Host Lattice Atoms Research Articles

Deep insight into structural and optoelectronic properties of mixed anion perovskites

Abstract Here we present the optoelectronic properties of pure inorganic lead-free halide perovskites in the form of Cs2AgBiX 6 (X = Br, Cl, F, I) using the density functional theory calculations on cubic phase (Fm 3 ̅ m) and tetragonal phase (I4/m). First, all the structures of the two phases were optimized at the PBE level. Structural, electronic, optical properties, phonon, and thermal properties of Cs2AgBiX 6 in cubic (Fm 3 ̅ m) and tetragonal phases (I4/m) were obtained using the VASP code. Tetragonal phases of all compounds of the form Cs2AgBiX 6, except Cs2AgBiBr6, are reported here for the very first time. Among all the Cs2AgBiX 6 (X = F, Cl, Br, I) structures, the cubic phase of Cs2AgBiBr6 was seen to have the highest absorption coefficient along with prominent electronic features that are favorable for optoelectronic applications. Thus, the cubic phase of Cs2AgBiBr6 was selected as the host lattice and bromine atoms were partly replaced with chlorine and iodine atoms. Electronic and optical properties of these mixed halide compounds of Cs2AgBiBr6−xFx, Cs2AgBiBr6−xClx, and Cs2AgBiBr6−xIx where x = 1, 2, 3, 4, 5 are investigated with hybrid functional HSE06 level. The electronic structure revealed that these mixed compounds exhibited indirect band gap nature regardless of the halide substitution (different x concentration) and the band gap of Cs2AgBiBr6 could be varied with the substitutions of fluorine, chlorine, and iodine atoms. Our in-depth analysis shows that Cs2AgBiBr6 and their mixed halides have the potential to become active double perovskite materials for photovoltaic applications and as photocatalysts for water splitting.

A full palette: Crystal chemistry, polymorphism, synthetic strategies, and functional applications of lanthanide oxyhalides

Ionic compounds wherein lanthanide cations are arrayed alongside anions adopt a wide range of crystal structures as a result of the variation of ionic radii and electronic configurations across the lanthanide series. Owing to the constricted nature of 4f orbitals, local coordination environments and structural preferences in such compounds are primarily dictated by electrostatic interactions and steric considerations with the primary driving force being the minimization of the crystal lattice energy. In this review, we examine a broad class of dianionic rare-earth compounds, lanthanide oxyhalides that present a multidimensional design space for tuning of functional properties by dint of the possibilities for extensive alloying on cationic and anionic sublattices; the considerable span of ionic radii and hardness across the lanthanide series and down the halide group, respectively; a large tolerance window for point defects such as oxide and halide vacancies; as well as multiple accessible polymorphs. In addition to their structural versatility, control over functional properties is accessible based on alteration of microstructure and surface chemistry. Synthetic strategies for accessing atomistic, nanoscale, and mesoscale control are discussed using illustrative examples placing particular emphasis on non-hydrolytic sol—gel processes that provide access to well-defined solid-solution nanocrystals. The reactivity and post-synthetic modification of these compounds is further delineated. Methods for defining color centers within these compounds, cooperativity of the optical response of the color centers with the host lattice (determined by overlap integrals specific to each structure type and the polarization and ligand field effects of different halide ions) and adjacent color centers (reliant on energy transfer schema), and their practical application in phosphors are further detailed. Mechanisms facilitating down- and up-conversion of energy absorbed from incident electromagnetic radiation or high-energy particles are delineated with particular emphasis on X-ray and electron-beam excitation. The accessible energy conversion mechanisms, efficacious radiative recombination channels, and opportunities for systematically tuning color through compositional modulation have led to the emergence of these materials as potential candidates for phosphors. These materials have potential applications in solid-state lighting, radiation detection, and as the active elements of imaging devices. The tolerance towards large concentrations of anion vacancies and the facile diffusivity of anions in these compounds further underpins the function of lanthanide oxyhalides as ion conductors, gas sensors, and heterogeneous catalysts. Increasing interest has focused on utilization of well-defined color centers for quantum information science and multiplexed sensing in biological systems. Such applications require additional control of the positioning of dopant atoms and understanding of their interactions with the host lattice and other dopant atoms.

