Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb1.5Nb2O6.5.

Lone pair-driven distortions are a hallmark of many technologically important lead (Pb)-based materials. The role of Pb2+ in polar perovskites is well understood and easily manipulated for applications in piezo- and ferroelectricity, but the control of ordered lone pair behavior in Pb-based pyrochlores is less clear. Crystallographically and geometrically more complex than the perovskite structure, the pyrochlore structure is prone to geometric frustration of local dipoles due to a triangular arrangement of cations on a diamond lattice. The role of vacancies on the O' site of the pyrochlore network has been implicated as an important driver for the expression and correlation of stereochemically active lone pairs in pyrochlores such as Pb2Ru2O6.5 and Pb2Sn2O6. In this work we report on the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb1.5Nb2O6.5 upon cooling. We find that local distortions are present at all temperatures that can be described by cristobalite-type cation ordering, and this ordering persists to longer length scales upon cooling. From a crystallographic perspective, the material remains disordered and does not undergo an observable phase transition. In combination with density function calculations, we propose that the stereochemical activity of the Pb2+ lone pairs is driven by proximity to O' vacancies, and the crystallographic site disorder of the O' vacancies prohibits long range correlation of lone pair-driven distortions. This in turn prevents a low-temperature phase transition and results in an elevated dielectric permittivity across a broad temperature range.

Versatility of Tellurium in Heteroanionic Ln2O2Te (Ln = La, Ce, Pr) and Tellurate Ln2TeO6 (Ln = La, Pr).

Heteroanionic compounds continue to gain interest in materials design because the expanded composition space provides opportunities to discover new phases and tune physical properties. Among heteroanionic materials, oxytellurides comprised of oxygen and tellurium anions are relatively underexplored despite the significant role of tellurium in emerging technologies. Herein, we present synthetic strategies toward oxytelluride Ln2O2Te (Ln = La-Pr), whose layered structure features square nets of Te2- anions. Upon heating in H2 or air, we find a reversible phase transition between the oxytelluride and tellurate Ln2TeO6 (Ln = La, Pr), wherein Te is octahedrally coordinated and a 6+ oxidation state is corroborated by bond valence analysis. We use X-ray diffraction along with thermogravimetric analyses to confirm the presence of oxytelluride and tellurate phases and emphasize key structural distinctions. In contrast, we find that Ce2O2Te decomposes to form CeO2 and demonstrate the instability of Ce2O2Te in ambient conditions by timelapse X-ray diffraction and diffuse-reflectance spectroscopy experiments. Band gaps of Ln2O2Te (Ln = La-Pr) were estimated from diffuse-reflectance spectroscopy in the semiconducting range ∼2.1-2.7 eV, while band gaps for La2TeO6 and Pr2TeO6 were much larger at ∼4.3 and ∼3.7 eV, respectively.

Structural Diversity of Rare-Earth Oxychalcogenides.

Mixed-anion systems have garnered much attention in the past decade with attractive properties for diverse applications such as energy conversion, electronics, and catalysis. The discovery of new materials through mixed-cation and single-anion systems proved highly successful in the previous century, but solid-state chemists are now embracing an exciting design opportunity by incorporating multiple anions in compounds such as oxychalcogenides. Materials containing rare-earth ions are arguably a cornerstone of modern technology, and herein, we review recent advances in rare-earth oxychalcogenides. We discuss ternary rare-earth oxychalcogenides whose layered structures illustrate the characters and bonding preferences of oxide and chalcogenide anions. We then review quaternary compounds which combine anionic and cationic design strategies toward materials discovery and describe their structural diversity. Finally, we emphasize the progression from layered two-dimensional compounds to three-dimensional networks and the unique synthetic approaches which enable this advancement.

Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate.

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu2WO4), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu2WO4, which was determined to be triclinic P1. Cu2WO4 was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV-visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu2+ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO4. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu2WO4 was contrasted with that of the Cu(II) counterpart, CuWO4 using spin-polarized density functional theory (DFT). Finally, the compound Cu2WO4 was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions.

Role of f Electrons in the Optical and Photoelectrochemical Behavior of Ca(La1- xCe x)2S4 (0 ≤ x ≤ 1).

This study focuses on a solid solution series, Ca(La1- xCe x)2S4 (0 ≤ x ≤ 1), where the f electron density is absent in CaLa2S4 and is progressively increased until it is maximized in CaCe2S4. Correspondingly, these samples, synthesized by a sealed ampule method, showed progressive variations in color ranging from gray for CaLa2S4 to orange-red for CaCe2S4. The crystal structural nuances of both the end members and three solid solutions with x = 0.25, 0.50, and 0.75 were established with the complementary use of synchrotron X-ray diffraction and neutron scattering. Interestingly, these data were consistent with a two-phase composition centered around each nominal solid solution stoichiometry. Optical characterization via diffuse reflectance spectroscopy and Tauc analyses showed a shrinking of the energy band gap (from the UV to vis range) when Ce was progressively introduced into the host CaLa2S4 structure. These data were in concert with electronic band structure calculations, using density functional theory, which showed the progressive formation of an intermediate f band when Ce was introduced intro the structure. Photoelectrochemical measurements in an aqueous redox electrolyte, as well as surface photovoltage and Kelvin probe measurements, revealed all samples to be n-type semiconductors. The valence and conduction band edge positions of the end members and the three solid solutions could be mapped, on both the redox and vacuum reference energy scales, by combining these measurements with the optical data.

