Heterovalent alloying is a promising strategy to lower the coercive field of wurtzite ferroelectrics, yet it is often presumed to introduce electronic conductivity. Here, we demonstrate that the insulating character of Al1−xMxN alloys (M is a heterovalent cation) is controlled by charge localization rather than charge compensation by Al vacancies. Hybrid DFT calculations of Al1−xHfxN show that Hf d orbitals hybridize with N(p) orbitals to form deep localized states that both preserve insulation and reduce the polarization switching barrier. This mechanism is indirectly supported by the predicted low switching barrier at x = 0.056, in excellent agreement with experimental reports of ferroelectric switching near ∼6 at.% Hf. The behavior generalizes across group-4 heterovalent cations: the localized state becomes deeper from 5d ≈ 4d → 3d, leading to progressively stronger localization for Hf, Zr, and Ti. In contrast, Mg and Si, which lack d orbitals, produce relatively shallower and more delocalized states that increases electronic leakage. These findings highlight that strict charge neutrality (isovalent alloying) is not required to maintain insulation in ferroelectric nitrides and establish charge localization as the key design principle for heterovalent alloying. They also provide a unifying explanation for the well-known challenges of doping wideand ultrawide-bandgap nitride semiconductors. This insight expands the accessible chemical design space for wurtzite ferroelectrics.

Wide-and ultrawide-band gap wurtzite III-nitrides are commonly alloyed with group-3 elements such as Sc, Y, and Gd to reduce the polarization switching coercive field ( E c ) for ferroelectrics. Because group-3 nitrides typically adopt the rocksalt structure with relatively small band gaps, alloying is generally expected to reduce both the band gap ( E g ) and dielectric breakdown field ( E b ). However, recent reports of band-gap widening in Ga 1−x Sc x N and In 1−x Sc x N challenge this assumption. Using DFT calculations, we show that band gaps in heterostructural III-nitride alloys is governed not only by the ground-state polymorph but also by structurally relevant polymorphs of the alloying end member. Extending the analysis to AlN-, GaN-, and InN-based alloys with Sc, Y, La, and B, we identify three distinct bandgap regimes that follow wurtzite-, hexagonal-BN-, or rocksalt-like trends. These behaviors are governed by bond ionicity and structural distortion, providing design guidelines for engineering E g and E b in wurtzite III-nitride alloys.

Ternary nitride alloys based on wurtzite AlN are a promising platform to realize functional materials, particularly ferroelectrics and optical emitters, that can smoothly integrate with conventional microelectronics. Here, a strategic design is presented to enable multifunctional materials by substituting multiple elements into AlN to create quaternary nitride alloys. By combining computational predictions and combinatorial thin film synthesis, the phase diagram of these quaternary Al-Sc-Gd-N alloys (or pseudo-ternary heterostructural AlN-ScN-GdN alloys) is successfully predicted as a function of effective temperature, and we experimentally grow Al 1 - x - y Sc x Gd y N ${\rm Al}_{1-x-y}{\rm Sc}_x{\rm Gd}_y{\rm N}$ thin films for the first time. It is revealed that Al 1 - x - y Sc x Gd y N ${\rm Al}_{1-x-y}{\rm Sc}_x{\rm Gd}_y{\rm N}$ crystallizes in a wurtzite-derived structure for x + y ≲ 0.35 $x+y \lesssim 0.35$ , consistent with the calculated phase diagram. The computational investigation explores whether co-substitution induces cooperative effects on these alloys' piezoelectric and ferroelectric properties, finding that it is beneficial for reducing the polarization switching barrier. We calculate that Al 1 - x - y Sc x Gd y N ${\rm Al}_{1-x-y}{\rm Sc}_x{\rm Gd}_y{\rm N}$ thin films should display ferroelectric switching. This is supported by our experimental measurements of a high optical bandgap, enhanced piezoelectric coefficient, and a change in the calculated polarization switching mechanism, and we achieve preliminary ferroelectric switching that experimentally realizes the prediction. Overall, our work sets the foundation toward quaternary wurtzite-nitride-based multifunctional materials, including piezoelectrics, ferroelectrics, and possibly even multiferroics.

