analyzing the DOS of HAP and CDHA, and the cracks originate from Ca vacancies. This work performs more realistic simulations of CDHA with calcium vacancy defects in actual bone tissue and directly reveals the development and progression of bone fragility at the nanometer scale.

Theoretical study on the mechanism of super-stable mineralization of LDHs in soil remediation

The problem of soil/water pollution by toxic heavy metals has become increasingly prominent, which pose a serious threat to human production and life. Layered double hydroxides (LDHs) have been used as an excellent mineralizer for the heavy metal cations to form a super-stable mineralization structure. In this work, the models of MII2Al-CO3-LDHs (M = Mg, Cu, Cd, Ca, Fe, Ni, Zn, Co) are built to investigate the driving force of LDHs super-stable mineralization and the mechanism of mineralization of heavy metal cations by Ca2Al-LDH using density functional theory (DFT) and molecular dynamics (MD) simulation methods. The calculation results exhibit that the reasons for the super-stable mineralization of LDHs are that they have the super-smaller solubility products than those of the corresponding hydroxides and carbonates. The chemical binding, electrostatic binding and isomorphic substitution mechanisms of CaAl-LDH for five common heavy metal cations (Cu2+, Ni2+, Zn2+, Co2+, Cd2+) are calculated, as well as the effects of the introduction of H vacancy, OH vacancy and Ca vacancy into (003) surface on the mechanism is further explored. The results show that the mechanism of LDHs removing heavy metal cations favor the electrostatic binding mechanism regardless of the introduction of vacancy. Compared with the chemical binding mechanism and electrostatic binding mechanism, the energy barrier of isomorphic substitution of heavy metal cations with Ca vacancy is reduced the most, indicating that the presence of Ca vacancy is conducive to the occurrence of isomorphic substitution reaction. This work provides useful theoretical understanding for the construction of LDHs mineralizer and understanding the mechanism for the removal of heavy metals.

DFT combined with XANES to investigate the sulfur fixation mechanisms of H2S on different CaO surfaces

In this study, Erdos (ED) coal with high inorganic sulfur content was selected to investigate the absorbed effects of H2S on the perfect or defective surfaces of CaO (001). Pyrolysis connected with gas chromatography (Py-GC) was used to analyze H2S release behavior during coal pyrolysis. S K-edge XANES spectrum can prove that CaO fixing sulfur mainly generates CaS. The O vacancy of CaO possesses the best adsorption effect of H2S. Through the electronic structure and orbital analysis, O atom can be confirmed to the active site on the surfaces of CaO (001), and the O vacancy belongs to a strong electron-donating system. Different paths of H2S fixation can undergo on both perfect surface and O vacancy of CaO. After 700 °C, the most possible paths would happen and generate CaS and clean energy H2 on the surface of the O vacancies. However, it is impossible for Ca vacancies to be generated and to further fix H2S during pyrolysis under inert atmosphere. These calculated results are very consistent with these experimental results. Thus, this study would provide theoretical support for the sulfur fixation mechanisms on the surface of CaO (001) and for the subsequent improvement of the CaO sulfur fixation agent.

Adsorption and solidification of cadmium by calcium sulfate dihydrate (gypsum) in an aqueous environment: a dispersion-corrected DFT and ab initio molecular dynamics study.

Phosphogypsum is a by-product of phosphoric acid production by a wet-process and can be used to prepare adsorption materials to treat Cd(II) in sewage. Its main component is calcium sulfate dihydrate (gypsum). In the present study, incorporation and adsorption of cadmium on gypsum in an aqueous environment have been investigated using dispersion-corrected density functional theory, and the effect of different vacancy defects on the adsorption and doping of Cd on gypsum was also studied. The results show that Cd impurity defects can form more easily in gypsum crystals with Ca vacancy defects. The increase in both calcium vacancy defects and sulfate vacancy defects is beneficial to the incorporation of Cd in calcium sulfate dihydrate. In addition, the calcium vacancy defects on the gypsum surface can promote the adsorption of Cd(II), while sulfate vacancy defects on the gypsum surface inhibit the adsorption of Cd(II). Therefore, increasing the Ca vacancy defects of gypsum will help in improving the adsorption and curing properties of phosphogypsum for cadmium.

The Electronic Structural and Defect-Induced Absorption Properties of a Ca2B10O14F6 Crystal

Comprehensive ab initio electronic structure calculations were performed for a newly developed deep-ultraviolet (DUV) non-linear optical (NLO) crystal Ca2B10O14F6 (CBOF) using the first principle method. Fifteen point defects including interstitial, vacancy, antisite, Frenkel, and Schottky of Ca, O, F, and B atoms in CBOF were thoroughly investigated as well as their effects on the optical absorption properties. Their formation energies and the equilibrium concentrations were also calculated by ab initio total energy calculations. The growth morphology was quantitatively analyzed using the Hartman–Perdok approach. The formation energy of interstitial F (Fi) and antisite defect OF were calculated to be approximately 0.33 eV and 0.83 eV, suggesting that they might be the dominant defects in the CBOF material. The absorption centers might be induced by the O and F vacancies (VF, VO), interstitial B and O (Oi, Bi), and the antisite defect O substitute of F (OF), which might be responsible for lowering the damage threshold of CBOF. The ionic conductivity might be increased by the Ca vacancy (Vca), and, therefore, the laser-induced damage threshold decreases.

