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Elpasolite- and cryolite-type oxyfluorides can be regarded as superstructures of perovskite and exhibit structural diversity. While maintaining a similar structural topology with the prototype structures, changes in the size, electronegativity, and charge of cation and/or anion inevitably lead to structural evolution. Therefore, the nominal one-to-one relation suggested by a doubled formula of perovskite does not guarantee a simple 2-fold superstructure for many cases. Herein, the commensurately modulated perovskite-like K3TiOF5 was refined at 100 K from single-crystal X-ray diffraction data by using a pseudotetragonal subcell with lattice parameters of a = b = 6.066(2) A and c = 8.628(2) A. The length of the modulation vector was refined to 0.3a* + 0.1b* + 0.25c*. In the commensurate supercell of K3TiOF5, the B-site Ti4+ and K+ cations in [TiOF5]3- and [KOF5]6- octahedral units were found to be significantly displaced from the average atomic positions refined in the subcell. The displacements of the K+ cations are ±0.76 A, and those for the Ti4+ cations are approximately ±0.13 A. One- and two-dimensional solid-state 19F NMR measurements revealed two tightly clustered groups of resonances in a ratio of ca. 4:1, assigned to equatorial and axial fluorine, respectively, consistent with local [TiOF5]3- units. S/TEM results confirmed the average structure. Electronic structure calculations of the idealized I4mm subcell indicate the instability to a modulated structure arises from soft optical modes that is controlled by the octahedrally coordinated B-site potassium ions in the cryolite-type structure.

The mixed cation compounds Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 crystallize in the polar noncentrosymmetric space group Cc. The AAsQ2 (A = alkali metals, Q = S, Se) family features one-dimensional (1D) 1/∞[AQ2-] chains comprising corner-sharing pyramidal AQ3 units in which the packing of these chains is dependent on the alkali metals. The parallel 1/∞[AQ2-] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe2 to 2.01 eV in Na0.35K0.65AsSe2, 2.07 eV in Na0.2K0.8AsSe2, and 2.18 eV in Na0.1K0.9AsS2. The Na1-xKxAsSe2 (x = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 suggest that Na0.2K0.8AsSe2 and Na0.1K0.9AsS2 have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na0.5K0.5AsSe2 confirm its SHG response with the highest value of d33 = 22.5 pm/V (χ333(2) = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be deff = 18.9 pm/V (χeff(2) = 37.8 pm/V), which is consistent with the experimentally obtained value of deff = 16.5 pm/V (χeff(2) = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with Na0.2K0.8AsSe2 exhibiting the highest stability.

The properties of crystalline materials tend to be strongly correlated with their structures, and the prediction of crystal structure from only the composition is a coveted goal in the field of inorganic materials. However, even for the simplest compositions, such prediction relies on a complex network of interactions, including atomic or ionic radii, ionicity, electronegativity, position in the periodic table, and magnetism, to name only a few important parameters. We focus here on the AB2X6 (AB2O6 and AB2F6) composition space with the specific goal of finding new oxide compounds in the trirutile family, which is known for unusual one-dimensional (1D) antiferromagnetic behavior. Through machine learning methods, we develop an understanding of how geometric and bonding constraints determine the crystallization of compounds in the trirutile structure as opposed to other ternary structures in this space. In combination with density functional theory (DFT) calculations, we predict 16 previously unreported candidate trirutile oxides. We successfully prepare one of these and show it forms in the disordered rutile structure, under the preparation conditions adopted here.

Incorporating bismuth, the heaviest element stable to radioactive decay, into new materials enables the creation of emergent properties such as permanent magnetism, superconductivity, and nontrivial topology. Understanding the factors that drive Bi reactivity is critical for the realization of these properties. Using pressure as a tunable synthetic vector, we can access unexplored regions of phase space to foster reactivity between elements that do not react under ambient conditions. Furthermore, combining computational and experimental methods for materials discovery at high-pressures provides broader insight into the thermodynamic landscape than can be achieved through experiment alone, informing our understanding of the dominant chemical factors governing structure formation. Herein, we report our combined computational and experimental exploration of the Mo-Bi system, for which no binary intermetallic structures were previously known. Using the ab initio random structure searching (AIRSS) approach, we identified multiple synthetic targets between 0-50 GPa. High-pressure in situ powder X-ray diffraction experiments performed in diamond anvil cells confirmed that Mo-Bi mixtures exhibit rich chemistry upon the application of pressure, including experimental realization of the computationally predicted CuAl2-type MoBi2 structure at 35.8(5) GPa. Electronic structure and phonon dispersion calculations on MoBi2 revealed a correlation between valence electron count and bonding in high-pressure transition metal-Bi structures as well as identified two dynamically stable ambient pressure polymorphs. Our study demonstrates the power of the combined computational-experimental approach in capturing high-pressure reactivity for efficient materials discovery.

