Heterovalent Dopants Research Articles

Effects of Sodium Vacancies and Concentrations in Na3SO4F Solid Electrolyte.

The sodium-rich solid electrolyte, Na3SO4F (NSOF), holds promise for eco-friendly and resource-abundant energy storage. While the introduction of heterovalent dopants has the potential to enhance its suitability for battery applications by creating Na vacancies, the effect of vacancies and sodium concentrations on sodium conduction remains unclear. In this work, Mg2+ was introduced into Na+ sites in Na3SO4F, generating sodium vacancies with different contents by using solid-state synthesis method. Among the resulting materials, Na2.96Mg0.02SO4F exhibited an ionic conductivity that is two-order-of-magnitude higher than NSOF at 298 K. Notably, as the sodium concentration decreased, the ionic conductivity also declined, revealing an equilibrium between Na vacancies and concentrations. To further investigate the influence of sodium concentration, excess Na+ was introduced into NaMgSO4F, which inherently possesses a lower sodium content by using solid-state synthesis method. However, this adjustment only led to an approximately one-order-of-magnitude enhancement in optimal ionic conductivity at 298 K. Combined with an in situ X-ray diffraction analysis, our findings underscore the greater sensitivity of sodium conduction to variations in sodium vacancies. This study paves the way for the development of ultrafast sodium ion conductors, offering exciting prospects for advanced energy storage solutions.

Charge-compensated co-doping stabilizes robust hafnium oxide ferroelectricity

Compensated co-doping not only achieves charge neutrality but is also highly favorable for the formation of polar phases and the absence of structural defects.

Induced p-type semiconductivity in Mg-doped Nd2Zr2O7 pyrochlore system

Heterovalent B-site MgO substitution in the Nd2Zr2O7-system (Nd2Zr2−xMgxO7−x) has been explored. The pyrochlores were synthesized by a polymeric sol-gel method and characterized by X-ray diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy to determine structure, phase composition and microstructure. Impedance Spectroscopy (IS) was employed to study the electrical behavior of the ceramics over the ranges 200–800 °C and under pure N2 and O2. The XRD showed that the solid solution limit was x > 0.02 and all the materials show a cubic Fd3̅m structure. The Raman results confirm the structural disorder created by the introduction of Mg2+ and the subsequent generation of oxygen vacancies. The IS data shows a dramatical increase of the oxide-ion conductivity when doping and that the conductivity depends strongly on the atmosphere, leading to p-type semiconductivity under pure O2 atmosphere. The present study highlights the use of heterovalent dopants to drastically increase the oxide-ion conductivity of pyrochlore-like materials.

Tuning Multicolor Emission of Manganese-Activated Gallogermanate Nanophosphors by Regulating Mn Ions Occupying Sites for Multiple Anti-Counterfeiting Application.

The ability to manipulate the luminescent color, intensity and long lifetime of nanophosphors is important for anti-counterfeiting applications. Unfortunately, persistent luminescence materials with multimode luminescent features have rarely been reported, even though they are expected to be highly desirable in sophisticated anti-counterfeiting. Here, the luminescence properties of Zn3Ga2GeO8:Mn phosphors were tuned by using different preparation approaches, including a hydrothermal method and solid-state reaction approach combining with non-equivalent ion doping strategy. As a result, Mn-activated Zn3Ga2GeO8 phosphors synthesized by a hydrothermal method demonstrate an enhanced red photoluminescence at 701 nm and a strong green luminescence with persistent luminescence and photostimulated luminescence at 540 nm. While Mn-activated Zn3Ga2GeO8 phosphors synthesized by solid-state reactions combined with a hetero-valent doping approach only exhibit an enhanced single-band red emission. Keeping the synthetic method unchanged, the substitution of hetero-valent dopant ion Li+ into different sites is valid for spectral fine-tuning. A spectral tuning mechanism is also proposed. Mn-activated Zn3Ga2GeO8 phosphors synthesized by a hydrothermal approach with multimodal luminescence is especially suitable for multiple anti-counterfeiting, multicolor display and other potential applications.

