Are Selenides the Same as Sulfides? Structure, Spectroscopy, and Properties of Narrow-Gap Rare-Earth Semiconductors RE2Sn(S1-xSex)5 (RE = La, Ce; x = 0-0.8).

The ternary rare-earth sulfides RE2SnS5 (RE = La-Nd) and the partial solid solutions RE2Sn(S1-xSex)5 (RE = La, Ce; x = 0-0.8) were prepared in the form of polycrystalline samples by reaction of the elements at 900 °C and as single crystals in the presence of KBr flux. They adopt the La2SnS5-type structure (orthorhombic, space group Pbam, Z = 2) consisting of chains of edge-sharing SnCh6 octahedra separated by RE atoms. Although the cell parameters evolve smoothly in RE2Sn(S1-xSex)5, detailed structural analysis by single-crystal X-ray diffraction revealed a pronounced preference for the Se atoms to occupy two out of the three chalcogen sites, which offers a rationalization for why the all-selenide end-members RE2SnSe5 do not form. Solid-state 119Sn NMR spectra confirmed the nonrandom distribution of SnS6-nSen local environments, which could be resolved into individual resonances. The Raman spectra of RE2SnS5 compounds show an intense peak at 307-320 cm-1 assigned to a symmetric A1g mode, which is dominated by Sn-S bonds; the Raman peak intensities varied with Se substitution in La2Sn(S1-xSex)5. Optical diffuse reflectance spectra, band structure calculations, and electrochemical impedance spectra indicated that these compounds are narrow band gap semiconductors; the optical band gaps are insensitive to RE substitution in RE2SnS5 (0.7 eV) but they gradually decrease with greater Se substitution in RE2Sn(S1-xSex)5 (0.7-0.4 eV).

Ahead by a Century: Discovery of Laves Phases Assisted by Machine Learning.

Laves phases AB2 form the most abundant group of intermetallic compounds, consisting of combinations of larger electropositive metals A with smaller metals B. Many practical applications of Laves phases depend on the ability to tune their physical properties through appropriate substitution of either the A or B component. Although simple geometrical and electronic factors have long been thought to control the formation of Laves phases, no single factor alone can make predictions accurately. Several machine learning models have been developed to discover new Laves phases, including variations caused by solid solubility, using elemental properties solely on the basis of chemical composition. These models were trained on a data set comprising about 3700 entries of experimentally known phases AB2 with Laves and non-Laves structures. Among these models, a decision tree algorithm gave very good performance (average recall of 95%, precision of 94%, and accuracy of 96% on the test set) by using only a small set of descriptors, the most important of which relates to the electron density at the boundary of the Wigner-Seitz cell for the B component. This model provides guidance for new experiments by making predictions on >400000 candidates very quickly. A chemically unintuitive candidate Cd(Cu1-xSbx)2 with a limited solid solubility of Sb for Cu was targeted; it was successfully synthesized and confirmed to adopt a cubic MgCu2-type Laves structure.

Interpretable Machine Learning in Solid-State Chemistry, with Applications to Perovskites, Spinels, and Rare-Earth Intermetallics: Finding Descriptors Using Decision Trees.

Machine-learning methods have exciting potential to aid materials discovery, but their wider adoption can be hindered by the opaqueness of many models. Even if these models are accurate, the inability to understand the basis for the predictions breeds skepticism. Thus, it is imperative to develop machine-learning models that are explainable and interpretable so that researchers can judge for themselves if the predictions are consistent with their own scientific understanding and chemical insight. In this spirit, the sure independence screening and sparsifying operator (SISSO) method was recently proposed as an effective way to identify the simplest combination of chemical descriptors needed to solve classification and regression problems in materials science. This approach uses domain overlap (DO) as the criterion to find the most informative descriptors in classification problems, but sometimes a low score can be assigned to useful descriptors when there are outliers or when samples belonging to a class are clustered in different regions of the feature space. Here, we present a hypothesis that the performance can be improved by implementing decision trees (DT) instead of DO as the scoring function to find the best descriptors. This modified approach was tested on three important structural classification problems in solid-state chemistry: perovskites, spinels, and rare-earth intermetallics. In all cases, the DT scoring gave better features and significantly improved accuracies of ≥0.91 for the training sets and ≥0.86 for the test sets.