The effect of vacancy defects on the adsorption of methane on calcite 104 surface

Density functional theory calculations are used to study the effect of vacancy defects (ionic Ca and charge neutral CO 3 ) on the adsorption properties of CH 4 molecule on different calcite surfaces. Both types of vacancy defect results in strong adsorption of the gas molecules as compared to defect-free sample. For example, for the 104 surface of the material the methane molecules gets adsorbed stronger by 63.6% and 24.4% for Ca and CO 3 vacancy defects, respectively. Such enhanced adsorption of the CH 4 molecule due to the presence of the vacancy defects is also obtained for the other surface symmetries. Electronic structure calculations show that the latter is due to the orbital-overlap/hybridization between the organic molecules and the substrate. Adsorption capacity of the calcite also increases due to the presence of the defect. Vacancy defects also create energy barriers for the migration of CH 4 on the surface of calcite as revealed in our nudged elastic band calculations. The effect on the CH 4 adsorption and migration becomes more pronounced in the case of Ca vacancy defect. These findings could be of practical importance for, e.g., estimating ultimate gas recovery.

Defects, diffusion and dopants in the ceramic mineral “Lime- Feldspar”

ABSTRACT Calcium aluminosilicate, CaAl2Si2O8, is a promising ceramic material that has found applications in several areas, such as glass production and petrology. Atomistic scale simulation techniques are used to study the intrinsic defects, Ca-ion migration paths and doping behavior in CaAl2Si2O8. The most favorable defect is the Al–Si anti-site in agreement with the experimental observation. The O-Frenkel is the second most favorable defect. The Ca-Frenkel is higher in energy only by 0.62 eV than the O-Frenkel. Long-range Ca-ion migration is observed in the ac plane with the activation energy of 2.84 eV suggesting that Ca-ion diffusion in this material is slow. The prominent isovalent dopant on the Ca site is the Sr, which was experimentally substituted on the Ca site to prevent phase transformation. The formation of Ca interstitials and oxygen vacancies is favored by Fe3+ doping on the Si site. The favorable tetravalent dopant on the Si site and the Al site to form Ca vacancies is the Ge.

Effects of La3+ or Ti4+ doping on dielectric and microwave absorption performance of CaMnO3 in the 8.2–18 GHz

In this paper, CaMnO3 and Ca0.97La (or Ti)0.03MnO3 powders were synthesized using mechanical ball milling technique and sintered at 1100°C for 12 h in an air atmosphere. Results showed that all the samples are single phase with an orthorhombic symmetry. Compared with the pure CaMnO3 sample, the Ca0.97La0.03MnO3 and Ca0.97Ti0.03MnO3 samples have higher complex permittivity and better microwave absorbing properties. These changes are caused by the doping-ion incorporating in CaMnO3 lattice and Ca vacancies, which could result in abundant dipoles and the local charge accumulation. The Ca0.97Ti0.03MnO3 sample exhibited the best microwave absorption performance, i.e. the minimum reflection loss was -46.9 dB with a matching thickness of 2.2 mm and effective absorption bandwidth reached 5.2 GHz with a thickness of only 1.8 mm. This study demonstrates that the CaMnO3 would be a potential material in the microwave absorbing field and needs further research.

Mechanistic study on calcium ion diffusion into fayalite: A step toward sustainable management of copper slag

Copper slag, which contains Fe-rich fayalite (Fe2SiO4), is a valuable solid waste that warrants further research in order to recover iron. Calcium oxide (CaO) can significantly enhance iron recovery from copper slag; however, the associated mechanism has not yet been explored. In this study, we investigated the interaction between CaO and Fe2SiO4 to obtain detailed understanding of the role of CaO in enhancing iron recovery. The presence of CaO was found to accelerate the decomposition of Fe2SiO4 via an ion-exchange-like process. Specifically, CaO dissociated into Ca(II) and a Ca-deficient Ca1-yO species at high temperatures. The Fe(II) ion at the M2 site of Fe2SiO4 was substituted by the released Ca(II) ion, resulting in the formation of [(Fe(2-x)Cax)SiO4]∙xFe(II). Subsequently, the substituted Fe(II) occupied the Ca vacancy in Ca1-yO to form (Ca(1-y)Fe(II)y)O. The disproportionation of Fe(II) and the combination reaction between CaO and the SiO2 separated from Fe2SiO4 led to the generation of the final products, viz. Fe2O3, Fe3O4, and CaSiO3. This study explains the specific role of CaO in decomposing Fe2SiO4. It would not only provide theoretical guidance for iron recovery from copper slag but also present a new perspective on the recycling of valuable resources from many other smelting slags (e.g., iron slag, lead slag, and nickel slag).