Understanding Oxygen Anion Transport in Nb-Doped La0.7Sr0.3CoO3 Perovskite Materials

Simple perovskite structured electrode materials of the series (LaxSr1-xCoO3- δ) are commonly utilised for high-temperature solid-state electrochemical cells and Li-air batteries with some limitations observed on prolong operations due to the structural instability associated with the segregation of strontium ions. In order to overcome this structural incompatibility, doping of heterovalent dopant at the B-site is proposed which could possibly improve upon the stability by controlling the segregation of surface cations. In addition, the doped structure is likely to provide faster oxygen anion transport through the vacancies in the structure, thereby improving its electrochemical performance. Herein, oxygen anion diffusion in pure La0.7Sr0.3CoO3-- δ (LSC) and Nb-doped La0.7Sr0.3Co0.95Nb0.05O3- δ (LCSN) perovskite structure is calculated using molecular dynamics (MD) simulations. Self diffusion coefficient of oxygen ions in the LSCN structure is observed to increase by an order of magnitude (1.356×10-8 cm2/s) on Nb doping as compared to the undoped LSC structure (6.499×10-9 cm2/s) at 1073 K. On varying the Nb concentration from 0.05 to 0.15 and optimum Nb concentration is estimated to be 0.05 corresponding to a maximum oxygen anion diffusivity in the structure (1.356×10-8 cm2/s) at 1073 K.

Cation Exchange Enabled Cu Dopants Location Tailoring and Photoelectric Properties Regulation in CdS Nanosheets

Doping-related point defect engineering in low-dimensional semiconductor nanostructures is important to regulate their optical and electronic properties. The substitutional or interstitial location of heterovalent dopants is critical and has not been controlled effectively yet. Herein, we carefully control the kinetics of reverse cation exchange between CuxS 2D nanosheets and ligand-coordinated Cd2+ cations to control the Cu doping sites in CdS nanosheets (NSs). The substitutional and interstitial Cu dopants were directly confirmed by spherical aberration-corrected TEM (SACTEM) and their X-ray absorption spectroscopy (XAS) coordination investigation. Density functional theory (DFT) calculations and their experimental conductivities and dopant luminescence performance demonstrated the dramatic differences that are due to the location of different Cu dopants. These findings provide deeper insights on dopants' location regulation in a nanostructured host semiconductor.

Defects and Lattice Instability in Doped Lead-Based Perovskite Antiferroelectrics: Revisited

This paper is a summary of earlier results that have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZrO3 single crystals doped with niobium and Pb(Zr0.70Ti0.30)O3 ceramics doped with different concentration of Bi2O3. It was found that these crystals undergo new phase transitions never observed before. To investigate all phase transitions, different experimental methods were used to characterize the crystal properties. Temperature and time dependencies have been tentatively measured in a wide range, including a region above Tc, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices. Also, coexistence of antiferroelectric phase and one of the intermediate phases could be observed in a wide temperature range. The phase transition mechanism in PbZrO3 is discussed, taking into account the local breaking of the crystal symmetry above Tc and the defects of crystal lattices, i.e., those generated during crystal growth, and intentionally introduced by preheating in a vacuum or doping with hetero-valent dopant.

Structural, electronic and magnetic properties of Ca, Sr and Ba heterovalent A-site ion substitution in BiFeO3 with different Fe oxidation states

A comparative study of structural, electronic and magnetic Ca2+, Sr2+ and Ba2+ A-site ion substitution of BiFeO3 with different Fe oxidation states was carried out. Calculation shows doped compounds favours energy stability in Fe oxidation states of 2+ and 4+ compared to 3+ charge. Electronic band gap makes significant changes with value of 1.32 eV, 1.36 eV and 1.31 eV for Ca, Sr and Ba doped respectively, compared to pure BiFeO3 of 1.41 eV. The net magnetization shows improvement with use of the dopant elements compared to pure BiFeO3. These changes indicate the role of the heterovalent dopant in BiFeO3.