Structure and Optical Properties of LixAg1-xGaSe2 and LixAg1-xInSe2.

Complete substitution of Li atoms for Ag atoms in AgGaSe2 and AgInSe2 was achieved, resulting in the solid solutions LixAg1-xGaSe2 and LixAg1-xInSe2. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state 7Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS2-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution LixAg1-xGaSe2, which is noteworthy because the end-member LiGaSe2 normally adopts the orthorhombic ß-NaFeO2-type structure (space group Pna21). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution LixAg1-xInSe2, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in LixAg1-xGaSe2 and from 1.2 to 2.5 eV in LixAg1-xInSe2, enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS2. All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.

La4Ga2Se6O3: A Rare-Earth Oxyselenide Built from One-Dimensional Strips.

The oxyselenide La4Ga2Se6O3 was obtained by reaction in NaCl flux. Its monoclinic crystal structure (space group C2/c, a = 21.2832(13) Å, b = 11.6272(7) Å, c = 6.0006(4) Å, ß = 106.3430(10)°, Z = 4), which is a new type, consists of strips of edge-sharing OLa4 tetrahedra and zigzag chains of corner-sharing GaSe4 tetrahedra. The separation into distinct structural blocks with mostly ionic vs covalent bonding character was supported by analysis of the electron localization function and crystal orbital bond index. An experimental band gap of 1.8 eV was extracted from optical diffuse reflectance spectra. First-principles calculations suggest that the thermoelectric power factor of this compound would be enhanced by n-doping.

Mercurial possibilities: determining site distributions in Cu2HgSnS4 using 63/65Cu, 119Sn, and 199Hg solid-state NMR spectroscopy.

Chalcogenides are an important class of materials that exhibit tailorable optoelectronic properties accessible through chemical modification. For example, the minerals kesterite, stannite, and velikite (Cu2MSnS4, where M = Zn, Cd, or Hg, respectively) are a series of Group 12 transition metal tin sulfides that readily exhibit optical bandgaps spanning the Shockley-Queisser limit; however, achieving consensus on their structure (space group I4̄ vs. I4̄2m) has been difficult. This study explores the average long-range and local structure of Cu2HgSnS4 and evaluates the parallels of M = Zn and Cd sister compounds using complementary X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. The 63/65Cu NMR spectra were acquired at multiple magnetic field strengths (B0 = 7.05, 11.75, and 21.1 T) to assess the unique chemical shift anisotropy and quadrupolar coupling contributions. They reveal two inequivalent sets of Cu sites in Cu2ZnSnS4, in contrast to only one set of sites in Cu2CdSnS4 and Cu2HgSnS4, clarifying structural assignments previously proposed through X-ray diffraction methods. The presence of these Cu sites was further supported by DFT calculations. The 119Sn and 199Hg NMR spectra suggest that an ordering phenomenon takes place in Cu2HgSnS4 when it undergoes annealing treatments. The trend in measured optical band gaps (1.5 eV for Cu2ZnSnS4, 1.2 eV for Cu2CdSnS4, and 0.9 eV for Cu2HgSnS4) was confirmed by electronic structure calculations, which show that the band gap narrows as the difference in electronegativity is diminished and that Hg-S bonds in Cu2HgSnS4 have greater covalent character.

Ternary Rare-Earth-Metal Nickel Indides RE23Ni7In4 (RE = Gd, Tb, Dy) with Yb23Cu7Mg4-Type Structure.

The ternary rare-earth-metal nickel indides RE23Ni7In4 (RE = Gd, Tb, Dy) were prepared by arc-melting mixtures of the elements followed by annealing at 870 K. They adopt the Yb23Cu7Mg4-type structure (space group P63/mmc, Pearson symbol hP68, Z = 2), as determined by laboratory and synchrotron powder diffraction methods for RE = Gd (a = 9.6435(10) Å, c = 22.118(3) Å) and Tb (a = 9.5695(8) Å, c = 21.983(3) Å), and single-crystal X-ray diffraction methods for RE = Dy (a = 9.533(5) Å, c = 21.890(13) Å). The centrosymmetric Yb23Cu7Mg4-type structure is closely related to the noncentrosymmetric Pr23Ir7Mg4-type structure. Triangular In3 clusters within RE23Ni7In4 represent a rare type of cluster found among metal-rich indides; the reasons for their formation were investigated by density functional theory methods.