Dental enamel is a principal component of teeth.[1] It has evolved to bear large masticatory forces, resist mechanical fatigue, and withstand wear over decades of use.[2] Functional impairment or loss, as a consequence of developmental defects or tooth decay (caries), has a dramatic impact on health and quality of life, with significant costs to society.[3] While the last decade has seen great progress in our understanding of enamel formation (amelogenesis) and the functional properties of mature enamel, attempts to repair enamel lesions or synthesize enamel in vitro have had limited success.[4–6] This is partly due to the highly hierarchical structure of enamel and the additional complexities arising from chemical gradients that we are only beginning to understand.[7–9] Herein, we show, using atomic-scale quantitative imaging and correlative spectroscopies, that the nanoscale crystallites of hydroxylapatite (OHAp; Ca5(PO4)3(OH)) that are the fundamental building blocks of enamel are comprised of two nanometric layers enriched in magnesium flanking a core rich in sodium, fluoride, and carbonate ions; this sandwich core is surrounded by a shell with lower concentration of substitutional defects. A mechanical model based on DFT calculations and X-ray diffraction data predicts that significant residual stresses arise as a consequence of the chemical gradients, in agreement with preferential dissolution of the crystallite core. Further, stresses may impact the mechanical resilience of enamel. Finally, the two additional layers of hierarchy suggest a new model for biological control over crystal growth during amelogenesis and have implications for the preservation of biomarkers during tooth development.

Complex crystal structures with subtle atomic-scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging...

Two polymorphs of ammonium trifluorooxovanadate (NH4VOF3) were synthesized as single crystals by the reaction of ammonium metavanadate with vanadium in hydrofluoric acid. The two phases crystalize in the same space group (Pbam), have similar unit cell volumes (α-NH4VOF3:371 A3, β-NH4VOF3:375 A3 at 100 K), and similar total energies (difference of ∼2.2 meV per formula unit at 0 K). Both structures contain the [VOF3]nn− ladder motif that has been reported in phases such as KVOF3 and RbVOF3. The polymorphs are differentiated by the angle between two neighboring ladder motifs. Although the two polymorphs are structurally similar, selective synthesis is possible as longer reaction times favor β-NH4VOF3.

The discovery of emergent materials lies at the intersection of chemistry and condensed matter physics. Synthetic chemistry offers a pathway to create materials with the desired physical and electronic structures that support fundamentally new properties. Metal–organic frameworks are a promising platform for bottom-up chemical design of new materials, owing to their inherent chemical predictability and tunability relative to traditional solid-state materials. Herein, we describe the synthesis and magnetic characterization of a new 2,5-dihydroxy-1,4-benzoquinone based material, (NMe2H2)3.5Ga2(C6O4Cl2)3 (1), which features radical-based electronic spins on the sites of a kagomé lattice, a geometric lattice known to engender exotic electronic properties. Vibrational and electronic spectroscopies, in combination with magnetic susceptibility measurements, revealed 1 exhibits mixed valency between the radical-bearing trianionic and diamagnetic tetraanionic oxidation states of the ligand. This unpaired electron density on the ligand forms a partially occupied kagomé lattice where approximately 85% of the lattice sites are occupied with an S = ½ spin. We found that gallium mediates ferromagnetic coupling between ligand spins, creating a ferromagnetic kagomé lattice. By modulation of the interlayer spacing via post-synthetic cation metathesis of 1 to (NMe4)3.5Ga2(C6O4Cl2)3 (2) and (NEt4)2(NMe4)1.5Ga2(C6O4Cl2)3 (3), we determined the nature of the magnetic coupling between neighboring planes is antiferromagnetic. Additionally, we determined the role of the metal in mediating this magnetic coupling by comparison of 2 with the In3+ analogue, (NMe4)3.5In2(C6O4Cl2)3 (4), and we found that Ga3+ supports stronger superexchange coupling between ligand-based spins than In3+. The combination of intraplanar ferromagnetic coupling and interplanar antiferromagnetic coupling exchange interactions suggests these are promising materials to host topological phenomena., 2D metal–organic frameworks provide insight into kagomé spin physics.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (TC). Here, we establish a predictive and quantitative relationship between TC and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden–Popper (RP) A3B2O7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1–xCax)3Sn2O7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-ord...

