In Pursuit of the Exceptional: Research Directions for Machine Learning in Chemical and Materials Science.

Exceptional molecules and materials with one or more extraordinary properties are both technologically valuable and fundamentally interesting, because they often involve new physical phenomena or new compositions that defy expectations. Historically, exceptionality has been achieved through serendipity, but recently, machine learning (ML) and automated experimentation have been widely proposed to accelerate target identification and synthesis planning. In this Perspective, we argue that the data-driven methods commonly used today are well-suited for optimization but not for the realization of new exceptional materials or molecules. Finding such outliers should be possible using ML, but only by shifting away from using traditional ML approaches that tweak the composition, crystal structure, or reaction pathway. We highlight case studies of high-Tc oxide superconductors and superhard materials to demonstrate the challenges of ML-guided discovery and discuss the limitations of automation for this task. We then provide six recommendations for the development of ML methods capable of exceptional materials discovery: (i) Avoid the tyranny of the middle and focus on extrema; (ii) When data are limited, qualitative predictions that provide direction are more valuable than interpolative accuracy; (iii) Sample what can be made and how to make it and defer optimization; (iv) Create room (and look) for the unexpected while pursuing your goal; (v) Try to fill-in-the-blanks of input and output space; (vi) Do not confuse human understanding with model interpretability. We conclude with a description of how these recommendations can be integrated into automated discovery workflows, which should enable the discovery of exceptional molecules and materials.

Twists and Puckers: Tuning Crystal Chemistry in the La(AuxGe1-x)2 Compositional Series.

The physical properties of solid-state materials are closely tied to their crystal structure, yet our understanding of how competing structural arrangements energetically compare is limited. In this work, we explore how small differences in composition affect the structure in the La(AuxGe1-x)2 series of compounds, which comprises four unique structure types between LaGe2 and LaAu2. This family includes the previously unknown AlB2-type compound with stoichiometry La(Au0.375Ge0.625)2 as well as La(Au0.25Ge0.75)2, an intergrowth of the AlB2 and ThSi2 structure types. We then study the chemical forces driving the structure changes and use phonon band structure calculations and DFT-Chemical Pressure to evaluate atomic-size effects. These calculations show that the parent AlB2 structure type is disfavored in Au-rich compounds due to soft atomic motions along the c axis. The instability of AlB2-type LaAuGe is confirmed by the presence of imaginary modes in the phonon band structure that correspond to a "puckering" of the hexagonal AlB2-type lattice, resulting in the experimentally observed LiGaGe structure type. The impact of size effects is less clear for Au-poor compositions; instead, twisting the AlB2 structure type to form the ThSi2 type opens a pseudogap at the Fermi level in the electronic density of states. This investigation demonstrates how crystal structure in solid-state materials can be compositionally tuned based on balancing size and electronics when multiple structure types are in close thermodynamic competition.

From Laves Phases to Quasicrystal Approximants in the Na-Au-Cd System.

The exploratory synthesis of gold-based polar intermetallic phases has revealed many new compounds with unprecedented crystal structures, unique bonding arrangements, and interesting electronic features. Here, we further understand the complexity of gold's crystal chemistry by studying the Na-Au-Cd ternary composition space. A nearly continuous structure transformation is observed between the seemingly simple binary NaAu2-NaCd2 phases, yielding three new intermetallic compounds with the compositions Na(Au0.89(5)Cd0.11(5))2, Na(Au0.51(4)Cd0.49(4))2, and Na8Au3.53(1)Cd13.47(1). Two compounds adopt different Laves phases, while the third crystallizes in a complex decagonal quasicrystal approximant. All three compounds are related through Friauf-Laves polyhedral building units with the gold/cadmium ratio found to control the transition among the unique crystal structures. Electronic structure calculations subsequently revealed the metallic nature of all three compounds with a combination of polar covalent Na-(Au/Cd) interactions and covalent (Au/Cd)-(Au/Cd) bonding interactions stabilizing each structure. These results highlight the crystal and electronic structure relationship among Laves phases and quasicrystal approximants enabled by the unique chemistry of gold.

Treating Superhard Materials as Anomalies.