Challenges in intermetallics: synthesis, structural characterization, and transitions.

Intermetallics that contain rare-earth elements are particularly interesting because of their temperature- and pressure-dependent structural and physical transitions that make them potential candidates for magnetic applications. This article highlights synthetic routes and structural characterization advancements used to investigate intermetallic materials. Experimental and theoretical examples of three intermetallic structure types--ThCr(2)Si(2), Heusler and Laves--are discussed to present a historical review and to illustrate the grand challenges in unravelling structure-property relationships of intermetallic compounds.

Structures and phase transitions of CePd3+xGa8-x: new variants of the BaHg11 structure type.

New distorted variants of the cubic BaHg11 structure type have been synthesized in Ga flux. Multiple phases of CePd3+xGa8-x, which include an orthorhombic Pmmn structure (x = 3.21(2)), a rhombohedral R3m structure (x = 3.13(4)), and a cubic Fm3m superstructure (x = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and in situ temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg11, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd3+xGa8-x, the rare earth analogues REPd3+xGa8-x, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups.

CsTe2O(6-x): novel mixed-valence tellurium oxides with framework-deficient pyrochlore-related structure.

Structures of CsTe2O(6-x) phases were investigated by single-crystal X-ray diffraction and neutron powder diffraction. Stoichiometric CsTe2O6 is a mixed-valence Cs2Te4⁺Te36⁺O12 compound with a rhombohedral pyrochlore-type structure where there is complete order of Te4⁺ and Te6⁺. On heating, this compound develops significant electrical conductivity. As CsTe2O6 becomes oxygen deficient above 600 °C, the rhombohedral pyrochlore-type structure is replaced by a cubic pyrochlore-type structure with disordered Te4⁺/Te6⁺ and oxygen vacancies. However, for CsTe2O(6-x) phases prepared at 500 °C, the observed pyrochlore-type structure has symmetry. The Te4⁺ and O vacancies are all on chains running along the b axis, and the maximum value of x observed is about 0.3. At still higher values of x a new compound was discovered with a structure related to that reported for Rb4Te34⁺Te56⁺O23.

4f-electron localization in CexLa 1-xM In5 with M=Co, Rh, or Ir.

de Haas-van Alphen measurements on Ce(x)La(1-x)MIn(5) yield contrasting types of behavior that depend on whether M=Co and Ir or M=Rh. A stronger x-dependent scattering in the case of M=Co and Ir is suggestive of a stronger relative coupling, J/W, of the conduction electrons to the 4f electrons, which would then account for the development of a heavy composite Fermi-liquid state as x-->1. The failure of a composite Fermi-liquid state to form for any x in the case of M= Rh is shown to be inconsistent with theoretical models that propose antiferromagnetism to result from spin-density-wave formation.

Rare beryllium icosahedra in the intermediate valence compound CeBe13.

Single-crystal X-ray diffraction experiments show that the Be atoms in CeBe13 form a Be12 icosahedra, which is a very unusual structural feature due, in part, to the remarkably low valence electron count of Be. Magnetization studies show that CeBe13 displays intermediate valence behavior, in which valence fluctuations between the Ce 4f0 and 4f1 states give rise to enhanced electronic specific heat and magnetic susceptibility. Calculations using ab initio theory were used to determine the electronic structure and bonding and to give insight into the relationship between the crystal structure, the bonding, and the intermediate valence behavior of CeBe13. The hybridization between the localized f electrons and the conduction electrons is responsible for the large values of the electronic specific heat coefficient (gamma approximately 100 mJ/mol K2) and magnetic susceptibility (chi approximately 1 x 10-3 emu/mol), which is in marked contrast to those of ordinary metals that have gamma approximately 1 mJ/mol K2 and chi approximately 1 x 10-5 emu/mol values. The magnetic susceptibility, chi = M/H versus T, of a single crystal of CeBe13 exhibits a broad maximum at Tmax approximately 130 K and is typical of intermediate valence systems with an unusually large energy scale (Kondo), TK approximately 500 K.

Synthesis, structure, and magnetoresistance of SmPd2Ga2.

Single crystals of a new ternary compound, SmPd(2)Ga(2), have been synthesized by flux growth methods. This compound adopts a tetragonal space group I4/mmm, Z = 2, with lattice parameters a = 4.2170(3) A and c = 10.4140(3) A. The crystal structure is composed of layers of isolated Sm atoms and layers of PdGa(4) edge-sharing tetrahedra alternating along the c-axis. The sample is metallic (d rho/dT > 0) with a weak temperature dependence above 100 K. This new material has physical properties similar to those of other Sm intermetallics and has, most notably, a large positive magnetoresistance at low temperatures. Magnetic measurements indicate that SmPd(2)Ga(2) is ferromagnetic with T(c) approximately 5 K.