Ruddlesden–Popper strontium titanates with composition Srn+1TinO3n+1, RP-n-STO, n = 1, 2, ..., ∞, are prototypical layered perovskites from which many crystal-chemistry principles can be assessed. In addition, their phase stabilities and lattice thermal expansions are essential factors for their usage in realistic devices. In this work, we use first-principles electronic structure methods to determine the n-layer dependent phase stabilities, including thermodynamic energies and pressure–critical temperature (P–Tc) phase diagrams and coefficients of thermal expansion. We correlate these properties with the perovskite (SrTiO3)n·SrO block thickness, n, to show that a change in materials behavior occurs at a critical thickness nc = 3, which we ascribe to the presence (absence) of octahedral layers exhibiting antiferrodistortions at n ≥ 3 (n < 3). Last, we attribute the absence of quasi-two-dimensional lattice anharmonicity in the layered RP titanates to the stiff (SrTiO3)n intrablock Ti–O bonds and strong int...

Borate halides are an ideal materials class from which to design high-performance nonlinear optical (NLO) materials. Currently, borate fluorides, chlorides, and bromides are extensively investigated while borate iodide materials discovery remains rare because of the perceived synthetic challenges. We report a new borate iodide, Pb2 BO3 I, synthesized by a straightforward hydrothermal method. The Pb2 BO3 I chemical formula conceals that the compound exhibits a structure similar to the well-established KBe2 BO3 F2 (KBBF), which we show supports the highest second-harmonic generation (SHG) at 1064 nm in the KBBF family, 10 × KH2 PO4 (KDP), arising from the inclusion of Pb2+ and I- and the crystal chemistry. Our work shows that KBBF-related compounds can be synthesized incorporating iodide and exhibit superior NLO responses.

The double perovskite CaMnTi2O6, is a rare A-site ordered perovskite oxide that exhibits a sizable ferroelectric polarization and relatively high Curie temperature. Using first-principles calculations combined with detailed symmetry analyses, we identify the origin of the ferroelectricity in CaMnTi2O6. We further explore the material properties of CaMnTi2O6, including its ferroelectric polarization, dielectric and piezoelectric responses, magnetic order, electronic structure, and optical absorption coefficient. It is found that CaMnTi2O6 exhibits room-temperature-stable ferroelectricity and moderate piezoelectric responses. Moreover, CaMnTi2O6 is predicted to have a semiconducting energy band gap similar to that of BiFeO3, and its band gap can further be tuned via distortions of the planar Mn–O bond lengths. CaMnTi2O6 exemplifies a new class of single-phase semiconducting ferroelectric perovskites for potential applications in ferroelectric photovoltaic solar cells.

Electronic materials that exhibit phase transitions between metastable states (e.g., metal-insulator transition materials with abrupt electrical resistivity transformations) are challenging to decode. For these materials, conventional machine learning methods display limited predictive capability due to data scarcity and the absence of features that impede model training. In this article, we demonstrate a discovery strategy based on multi-objective Bayesian optimization to directly circumvent these bottlenecks by utilizing latent variable Gaussian processes combined with high-fidelity electronic structure calculations for validation in the chalcogenide lacunar spinel family. We directly and simultaneously learn phase stability and bandgap tunability from chemical composition alone to efficiently discover all superior compositions on the design Pareto front. Previously unidentified electronic transitions also emerge from our featureless adaptive optimization engine. Our methodology readily generalizes to optimization of multiple properties, enabling co-design of complex multifunctional materials, especially where prior data is sparse.

