Machine Learning-Guided Discovery of Ternary Compounds Containing La, P, and Group 14 Elements.

We integrate a deep machine learning (ML) method with first-principles calculations to efficiently search for the energetically favorable ternary compounds. Using La-Si-P as a prototype system, we demonstrate that ML-guided first-principles calculations can efficiently explore crystal structures and their relative energetic stabilities, thus greatly accelerate the pace of material discovery. A number of new La-Si-P ternary compounds with formation energies less than 30 meV/atom above the known ternary convex hull are discovered. Among them, the formation energies of La5SiP3 and La2SiP phases are only 2 and 10 meV/atom, respectively, above the convex hull. These two compounds are dynamically stable with no imaginary phonon modes. Moreover, by replacing Si with heavier-group 14 elements in the eight lowest-energy La-Si-P structures from our ML-guided predictions, a number of low-energy La-X-P phases (X = Ge, Sn, Pb) are predicted.

Add a Pinch of Tetrel: The Transformation of a Centrosymmetric Metal into a Nonsymmorphic and Chiral Semiconductor.

Centrosymmetric skutterudite RhP3 was converted to a nonsymmorphic and chiral compound RhSi0.3 P2.7 (space group P21 21 21 ) by means of partial replacement of Si for P. The structure, determined by a combination of X-ray crystallography and solid state 31 P NMR, exhibits branched polyanionic P/Si chains that are unique among metal phosphides. A driving force to stabilize the locally noncentrosymmetric cis-RhSi2 P4 and fac-RhSi3 P3 fragments is π-electron back-donation between the Rh t2g -type orbitals and the unoccupied antibonding Si/P orbitals, which is more effective for Si than for P. In situ studies and total energy calculations revealed the metastable nature of RhSi0.3 P2.7 . Electronic structure calculations predicted centrosymmetric cubic RhP3 to be metallic which was confirmed by transport properties measurements. In contrast, the electronic structure for chiral orthorhombic RhSi0.3 P2.7 contained a bandgap, and this compound was shown to be a narrow gap semiconductor.

Synthesis-enabled exploration of chiral and polar multivalent quaternary sulfides.

An innovative method of synthesis is reported for the large and diverse (RE)6(TM) x (Tt)2S14 (RE = rare-earth, TM = transition metals, Tt = Si, Ge, and Sn) family of compounds (∼1000 members, ∼325 contain Si), crystallizing in the noncentrosymmetric, chiral, and polar P63 space group. Traditional synthesis of such phases involves the annealing of elements or binary sulfides at elevated temperatures. The atomic mixing of refractory components technique, presented here, allows the synthesis of known members and vastly expands the family to nearly the entire transition metal block, including 3d, 4d, and 5d TMs with oxidation states ranging from 1+ to 4+. Arc-melting of the RE, TM, and tetrel elements of choice forms an atomically-mixed precursor, which readily reacts with sulfur providing bulk powders and large single crystals of the target quaternary sulfides. Detailed in situ and ex situ experiments show the mechanism of formation, which involves multiphase binary sulfide intermediates. Crystal structures and metal oxidation states were corroborated by a combination of single crystal X-ray diffraction, elemental analysis, EPR, NMR, and SQUID magnetometry. The potential of La6(TM) x (Tt)2S14 compounds for non-linear optical applications was also demonstrated.

Third time's the charm: intricate non-centrosymmetric polymorphism in LnSiP3 (Ln = La and Ce) induced by distortions of phosphorus square layers.