Quaternary Arsenides REHfCu2+δAs3 (RE = La-Nd; δ ≈ 0.17): Superstructures of the Zr2Ni3P3-Type Structure.

The quaternary rare-earth-metal hafnium copper arsenides REHfCu2+δAs3 (where RE = La-Nd; δ ≈ 0.17) were obtained by direct reactions of the elements at 1073 K. They adopt a noncentrosymmetric orthorhombic crystal structure (space group Pmn21; a = 397.73(3)-392.37(2) pm, b = 1062.68(8)-1055.23(6) pm, c = 1307.25(9)-1291.40(7) pm for RE = La-Nd), which can be considered to be a Cu-deficient superstructure of the ternary Zr2Ni3P3-type structure found among metal-rich pnictides with a metal-to-nonmetal ratio close or equal to 2:1. The RE atoms occupy trigonal prismatic (CN6) and monocapped trigonal prismatic (CN7) sites, the latter resulting from the occurrence of Cu vacancies, the Hf atoms occupy octahedral sites (CN6), and the Cu atoms occupy tetrahedral sites (CN4). Band structure calculations on idealized models LaHfCu2As3 and LaHfCu2.5As3 suggest that the bonding situation improves by fractional occupation of the Cu3 site via depopulation of antibonding Cu-As states.

Discovery of Intermetallic Compounds from Traditional to Machine-Learning Approaches.

Intermetallic compounds are bestowed by diverse compositions, complex structures, and useful properties for many materials applications. How metallic elements react to form these compounds and what structures they adopt remain challenging questions that defy predictability. Traditional approaches offer some rational strategies to prepare specific classes of intermetallics, such as targeting members within a modular homologous series, manipulating building blocks to assemble new structures, and filling interstitial sites to create stuffed variants. Because these strategies rely on precedent, they cannot foresee surprising results, by definition. Exploratory synthesis, whether through systematic phase diagram investigations or serendipity, is still essential for expanding our knowledge base. Eventually, the relationships may become too complex for the pattern recognition skills to be reliably or practically performed by humans. Complementing these traditional approaches, new machine-learning approaches may be a viable alternative for materials discovery, not only among intermetallics but also more generally to other chemical compounds. In this Account, we survey our own efforts to discover new intermetallic compounds, encompassing gallides, germanides, phosphides, arsenides, and others. We apply various machine-learning methods (such as support vector machine and random forest algorithms) to confront two significant questions in solid state chemistry. First, what crystal structures are adopted by a compound given an arbitrary composition? Initial efforts have focused on binary equiatomic phases AB, ternary equiatomic phases ABC, and full Heusler phases AB2C. Our analysis emphasizes the use of real experimental data and places special value on confirming predictions through experiment. Chemical descriptors are carefully chosen through a rigorous procedure called cluster resolution feature selection. Predictions for crystal structures are quantified by evaluating probabilities. Major results include the discovery of RhCd, the first new binary AB compound to be found in over 15 years, with a CsCl-type structure; the connection between "ambiguous" prediction probabilities and the phenomenon of polymorphism, as illustrated in the case of TiFeP (with TiNiSi- and ZrNiAl-type structures); and the preparation of new predicted Heusler phases MRu2Ga and RuM2Ga (M = first-row transition metal) that are not obvious candidates. Second, how can the search for materials with desired properties be accelerated? One particular application of strong current interest is thermoelectric materials, which present a particular challenge because their optimum performance depends on achieving a balance of many interrelated physical properties. Making use of a recommendation engine developed by Citrine Informatics, we have identified new candidates for thermoelectric materials, including previously unknown compounds (e.g., TiRu2Ga with Heusler structure; Mn(Ru0.4Ge0.6) with CsCl-type structure) and previously reported compounds but counterintuitive candidates (e.g., Gd12Co5Bi). An important lesson in these investigations is that the machine-learning models are only as good as the experimental data used to develop them. Thus, experimental work will continue to be necessary to improve the predictions made by machine learning.