Bi2SeO2 is a promising n-type semiconductor to pair with p-type BiCuSeO in a thermoelectric (TE) device. The TE figure of merit zT and, therefore, the device efficiency must be optimized by tuning the carrier concentration. However, electron concentrations in self-doped n-type Bi2SeO2 span several orders of magnitude, even in samples with same nominal compositions. Such unsystematic variations in the electron concentration has a thermodynamic origin related to the variations in native defect concentrations. In this study, we use first-principles calculations to show that the selenium vacancy, which is the source of n-type conductivity in Bi2SeO2, varies by 1-2 orders of magnitude depending on the thermodynamic conditions. We predict that the electron concentration can be enhanced by synthesizing under more Se-poor conditions and/or at higher solid-state reaction temperatures (T_SSR), which promote the formation of selenium vacancies without introducing extrinsic dopants. We validate our computational predictions through solid-state synthesis of Bi2SeO2. We observe more than two orders of magnitude increase in the electron concentration simply by adjusting the synthesis conditions. Additionally, we reveal the significant effect of grain boundary scattering on electron mobility in Bi2SeO2, which can also be controlled by adjusting T_SSR. By simultaneously optimizing the electron concentration and mobility, we achieve a zT of ~0.2 at 773 K for self-doped n-type Bi2SeO2. Our study highlights the need for careful control of thermodynamic growth conditions and demonstrates TE performance improvement by varying synthesis parameters according to thermodynamic guidelines.

The Na3SbS4 solid electrolyte has achieved high ionic conductivity (sigma ion ) exceeding 10 mS/cm through aliovalent doping. While sigma ion is enhanced through aliovalent doping, a rigorous atomic-scale mechanistic explanation is needed. Doping also affects sigma ion by modifying ion mobility-an effect that is not well understood and often overlooked. We use first-principles defect calculations to mechanistically explain and quantify the increase/decrease in Na vacancy concentration due to aliovalent doping of Na3SbS4. By focusing on isovalent doping, we reveal local and global structural effects of doping on the migration barrier and, therefore, ion mobility. In conjunction with experiments, we demonstrate the interplay between the local and global effects. Doping with heavier anions to achieve more polarizable frameworks is a common approach to enhancing sigma ion . Our findings present a unique approach to enhancing sigma ion by doping with smaller and lighter cations that soften the parent framework.

The 2019 report of ferroelectricity in (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] broke a long-standing tradition of considering AlN the textbook example of a polar but non-ferroelectric material. Combined with the recent emergence of ferroelectricity in HfO2-based fluorites [Böscke et al., Appl. Phys. Lett. 99, 102903 (2011)], these unexpected discoveries have reinvigorated studies of integrated ferroelectrics, with teams racing to understand the fundamentals and/or deploy these new materials—or, more correctly, attractive new capabilities of old materials—in commercial devices. The five years since the seminal report of ferroelectric (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] have been particularly exciting, and several aspects of recent advances have already been covered in recent review articles [Jena et al., Jpn. J. Appl. Phys. 58, SC0801 (2019); Wang et al., Appl. Phys. Lett. 124, 150501 (2024); Kim et al., Nat. Nanotechnol. 18, 422–441 (2023); and F. Yang, Adv. Electron. Mater. 11, 2400279 (2024)]. We focus here on how the ferroelectric wurtzites have made the field rethink domain walls and the polarization reversal process—including the very character of spontaneous polarization itself—beyond the classic understanding that was based primarily around perovskite oxides and extended to other chemistries with various caveats. The tetrahedral and highly covalent bonding of AlN along with the correspondingly large bandgap lead to fundamental differences in doping/alloying, defect compensation, and charge distribution when compared to the classic ferroelectric systems; combined with the unipolar symmetry of the wurtzite structure, the result is a class of ferroelectrics that are both familiar and puzzling, with characteristics that seem to be perfectly enabling and simultaneously nonstarters for modern integrated devices. The goal of this review is to (relatively) quickly bring the reader up to speed on the current—at least as of early 2025—understanding of domains and defects in wurtzite ferroelectrics, covering the most relevant work on the fundamental science of these materials as well as some of the most exciting work in early demonstrations of device structures.

Point defects are ubiquitous in materials and play a critical role in determining their electrical, optical, ion-transport, and other properties, making them key to the performance of materials in energy applications. First-principles defect calculations, particularly those based on density functional theory, are powerful tools for predicting defect energetics and associated electronic properties. However, the results are often communicated through defect formation energy diagrams and defect level diagrams, which can be challenging to interpret for researchers who do not specialize in defect physics. This Tutorial provides a practical accessible guide for understanding and interpreting these diagrams, with a focus on materials with a band gap—semiconductors and insulators. Aimed at experimentalists and computationalists new to defect analysis, the Tutorial explains how to extract key physical insights from defect diagrams, such as defect and carrier concentrations, dopability, midgap states, and charge localization. By demystifying the language and visuals of defect calculations, this guide seeks to bridge the gap between theory and experiments, enabling broader use of defect data in the rational design of functional materials for energy applications and beyond.

