Crossover Composition Research Articles

The ferromagnetic to antiferromagnetic crossover in chromium-based pyroxenes tuned via the Ge to Si ratio

For the isotropic spin Cr3+-based (S = 3/2) chain-structure pyroxenes, NaCrGe2O6 is a ferromagnet while NaCrSi2O6 is an antiferromagnet, offering a platform for studying one-dimensional (1D) magnetic interactions. Here we describe our characterization of the NaCrGe2-xSixO6 (0 ≤ x ≤ 2) solid solution series, locating the ferromagnetic-to-antiferromagnetic crossover composition at x = 1.40. NaCrGe0.6Si1.4O6 has a close-to-zero Weiss temperature, which we attribute to a balance between the ferromagnetic and antiferromagnetic interactions in this pseudo-1D isotropic spin system.

The martensite/strain-glass crossover composition region of Ni-Co-Mn-Ga ferromagnetic strain glass system

The ferromagnetic strain glass within the martensite/strain-glass crossover composition region has been reported to exhibit optimal damping performance. The construction of a crossover composition region in ferromagnetic strain glass systems has great potential to optimize their functional properties. The transforming properties of the Ni55-xCoxMn20Ga25 (x = 8.2–9.2) system is detected to construct the crossover composition region. The results reveal that Ni55-xCoxMn20Ga25 (x = 8.4, 8.6, 8.8, 9.0, and 9.2) alloys undergo two-step transitions, i.e., austenite to strain glass and strain glass to martensite transition. Based on the experimental results, the phase diagram with a crossover composition region of Ni55-xCoxMn20Ga25 ferromagnetic strain glass system was established. This guide explores the relevant functional properties of alloys within the martensite/strain-glass crossover composition region.

Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR.

Efficient white-light-emitting single-material sources are ideal for sustainable lighting applications. Though layered hybrid lead-halide perovskite materials have demonstrated attractive broad-band white-light emission properties, they pose a serious long-term environmental and health risk as they contain lead (Pb2+) and are readily soluble in water. Recently, lead-free halide double perovskite (HDP) materials with a generic formula A(I)2B'(III)B″(I)X6 (where A and B are cations and X is a halide ion) have demonstrated white-light emission with improved photoluminescence quantum yields (PLQYs). Here, we present a series of Bi3+/In3+ mixed-cationic Cs2Bi1-xInxAgCl6 HDP solid solutions that span the indirect to direct band-gap modification which exhibit tailorable optical properties. Density functional theory (DFT) calculations indicate an indirect-direct band-gap crossover composition when x > 0.50. These HDP materials emit over the entire visible light spectrum, centered at 600 ± 30 nm with full-width at half maxima of ca. 200 nm upon ultraviolet light excitation and a maximum PLQY of 34 ± 4% for Cs2Bi0.085In0.915AgCl6. Short-range structural insight for these materials is crucial to unravel the unique atomic-level structural properties which are difficult to distinguish by diffraction-based techniques. Hence, we demonstrate the advantage of using solid-state nuclear magnetic resonance (NMR) spectroscopy to deconvolute the local structural environments of these mixed-cationic HDPs. Using ultrahigh-field (21.14 T) NMR spectroscopy of quadrupolar nuclei (115In, 133Cs, and 209Bi), we show that there is a high degree of atomic-level B'(III)/B″(I) site ordering (i.e., no evidence of antisite defects). Furthermore, a combination of XRD, NMR, and DFT calculations was used to unravel the complete atomic-level random Bi3+/In3+ cationic mixing in Cs2Bi1-xInxAgCl6 HDPs. Briefly, this work provides an advance in understanding the photophysical properties that correlate long- to short-range structural elucidation of these newly developed solid-state white-light emitting HDP materials.