Solving the Coloring Problem in Half-Heusler Structures: Machine-Learning Predictions and Experimental Validation.

The site preferences within the structures of half-Heusler compounds have been evaluated through a machine-learning approach. A support-vector machine algorithm was applied to develop a model which was trained on 179 experimentally reported structures and 23 descriptors based solely on the chemical composition. The model gave excellent performance, with sensitivity of 93%, selectivity of 96%, and accuracy of 95%. As an illustration of data sanitization, two compounds (GdPtSb, HoPdBi) flagged by the model to have potentially incorrect site assignments were resynthesized and structurally characterized. The predictions of the correct site assignments from the machine-learning model were confirmed by single-crystal and powder X-ray diffraction analysis. These site assignments also corresponded to the lowest total energy configurations as revealed from first-principles calculations.

AHgSnQ4 (A = Sr, Ba; Q = S, Se): A Series of Hg-Based Infrared Nonlinear-Optical Materials with Strong Second-Harmonic-Generation Response and Good Phase Matchability.

Four Hg-based IR nonlinear-optical materials, AHgSnQ4 (A = Sr, Ba; Q = S, Se), were discovered and investigated systematically. Their structures are built of two-dimensional [HgSnQ4]2- layers, which are assembled alternately by distorted (HgQ4 and SnQ4) tetrahedra and separated by eight-coordinated A2+ cations. The two sulfides AHgSnS4 (A = Ba, Sr) exhibit large second-harmonic-generation (SHG) responses (2.8 and 1.9 × AgGaS2 at 2.09 µm), as well as large band gaps (2.77 and 2.72 eV). The two selenides AHgSnSe4 (A = Ba, Sr) show even stronger SHG responses, about 5 times that of AgGaS2. Furthermore, all four compounds show phase-matching behavior, and the results of first-principles calculation elucidate the key role of the HgQ4 group in the enhanced SHG effect in ß-BaHgSnS4 and BaHgSnSe4.

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.

Not Just Par for the Course: 73 Quaternary Germanides RE4 M2 XGe4 ( RE = La-Nd, Sm, Gd-Tm, Lu; M = Mn-Ni; X = Ag, Cd) and the Search for Intermetallics with Low Thermal Conductivity.

A total of 73 new quaternary rare-earth germanides RE4 M2 XGe4 ( RE = rare-earth metal; M = Mn-Ni; X = Ag, Cd) were prepared through reactions of the elements. The solid solution Nd4Mn2Cd(Ge1- ySi y)4 was also prepared under the same conditions and found to be complete over the entire range. All of these compounds adopt the monoclinic Ho4Ni2InGe4-type structure (space group C2/ m, a = 14.2-16.7 Å, b = 4.0-4.6 Å, c = 6.8-7.5 Å, ß = 106-109°), as revealed by powder X-ray diffraction analysis and single-crystal X-ray diffraction analysis on selected members. The structure determination of Nd4(Mn0.78(1)Ag0.22(1))2Ag0.83(1)Ge4 disclosed disorder of Mn and Ag atoms within the tetrahedral site and Ag deficiencies within the square planar site. Within the solid solution Nd4Mn2Cd(Ge1- ySi y)4, the end-members and two intermediate members were structurally characterized; as the Si content increases, the Cd sites become less deficient and the individual [Mn2 Tt2] layers contract but become further apart from each other. Electronic band structure calculations confirm that the Ag-Ge or Cd-Ge bonds are the weakest in the structure and thus prone to distortion. Thermal property measurements confirm expectations from machine-learning predictions that these quaternary germanides should exhibit low thermal conductivity, which was found to be <10 W m-1 K-1 for Nd4Mn2AgGe4.

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.