Superhard materials are among the most scarce functional inorganic solids in existence. Indeed, recent research suggested that less than 0.1% of all known materials are likely to have a Vickers hardness ≥40 GPa. Here, an anomaly detection framework is created to treat these materials as rare occurrences by encoding and reconstructing the input composition and crystal structure information without supervision. The resulting model can quantitatively identify outliers from "normal" behaving materials, leading to the discovery of materials with exceptional properties such as a superhard response. Moreover, examining the difference between the encoded and decoded crystal structure provides fundamental insights into the crystal-chemical origin of hardness. The presented methodology is ultimately generalizable, enabling the design of other outlier materials with rare and unexpected physical properties.

Spectral Design of Phosphor-Converted LED Lighting Guided by Color Theory.

In the race to develop new luminescent materials for the next generation of light-emitting-diode (LED)-based solid-state lighting and display applications, it is often forgotten that color theory and human perception should be some of the principal factors guiding materials design. In this Viewpoint, we explore some of the antiquated colorimetrics established originally for incandescent and fluorescent lighting and discuss how they are still widely applied in the literature today to interpret the color quality of luminescent materials, like inorganic phosphors and quantum dots, and to analyze prototype devices, despite their shortcomings. We then shift our analysis toward contemporary ideas in color theory that more accurately describe the color quality of modern LED light bulbs and flat-panel displays. Finally, the perspective examines the opportunities and challenges of applying these new concepts to guide the design of luminescent materials used in LED-based applications.

The Role of the Bi3+ Lone Pair Effect in Bi(H3O)(SO4)2, Bi(HSO4)3, and Bi2(SO4)3.

Three new members in the Bi2O3-SO3-H2O system are identified by single crystal X-ray diffraction and Rietveld refinement after a fundamental examination of this phase space. Bi(H3O)(SO4)2 crystallizes in space group P21/c (no. 14, a = 1203.5(4), b = 682.9(2), c = 821.2(2) pm, ß = 102.99(1)°, 861 independent reflections, 88 refined parameters, wR2 = 0.14) homeotypic with Nd(H3O)(SO4)2 featuring edge-sharing BiO9 polyhedra. Bi(HSO4)3 crystallizes in a new structure type in space group P1 (no. 2, a = 492.04(7), b = 910.8(1), c = 1040.8(2) pm, α = 85.443(5)°, ß = 86.897(5)°, γ = 74.542(4)°, 3227 independent reflections, 154 refined parameters, wR2 = 0.05) comprising dimers of edge-sharing BiO8 polyhedra. For Bi2(SO4)3, a new modification crystallizing in space group P21/n (no. 14, a = 1308.03(7), b = 473.25(3), c = 1452.61(8) pm, ß = 100.886(2)°, 3189 independent reflections, 155 refined parameters, wR2 = 0.03) isotypic to Sb2(SO4)3 with noncondensed BiO7 polyhedra is presented. The role of the Bi3+ lone pair effect as elucidated by density functional theory (DFT) calculations is discussed for all three compounds with respect to their structural and optical properties. Additionally, the Bi3+ lone pair activity is compared to the recently reported borosulfates Bi(H3O)[B(SO4)2]4 and Bi2[B2(SO4)6]. Geometrical calculations based on structural data are correlated with electron localization function (ELF) calculations to establish the origin of the direction and strength of the lone pair stereoactivity of Bi3+ in oxidic compounds. Finally, the thermal properties of the three compounds are reported.

Phosphorescence in Mn4+-Doped R+/R2+ Germanates (R+ = Na+ or K+, R2+ = Sr2+).