Phase behavior of aqueous solutions of ethaline deep eutectic solvent

We have established a detailed phase diagram of a prototypical DES as a function of the hydration level. Two distinct thermal phase behaviors are observed depending on the water content with respect to a cross-over composition Wg’ = 30%. For W < Wg’, the formation of ice is not observed under the experimental conditions used in this study, and the solution falls in the category of glassforming systems. Fully vitreous states could also be obtained between 30% and 50%, but they are metastable with respect to water crystallization. For W > Wg’, ice crystallization occurs but the residual DES solution remains amorphous (liquid or glassy). In the latter case, whatever the initial water fraction, this transformation finally ends at the fixed composition Wg’ corresponding to 6 to 10 water molecules per choline ion for the two studied DESs. We infer that the residual liquid water molecules forming this maximally freeze-concentrated solution are strongly interacting with DES molecular units. This situation is also known as the “water-in-DES” case. Conversely, ice crystallization concerns free water molecules, provided that W > Wg’, also known as the “DES-in-water” case. This entire phase behavior is explained in the context of maximally freeze-concentrated solutions and attributed to the concomitant effects of ice freezing depression, glassforming ability of weakly hydrated DES (W < Wg’) and water structure distortion. This study also highlights the potential of DESs for their uses in freeze-drying processes and biopreservative applications.

Structural properties of Sm-doped ceria electrolytes at the fuel cell operating temperatures

A high temperature structural study (673–1073K) was performed by means of synchrotron x-ray diffraction and μ-Raman spectroscopy on several compositions belonging to the Ce1−xSmxO2−x/2 system with the aim to investigate the crystallographic features of Sm-doped ceria electrolytes at the fuel cell operating temperatures; ionic conductivity of samples with x ranging between 0.1 and 0.4 was measured too in order to correlate the main structural features with transport properties. A slight shift toward lower x values of the fluorite-based/hybrid region boundary is observed with increasing temperature; moreover, the coefficient of thermal expansion reveals a strong slope change close to x=0.3, which represents the crossover composition between the two atomic arrangements. The presence of C-structured RE2O3 nanodomains within the fluorite structure starting from x~0.2 is revealed by μ-Raman spectroscopy and confirmed by the behaviour of total conductivity. The ordering effect exerted at each temperature by the Sm-vacancies aggregates on the fluorite structure within the hybrid region influences the behaviour of both the intensity and the full width at half maximum of the Raman signal typical of the CeO2 structure. The obtained results are discussed in comparison to the ones deriving from the Gd-doped ceria system.

Band gap bowing and crossing of BxGa1−xN alloy investigated by hybrid functional method**Project supported by the Fundamental Research Funds for the Central Universities (No. 2010LKWL03), the Special Fund for Theoretical Physics (No. 11047130), and the National Natural Science Foundation of China (No. 11104345).

The electronic properties of zinc-blende BxGa1–xN alloys are comparatively investigated by employing both the Perdewe-Burkee–Ernzerhof generalized-gradient approximation (PBE-GGA) and the Heyd-Scuseria-Ernzerhof screened hybrid functional methods (HSE06). HSE06 reproduced much closer ground-state properties to experiments. Large and composition-dependent bowing parameters bγ for the direct band gaps were obtained from both PBE and HSE06. The crossover composition where alloy switches from direct to indirect was predicted to occur at very similar x from PBE and HSE06. We can obtain direct gap BxGa1–xN with a gap value much larger than that of GaN by alloying x < 0.557 boron into GaN.

Doping of Direct Gap Ge1 -y Sn y Alloys to Attain Electroluminescence and Enhanced Photoluminescence