Size-Induced Enhancement of Carrier Density, LSPR Quality Factor, and Carrier Mobility in Cr–Sn Doped In2O3 Nanocrystals

Heterovalent dopant ions, such as Sn4+, in In2O3 nanocrystals (NCs) provide free electrons for localized surface plasmon resonance (LSPR). But the same heterovalent dopants act as electron scattering centers, both independently and by forming complexes with interstitial oxygen, thereby increasing LSPR line width. Also, such complexes decrease free carrier density. These detrimental effects diminish the figure-of-merit of LSPR known as the quality factor (Q-factor). Herein, we designed colloidal Cr–Sn codoped In2O3 NCs, where both high carrier density and low carrier scattering can be achieved simultaneously, yielding a high LSPR Q-factor of 7.2, which is a record high number compared to prior reports of doped In2O3 NCs. Q-factors increase systematically from 3.2 for 6.6% Sn doped In2O3 NCs to 7.2 for 23.8% Cr–6.6% Sn codoped In2O3 NCs by increasing the Cr codoping concentration, which is also accompanied by an increase in NC size from 6.7 to 22.1 nm. Detailed characterization and analysis of LSPR spectra ...

Heterovalent Doping in Colloidal Semiconductor Nanocrystals: Cation-Exchange-Enabled New Accesses to Tuning Dopant Luminescence and Electronic Impurities.

Heterovalent doping in colloidal semiconductor nanocrystals (CSNCs), with provisions of extra electrons (n-type doping) or extra holes (p-type doping), could enhance their performance of optical and electronical properties. In view of the challenges imposed by the intrinsic self-purification, self-quenching, and self-compensation effects of CSNCs, we outline the progress on heterovalent doping in CSNCs, with particular focus on the cation-exchange-enabled tuning of dopant luminescence and electronic impurities. Thus, the well-defined substitutional or interstitial heterovalent doping in a deep position of an isolated nanocrystal has been fulfilled. We also envision that new coordination ligand-initiated cation exchange would bring about more choices of heterovalent dopants. With the aid of high-resolution characterization methods, the accurate atom-specific dopant location and distribution could be confirmed clearly. Finally, new applications, some of the remaining unanswered questions, and future directions of this field are presented.

Scanning tunneling spectroscopy to probe site-selection in heterovalent doping: Zn(II)-doped Cu(I)In(III)S2 as a case study

We report scanning tunneling spectroscopy (STS) of a heterovalent-doped ternary compound semiconductors and their binary counterparts. The effect of dopants in the semiconductors that yielded a shift in Fermi energy has been found to be manifested in the density of states (DOS) spectrum. The shift infers the nature of doping, which the heterovalent dopants induce, and hence the site of the ternary system that the dopants occupy. For example, in the present case with Zn(II)-doped Cu(I)In(III)S2, the DOS spectra showed a shift in Fermi energy towards the conduction band and hence a n-type doping due to the introduction of electrons. Such a shift inferred that the bivalent dopants occupied the cuprous site. The results have been substantiated by STS studies of doped binary components, namely, Cu2S and In2S3 and shift in Fermi energy thereof. With the tuning in the Fermi energy, the homojunctions between undoped and doped semiconductors have a type-II band-alignment at the interface resulting in current rectification through the junctions. The band-diagram of the homojunctions formed through STS substantiated the direction of current-rectification in the junctions.

Unconventional interplay between heterovalent dopant elements: Switch-and-modulator band-gap engineering in (Y, Co)-Codoped CeO2 nanocrystals.

We report the experimental observation and theoretical explanation of an unconventional interplay between divalent Co and trivalent Y dopants, both of which incur oxygen vacancies in the CeO2 host that has predominantly tetravalent Ce cations. The Co dopant atoms were experimentally found to act as a switch that turns on the dormant effect of Y-modulated band-gap reduction. As revealed by density functional theory (DFT) calculations with structures verified by synchrotron-radiation x-ray measurements, a Co 3d band that hybridizes with Ce 4f band was lowered due to reduced O 2p repulsion arising from oxygen vacancies incurred by Y doping and therefore gave rise to the observed band-gap narrowing effect. Such switch-and-modulator scheme for band-gap engineering in nanocrystal materials can lead to important applications in environmental protection and solar energy harvesting technologies.