Soft x-ray absorption spectroscopy on Co doped ZnO: structural distortions and electronic structure

We present soft x-ray absorption spectra from a series of Co doped ZnO films. We discuss systematic variations of the Co L-edge white line intensity and multiplet features for this series of samples. We document sizeable differences in the electronic state of the Co ionic cores, as well as in the local environment of the host lattice atoms, characterised by means of x-ray absorption spectra at the O K-edge and Zn L-edges. Model calculations allow to correlate the observed effects to small structural distortions of the ZnO lattice.

The Use of the “+<i>U</i>” Correction in Describing Defect States at Metal Oxide Surfaces: Oxygen Vacancies on CeO<sub>2</sub> and TiO<sub>2</sub>, and Li-doping of MgO

There are many examples of defects in strongly-correlated metal oxides for which density functional theory predicts electronic structures that qualitatively disagree with experimental data. This behaviour arises from the self-interaction error inherent to standard density functionals, and is demonstrated by both p- and n-type systems where the defect state is a small polaron associated with host lattice atoms. An approximate correction is to describe the electron—electron interactions in the orbitals of interest within the DFT+U formalism. This gives improved descriptions for systems where the states of interest are well represented by atomic-like orbitals. The qualitative failure of standard DFT and corresponding improvement achieved with DFT+U is illustrated for cases where the defect state is primarily associated with localised cation f and d states (O vacancies in CeO2 and TiO2) and anion p states (Li-doped MgO). [DOI: 10.1380/ejssnt.2009.389]

A computational method to identify interstitial sites in complex materials

We propose a general computational method to replace the current practice of identifying interstitial sites through visual inspection and symmetry consideration. Our method employs a repulsive pair potential to model interatomic interactions between an interstitial atom and its neighboring host lattice atoms, and can be used to systematically determine interstices in materials with arbitrary symmetry and complexity. We demonstrate the usefulness of our method using alpha-uranium and cementite (Fe3C) as examples.

Large disparity between gallium and antimony self-diffusion in gallium antimonide.

The most fundamental mass transport process in solids is self-diffusion. The motion of host-lattice ('self-') atoms in solids is mediated by point defects such as vacancies or interstitial atoms, whose formation and migration enthalpies determine the kinetics of this thermally activated process. Self-diffusion studies also contribute to the understanding of the diffusion of impurities, and a quantitative understanding of self- and foreign-atom diffusion in semiconductors is central to the development of advanced electronic devices. In the past few years, self-diffusion studies have been performed successfully with isotopically controlled semiconductor heterostructures of germanium, silicon, gallium arsenide and gallium phosphide. Self-diffusion studies with isotopically controlled GaAs and GaP have been restricted to Ga self-diffusion, as only Ga has two stable isotopes, 69Ga and 71Ga. Here we report self-diffusion studies with an isotopically controlled multilayer structure of crystalline GaSb. Two stable isotopes exist for both Ga and Sb, allowing the simultaneous study of diffusion on both sublattices. Our experiments show that near the melting temperature, Ga diffuses more rapidly than Sb by over three orders of magnitude. This surprisingly large difference in atomic mobility requires a physical explanation going beyond standard diffusion models. Combining our data for Ga and Sb diffusion with related results for foreign-atom diffusion in GaSb (refs 8, 9), we conclude that the unusually slow Sb diffusion in GaSb is a consequence of reactions between defects on the Ga and Sb sublattices, which suppress the defects that are required for Sb diffusion.

First-Principles Band-Structure Calculations of p- and n-Type Substitutional Impurities in Zinc-Blende Aluminum Nitride

The tight-binding linearized muffin-tin orbitals technique is used in the atomic-sphere approximation (TB-LMTO-ASA) to study electronic properties of zinc-blende aluminum nitride (c-AlN) doped with various substitutional impurities. p-type impurities include Mg and Zn substituted at Al sites of the host lattice, and C and Si atoms substituted at N sites. The n-type impurities considered here include C and Si at Al sites and O substituted at N sites. To mitigate the shortcomings of the local density approximation (LDA) in accurately predicting energy differences between impurity bands and the bottom of the conduction band, the predictions of Zn and O substitutional impurities are compared with experimental results for these impurities. Based on these electronic-structure comparisons, we suggest that Mg is the best candidate among the four elements investigated as p-type dopants, and that Si is better than C for n-type doping.