Alloying of elemental Ge with Sn permits tuning of the band gap, as well as a transition towards direct gap behavior. Epitaxial deposition of direct gap Ge 1-y Sn y alloys (y≥0.09) requires ultra-low temperatures of ~300°C.1–3 In order to fully realize the electroluminescence and photoluminescence (PL) potential of this class of materials, strategies for p- and n-type doping that are compatible with this temperature range are needed. In our previous work, combination of both traditional and novel dopant sources were utilized for doping of materials ranging from pure Ge to Ge 1-y Sn y alloy at the direct to indirect crossover composition of y=0.09.4,5 Presented here are the extensions of these methods to include alloys beyond the crossover composition. The n-type doped alloys were grown on virtual Ge substrates in a hot wall UHV-CVD reactor using trigermane (Ge3H8) and deuterostannane (SnD4) sources to provide the respective elements. Trisilylphosphine (P(SiH3)3) was used as a P source for n-type doping alloys in the composition range y=0.09-0.13, which required growth temperatures of 300°C-275°C. Choice of this doping agent was due to its greater stability (which facilitates long term storage) and higher vapor pressure (which allows straightforward use in mixture preparation) when compared to trigermylphosphine (P(GeH3)3), another agent capable of delivering donor P atoms at low temperatures. The use of these precursor combinations has enabled the deposition of direct gap Ge 1-y Sn y alloy films with 300-500 nm thicknesseswith dopant concentrations 5x1018-3x1019 cm-3, as determined by IR spectroscopic ellipsometry. RBS channeling spectra and (004) rocking curves show these materials to have excellent crystallinity. Furthermore, analysis using SIMS shows that the incorporated donor atoms are fully activated, and the amount of Si incorporated into the materials from using P(SiH3)3 as the doping precursor is at impurity levels which does not affect the band gap properties. These observations are in agreement with previous experiments in which P(SiH3)3 was used as a doping agent, albeit at higher temperatures.4,5 Using identical protocols but removing P(SiH3)3 from the precursor mixtures, it was possible to obtain intrinsic Ge 1-y Sn y alloys with concentrations up to y=0.14. By choosing n-type doped virtual Ge substrates for these depositions, it was possible to investigate electroluminescence, after growth of appropriate p-type layers to form n-i-p diodes, as described below. The p-type doped alloys were grown on either intrinsic or n-type doped Ge­1-y Sn y alloys with y=0.09-0.14 in order to fabricate n-p or n-i-p device stacks. The depositions were carried out in a hot wall UHV-CVD reactor with a temperature profile that enables the pre-activation of the precursor molecules prior to reaching the growth surface. The precursors used for the depositions were digermane (Ge2H6), SnD4 and diborane (B2H6). The films obtained were 100-300 nm thick, with active dopant concentrations in the 2-3×1019 cm-3 range, and compositions of y=0.08-0.12. As shown in Fig.1, n-type doping of the Ge 1-y Sn y alloys enhances PL in comparison to intrinsic counterparts. Furthermore, prototype LEDs produced with the aid of above doping strategies exhibit strong electroluminescence,2 paving the way for obtaining emission for a broad spectral range, from pure Ge at 1550 nm up to 2700 nm and beyond. References (1) Gallagher, J. D.; Senaratne, C. L.; Kouvetakis, J.; Menéndez, J. Appl. Phys. Lett. 2014, 105, 142102. (2) Gallagher, J. D.; Senaratne, C. L.; Sims, P.; Aoki, T.; Menéndez, J.; Kouvetakis, J. Appl. Phys. Lett. 2015, 106, 091103. (3) Senaratne, C. L.; Gallagher, J. D.; Jiang, L.; Aoki, T.; Menéndez, J.; Kouvetakis, J. J. Appl. Phys. 2014, 116 (13), 133509. (4) Senaratne, C. L.; Gallagher, J. D.; Aoki, T.; Kouvetakis, J.; Menéndez, J. Chem. Mater. 2014, 26 (20), 6033–6041. (5) Xu, C.; Gallagher, J. D.; Sims, P.; Smith, D. J.; Menéndez, J.; Kouvetakis, J. Semicond. Sci. Technol. 2015, 30 (4), 045007. Fig. 1 – PL spectra obtained from a liquid nitrogen cooled InGaAs detector for n-type doped (solid line) and intrinsic (dashed line) Ge 1-y Sn y samples with y=0.09. Both samples show strong luminescence as expected for direct gap materials. However the doped sample, which has an active carrier concentration of 1×1019 cm-3, shows clear enhancement of PL compared to the intrinsic sample. The inset shows the same spectra obtained from a thermoelectrically cooled PbS detector with extended spectral range beyond the cutoff point of the InGaAs detector, albeit at expense of poorer signal-to-noise ratio. Figure 1

Bernstein meets Broadway: collaborative art in a time of war

Introduction Section I: Ballet, Nightclubs, Broadway 1.Youthful Celebrity and Progressive Visions: Breaking Out with Fancy Free 2. From Nightclubs to Broadway: The Revuers, Comedy Skits, and Progressive Politics 3. Creating a Broadway Musical: The Conception and Debut of On the Town Section II: Staging Racial Politics 4. A Japanese American Star on Broadway: Sono Osato and 5. Desegregating Broadway: On the Town and Race 6. Biographies on Stage: On the Town's Black Conductor, Dancers, and Singers Section III: Musical Style 7. Crossover Composition: The Musical Styles of On the Town 8. On the Town After Dark: The Nightclub Scene Section IV: Epilogue Appendices A. Discography and Videography of Fancy Free, On the Town, and The Revuers B. Scenario for Fancy Free C. Cast List for On the Town on Opening Night Index