We report a workflow for heteroanionic materials discovery using Pauling's second rule to filter for and predict new candidate materials for synthesis with reduced computational overhead. Using oxyfluoride and oxynitride n = 1 Ruddlesden-Popper compounds as a use-case, we show that a minimization scheme based on the global instability index (GII) efficiently filters up to 50% of highly unstable candidate compositions based on crystal-chemistry grounds. We then validate the minimization scheme using density functional theory (DFT) calculations and find that unexpectedly the GII of stable heteroanionic materials is higher than that of homoanionic oxides owing to significant charge redistribution in compounds containing more than one anionic species. Using this workflow, we predict Sr2AlO3F to be stable and describe our attempts to synthesize a phase-pure material.

The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{)}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Prior work has established site disorder by the ${\mathrm{Bi}}^{3+}$ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state.

Author(s): Yang, W; Chandrasena, RU; Gu, M; Dos Reis, RMS; Moon, EJ; Arab, A; Husanu, MA; Nemsak, S; Gullikson, EM; Ciston, J; Strocov, VN; Rondinelli, JM; May, SJ; Gray, AX | Abstract: Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3]×15 wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.

We report the high-pressure, high-temperature synthesis of $\mathrm{BiV}{\mathrm{O}}_{3}$, a cubic perovskite that thus far has remained inaccessible under ambient pressure conditions. We created this material at $\ensuremath{\sim}25$ GPa and 1500 K in a laser-heated diamond-anvil cell and recovered it to ambient pressure and temperature. Our synthetic approach circumvents the oxidative chemistry that, at ambient pressures, previously rendered the cubic $\mathrm{BiV}{\mathrm{O}}_{3}$ perovskite inaccessible. We find through density-functional theory calculations that this material is a unique metallic and antiferromagnetic transition-metal oxide.

A new beryllium-free deep-ultraviolet (DUV) nonlinear optical (NLO) material, β-Rb2Al2B2O7 (β-RABO), has been synthesized and characterized. The chiral nonpolar acentric material shows second-harmonic generation (SHG) activity at both 1064 and 532 nm with efficiencies of 2×KH2PO4 and 0.4×β-BaB2O4, respectively, and exhibits a short absorption edge below 200 nm. β-Rb2Al2B2O7 has a three-dimensional structure of corner-shared Al(BO3)3O polyhedra. The discovery of β-RABO shows that through careful synthesis and characterization, replacement of KBe2BO3F2 (KBBF) by a Be-free DUV NLO material is possible.

Noncentrosymmetric mixed-metal carbonate fluorides are promising materials for deep-ultraviolet (DUV) nonlinear optical (NLO) applications. We report on the synthesis, characterization, structure–property relationships, and electronic structure calculations on two new DUV NLO materials: KMgCO3F and Cs9Mg6(CO3)8F5. Both materials are noncentrosymmetric (NCS). KMgCO3F crystallizes in the achiral and nonpolar NCS space group P62m, whereas Cs9Mg6(CO3)8F5 is found in the polar space group Pmn21. The compounds have three-dimensional structures built up from corner-shared magnesium oxyfluoride and magnesium oxide octahedra. KMgCO3F (Cs9Mg6(CO3)8F5) exhibits second-order harmonic generation (SHG) at both 1064 and 532 nm incident radiation with efficiencies of 120 (20) × α-SiO2 and 0.33 (0.10) × β-BaB2O4, respectively. In addition, short absorption edges of

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI3 has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic–inorganic lead halide perovskite CH3NH3PbI3, CsSnI3 solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI3. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI3, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)Sn2I6. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI6 network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI3. More importantly, unlike nonpolar CsSnI3, the e...

The hybrid two-dimensional (2D) halide perovskites have recently drawn significant interest because they can serve as excellent photoabsorbers in perovskite solar cells. Here we present the large scale synthesis, crystal structure, and optical characterization of the 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, 4, ∞) perovskites, a family of layered compounds with tunable semiconductor characteristics. These materials consist of well-defined inorganic perovskite layers intercalated with bulky butylammonium cations that act as spacers between these fragments, adopting the crystal structure of the Ruddlesden–Popper type. We find that the perovskite thickness (n) can be synthetically controlled by adjusting the ratio between the spacer cation and the small organic cation, thus allowing the isolation of compounds in pure form and large scale. The orthorhombic crystal structures of (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 (n = 2, Cc2m; a = 8.9470(4), b = 39.347(2) A, c = 8.8589(6)), (CH3(CH2)3NH3)2(CH3NH3)2Pb3I10 (...