Highly alloyed (Al,Gd)N is of potential interest in a variety of applications, including neutron detection and in devices such as non-volatile memory. Gd has been shown to have very low equilibrium solubility in AlN at room temperature; however, non-equilibrium deposition techniques such as sputtering are able to deposit thin films, which incorporate large amounts of Gd. Here, we characterize a highly-alloyed (Al,Gd)N combinatorial thin film grown by RF sputtering on a GaN substrate, looking for any evidence of chemical or phase segregation or structural disorder in the films. Compositions with between 13% and 32% Gd (on a cation basis) were studied. No evidence was found for chemical or phase segregation in any studied composition. Higher degrees of Gd incorporation led to greater structural disorder in the film and a tendency toward amorphization; however, electron diffraction shows that the film does not become fully amorphous at any of the studied compositions, instead retaining textured local order even at 32% Gd. Electron energy loss spectra suggest that the material retains a locally wurtzite-like tetrahedral bonding environment at all studied compositions.

Understanding the structure-property relationship in glass solid electrolytes (SEs) remains a major challenge due to their inherent disorder and the difficulty of probing local structures, particularly in relation to oxygen incorporation. Despite recent interest in multianion halide solid electrolytes, there are few systematic studies on how varying the oxygen content affects the local structure and ion transport. Here, we investigate a series of amorphous sodium oxychloride SEs with the composition xNa2O-TaCl5 (0.1 ≤ x ≤ 1.5), revealing three distinct conductivity regimes and achieving a maximum of 4.1 mS cm-1 at room temperature. Synchrotron and lab X-ray total scattering and Raman spectroscopy indicate the gradual formation of Ta-O-Ta bonds that bridge the two or more metal chloride polyhedra, while ab initio molecular dynamics simulations clarify the distinct roles of bridging and nonbridging O2- species. These findings not only provide mechanistic insights into oxygen-mediated glass formation but also establish guiding principles for multianion engineering in the design of next-generation solid electrolytes.

A large number of Zintl phases have been discovered by solid-state chemists driven by empirical knowledge, chemical intuition and in some cases, through serendipitous accidents. These discoveries have only scratched the surface, given the vast compositional and structural diversity that Zintl phases can accommodate. The large chemical space of Zintl phases, as well as intermetallic compounds in general, remain under-explored. Here, we use graph neural networks and the upper bound energy minimization approach to efficiently scan a large chemical space of >90 000 hypothetical Zintl phases and accurately discover 1810 new thermodynamically stable phases with 90% precision, as validated with first-principles calculations. We show that our approach is more than 2x more accurate in predicting DFT stability than M3GNet (40% precision) on the same dataset. Using a random forest model and SHAP analysis, we demonstrate the critical role of ionic bonding in the thermodynamic stability of Zintl phases. Our results not only expand the known chemical landscape of Zintl phases but also highlight the efficacy of machine learning frameworks combined with domain knowledge in uncovering chemically meaningful insights across complex intermetallics.

Wurtzite ferroelectrics (FEs) are promising candidates for next-generation memory and computing devices due to their compatibility with semiconductor processing. However, their adoption is limited by large coercive electric fields (E-c), often approaching the dielectric breakdown field (E-b), raising concerns about energy efficiency and device reliability. The discovery of wurtzite FEs has also been constrained by a narrow chemical design space, with few known examples to date. Ferroelectricity in AlN-based alloys, particularly Al1-xMxN with trivalent M3+ cations, has been a focal point of recent work. Building on studies of co-alloyed AlN for enhanced piezoelectricity, we computationally investigate ferroelectricity in Al1-x(M1;M2)(x)N alloys, where M1(2+)and M2(4+) are non-trivalent cations. Using density functional theory, solid-state nudged elastic band method, and structural analysis, we predict switchable polarization in Al1-x(Mg,Hf)(x)N. Compared to the prototypical Al1-xScxN, this co-alloy exhibits a more rapid decrease in both the switching barrier and bandgap with increasing x, suggesting a simultaneous reduction in E-c and E-b. This reduction in E-c is attributed to enhanced structural distortions introduced by co-alloying. By using bandgap and distortion as design metrics, we identify several other promising M1-M2 combinations, and highlight Al1-x(Ca,Si)(x)N as a strong candidate for experimental validation. Our work introduces a co-alloying strategy to access new wurtzite FEs and expands the design space to include earth-abundant elements.