Spectroscopic determination of the bandgap crossover composition in MBE-grown AlxGa1−xAs

The aluminum concentration dependence of the energies of the direct and indirect bandgaps arising from the Γ and X conduction bands are measured at 1.7 K in the semiconductor alloy AlxGa1−xAs. The composition at which the bands cross is determined from photoluminescence of samples grown by molecular-beam epitaxy very close to crossover at x ≈ 0.4. The use of resonant laser excitation and the improved sample linewidth allows excitation intensities as low as 10−2 W/cm2, giving a precise determination of the bound exciton transition energies and their Γ and X crossover. Photoluminescence excitation spectroscopy is then used to measure the binding energies of the donor-bound excitons and the Γ free exciton binding energy. After correcting for the Γ- and X-dependence of these quantities, the crossover of the bandgap is determined to be at x = 0.401 and E = 2.086 eV.

Determination of the direct to indirect bandgap transition composition in AlxIn1−xP

AlxIn1−xP semiconductor alloys grown by metalorganic chemical vapor deposition on InGaAs graded buffer layers with varied aluminum compositions that span the transition from a direct to indirect semiconductor alloy are explored. The direct and indirect band gap transitions are observed in a single AlxIn1−xP sample with 40.8% allowing for a precise determination of the direct-indirect cross-over composition, xc. The direct and indirect nature of observed luminescence peaks is verified using time-resolved photoluminescence. At low temperatures, xc is determined to be 40.5% at a corresponding direct band gap energy of 2.34 eV.

Time-dependent ferroelectric transition in Pb(1−x)(Zr0.4Ti0.6)(1−x/4)O3 − xLa system

Ferroelectric transition involves tiny shift of ions within unit cell, thus being intrinsically a very fast process without apparent time-dependence. Contrary to this conventional wisdom, here we report a time-dependent ferroelectric transition, which occurs in hours. The system studied was Pb(1−x)(Zr0.4Ti0.6)(1−x/4)O3 − xLa system with relaxor-forming dopant La3+. The time-dependent ferroelectric transition occurs at the ferroelectric/relaxor crossover composition range of 0.09 &amp;lt; x ≤ 0.16. In these compositions, in situ Raman spectroscopy and transmission electron microscopy reveal very slow growth of ferroelectric phase. Dielectric measurement shows isothermal kinetics of the transition. The slow ferroelectric transition can be understood as being caused by the slowing-down of the otherwise fast growth of polar nano-domains due to the random local field caused by La3+, so that long time is needed to achieve long-range order macroscopic ferroelectric phase.

Evidence for crossover martensite in Ti50Ni45Fe5: An intermediate state between normal martensite and strain glass

We report that the crossover from normal martensite to strain glass in the TiNi-Fe alloy system does not proceed directly as indicated by previous studies but is actually mediated by an intermediate state named crossover martensite, appearing in the crossover composition of Ti50Ni45Fe5. The martensitic transition from the parent phase to the crossover martensite is a thermodynamic phase transition, being similar to a normal martensitic transition. However, its produced crossover martensite shows an obvious kinetic feature such as a time-dependent behavior in internal friction (aging effect), resembling that of the strain glass. The microstructure of the crossover martensite exhibits a special morphology of coexisting macro-sized martensite plates and nano-sized martensitic domains. The displaying of both thermodynamic and kinetic features in the crossover martensite demonstrates that it is an intermediate state between normal martensite and strain glass; this finding provides a complete physical picture for the crossover phenomenon from normal martensite to strain glass.

Direct-indirect crossover in GaxIn1-xP alloys

The energy and composition of the direct to indirect bandgap crossover in GaxIn1-xP significantly influences its potential for optoelectronic devices, such as solar cells and light emitting diodes, however considerable discrepancies still remain in the literature with regard to the precise value of the crossover composition xc. We revisit this issue in GaxIn1-xP films epitaxially grown on GaAs substrates. Observation of concurrent yet distinct direct and indirect transitions in Ga0.719In0.281P at 2 K using time integrated and time resolved photoluminescence studies places the crossover very near the composition xC = 0.71.