A family of six nonlinear optical (NLO) materials, A3B3CD2O14 (A = Sr, Ba, or Pb; B = Mg or Zn; C = Te or W; and D = P or V), has been synthesized and characterized. In addition to the synthesis and crystal structures, comprehensive characterization of these compounds includes second harmonic generation (SHG) measurements, theoretical calculations, infrared and diffuse reflectance spectroscopies, and thermogravimetric measurements. We find that all of the reported materials are SHG-active at 1064 nm, with responses ranging from 2.8 to 13.5 × KDP, and exhibit absorption edges in the mid- to deep-ultraviolet regime. By systematically replacing the A, B, C, and D cations, we are able to tune these properties and investigate the role of different NLO-active structural units in producing the SHG responses. Specifically, our electronic structure calculations reveal that the presence of Pb(2+) on the A-site and Te(6+) on the C-site is critical for generating a large SHG response. The synthesis and structure-property relationships described in this family of materials will enable the design and discovery of new NLO materials.

The burgeoning field of anion engineering in oxide-based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent-bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide-only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion-engineering principles in heteroanionic materials are reviewed.

We report the synthesis of $\mathrm{SrMn}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}{\mathrm{F}}_{\ensuremath{\gamma}}$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown $\mathrm{SrMn}{\mathrm{O}}_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF), and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene to isolate the role of carbon in the vapor transport process. The F distribution was probed by x-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The $c$-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using x-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F substitutes on O sites in the lattice as opposed to filling anion vacancy sites, a finding further supported by calculated formation energies of different F site occupancies. These results provide insights into topochemical fluorination of perovskite oxides, which should enable future synthesis and design efforts focused on oxyfluoride heterostructures.

Phase transitions in ABX3 perovskites are often accompanied by rigid rotations of the corner-connected BX6 octahedral network. Although the mechanisms for the preferred rotation patterns of perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a(0)a(0)c(+) in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the A-site cations, and enhances the covalency between the A-site metal and Br(-) or I(-) ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors.

Photovoltaic (PV) materials, which directly convert solar radiation into electrical energy, are one of the most important renewable energy materials platforms. The power conversion efficiency of many candidate PV compounds, however, is often too low for broad commercial use. In general, there are three main features to optimize for efficient power conversion: (i) absorption of solar light, (ii) separation of photon-excited electron and hole pairs, and (iii) transportation of electron and hole carriers from bulk to surface. First, the band gap should be smaller than 2 eV to match the energy distribution of the solar spectrum and large enough to generate sufficient open circle voltages, which makes 1 to 2 eV the optimal band gap range. Second, a built-in electric field can improve charge separation and is usually achieved via a p–n junction. Finally, the charge carrier diffusion length should exceed the absorption depth, and this can be supported by selecting chemistries that give large band dispersions (small electron and hole effective masses) near the Fermi level.

A new ultraviolet nonlinear optical (NLO) material, Pb3Mg3TeP2O14 (PMTP), has been synthesized and characterized. The chiral material exhibits a large second harmonic generation (SHG) response of 13.5 × KDP (600 × α-SiO2), and the shortest absorption edge (250 nm) of reported materials with a strong SHG response (10 × KDP). PMTP has a three-dimensional crystal structure of corner-shared MgO4, PO4, and TeO6 polyhedra, which form a [TeMg3P2O14]∞ framework. Electronic structure calculations revealed that the stereoactive lone pair on the Pb(2+) cation is critical to producing the substantial NLO response and that the NLO activity is further enhanced by the presence of triply bidentate Te(6+) cations found in Te-O-O-Pb rings.

Access to the elusive deep-ultraviolet by direct second harmonic generation (SHG) enabled by a new beryllium-free zincoborate-phosphate crystal is reported. Ba3(ZnB5O10)PO4, exhibits large SHG responses at 1064 and 532 nm and a short 180 nm absorption edge. Centimeter-size crystals are grown, and quantum mechanical calculations demonstrate the key role played by ZnO4 tetrahedra in the enhanced optical responses.