Complex polymorphic relationships in the LnSiP3 (Ln = La and Ce) family of compounds are reported. An innovative synthetic method was developed to overcome differences in the reactivities of the rare-earth metal and refractory silicon with phosphorus. Reactions of atomically mixed Ln + Si with P allowed for selective control over the reaction outcomes resulting in targeted isolation of three new polymorphs of LaSiP3 and two polymorphs of CeSiP3. In situ X-ray diffraction studies revealed that the developed method bypasses formation of the thermodynamic dead-end, the binary SiP. Careful re-determination of the crystal structure ruled out the previously reported ordered centrosymmetric structure of CeSiP3 and showed that the main LnSiP3 polymorphs crystallize in the non-centrosymmetric Pna21 and Aea2 space groups featuring distinct distortions of the regular P square net to yield either cis-trans 1D phosphorus chains (Pna21) or disordered-2D phosphorus layers (Aea2). The disordered 2D nature of the P layers in the Aea2 LaSiP3 polymorph was confirmed by Raman spectroscopy. A unique centrosymmetric P21/c polymorph was observed for LaSiP3 and has a completely different crystal structure lacking P layers. Consecutive polymorphic transformations at increasing temperatures for LaSiP3(Pna21 → P21/c → Aea2) were derived from optimized synthetic profiles and confirmed by a combination of phonon computations and experimental in situ and ex situ annealings. Crystal structures of the LaSiP3 polymorphs were verified via advanced solid state NMR analysis using 31P MAS and 31P{139La} double resonance techniques. A combination of phonon and electronic structure calculations, NMR T1 relaxation times, UV/Vis/NIR spectroscopy, and resistivity measurements revealed that all the reported polymorphs are semiconductors with resistivities and thermal conductivities strongly dependent on the degree of distortion of P square layers in the crystal structure. Reported here, non-centrosymmetric LnSiP3 polymorphs with tunable resistivity and thermal conductivity provide a platform for the development of novel functional materials with a wide range of applications.

New Noncentrosymmetric Tetrel Pnictides Composed of Square-Planar Gold(I) with Peculiar Bonding.

Three novel isostructural equiatomic gold tetrel pnictides, AuSiAs, AuGeP, and AuGeAs, were synthesized and characterized. These phases crystallize in the noncentrosymmetric (NCS) monoclinic space group Cc (no. 9), featuring square-planar Au within cis-[AuTt2 Pn2 ] units (Tt=tetrel, Si, Ge; Pn=pnictogen, P, As). This is in drastic contrast to the structure of previously reported AuSiP, which exhibits typical linear coordination of Au with Si and P. Chemical bonding analysis through the electron localization function suggests covalent two-center two-electron Tt-Pn bonds, and three-center Au-Tt-Au and Au-Pn-Au bonds with 1.6 e- per bond. X-ray photoelectron spectroscopy studies support the covalent and nonionic nature of Au-Pn and Au-Tt bonds. The title materials were found to be n-type narrow-gap semiconductors or semimetals, with nearly temperature-independent electrical resistivities and low thermal conductivities. A combination of the semimetallic properties with tunable NCS structure provides opportunities for the development of materials based on gold tetrel pnictides.

Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr1- xY xB12 and Zr1- xU xB12.

Single-phase metal dodecaboride solid solutions, Zr0.5Y0.5B12 and Zr0.5U0.5B12, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and 10B and 11B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr1- xY xB12 were studied and it was found that carbon causes fast cooling and as a result rapid nucleation of grains, as well as "templating" and patterning effects of the surface morphology. The hardness of the Zr0.5Y0.5B12 phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ∼17% higher than that of its parent compounds, ZrB12 and YB12, with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr0.5U0.5B12 is ∼54% higher than that of its UB12 parent. The dodecaborides were confirmed to be metallic by band structure calculations, diffuse reflectance UV-vis, and solid-state NMR spectroscopies. The nature of the dodecaboride colors-violet for ZrB12 and blue for YB12-can be attributed to charge-transfer. XPS indicates that the metals are in the following oxidation states: Y3+, Zr4+, and U5+/6+. The superconducting transition temperatures ( Tc) of the dodecaborides were determined to be 4.5 and 6.0 K for YB12 and ZrB12, respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The Tc of the Zr0.5Y0.5B12 solid solution was suppressed to 2.5 K.

Superhard Tungsten Diboride-Based Solid Solutions.