Single crystals of three new compounds, Na0.36Sr0.82Ge4O9 (1, proposed composition), Na2SrGe6O14 (2), and K2SrGe8O18 (3), were obtained and characterized using single-crystal X-ray diffraction. Their structures contain three-dimensional (3D) anionic frameworks built from GeO4 and GeO6 polyhedra. The presence of octahedral Ge4+ sites makes the new phases suitable for Mn4+ substitution to obtain red-emitting phosphors with a potential application for light conversion. Photoluminescence properties of Mn4+-substituted Na2SrGe6O14 (2) and K2SrGe8O18 (3) samples were studied over a range of temperatures, and red light photoluminescence associated with the electronic transitions of tetravalent manganese was observed. The Na2SrGe6O14 (2) phase was also substituted with Pr3+ on the mixed Na-Sr site similar to the previously studied Na2CaGe6O14:Pr3+. The red emission peak of the Pr3+ activator occurs at a shorter wavelength (610 nm) compared to that of Mn4+ (662-663 nm). Additionally, second harmonic generation (SHG) data were collected for the noncentrosymmetric Na2SrGe6O14 (2) phase, indicating weak SHG activity. Diffuse reflectance spectroscopy and density of states calculations were performed to estimate the band gap values for pristine Na2SrGe6O14 (2) and K2SrGe8O18 (3) phases.

Sensitive and Reliable Fluorescent Thermometer Based on a Red-Emitting Li2MgHfO4:Mn4+ Phosphor.

Contactless fluorescent thermometers are rapidly gaining popularity due to their sensitivity and flexibility. However, the development of sensitive and reliable non-rare-earth-containing fluorescent thermometers remains a significant challenge. Here, a new rare-earth-free, red-emitting phosphor, Li2MgHfO4:Mn4+, was developed for temperature sensing. An experimental analysis combined with density functional theory and crystal field calculations reveals that the sensitive temperature-dependent luminescence arises from nonradiative transitions induced by lattice vibration. Li2MgHfO4:Mn4+ also exhibits reliable recovery performance after 100 heating-cooling cycles due to the elimination of surface defects, which is rare but vital for practical application. This study puts forward a new design strategy for fluorescent thermometers and sheds light on the fundamental structure-property relationships that guide sensitive temperature-dependent luminescence. These considerations are crucial for developing next-generation fluorescence-based thermometers.

The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn4V2O9.

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn3(OH)2V2O7·nH2O and Zn5(OH)6(CO3)2 are obtained by simply introducing deionized H2O to the DES solution containing dissolved ZnO and V2O5. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn3V2O8 and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn4V2O9. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V4+) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.

Discovering Intermetallics Through Synthesis, Computation, and Data-Driven Analysis.

Intermetallics adopt an array of crystal structures, boast diverse chemical compositions, and possess exotic physical properties that have led to a wide range of applications from the biomedical to aerospace industries. Despite a long history of intermetallic synthesis and crystal structure analysis, identifying new intermetallic phases has remained challenging due to the prolonged nature of experimental phase space searching or the need for fortuitous discovery. In this Minireview, new approaches that build on the traditional methods for materials synthesis and characterization are discussed with a specific focus on realizing novel intermetallics. Indeed, advances in the computational modeling of solids using density functional theory in combination with structure prediction algorithms have led to new high-pressure phases, functional intermetallics, and aided experimental efforts. Furthermore, the advent of data-centered methodologies has provided new opportunities to rapidly predict crystal structures, physical properties, and the existence of unknown compounds. Describing the research results for each of these examples in depth while also highlighting the numerous opportunities to merge traditional intermetallic synthesis and characterization with computation and informatics provides insight that is essential to advance the discovery of metal-rich solids.

Thermally Robust and Color-Tunable Blue-Green-Emitting BaMgSi4O10:Eu2+,Mn2+ Phosphor for Warm-White LEDs.

The dual emission produced from Mn2+ when codoped with rare earth ions like Eu2+ or Ce3+ in inorganic compounds makes these materials attractive as efficient, color-tunable phosphors for warm-white solid-state lighting. Here, a series of efficient blue-green-emitting BaMgSi4O10:Eu2+,Mn2+ phosphors with thermally robust, tunable luminescence are reported. Steady-state and time-resolved photoluminescence spectroscopy reveal that Eu2+ and Mn2+ each occupy a single crystallographic site and confirm that energy transfer occurs from Eu2+ to Mn2+. The internal and external quantum efficiency of BaMgSi4O10:Eu2+,Mn2+ can reach as high as 69.0 and 47.5%, respectively, upon 360 nm excitation. Moreover, this phosphor possesses nearly zero-thermal quenching up to 440 K due to thermally induced electron detrapping. A fabricated UV-excited white LED device incorporating the blue-green-emitting BaMgSi4O10:Eu2+,Mn2+ and the red-emitting Sr2Si5N8:Eu2+ phosphors exhibits an excellent CRI of 94.3 with a correlated color temperature of 3967 K. These results prove the potential applications of Eu2+,Mn2+ codoped BaMgSi4O10 phosphor for generating warm-white light.