Low-temperature infrared spectrum and atomic-scale structure of hydrous defects in diopside

Abstract. Hydrous defects in diopside (CaMgSi2O6) play an important role in the water budget of the Earth's mantle. Related OH-stretching modes lead to a variety of infrared absorption bands observed in natural or experimental samples. In the present study, we report new low-temperature infrared spectra of reference natural diopside samples in the OH-stretching range. In parallel, the structure and vibrational properties of a series of OH-bearing defects in diopside are theoretically determined at the density functional theory level. The infrared spectra make it possible to resolve additional bands in the region above 3600 cm−1 and reveal that their anharmonic behavior differs from that of the bands at lower frequency. A comparison of theoretical results with experimental data makes it possible to propose atomic-scale geometries corresponding to observed OH-stretching bands. It confirms that the bands observed at 3620–3651 cm−1 are related to M3+ ions substituted for Si in tetrahedral sites, while the 3420 cm−1 band is associated with the Na+ for Ca2+ substitution. In both cases, H+ incorporation compensates the charge deficit due to the heterovalent substitution. The other major mechanism of water incorporation in diopside relates to the charge compensation of cationic vacancies, among which Ca vacancies play a central role. The 3357 cm−1 band corresponds to doubly protonated Ca vacancies in pure diopside. In experimental diopside-bearing trivalent cations, the bands at 3432–3460 cm−1 correspond to singly protonated Ca vacancies with a nearby octahedral M3+ ion, while the 3310 cm−1 band likely involves a more remote charge compensation by M3+ ions. More complex defects associating Ca vacancies with tetrahedral M3+ and octahedral Ti4+ ions are proposed for the bands observed between 3500 and 3600 cm−1 in natural diopside. The Fe2+ for Mg2+ and Fe2+ for Ca2+ substitutions are also found to affect nearby OH-bearing defects, causing a shift and broadening of OH stretching bands in chemically more complex diopside samples.

Defects and Dopants in CaFeSi2O6: Classical and DFT Simulations

Calcium (Ca)-bearing minerals are of interest for the design of electrode materials required for rechargeable Ca-ion batteries. Here we use classical simulations to examine defect, dopant and transport properties of CaFeSi2O6. The formation of Ca-iron (Fe) anti-site defects is found to be the lowest energy process (0.42 eV/defect). The Oxygen and Calcium Frenkel energies are 2.87 eV/defect and 4.96 eV/defect respectively suggesting that these defects are not significant especially the Ca Frenkel. Reaction energy for the loss of CaO via CaO Schottky is 2.97 eV/defect suggesting that this process requires moderate temperature. Calculated activation energy of Ca-ion migration in this material is high (>4 eV), inferring very slow ionic conductivity. However, we suggest a strategy to introduce additional Ca2+ ions in the lattice by doping trivalent dopants on the Si site in order to enhance the capacity and ion diffusion and it is calculated that Al3+ is the favourable dopant for this process. Formation of Ca vacancies required for the CaO Schottky can be facilitated by doping of gallium (Ga) on the Fe site. The electronic structures of favourable dopants were calculated using density functional theory (DFT).

Defect-induced diverse electronic, magnetic and optical properties in monolayer CaP3

Monolayer CaP3 (mCaP3), a semiconductor predicted theoretically with suitable band gap and high electron mobility, shows potential applications in nanoelectronics and optoelectronics. In this work, using first-principles calculations, diverse electronic, magnetic and optical properties of mCaP3 were explored by substituting P atoms by B, C, N, O and F or introducing P and Ca vacancy, respectively. The results show that B and C prefer to substitute P atoms (denoted by P1) bonding with four adjacent P atoms, while N, O and F tend to substitute P (denoted by P2) bonding with two P and two Ca atoms. These two types of systems, expressed by X(P1)-mCaP3 (XB and C) and X(P2)-mCaP3 (XN, O and F), are favorable in energy. The systems of B(P1)-mCaP3, N(P2)-mCaP3 and F(P2)-mCaP3 with four different doping concentrations are found to be spin-non-polarized semiconductors. Notably, in N(P2)-mCaP3 and F(P2)-mCaP3, the band-gap is monotonically tuned by changing doping concentrations. In contrast, in C(P1)-mCaP3 and O(P2)-mCaP3, the spin-polarized semiconducting characters are obtained at the two low doping concentrations, and the transition from semiconducting to metallic states appears at the highest doping concentrations. Besides the tunable band gaps obtained, good optical absorption features in entire visible-light range are kept in all doped systems as the pristine one. In particular, an additional absorption peak in low energy region is induced by Ca vacancy and P substitution by C. These diverse electronic, magnetic and optical properties obtained by selective doping in mCaP3 will expand its potentials in future applications.