Randomized self-stabilizing and space optimal leader election under arbitrary scheduler on rings

We present a randomized self-stabilizing leader election protocol and a randomized self-stabilizing token circulation protocol under an arbitrary scheduler on anonymous and unidirectional rings of any size. These protocols are space optimal. We also give a formal and complete proof of these protocols. To this end, we develop a complete model for probabilistic self-stabilizing distributed systems which clearly separates the non deterministic behavior of the scheduler from the randomized behavior of the protocol. This framework includes all the necessary tools for proving the self- stabilization of a randomized distributed system: definition of a probabilistic space and definition of the self-stabilization of a randomized protocol. We also propose a new technique of scheduler management through a self-stabilizing protocol composition (cross-over composition). Roughly speaking, we force all computations to have a fairness property under any scheduler, even under an unfair one.

Melt Crystallization of Nanocomposites of Poly(Vinylidene Fluoride) with OMS

ABSTRACTIn our previous work [1] we investigated the impact of cold-crystallization on the structure of nanocomposites of PVDF with Lucentite STN™ OMS and observed the crossover composition in which the crystallographic beta phase dominated over alpha phase. Here, melt crystallization of PVDF/OMS nanocomposites was studied in the range of 0 to 1.0 wt% of OMS. In crystallization from melt, a decrease in crystallinity index occurs as a result of an increase in OMS. While beta phase fraction increased with an increase of OMS content in the range of interest, the amount of alpha crystals was found to be dominant even at high OMS compositions. At 1.0 wt% of OMS, beta crystals had reached at most 30% of the total crystallinity. Polarizing optical microscopy (POM) studies showed smaller, less birefringent spherulites with higher melting temperature compared to alpha spherulites appeared with OMS addition. Growth rate of these weakly birefringent spherulites is smaller than that of the strongly birefringent alpha spherulites. The increase in size and amount of the weakly birefringent spherulites with an increase of OMS and their higher melting point suggests these spherulites may be in the beta crystallographic phase.

Mixed-alkali effect in Na–Rb borate glasses: A tracer diffusion and electrical conductivity study

The ionic transport in a series of 0.2 [X Na2O·(1 − X)Rb2O]·0.8B2O3 glasses has been studied by measuring both the electrical conductivity of these glasses and the tracer diffusivities of 22Na and 86Rb as functions of composition and temperature. At constant temperature (380 °C) the tracer diffusivities of 22Na and 86Rb decrease monotonically with decreasing Na- or Rb-content, respectively. The tracer diffusivities as functions of the composition parameter X intersect near X = 0.2 (‘crossover composition’). This behaviour gives rise to a minimum in the conductivity which appears near X = 0.4. At the crossover composition X = 0.2 the diffusivities of 22Na and 86Rb are practically the same within the measured temperature range resulting in similar Arrhenius parameters. For each other investigated composition the Arrhenius parameters for 22Na and 86Rb diffusion differ significantly. Comparing the 22Na-diffusivity in the pure Na-borate glass with the corresponding conductivity diffusion coefficient calculated via the Nernst–Einstein equation from the measured conductivity yields a temperature-independent Haven ratio. In contrast, a temperature-dependent Haven ratio was found in the pure Rb-borate glass. In order to compare the tracer diffusivities in mixed-alkali glasses with their ionic conductivity we introduce a ‘common Haven ratio’ for the mixed-alkali glasses which coincides with the conventional definition of the Haven ratio for single-alkali glasses. On the Rb-rich side the common Haven ratio decreases with temperature for Na–Rb borate glasses (X = 0.2 and X = 0.4) similar to the Haven ratio in the pure Rb borate glass.

Compositional evolution of structural phase transitions in sodium niobates

Li x Na 1− x NbO 3 ceramics with 5⩽ x⩽12% have been studied in the temperature range RT–773 K through elastic, thermal expansion and dielectric measurements as a function of the temperature. Thermal expansion behaviour shows single, (for x⩽8%) or double, (for x⩾10%), reversible anomalies in the complete thermal cycle. The elastic behaviour shows single, for x=5%, or double and reversible anomalies, for x⩾8%. In both cases the thermal behaviours scarcely depend on the thermal rate. The dielectric permitivity shows reversible single anomalies in the phase transition temperature for x⩽8% and double non-reversible anomalies for x⩾8% that are very dependent on the thermal rate and thermal history of the sample. The crossover composition for the appearance of a double anomaly can be established close to x=8%. The main (FE–PE) phase transitions and their temperatures have been established using three measuring techniques.