Combining experiment and computation to elucidate the optical properties of Ce3+ in Ba5Si8O21.

Complex alkaline earth silicates have been extensively studied as rare-earth substituted phosphor hosts for use in solid-state lighting. One of the biggest challenges facing the development of new phosphors is understanding the relationship between the observed optical properties and the crystal structure. Fortunately, recent improvements in characterization techniques combined with advances in computational methodologies provide the research tools necessary to conduct a comprehensive analysis of these systems. In this work, a new Ce3+ substituted phosphor is developed using Ba5Si8O21 as the host crystal structure. The compound is evaluated using a combination of experimental and computational methods and shows Ba5Si8O21:Ce3+ adopts a monoclinic crystal structure that was confirmed through Rietveld refinement of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence spectroscopy reveals a broad-band blue emission centered at ∼440 nm with an absolute quantum yield of ∼45% under ultraviolet light excitation (λex = 340 nm). This phosphor also shows a minimal chromaticity-drift but with moderate thermal quenching of the emission peak at elevated temperatures. The modest optical response of this phase is believed to stem from a combination of intrinsic structural complexity and the formation of defects because of the aliovalent rare-earth substitution. Finally, computational modeling provides essential insight into the site preference and energy level distribution of Ce3+ in this compound. These results highlight the importance of using experiment and computation in tandem to interpret the relationship between observed optical properties and the crystal structures of all rare-earth substituted complex phosphors.

Single-Crystal Automated Refinement (SCAR): A Data-Driven Method for Determining Inorganic Structures.

Single-crystal diffraction is one of the most common experimental techniques in chemistry for determining a crystal structure. However, the process of crystal structure determination and refinement is not always straightforward. Methods for simplifying and rationalizing the path to the most optimal crystal structure model have been incorporated into various data processing and crystal structure solution software, with the focus generally on aiding macromolecular or protein structure determination. In this work, we propose a new method that uses single-crystal data to determine the crystal structures of inorganic, extended solids called "single-crystal automated refinement" (SCAR). The approach was developed using data mining and machine learning methods and considers several structural features common in inorganic solids, like atom assignment based on physically reasonable distances, atomic statistical mixing, and crystallographic site deficiency. The output is a tree of possible solutions for the data set with a corresponding fit score indicating the most reasonable crystal structure. Here, the foundation for SCAR is presented followed by the implementation of SCAR to determine two newly synthesized and previously unreported phases, ZrAu0.5Os0.5 and Nd4Mn2AuGe4. The structure solutions are found to be comparable with those produced by manually solving the data set, including the same refined mixed occupancies and atomic deficiency, supporting the validity of this automatic structure solution method. The proposed SCAR program is thus verified as being a fast and reliable assistant in determining even complex single-crystal diffraction data for extended inorganic solids.

Unlocking the key to persistent luminescence with X-ray absorption spectroscopy: a local structure investigation of Cr-substituted spinel-type phosphors.

Developing new persistent luminescent phosphors, a unique class of inorganic materials that can produce a visible light emission lasting minutes to hours requires improving our understanding of their fundamental structure-property relationships. Research has shown that one of the most critical components governing persistent luminescence is the existence of lattice defects in a material. Specifically, vacancies and anti-site defects that coincide with substitution of the luminescent center, e.g., Eu2+ or Cr3+, are generally considered essential to generate the ultra-long luminescent lifetimes. This research solidifies the connection between defects and the remarkable optical properties. The persistent luminescent compound Zn(Ga1-xAlx)2O4 (x = 0-1), which adopts a spinel-type structure, is investigated by examining the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) at the Cr K and Zn K edges. This investigation reveals a structural distortion of the octahedrally coordinated main group metal site concurrent with increasing Al3+ content. Moreover, these results suggest there is a dependence between the local crystallographic distortions, the presence of defects, and a material's persistent luminescence. In combination, this work provides an avenue to understand the connection between the structure-defect-property relationships that govern the properties of many functional inorganic materials.