Trapping of Guests in Rare Gas Matrices: Simulation and Experiment

AbstractMolecular dynamics simulations of the trapping of molecules in a rare gas matrix are presented. In distinction with most previous simulations, a site's structure is not obtained by a guess and optimize method, but is found by allowing the lattice to grow by adding atoms from the vapor onto a “seed” crystal. Simple pairwise potentials are used to simulate the growth of rare gas matrices containing some small guests, including diatomic and triatomic molecules. Changes in the structure and energy of the solid are calculated, as well as the vibrational frequencies of the trapped molecules, providing an estimate for the matrix shift. The method also allows the estimate of small changes in the interatomic distances in the trapped molecules, as compared to the gas phase values. Novel site geometries, in which the host lattice atoms are displaced from their equilibrium crystal positions, are observed for guest molecules that can strongly distort the lattice because of asymmetry or size mismatches.

Molecular dynamics simulations of rare gas matrix deposition

Molecular dynamics simulations of the trapping of molecules in a rare gas matrix based on a fcc lattice is presented. In distinction with most previous simulations, a site's structure is not obtained by a guess and optimize method, but is found by allowing the lattice to grow by adding atoms from the vapor onto the potential field of a “seed” crystal. Simple pairwise potentials are used to simulate the growth of rare gas matrices containing some small guests, including the diatomic molecules N2, NBr and Br2. Changes in the structure and energy of the solid are calculated, as well as the vibrational frequencies of the trapped molecules, yielding the matrix shift. Novel site geometries, in which the host lattice atoms are displaced from their equilibrium crystal positions, are observed for guest molecules that can strongly distort the lattice because of asymmetry or size mismatches. In these cases, rapid cooling of the incoming atom is essential to the formation of several distinct trapping sites.

Self-consistent calculation of the phonon self-energy and thermal conductivity for disordered solids.

We present a self-consistent calculation of the phonon self-energy as a function of frequency for disordered solids by using a Green's-function approach. The disordered solids consist of two species of atoms (the host lattice and defect atoms), which are arranged randomly in the three-dimensonal lattice sites. The present calculation contains the contributions to second order of the defect density. The previous calculation gives exactly the phonon self-energy to first order of the defect density. We show that for small defect density the present calculation only modifies the previous calculation slightly, but for large defect density the present calculation differs appreciably from the previous calculation. Our numerical calculations are done in the Debye model. We calculate the thermal conductivity as a function of temperature. We find that the present calculation is able to explain the observed ${\mathit{T}}^{2}$ behavior of the thermal conductivity for disordered solids at low temperatures.

The Luminescence and EPR Characterisation of Neutron Transmutation Doped Gallium Phosphide

Nuclear Transmutation Doping (NTD) enables doping of semiconductor crystals, independent of their growth conditions [1]. In this method the dopand is generated in the crystal volume as a result of thermal neutron irradiation. Owing to the low capture cross-section of thermal neutrons by host lattice atoms, the distribution of transmutation-induced dopants is homogeneous and nearly independent of the diameter and size of the semiconductor crystals. In this paper we present the results of photoluminescence (PL) and EPR measurements on thermal neutron irradiated and annealed GaP. When GaP is irradiated with thermal neutrons, both of its elements are transmuted and the isotopeS 70 Ge and 32 S are formed [2]. The cross-section (σ th) for the formation of the Ge isotopes is much larger than for the S isotope creation. Therefore, during the neutron irradiation process, mainly Ge dopants are generated. The transmuted Ge and S nuclei experience the recoil due to the subsequent β and γ emissions. Thus, the position of Ge or S atoms in the unit cell is not necessarily a substitutional

Rutherford backscattering and luminescence characteristics of neodymium implanted GaP, GaAs, and AlGaAs

GaP, GaAs, and Al0.25Ga0.75As samples implanted with neodymium were characterized by Rutherford backscattering spectroscopy and by photoluminescence. It is shown that recrystallization of room-temperature-implanted GaP and GaAs is retarded by the presence of Nd. However, the intra-4f shell luminescence spectra of Nd are independent of the crystalline state of GaAs. This suggests that local bonding of Nd to host lattice atoms is the basic factor determining the optical properties of Nd in GaAs. It is also shown that recrystallization of AlGaAs is only weakly affected by the presence of Nd in the layers. This shows AlGaAs:Nd to be a very promising material for optoelectronic applications.