A new deep-ultraviolet nonlinear optical material, RbMgCO3F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 × α-SiO2 and 0.6 × β-BaB2O4, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO3F possesses a three-dimensional structure of corner-shared Mg(CO3)2F2 polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg2+ creates pentagonal channels where the Rb+ resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response.

Controlling interfaces between transition-metal oxides and dissimilar structures is crucial for practical applications, yet has remained a quandary. Now, a coherent interface that bridges a perovskite and a fluorite structure has been formed using judiciously chosen metal cations.

The local symmetry, beyond the averaged crystallographic structure, tends to bring unusual performances. Negative thermal expansion is a peculiar physical property of solids. Here, we report the delicate design of the localized symmetry breaking to achieve controllable thermal expansion in ScF3 nanoscale frameworks. Intriguingly, an isotropic zero thermal expansion is concurrently engineered by localized symmetry breaking, with a remarkably low coefficient of thermal expansion of about +4.0 × 10–8/K up to 675 K. This mechanism is investigated by the joint analysis of atomic pair distribution function of synchrotron X-ray total scattering and extended X-ray absorption fine structure spectra. A localized rhombohedral distortion presumably plays a critical role in stiffening ScF3 nanoscale frameworks and concomitantly suppressing transverse thermal vibrations of fluorine atoms. This physical scenario is also theoretically corroborated by the extinction of phonon modes with negative Gruneisen parameters in rh...

We use a symmetry-based structural analysis combined with an electronic descriptor for bond covalency to explain the origin of the second-order nonlinear optical response (second harmonic generation, SHG) in noncentrosymmetric nonpolar ATeMoO6 compounds (where A = Mg, Zn, or Cd). We show that the SHG response has a complex dependence on the asymmetric geometry of the AO6 and AO4 functional units and the orbital character at the valence band edge, which we are able to distinguish using an A–O bond covalency descriptor. The degree of covalency between the divalent A-site cation and the oxygen ligands dominates over the geometric contributions to the SHG arising from the acentric polyhedra, and this can be understood from considerations of the local static charge density distribution. The use of a local dipole model for the polyhedral moieties (AO4/AO6, MoO4, and TeO4) can account for a nonzero SHG response, even though the materials exhibit nonpolar structures; however, it is insufficient to explain the cha...

The 3 d transition metal ( M ) perovskite oxides exhibit a remarkable array of properties, including novel forms of superconductivity, magnetism and multiferroicity. Strain can have a profound effect on many of these properties. This is due to the localized nature of the M 3 d orbitals, where even small changes in the M –O bond lengths and M –O–M bond angles produced by strain can be used to tune the 3 d – O 2 p hybridization, creating large changes in electronic structure. A new route is presented to strain the M–O bonds in epitaxial two-dimensional perovskite fi lms by tailoring local electrostatic dipolar interactions within every formula unit via atomic layer-by-layer synthesis. The response of the O anions to the resulting dipole electric fi elds distorts the M –O bonds by more than 10%, without changing substrate strain or chemical composition. This distortion is largest for the apical oxygen atoms (O ap ), and alters the transition metal valence state via self-doping without chemical substitution.

It is shown that there is a dynamic lattice instability in the aristotypeP63/mstructure ofA10(PO4)6F2apatites containing divalentA-site Cd or Hg cations with (n − 1)d10ns0electronic configurations. The distortion to a low-symmetry P\bar{1} triclinic structure is driven by an electronic mechanism rather than from ionic size mismatch. Our theoretical work provides key insights into the role of the electronic configurations ofAcations in fluorapatites.