Trigonal Planar [HgSe3](4-) Unit: A New Kind of Basic Functional Group in IR Nonlinear Optical Materials with Large Susceptibility and Physicochemical Stability.

A new mercury selenide BaHgSe2 was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe3](4-) anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.

Gd12Co5.3Bi and Gd12Co5Bi, Crystalline Doppelgänger with Low Thermal Conductivities.

Attempts to prepare Gd12Co5Bi, a member of the rare-earth (RE) intermetallics RE12Co5Bi, which were identified by a machine-learning recommendation engine as potential candidates for thermoelectric materials, led instead to formation of the new compound Gd12Co5.3Bi with a very similar composition. Phase equilibria near the Gd-rich corner of the Gd-Co-Bi phase diagram were elucidated by both lab-based and variable-temperature synchrotron powder X-ray diffraction, suggesting that Gd12Co5.3Bi and Gd12Co5Bi are distinct phases. The higher symmetry structure of Gd12Co5.3Bi (cubic, space group Im3̅, Z = 2, a = 9.713(6) Å), as determined from single-crystal X-ray diffraction, is closely related to that of Gd12Co5Bi (tetragonal, space group Immm). Single Co atoms and Co-Co dumbbells are disordered with occupancies of 0.78(2) and 0.22(2), respectively, in Gd12Co5.3Bi, but they are ordered in Gd12Co5Bi. Consistent with this disorder, the electrical resistivity shows less dependence on temperature for Gd12Co5.3Bi than for Gd12Co5Bi. The thermal conductivity is low and reaches 2.8 W m(-1) K(-1) at 600 °C for both compounds; however, the temperature dependence of the thermal conductivity differs, decreasing for Gd12Co5.3Bi and increasing for Gd12Co5Bi as the temperature increases. The unusual trends in thermal properties persist in the heat capacity, which decreases below 2R, and in the thermal diffusivity, which increases at higher temperatures.

Many metals make the cut: quaternary rare-earth germanides RE4M2InGe4 (M = Fe, Co, Ni, Ru, Rh, Ir) and RE4RhInGe4 derived from excision of slabs in RE2InGe2.

The formation of quaternary rare-earth (RE) germanides containing transition metals (M's) from groups 6 to 10 was investigated through arc-melting and annealing reactions at 800 °C; about 50 new compounds were obtained. These include several new series of quaternary germanides RE4M2InGe4 (M = Fe, Co, Ru, Rh, Ir), previously known only for M = Mn and Ni; additional members of RE4Ni2InGe4 extended to other RE substituents; and a different but closely related series RE4RhInGe4. Detailed crystal structures were determined by single-crystal X-ray diffraction studies for 20 compounds. Monoclinic structures in space group C2/m are adopted by RE4M2InGe4 (Ho4Ni2InGe4-type, a = 15.1-16.5 Å, b = 4.1-4.4 Å, c = 6.9-7.3 Å, ß = 106.2-108.6°) and RE4RhInGe4 (own type, a = 20.0-20.2 Å, b = 4.2-4.3 Å, c = 10.1-10.2 Å, ß = 105.0-105.3°). Both structures contain frameworks built from MGe4 tetrahedra, InGe4 square planes, and Ge2 dimers, delimiting tunnels occupied by RE atoms. These structures can also be derived by cutting slabs along different directions from the more symmetrical RE2InGe2 structure. Although the Ge2 dimers are relatively invariant, the InGe4 square planes can undergo distortion to form two sets of short versus long In-Ge distances. This distortion results from a competition between M-Ge bonding in the MGe4 tetrahedra and In-Ge bonding in the InGe4 square planes.

Rare-earth manganese copper phosphides REMnCu4P3 (RE = Gd-Ho): the first quaternary ordered variants of the YCo5P3-type structure.