Extraordinary Doping Effects on Quasiparticle Scattering and Bandwidth in Iron-Based Superconductors

The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy, we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. Beyond the Fermi-surface alteration that has been reported most often in the past, we found two more extraordinary effects of doping: (1) the site and band dependencies of quasiparticle scattering and, more importantly, (2) the ubiquitous and significant change of electronic correlation by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the electronic correlation could be suppressed by applying either the chemical pressure or doping electrons but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors.1 MoreReceived 28 April 2014DOI:https://doi.org/10.1103/PhysRevX.4.031041This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Thermal expansion of solid solutions based on calcium and barium fluorides

The thermal expansion of the Ca0.90Y0.10F2.10, Ca0.97Yb0.03F2.03, Ca0.97Co0.03F2.00, Ba0.70La0.30F2.30, and Ba0.80Nd0.20F2.20 solid solutions has been studied experimentally from below liquid-nitrogen temperature to room temperature. The results demonstrate that both the iso- and heterovalent dopants have a weak effect on the linear thermal expansion coefficient of the fluorite-structure crystals.

ChemInform Abstract: From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2—BaF2 System.

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).

From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2–BaF2 System

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent "dopants" can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).

Role of trivalent Sr substituents and Sr vacancies in tetragonal and polar states of SrTiO 3

Inelastic light scattering is used to study lattice dynamics of strontium titanate (STO) ceramics with several heterovalent dopants (La 3+, Gd 3+, Y 3+), which substitute Sr 2+ ions. An extraordinary shift of the antiferrodistortive transition temperature ( T a) is ascertained when just a small percentage of any of the dopants is used. T a is dependent on the tolerance factor ( t). In this work, it is clearly shown that, regardless of the dopant used, a common linear dependence of T a vs. t is obtained if strontium vacancies are taken into account. A vacancy size of ∼1.547 Å was estimated, which is ∼7% larger than the Sr 2+ radius. The vacancy size obtained can directly explain the increase in lattice parameter with increasing Bi 3+ content in Bi-doped STO, as opposed to the dopants referred to above. Furthermore, the introduction of La 3+, Gd 3+ or Y 3+ ions at the Sr site causes a considerable stiffening of the transverse optic TO 1 mode at low temperatures, thereby decreasing the phonon contribution to the dielectric permittivity. Thus, no traces of a ferroelectric phase are found for any of the dopants used in this work.

Valence and spin states in perovskites LaCo 0.95M 0.05O 3 (M=Mg, Ga, Ti)

The samples LaCoO 3 with dilute substitutions on cobalt sites have been studied using the resistivity, thermopower and magnetic susceptibility measurements over the temperature range up to ∼900 K. The Co-site substitution does not affect the magnetic transition at ∼100 K and the onset of massive population of hole carriers at ∼500 K, characteristic for undoped LaCoO 3. On the other hand, the low-temperature transport and magnetism is markedly distinct for samples with extra charge on cobalt ions introduced by the heterovalent dopants (Mg 2+, Ti 4+) compared to samples with minor non-stoichiometry (LaCoO 3, Ga 3+-doped sample). Magnetic properties suggest that these extra charges create thermally stable magnetic polarons of total S∼2–3. Common features of Co-site doped and La-site doped samples (La 1− x Sr x CoO 3) are discussed.

Effect of heterovalent ion substitution on the crystalline structure and magnetic ordering of SrRuO3

Structural and magnetic studies have been carried out on SrRu1−xMxO3 (M=Ga:x=0.1; Cu and Zn: x=0.1 and 0.2) in order to understand the dependence of the ferromagnetic transition temperature of SrRuO3 on heterovalent magnetic and nonmagnetic cation substitution at the Ru site. Powder x-ray diffraction has been carried out to analyze the crystal structure using the Rietveld refinement procedure. Tc decreases more rapidly for divalent cation dopants as compared to the trivalent cations. The refinement results show that there are distortions of the RuO6 octahedra in the doped compounds, with little change in the lattice parameters. However, the direction and magnitudes of the tilt of the octahedra for all the compounds do not show correlation with the Tc. Therefore, the change of Tc is attributed to the charge and spin disorders introduced by the heterovalent dopant cations. The filled eg orbitals in Ga, Cu, and Zn ions are expected to have weak hybridization with the oxygen p orbitals as compared to the pristine compound, leading to a weakening of the overall ferromagnetic interactions between the Ru ions.