Solid solutions of tungsten diboride (WB2) with increasing substitution of tungsten (W) by tantalum (Ta) and niobium (Nb)-ranging from 0 to 50 at. % on a metals basis-were synthesized through resistive arc melting. Samples were characterized using a combination of powder X-ray diffraction (PXRD) for phase identification, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy for elemental composition, Vickers microindentation for hardness measurements, and thermogravimetric analysis for thermal stability. The solubility limit was found to be less than 8 at. % for Nb and less than 10 at. % for Ta, as determined by PXRD. Vickers hardness ( Hv) values were measured to be 40.3 ± 1.6 and 41.0 ± 1.2 GPa at 0.49 N for 6 at. % Nb and for 8 at. % Ta substitution, respectively. In addition, the hardest solid solution (W0.92Ta0.08B2) showed oxidation resistance up to ∼570 °C, approximately 70 °C higher than that of tungsten carbide (WC). Although pure WB2 is known not to be superhard, these results demonstrate the formation of superhard solid solutions through the substitution of tungsten by small amounts of transition metals. This increase in hardness can be attributed to solid solution hardening.

Investigation of ternary metal dodecaborides (M1M2M3)B12 (M1, M2 and M3 = Zr, Y, Hf and Gd).

Samples of metal borides with a nominal composition of ((M1)(1-x-z)(M2)(x)(M3)(z)) : 20B (M1, M2 and M3 = Zr, Y, Hf and Gd) were prepared by arc-melting and studied for phase composition (using powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS)) and mechanical properties (Vickers hardness). Ternary metal dodecaboride phases were successfully synthesized for the majority of compositions, including stabilization of two high-pressure (6.5 GPa) phases (cubic-UB12 structure), HfB12 and GdB12, in (Zr1-x-zHfxGdz) : 20B and (Y1-x-zHfxGdz) : 20B nominal alloy compositions. Unit cell refinement for the samples showed solid solution formation in most cases. Vickers hardness measurements indicated that most samples possess enhanced hardness in comparison to their parent phases, with the alloy (Zr0.50Y0.25Gd0.25) : 20B having a hardness of 46.9 ± 2.4 GPa compared to 41.3 ± 1.1 and 41.6 ± 1.3 GPa for alloy compositions of 1.0 Zr : 20B and 1.0 Y : 20B, respectively, at 0.49 N of applied load. Using the data from this manuscript as well as previous work, pseudo-ternary phase diagrams (at a constant boron content) have been constructed.

Effects of Variable Boron Concentration on the Properties of Superhard Tungsten Tetraboride.

Tungsten tetraboride is an inexpensive, superhard material easily prepared at ambient pressure. Unfortunately, there are relatively few compounds in existence that crystallize in the same structure as tungsten tetraboride. Furthermore, the lack of data in the tetraboride phase space limits the discovery of any new superhard compounds that also possess high incompressibility and a three-dimensional boron network that withstands shear. Thus, the focus of the work here is to chemically probe the range of thermodynamically stable tetraboride compounds with respect to both the transition metal and the boron content. Tungsten tetraboride alloys with a variable concentration of boron were prepared by arc-melting and investigated for their mechanical properties and thermal stability. The purity and phase composition were confirmed by energy dispersive X-ray spectroscopy and powder X-ray diffraction. For variable boron WBx, it was found that samples prepared with a metal to boron ratio of 1:11.6 to 1:9 have similar hardness values (∼40 GPa at 0.49 N loading) as well as having a similar thermal oxidation temperature of ∼455 °C. A nearly single phase compound was successfully stabilized with tantalum and prepared with a nearly stoichiometric amount of boron (4.5) as W0.668Ta0.332B4.5. Therefore, the cost of production of WB4 can be decreased while maintaining its remarkable properties. Insights from this work will help design future compounds stable in the adaptable tungsten tetraboride structure.

Rediscovering the Crystal Chemistry of Borides.

For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Stabilization of LnB12 (Ln = Gd, Sm, Nd, and Pr) in Zr1-xLnxB12 under Ambient Pressure.