The quaternary rare-earth phosphides REMnCu(4)P(3) (RE = Gd-Ho) were obtained from direct reactions of the elements at 800 °C. They are the first examples in which ordering of two different transition-metal atoms takes place within the orthorhombic YCo(5)P(3)-type structure [Pearson symbol oP36, space group Pnma, Z = 4; a = 12.667(2)-12.6489(4) Å, b = 3.8119(7)-3.7755(1) Å, and c = 10.895(2)-10.8632(4) Å for RE = Gd-Ho]. Columns of trigonal prisms centered by P atoms are connected in propellor-shaped units in zigzag arrangements to generate square-pyramidal (CN5) sites that are occupied by Mn atoms and tetrahedral sites (CN4) that are occupied by Cu atoms. Spin-polarized band-structure calculations predict that the hypothetical compound YMnCu(4)P(3) will exhibit magnetic ordering. Electrical resistivity measurements on TbMnCu(4)P(3) indicate a poor metal.

Quaternary arsenides ACdGeAs2 (A = K, Rb) built of ethane-like Ge2As6 units.

Reactions of the elements at high temperature resulted in the quaternary arsenides KCdGeAs2 (650 °C) and RbCdGeAs2 (600 °C). Single-crystal X-ray diffraction analysis reveals that they adopt a new triclinic structure type (space group P1̅, Pearson symbol aP20, Z = 4; a = 8.0040(18) Å, b = 8.4023(19) Å, c = 8.703(2) Å, α = 71.019(3)°, ß = 75.257(3)°, γ = 73.746(3)° for KCdGeAs2; a = 8.2692(13) Å, b = 8.4519(13) Å, c = 8.7349(13) Å, α = 71.163(2)°, ß = 75.601(2)°, γ = 73.673(2)° for RbCdGeAs2). Two-dimensional anionic layers [CdGeAs2](-) are separated by A(+) cations and are built from ethane-like Ge2As6 units forming infinite chains connected via three- and four-coordinated Cd atoms. Being Zintl phases, these compounds satisfy charge balance and are expected to be semiconducting, as confirmed by band structure calculations on KCdGeAs2, which reveal a band gap of 0.8 eV. KCdGeAs2 is diamagnetic.

Manganese-substituted rare-earth zinc arsenides RE(1-y)Mn(x)Zn(2-x)As2 (RE = Eu-Lu) and RE(2-y)Mn(x)Zn(4-x)As4 (RE = La-Nd, Sm, Gd).

Two series of Mn-substituted rare-earth zinc arsenides RE(1-y)Mn(x)Zn(2-x)As2 (RE = Eu-Lu) and RE(2-y)Mn(x)Zn(4-x)As4 (RE = La-Nd, Sm, Gd) were prepared by reaction of the elements at 750 °C. Both series are derived from ideal empirical formula REM2As2 (M = Mn, Zn) and adopt crystal structures related to the trigonal CaAl2Si2-type (space group P3m1) in which hexagonal nets of RE atoms and [M2As2] slabs built up of edge-sharing M-centered tetrahedra are alternately stacked along the c-direction. For compounds with divalent RE components (Eu, Yb), the fully stoichiometric and charge-balanced formula REM2As2 is obtained, with Mn and Zn atoms statistically disordered within the same tetrahedral site. For compounds with trivalent RE components, the RE sites become deficient, and the Mn atoms are segregated from the Zn atoms in separate tetrahedral sites. Within the series RE(1-y)Mn(x)Zn(2-x)As2 (Gd-Tm, Lu), the parent CaAl2Si2-type structure is retained, and the Mn atoms are disordered within partially occupied interstitial sites above and below [Zn(2-x)As2] slabs. Within the series RE(2-y)Mn(x)Zn(4-x)As4 (RE = La-Nd, Sm, Gd), the c-axis becomes doubled as a result of partial ordering of Mn atoms between every other pair of [Zn(2-x)As2] slabs. Attempts to synthesize Gd-containing solid solutions with the charge-balanced formula Gd0.67Mn(x)Zn(2-x)As2 suggested that these phases could be formed with up to 50% Mn substitution. Band structure calculations reveal that a hypothetical superstructure model with the formula La1.33MnZn3As4 would have no gap at the Fermi level and that slightly lowering the electron count alleviates antibonding Mn-As interactions; a spin-polarized calculation predicts nearly ferromagnetic half-metallic behavior. X-ray photoelectron spectroscopy confirms the presence of divalent Mn in these compounds.