Two lead fluorocarbonates, RbPbCO3F and CsPbCO3F, were synthesized and characterized. The materials were synthesized through solvothermal and conventional solid-state techniques. RbPbCO3F and CsPbCO3F were structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of corner-shared PbO6F2 polyhedra. For RbPbCO3F, infrared and ultraviolet–visible spectroscopy and thermogravimetric and differential thermal analysis measurements were performed. RbPbCO3F is a new noncentrosymmetric material and crystallizes in the achiral and nonpolar space group P6̅m2 (crystal class 6̅m2). Powder second-harmonic generation (SHG) measurements on RbPbCO3F and CsPbCO3F using 1064 nm radiation revealed an SHG efficiency of approximately 250 and 300 × α-SiO2, respectively. Charge constants d33 of approximately 72 and 94 pm/V were obtained for RbPbCO3F and CsPbCO3F, respectively, through converse piezoelectric measurements. Electronic structure calculations indicate that the nonlinear optical response originates from the distorted PbO6F2 polyhedra, because of the even–odd parity mixing of the O 2p states with the nearly spherically symmetric 6s electrons of Pb2+. The degree of inversion symmetry breaking is quantified using a mode-polarization vector analysis and is correlated with cation size mismatch, from which it is possible to deduce the acentric properties of 3D alkali-metal fluorocarbonates., Two lead fluorocarbonates, RbPbCO3F and CsPbCO3F, were synthesized and characterized. Both materials exhibit three-dimensional structures consisting of corner-sharing Pb(CO3)3F2 polyhedra. The bridging F atom in RbPbCO3F is statistically disordered in the ab-plane, whereas that in CsPbCO3F is clearly disordered along the c-axis. RbPbCO3F and CsPbCO3F are noncentrosymmetric and crystallize in the achiral and nonpolar space group P6̅m2. Additionally, an improved structural understanding of the acentric properties was developed through the specific acentric-mode displacements analyses.

We investigate structural coupling of the MnO6 octahedra across a film/substrate interface and the resultant changes of the physical properties of ultrathin La2/3Sr1/3MnO3 (LSMO) films. In order to isolate the effect of interfacial MnO6 octahedral behavior from that of epitaxial strain, LSMO films are grown on substrates with different symmetry and similar lattice parameters. Ultrathin LSMO films show an increased magnetization and electrical conductivity on cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) compared to those grown on orthorhombic NdGaO3 (NGO) substrates, an effect that subsides as the thickness of the films is increased. This study demonstrates that interfacial structural coupling can play a critical role in the functional properties of oxide heterostructures.

The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O K-edge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bu...

Noncentrosymmetric (NCS) phases are seldom seen in layered A2BO4 Ruddlesden-Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of two distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a "compositional" A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A(2+) cations, identifying RP-structured Ca2IrO4 as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca2IrO4 and suggest pathways to stabilize the NCS structure.

Demands for low-power and high-efficiency electronic devices have spurred an increased interest in new ferroelectric oxides, which display spontaneous electric polarizations. There are only a few mechanisms, however, capable of producing ordered dipoles in solid-state materials. Using first-principles density functional calculations, we extend the current repertoire and identify the required rotational patterns conducive to “geometric” ferroelectricity in (A,A′)B2O6 perovskite oxides with A cation order along [001]-, [111]-, and [110]-directions. For the polar oxides, we show that electric polarizations arise through a geometric, “rotation-induced” mechanism and are greater than those induced by spin-driven mechanisms. We also discuss the energetics of each ordered arrangement and explain how competing centrosymmetric phases can lead to potential complications in thin-film growth of these materials. Finally, we generalize these results to a simple set of structural chemistry guidelines, which may be used ...

The generation of intense coherent deep-UV light from nonlinear optical materials is crucial to applications ranging from semiconductor photolithography and laser micromachining to photochemical synthesis. However, few materials with large second harmonic generation (SHG) and a short UV-cutoff edge are effective down to 200 nm. A notable exception is KBe2BO3F2, which is obtained from a solid-state reaction of highly toxic beryllium oxide powders. We designed and synthesized a benign polar material, Ba4B11O20F, that satisfies these requirements and exhibits the largest SHG response in known borates containing neither lone-pair-active anions nor second-order Jahn-Teller-active transition metals. We developed a microscopic model to explain the enhancement, which is unexpected on the basis of conventional anionic group theory arguments. Crystal engineering of atomic displacements along the polar axis, which are difficult to attribute to or identify within unique anionic moieties, and greater cation polarizabilities are critical to the design of next-generation SHG materials.

One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegard’s principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca2Fe2O5 and Sr2Fe2O5. We then realize epitaxial (SrFeO2.5)1/(CaFeO2.5)1 thin film superlattices possessing both anion-vacancy order and Sr and Ca ...