Hydrogen incorporation behaviour and radiation damage in proton bombarded InP single crystals

Abstract In proton bombarded InP single crystals the incorporation behaviour of different hydrogen isotopes is studied in relation to implantation induced radiation defects. Investigations of the fluence dependence (D = 1016-1018 cm−2), of the depth profile and of the annealing behaviour (T an = 300–1000 K) of hydrogen incorporation and of damage density indicate that only a small fraction of the implanted hydrogen is chemically bonded to host lattice atoms. These bonded hydrogen atoms saturate dangling bonds at defect sites.

Injection of self-interstitials into a single crystal of nickel: One-interstitial vs. two-interstitial model

"In case the reader is not aware of it, it should be pointed out that there is, and has been, a long-standing controversy on the interpretation of the recovery stages in fcc metals. I suspect that after this conference there will still be a controversy!" Twenty years after these prophetic words were spoken by J.W. Corbett [i], Frank and Seeger [2] addressed the configuration of a cubic lattice defect in Ni that was discovered in a DPAC measurement on ~11In [3]. They argue that the defect must be a mixed interstitial, contrary to the original interpretation according to which the defect consists of an 111In atom at the centre of a tetrahedron of four nearest-neighbour vacancies. They further argue that, since the defect is formed in recovery stage III, its structure is a sensitive test of the existing defect models: the one-interstitial model according to which recovery stage III is associated with vacancy migration, and the two-interstitial model which denies any vacancy mobility in recovery stage III. In the following we report on the annihilation of C-defects by self-interstitial injection. We prepared a single crystal of Ni so that about 30% of the implanted *11In atoms were associated with the cubic defect, and bombarded it subsequently with i00 eV Xe ions to stepwise increasing doses ranging from 1.10'5-1.1017 cm -2. The Xe ions transfer at most 3.7 times the displacement energy to the host lattice atoms, and eventually initiate a displacement collision sequence leaving a vacancy at the surface. The self-interstitials injected in this way into the Ni lattice will move on

Electrical properties of alpha-cyclodextrin metal iodide inclusion compounds

In order to build molecular scale mechanical or electronic devices it will be necessary to understand in atomic detail the general principles of molecular recognition and molecular self-assembly. Host-guest chemistry is essential for this understanding. The following short paper is a survey of electronic properties of molecular self-assembled cyclodextrin-metal iodide host-guest compounds. Alpha-cyclodextrin has the ability to crystallize into stacked channels which can act as hosts for the confinment of small guest molecules. The results of electrical studies on fifteen metal iodide cyclodextrin complexes are reported. These complexes consist of chains of polyiodide in the host lattice and metal atoms in the interspace regions. The conductivity is shown to vary, depending on the metal, between 10 −10 to 10 −4 S/cm at room temperature. When the samples are doped with excess iodine the conductivity decreases by about one order of magnitude for each metal iodide complex.

Amorphization process in manganese-implanted aluminium thin films and single crystals: Effects of strains and target temperature

The amorphization of aluminium thin films and single crystals by implantation at liquid nitrogen temperature (LNT) and room temperature (RT) was studied by X-ray diffraction, Rutherford backscattering and channelling experiments. Structural changes, the lattice site occupation of the implanted manganese atoms, static displacements of the lattice atoms, the ion implantation-induced reduction of grain size and the accumulation of strain could be directly observed as a function of manganese concentration. For the LNT implants it was found that at low manganese concentrations (i.e. less than 5 at.%) the impurities occupy mainly substitutional lattice sites ( f s ≅ 90%). In this concentration range the atomic size mismatch leads to static displacements of the host lattice atoms. Measurements further suggest that the regions in the immediate vicinity of the manganese impurity atoms are severely distorted. When the local manganese concentration and thus the local distortions exceed a threshold value the matrix becomes unstable and amorphous clusters are formed throughout the sample. The critical local manganese concentration c c for amorphization was found to be 8.5 at.% and the minimum volume v c of the amorphous clusters was found to be 2 × 10 −21 cm 3 which is the volume of a sphere with a radius of three interatomic distances. The RT implants showed a drastically reduced solubility, small lattice parameter changes and negligible homogeneous strains. Moreover, a non-zero amorphous fraction is observed at very small manganese concentrations, i.e. no threshold effect is observed in the amorphization curve. The differences of the amorphization process compared with the LNT case are discussed in terms of thermally activated short-range migration of manganese atoms which leads to the formation of amorphous clusters and to a relaxation of strains at lower manganese concentrations.