We report ambient pressure stabilization of a previously synthesized high-pressure (6.5 GPa) phase, GdB12, in a Zr1-xGdxB12 solid solution (with ∼54 at. % Gd solubility, as determined by both powder X-ray diffraction and energy-dispersive spectroscopy). Limited solubilities of Sm (∼15 at. % Sm), Nd (∼7 at. % Nd), and Pr (∼4 at. % Pr), in ZrB12 were also achieved. Previous attempts at preparing these rare-earth borides were unsuccessful even under high pressure. On the basis of insights provided from the unit cell sizes observed via solid solutions, at least 6.5 GPa of pressure would be needed to synthesize these rare-earth borides since Sm, Nd, and Pr atomic radii are larger than that of Gd. The solid-solution formation for Zr1-xGdxB12 and Zr1-xSmxB12 can be seen in the change of the unit cell of each of the solid solutions relative to their pure parent compounds as well as in the change of color of the respective alloys. For Zr0.45Gd0.55B12 and Zr0.70Sm0.30B12, the cubic unit cell parameter (a) reached a value of 7.453 and 7.428 Å, respectively, compared to 7.412 Å for pure ZrB12.

Stabilization of HfB12 in Y1-xHfxB12 under Ambient Pressure.

Alloys of metal dodecaborides-YB12 with HfB12-were prepared via arc-melting in order to stabilize the metastable HfB12 high-pressure phase under ambient pressure. Previously, HfB12 had been synthesized only under high-pressure (6.5 GPa). Powder X-ray diffraction (PXRD) and energy-dispersive X-ray spectroscopy (EDS) were used to confirm the purity and phase composition of the prepared samples. The solubility limit for HfB12 in Y1-xHfxB12 (cubic UB12 structure type) was determined to be ∼35 at. % Hf by PXRD and EDS analysis. The value of the cubic unit cell parameter (a) changed from 7.505 Å (pure YB12) to 7.454 Å across the solid solution range. Vickers hardness increased from 40.9 ± 1.6 GPa for pure YB12 to 45.0 ± 1.9 GPa under an applied load of 0.49 N for the Y1-xHfxB12 solid solution composition with ∼28 at. % Hf, suggesting both solid solution hardening and extrinsic hardening due to the formation of secondary phases of hafnium.

Extrinsic Hardening of Superhard Tungsten Tetraboride Alloys with Group 4 Transition Metals.

Alloys of tungsten tetraboride (WB4) with the group 4 transition metals, titanium (Ti), zirconium (Zr), and hafnium (Hf), of different concentrations (0-50 at. % on a metals basis) were synthesized by arc-melting in order to study their mechanical properties. The phase composition and purity of the as-synthesized samples were confirmed using powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). The solubility limit as determined by PXRD is 20 at. % for Ti, 10 at. % for Zr, and 8 at. % for Hf. Vickers indentation measurements of WB4 alloys with 8 at. % Ti, 8 at. % Zr, and 6 at. % Hf gave hardness values, Hv, of 50.9 ± 2.2, 55.9 ± 2.7 and 51.6 ± 2.8 GPa, respectively, compared to 43.3 GPa for pure WB4 under an applied load of 0.49 N. Each of the aforementioned compositions are considered superhard (Hv > 40 GPa), likely due to extrinsic hardening that plays a key role in these superhard metal borides. Furthermore, these materials exhibit a significantly reduced indentation size effect, which can be seen in the plateauing hardness values for the W1-xZrxB4 alloy. In addition, W0.92Zr0.08B4, a product of spinoidal decomposition, possesses nanostructured grains and enhanced grain hardening. The hardness of W0.92Zr0.08B4 is 34.7 ± 0.65 GPa under an applied load of 4.9 N, the highest value obtained for any superhard metal at this relatively high loading. In addition, the WB4 alloys with Ti, Zr, and Hf showed a substantially increased oxidation resistance up to ∼460 °C, ∼510 °C, and ∼490 °C, respectively, compared to ∼400 °C for pure WB4.