Machine Learning Directed Search for Ultraincompressible, Superhard Materials.

In the pursuit of materials with exceptional mechanical properties, a machine-learning model is developed to direct the synthetic efforts toward compounds with high hardness by predicting the elastic moduli as a proxy. This approach screens 118 287 compounds compiled in crystal structure databases for the materials with the highest bulk and shear moduli determined by support vector machine regression. Following these models, a ternary rhenium tungsten carbide and a quaternary molybdenum tungsten borocarbide are selected and synthesized at ambient pressure. High-pressure diamond anvil cell measurements corroborate the machine-learning prediction of the bulk modulus with less than 10% error, as well as confirm the ultraincompressible nature of both compounds. Subsequent Vickers microhardness measurements reveal that each compound also has an extremely high hardness exceeding the superhard threshold of 40 GPa at low loads (0.49 N). These results show the effectiveness of materials development through state-of-the-art machine-learning techniques by identifying functional inorganic materials.

Polyanionic Gold-Tin Bonding and Crystal Structure Preference in REAu1.5Sn0.5 (RE = La, Ce, Pr, Nd).

During a systematic search of the RE-Au-Sn (RE = La, Ce, Pr, Nd) ternary phase space, a series of compounds with the general formula REAu1.5Sn0.5 have been identified. These phases can be synthesized by arc melting the elemental metals, followed by annealing. The crystal structures were solved using single-crystal X-ray diffraction, with the composition confirmed by energy-dispersive X-ray spectroscopy. All four compounds crystallize in orthorhombic space group Imma with the CeCu2-type structure. Most notable in these compounds is the polyanionic backbone composed of a single statistically mixed Au/Sn position, which creates a puckered hexagonal bonding network separated by the rare-earth atoms. Electronic structure calculations indicate that the Au 5d bands are dominant in the density of states, while the crystal orbital Hamilton population (-COHP) curves demonstrate Au-Au and Au-Sn interactions, which stabilize the crystal structure. Likewise, a qualitative electron localization function analysis supports the existence of a polyanionic network, and a Bader charge analysis implies anionic character on Au and Sn. The preference for these compounds to adopt the simple CeCu2-type structure is also determined using density functional theory calculations and compared to related compounds to establish a better picture of the unusual behavior of Au in polar intermetallic compounds.

Structure Transformation and Cerium-Substituted Optical Response across the Carbonitridosilicate Solid Solution (LaδY1-δ)2Si4N6C (δ = 0-0.5).

Following an investigation proving La2Si4N6C crystallizes in a monoclinic space group, isostructural to Y2Si4N6C, the reportedly hexagonal (La0.5Y0.5)2Si4N6C was reinvestigated to examine the apparent crystal structure change across the solid solution. Initially, calculating the electronic structure and phonon density of states of (La0.5Y0.5)2Si4N6C in the P63mc space group revealed an imaginary phonon mode, which is indicative of a structural instability. Displacing the atoms along the pathway of the imaginary vibration led to a previously unreported space group for carbonitridosilicates, trigonal P31c. The assignment of the trigonal space group was subsequently confirmed by synthesizing (La0.5Y0.5)2Si4N6C using high-temperature, solid state synthesis and analyzing the crystal structure with high-resolution synchrotron X-ray powder diffraction. Preparing the solid solution, (LaδY1-δ)1.98Ce0.02Si4N6C (δ = 0-0.5), showed that the crystal structure changes from the monoclinic to the trigonal space group at δ ≈ 0.25. Finally, substituting Ce3+ in the crystal structure to investigate the optical response via steady-state luminescent and photoluminescent quantum yield measurements reveals severe luminescent quenching with increasing La3+ content, due to a combination of absorption of luminescence by the host structure and thermal quenching. These results display the virtue of combining computational and experimental techniques to solve inorganic crystal structures and assess potential phosphor hosts.