Electron transport across bulk (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P barriers determined from the I-V characteristics of n-i-n diodes measured between 60 and 310 K

The electron transport characteristics of five n-i-n diodes with (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P intrinsic barrier regions of various aluminum composition x were determined from the measured I-V characteristics between 60 and 310 K. From these measurements, three different transport regimes were identified. Fowler-Nordheim tunneling was observed at temperatures below 215, 260, 110, 150, and 120 K for aluminum compositions of x=0.4, 0.5, 0.6, 0.7, and 1.0, respectively, with applied electric fields in excess of 5 MV/m. The temperature dependence of the Fowler-Nordheim tunneling currents is shown in AlGaInP for the first time with direct bandgap AlGaInP exhibiting a strong linear decrease in apparent barrier height with increasing temperature. The measured barrier height using the thermionic emission model yields values close to the expected conduction band offset between the GaInP spacer layers and the AlGaInP intrinsic barriers, as measured using high-pressure photoluminescence, and provides a novel technique for measuring the direct-indirect crossover composition in AlGaInP. It is shown that the lowest lying conduction band in AlGaInP is the dominant barrier to electron transport. This has important implications for the design of AlGaInP laser diodes.

OptimalTCin Layered Manganites: Different Roles of Coherent and Incoherent Lattice Distortions

Neutron powder diffraction experiments on layered manganites ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}\phantom{\rule{0ex}{0ex}}(0.32\ensuremath{\le}x\ensuremath{\le}0.40)$ show that coherent lattice anomalies observed at the Curie temperature (lattice parameters, Mn-O bond lengths) reverse sign at $x\ensuremath{\approx}0.36$, coinciding with the maximum ${T}_{C}\ensuremath{\approx}131\mathrm{K}$. Remarkably, the anomalies are completely suppressed at this crossover composition. Incoherent distortions, as measured by anisotropic Debye-Waller factors, undergo an abrupt decrease below ${T}_{C}$. The magnitude of this anomaly changes very little with $x$ and does not display any sign reversal for $x\ensuremath{\approx}0.36$.

Doping properties of C, Si, and Ge impurities in GaN and AlN

Doping properties of substitutional C, Si, and Ge impurities in wurtzite GaN and AlN were studied by quantum molecular dynamics. We considered incorporation of impurities on both cation and anion sublattices. When substituting for cations, Si and Ge are shallow donors in GaN, while Ge becomes a deep donor in AlN. Both impurities are deep acceptors on the N site. Substitutional C${}_{\mathrm{cation}}$ is a shallow donor in GaN, but a deep one in AlN; C${}_{\mathrm{N}}$ is a relatively shallow acceptor in both materials. Two effects that potentially quench doping efficiency were investigated. The first one is the transition of a donor from a substitutional position to a $\mathrm{DX}$-like configuration. In crystals with a wurtzite symmetry, there are two possible variants of a $\mathrm{DX}$-like state, and they have substantially different properties. In GaN, ${\mathrm{DX}}^{\ensuremath{-}}$ states of both Si and Ge are unstable, or metastable, and thus they do not affect doping efficiency. In contrast, they are stable in AlN, and therefore neither Si nor Ge is a dopant in this material. Estimates obtained for Al${}_{x}$Ga${}_{1\ensuremath{-}x}$N alloys show that the crossover composition for $\mathrm{DX}$ stability is much lower for Ge $(x\ensuremath{\simeq}0.3)$ than for Si $(x\ensuremath{\simeq}0.6)$. The second effect quenching the doping efficiency is self-compensation, i.e., simultaneous incorporation of impurity atoms on both cation and anion sublattice. This effect may be enhanced by the formation of nearest-neighbor donor-acceptor pairs. The calculated binding energies of such pairs are large, about 1 eV, influencing self-compensation in some cases. Finally, the computed formation energies are used to identify growth conditions under which all these impurities may be efficient dopants in wide-band-gap nitrides.