The applications of transition metal oxides as photovoltaic and photocatalytic materials are mainly impeded by their poor visible light absorption, low photogenerated carrier mobility, and low valence band position, which originate from the generally large band gap (≥3 eV), narrow transition metal d states, and deep oxygen 2p states. Here, we conceive a design strategy to realize small band gap polar oxides with high carrier mobilities by combining small radii A cations with Bi3+/Bi5+ charge disproportion. We show that these cation sizes and chemical features shift the valence band edge to higher energies and therefore reduce the band gap, promoting the formation of highly dispersive Bi 6s states near the Fermi level as a byproduct. By means of advanced many-electron-based first-principles calculations, we predict a new family of thermodynamically stable/metastable polar oxides ABiO3 (A = Ca, Cd, Zn, and Mg), which adopt the Ni3TeO6-type (space group R3) structure and exhibit optical band gaps of ∼2.0 eV, as promising single phase photovoltaic and photocatalytic materials operating in the visible light spectrum.

Deep ultraviolet (absorption edge 6.2 eV) nonlinear optical (NLO) materials are of current interest owing to their technological applications and materials design challenges. Technologically, the materials are used in laser systems, atto-second pulse generation, semiconductor manufacturing, and photolithography. Designing and synthesizing a deep UV NLO material requires crystallographic non-centrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient (dij > 0.39 pm/V), moderate birefringence (Δn ∼ 0.07), chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. This review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence. Finally, future directions with respect to new deep UV NLO materials are discussed.

Phase transitions are ubiquitous in structurally complex transition metal compounds composed of homoanionic polyhedra, including nitrides, oxides, and fluorides. The symmetry breaking that occurs across polymorphic transitions is often achieved by small atomic displacements, rendering these displacive transitions reversible. In contrast, elemental crystals, alloys, and simple minerals will exhibit reconstructive "bond-breaking" transitions. Here we show that a reconstructive transition occurs in the heteroanionic compound KNaNbOF5, owing to reorientations of the [NbOF5](2-) units that trigger a reconfiguration of the cation lattice. Using a combination of synchrotron-based measurements, empirical dynamic simulations, and ab initio calculations, we report structure changes across the transition and formulate an atomistic minimum energy transition path to explain its irreversible nature. Our results indicate that multianionic compounds are likely to host reconstructive transitions that are frequently difficult to study and functionalize in simpler compounds. We anticipate that our insight into the forces that drive the transition will also lead to novel methods to control the assembly of structures in the solid state.

The new UV nonlinear optical title material is prepared by solid state reaction of a stoichiometric mixture of PbO, MgO, H2TeO4·2H2O, and NH4H2PO4 (Pt plate, 400 °C for 20 h followed by 850 °C for 5 d).

Research in ABO3 perovskite oxides ranges from fundamental scientific studies in superconductivity and magnetism to technologies for advanced low-power electronics, energy storage, and conversion. The breadth in functionalities observed in this versatile materials class originates, in part, from the ability to control the local and extended crystallographic structure of corner-connected octahedral units. While an established paradigm exists to alter the size, shape, and connectivity of the octahedral building blocks in bulk materials, these approaches are often limited to certain subsets of the allowed perovskite archetypes and chemistries. In this article, we describe emerging routes in thin films and multilayer superlattices enabled by epitaxial synthesis aimed at engineering the octahedral connectivity—rotational magnitudes and patterns—to reach unexplored portions of the crystallographic structure–property phase space for rational materials design. We review three promising chemistry-independent strategies that provide a handle to tune the octahedral connectivity: epitaxial strain, interfacial control at perovskite/perovskite heterojunctions, and rotation engineering in short-period superlattices. Finally, we touch upon potential new functionalities that could be attained by extending these approaches to static and dynamic manipulation of the perovskite structure through external fields and highlight unresolved questions for the deterministic control of octahedral rotations in perovskite-structured materials.

A new deep-ultraviolet nonlinear optical material, RbMgCO3F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 × α-SiO2 and 0.6 × β-BaB2O4, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO3F possesses a three-dimensional structure of corner-shared Mg(CO3)2F2 polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg2+ creates pentagonal channels where the Rb+ resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response.