Amorphous phase formation in Mn-implanted Al films and single crystals.

The amorphization of Al thin films and single crystals by Mn implantation at liquid-nitrogen temperature was studied by x-ray diffraction, Rutherford-backscattering and channeling experiments. Structural changes, the lattice site occupation of the implanted Mn atoms, static displacements of the lattice atoms, and the accumulation of strain could be directly observed as a function of manganese concentration. It was found that at low Mn concentrations (5 at. %) the impurities occupy mainly substitutional lattice sites (${f}_{s}$\ensuremath{\simeq}90%). In this concentration range the atomic-size mismatch leads to static displacements of the host lattice atoms. Measurements further suggest that the regions in the immediate vicinity of the Mn impurity atoms are severely distorted. When the local Mn concentration and thus the local distortions exceed a threshold value, the matrix becomes unstable and amorphous clusters are formed throughout the sample. The critical local Mn concentration for amorphization was found to be ${c}_{c}$=8.5 at. % and the minimum volume of the amorphous clusters ${v}_{c}$=2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ ${\mathrm{cm}}^{3}$ which is the volume of a sphere with a radius of 3 interatomic distances.

Theory of impurity vibrations due to isolated interstitials and interstitial-substitutional pair defects in semiconductors.

Dynamical properties of isolated interstitials and ``interstitial substitutional'' pair defects in semiconductors are reported with use of the well-known Green's-function technique. To make the problem amenable to calculations, we exploit the symmetry properties of the defect environment and assume the interstitial site to be tetrahedral. For the isolated-impurity case, the perturbation is restricted to the impurity interacting with the host lattice atoms up to and including second-nearest neighbors. Similar to the substitutional-impurity vibrations, the contribution of the nearest-neighbor interactions to the dynamical properties of interstitials in semiconductors is found to be appreciable. For light defects (e.g., ${\mathrm{Li}}_{\mathrm{int}}$ in Si and CdTe), a single dimensionless nearest-neighbor perturbation parameter \ensuremath{\tau} has provided the isotopic frequency shifts of localized vibrational modes, in excellent agreement with the observed optical data. To explain the observed isotopic shifts of inband resonance modes due to heavier impurities (e.g., $^{63}\mathrm{Cu}$, $^{65}\mathrm{Cu}$ in Si, etc.) the effect of second-nearest-neighbor impurity-host interaction is found to be important. Finally, the calculated force-constant changes due to isolated substitutional and interstitial impurities are used to understand the ``interstitial-substitutional'' pair defect vibrations in Si and CdTe.

Generalized Langevin-equation approach to impurity diffusion in solids: Perturbation theory

We derive, from a microscopic viewpoint, a nonlinear and non-Markovian Langevin equation to describe impurity diffusion in solids, assuming weak interaction between an impurity and the host-lattice atoms. The memory function and the periodic potential in the Langevin equation are calculated for the isotropic Debye lattice which models the host crystalline solid. The diffusion constant is obtained in a closed form with the use of a theory for noise-induced activation processes.

Plastic deformation effect in high dose ion implanted systems

The rotation of the implanted surface region with respect to the orientation of the bulk crystal during high dose ion implantation has been observed for the systems V Sn , V Te , V Cs and N bCs . The critical foreign atom concentration at which the rotation occur is different for different systems and can be correlated to the size-mismatch energy between the foreign and the host lattice atoms. Amorphization of the implanted region stabilizes the angle of rotation for the system V Sn .