Searching for Missing Binary Equiatomic Phases: Complex Crystal Chemistry in the Hf-In System.

There remain 21 systems (out of over 3500 possible combinations of the elements) in which the existence of the simple binary equiatomic phases AB has not been established experimentally. Among these, the presumed binary phase HfIn is predicted to adopt the tetragonal CuAu-type structure (space group P4/ mmm) by a recently developed machine-learning model and by structure optimization through global energy minimization. To test this prediction, the Hf-In system was investigated experimentally by reacting the elements in a 1:1 stoichiometry at 1070 K. Under the conditions investigated, the bulk and surface of the sample correspond to different crystalline phases but have nearly the same equiatomic composition, as revealed by energy-dispersive X-ray analysis. The structure of the bulk sample, which was solved from powder X-ray diffraction data through simulated annealing, corresponds to the γ-brass (Cu5Zn8) type (space group I4̅3 m) with Hf and In atoms disordered over four sites. The structure of crystals selected from the surface, which was solved using single-crystal X-ray diffraction data, corresponds to the CuPt7 type (space group Fm3̅ m) with Hf and In atoms partially disordered over three sites. The discrepancy between the predicted CuAu-type structure and the two experimentally refined crystal structures is reconciled through close inspection of structural relationships, which reveal that the γ-brass-type structure of the bulk HfIn phase is indeed derived through small distortions and defect formation within the CuAu-type structure.

Disentangling Structural Confusion through Machine Learning: Structure Prediction and Polymorphism of Equiatomic Ternary Phases ABC.

A method to predict the crystal structure of equiatomic ternary compositions based only on the constituent elements was developed using cluster resolution feature selection (CR-FS) and support vector machine (SVM) classification. The supervised machine-learning model was first trained with 1037 individual compounds that adopt the most populated ternary 1:1:1 structure types (TiNiSi-, ZrNiAl-, PbFCl-, LiGaGe-, YPtAs-, UGeTe-, and LaPtSi-type) and then validated using an additional 519 compounds. The CR-FS algorithm improves class discrimination and indicates that 113 variables including size, electronegativity, number of valence electrons, and position on the periodic table (group number) influence the structure preference. The final model prediction sensitivity, specificity, and accuracy were 97.3%, 93.9%, and 96.9%, respectively, establishing that this method is capable of reliably predicting the crystal structure given only its composition. The power of CR-FS and SVM classification is further demonstrated by segregating the crystal structure of polymorphs, specifically to examine polymorphism in TiNiSi- and ZrNiAl-type structures. Analyzing 19 compositions that are experimentally reported in both structure types, this machine-learning model correctly identifies, with high confidence (>0.7), the low-temperature polymorph from its high-temperature form. Interestingly, machine learning also reveals that certain compositions cannot be clearly differentiated and lie in a "confused" region (0.3-0.7 confidence), suggesting that both polymorphs may be observed in a single sample at certain experimental conditions. The ensuing synthesis and characterization of TiFeP adopting both TiNiSi- and ZrNiAl-type structures in a single sample, even after long annealing times (3 months), validate the occurrence of the region of structural uncertainty predicted by machine learning.

Determining a Structural Distortion and Anion Ordering in La2Si4N6C via Computation and Experiment.

A structural instability in the orthorhombic carbonitridosilicate La2Si4N6C arises when calculating the ab initio phonon dispersion curves. The presence of imaginary modes indicates the compound reported in space group Pnma is dynamically unstable with the eigenvectors showing a monoclinic distortion pathway leading to space group P21/c. Synthesizing La2Si4N6C using a high-temperature route and conducting a co-refinement with high-resolution synchrotron X-ray and neutron powder diffraction shows the predicted peak splitting confirming the predicted lower symmetry crystal structure. Further, the combination of ab initio computation, neutron diffraction, and a total scattering analysis based on a neutron pair distribution function analysis supports that the anions are fully ordered and that carbon is only found on the central position of a star-shaped C(SiN3)4 unit. These results illustrate the power of combining computation and experiment to unequivocally solve crystal structures from polycrystalline powders.