Optical properties of insulators can be dominated by trace-level impurities and/or other defects that maybe difficult to identify using direct chemical analysis. The present work investigates the origins of coloration in BaSO4 powders following exposure to high energy photons (1.33 and 1.17 MeV gamma from a 60 Co source)using a through the combination ofexperimental and computational predictions of defect energies and a suite of complementary experimental techniques. We conclude that a slight greying observed in some BaSO4 powders is caused by activation of sulfate radicals that are stabilized by nearby barium vacancies or trace amounts of sodium substitution on the barium site. Electron-hole pairs activated by exposure to gamma irradiation and trapped by these defect couples lead to visible absorption centered around similar to 650 nm. Such trapped electronic defects are stable at room temperature but can be 'healed' byeither thermal annealing above 300 degrees Cor ultraviolet bleaching. Such gamma-induced colorationwill therefore have has no impact on the mass density that dominates radiopacity nor on the chemical or physical properties of BaSO4and is not an indication of any sort of toxic contamination.

Understanding the structure-property relationship in glass solid electrolytes (SEs) remains a major challenge due to their inherent disorder and the difficulty of probing local structures, particularly in relation to oxygen incorporation. Despite recent interest in multianion halide solid electrolytes, there are few systematic studies on how varying the oxygen content affects the local structure and ion transport. Here, we investigate a series of amorphous sodium oxychloride SEs with the composition xNa2O-TaCl5 (0.1 ≤ x ≤ 1.5), revealing three distinct conductivity regimes and achieving a maximum of 4.1 mS cm-1 at room temperature. Synchrotron and lab X-ray total scattering and Raman spectroscopy indicate the gradual formation of Ta-O-Ta bonds that bridge the two or more metal chloride polyhedra, while ab initio molecular dynamics simulations clarify the distinct roles of bridging and nonbridging O2- species. These findings not only provide mechanistic insights into oxygen-mediated glass formation but also establish guiding principles for multianion engineering in the design of next-generation solid electrolytes.

Thin films of aluminum hafnium nitride (Al_1-xHf_xN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard X-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs. electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d_33,f,meas measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent–and not just trivalent–alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.

Thin films of aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) were synthesized via reactive magnetron sputtering for Hf contents up to $x$ = 0.13. X-ray diffraction showed a single $c$-axis oriented wurtzite phase for all films. Hard X-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs. electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d$_{33,f,meas}$ measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent--and not just trivalent--alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors, such as Si, SiC, and III–Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al1−xGdxN alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction x with x > 0.10–0.15. Experimentally, ferroelectric switching is observed at room temperature in Al1−xGdxN films with x > 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously [Lee et al., Sci. Adv. 10, eadl0848 (2024)]. This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials.

Thin films of aluminum hafnium nitride (Al1-xHfxN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard x-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d(33,f,meas) measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent-and not just trivalent-alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/). https://doi.org/10.1063/5.0271563

Ferroelectricity was long considered incompatible with the wurtzite structure, but the recent discovery of switchable polarization in wurtzite alloys has renewed interest in these materials for integrated electronic and memory applications. The development of wurtzite ferroelectrics faces significant technological challenges, which can be addressed through a fundamental physical understanding of their dielectric and ferroelectric properties. This article focuses on the physics that govern the polarization switching behavior, emphasizing the atomic- and meso-scale (domain) mechanisms involved in the transition between polarization states. A distinguishing feature of this article is a deep dive into the role of intrinsic and extrinsic defects—an area that has received limited attention in prior reviews, but is increasingly recognized as central to polarization switching, coercive fields, leakage, and fatigue. We highlight how defect behavior evolves during processing and electrical cycling, often contributing to long-term degradation. We also introduce powerful first-principles defect calculations, common in semiconductors but not yet widespread in ferroelectrics, as tools to understand and design materials. By integrating recent theoretical and experimental insights, we aim to provide a framework for advancing wurtzite ferroelectrics.

Raman spectroscopy is widely applied in identifying local structures in materials, but the interpretation of Raman spectra is non-trivial. An accurate computational database of reference spectra calculated with a consistent level of theory can significantly aid in interpreting measured Raman spectra. Here, we present a database of Raman spectra of inorganic compounds calculated with accurate hybrid functionals in density functional theory. Raman spectra were obtained by calculating dynamical matrices and polarizability tensors for structures from the Inorganic Crystal Structure Database. The calculated Raman spectra and other phonon properties (e.g., infrared spectra) are stored in a MongoDB database publicly shared through a web application. We assess the accuracy of our Raman calculations by statistically comparing ~80 calculated spectra with an existing experimental Raman database. To date, the database contains 161 compounds and is continuously growing as we add more materials computed with our automated workflow.