Discovery of a Low Thermal Conductivity Oxide Guided by Probe Structure Prediction and Machine Learning.

We report the aperiodic titanate Ba10 Y6 Ti4 O27 with a room-temperature thermal conductivity that equals the lowest reported for an oxide. The structure is characterised by discontinuous occupancy modulation of each of the sites and can be considered as a quasicrystal. The resulting localisation of lattice vibrations suppresses phonon transport of heat. This new lead material for low-thermal-conductivity oxides is metastable and located within a quaternary phase field that has been previously explored. Its isolation thus requires a precisely defined synthetic protocol. The necessary narrowing of the search space for experimental investigation was achieved by evaluation of titanate crystal chemistry, prediction of unexplored structural motifs that would favour synthetically accessible new compositions, and assessment of their properties with machine-learning models.

Identification of (Tb,Eu)9.43(SiO4)6O2-δ Oxy-Apatite Structures as Nanometric Inclusions in Annealed (Eu,Tb)-Doped ZnO/Si Junctions: Combined Electron Diffraction and Chemical Contrast Imaging Studies.

(Tb,Eu)-doped ZnO-annealed films at 1100 °C showed intense photoluminescense (PL) emission from Eu and Tb ions. The high-temperature annealing led to a chemical segregation and a secondary Zn-free phase formation that is suspected to be responsible for the high PL intensity. Large faceted inclusions of rare-earth (RE) silicates of a size of few hundred nanometers were observed. Owing to various advanced electron microscopy techniques, a detailed microstructural study of these nanometric inclusions combining atomic Z contrast imaging (STEM) and precession electron diffraction tomography (PEDT) data was carried out and resulted in the determination of a hexagonal P63/m-type (Tb,Eu)9.43(SiO4)6O2-δ structure related to an oxy-apatite structure. Chemical analyses from spectroscopic data (energy-dispersive X-ray mapping and electron energy loss spectroscopy) at the atomic scale showed that both RE elements sitting on two independent (4f) and (6h) atomic sites have three-fold oxidation states, while refinements of their occupancy sites from PEDT data have evidenced preferential deficiency for the first one. The deduced RE-O distances and their corresponding bond valences are listed and discussed with the efficient energy transfer from Tb3+ toward Eu3+.

Exotic Compositional Ordering in Manganese-Nickel-Arsenic (Mn-Ni-As) Intermetallics.

In this work we benefited from recent advances in tools for crystal-structure analysis that enabled us to describe an exotic nanoscale phenomenon in structural chemistry. The Mn0.60 Ni0.40 As sample of the Mn1-x Nix As solid solution, exhibits an incommensurate compositional modulation intimately coupled with positional modulations. The average structure is of the simple NiAs type, but in contrast to a normal solid solution, we observe that manganese and nickel segregate periodically at the nano-level into ordered MnAs and NiAs layers with thickness of 2-4 face-shared octahedra. The detailed description was obtained by combination of 3D electron diffraction, scanning transmission electron microscopy, and neutron diffraction. The distribution of the manganese and nickel layers is perfectly described by a modulation vector q=0.360(3) c*. Displacive modulations are observed for all elements as a consequence of the occupational modulation, and as a means to achieve acceptable Ni-As and Mn-As distances. This modulated evolution of magnetic MnAs and non-magnetic NiAs-layers with periodicity at approximately 10 Šlevel, may provide an avenue for spintronics.

Tetrahedral chain ordering and low dimensional magnetic lattice in a new brownmillerite Sr2ScFeO5.

We report the synthesis, structure and physical properties of a hitherto unreported brownmillerite compound Sr2ScFeO5. We have shown a new ordering sequence of the interlayer iron tetrahedral chains. Reduced dimensionality of the magnetic lattice and the frustration in the two dimensional iron tetrahedral chains originate complex magnetic and magneto-dielectric effects. Our study highlights a novel approach to tailor the magnetic lattice in bulk oxides.

Low Dimensional Magnetic Lattice and Room Temperature Magneto(di)electric Effect in Polyanion Ruddlesden-Popper Iron Oxides.

We have shown a new design strategy which exploits different oxyanions in a Ruddlesden-Popper (RP)-type phase to modulate the local crystal structure and magnetic lattice. Material (Sr4Fe2(SO4)0.5O6.5) with the larger voluminous oxyanion (SO4, S-O distance = 1.49 Å) as separating blocks between magnetic FeO layers shows a two-dimensional magnetic lattice. A three-dimensional magnetic lattice and spin reorientation transition is observed for the Sr4Fe2(CO3)O6, having CO3 (C-O distance = 1.25 Å), a smaller oxyanion, as a separating layer. Using mixed oxyanions (SO4 and CO3) in the central perovskite block of the RP3 phase, we have demonstrated a facile strategy to modulate the local crystal structure. The modulated displacement of the magnetic cations, which can break the local centrosymmetry, is suggested to originate the magnetodielectric effect near the magnetic ordering temperatures (higher than room temperature). Further, all CO3 containing samples show magnetodielectric coupling below room temperature due to the spin reorientation transition. The room temperature magnetodielectric effect coupled to the targeted local modulation of the crystal structure by oxyanions (in the absence of second-order Jahn-Teller active "distortion centers") opens a new door to the design of new multifunctional materials with the possibility for the room temperature application.

Impact of the iron substitution on the thermoelectric properties of Co1- xFe xS2 ( x ≤ 0.30).

The strong interplay between magnetism and transport can tune the thermoelectric properties in chalcogenides and oxides. In the case of ferromagnetic CoS2 pyrite, it was previously shown that the power factor is large at room temperature, reaching 1 mW m-1 K-2 and abruptly increases for temperatures below the Curie transition ( TC), an increase potentially due to a magnonic effect on the Seebeck ( S) coefficient. The too large thermal conductivity approximately equal to 10.5 W m-1 K-1 at room temperature prevents this pyrite from being a good thermoelectric material. In this work, samples belonging to the Co1- xFe xS2 pyrite family ( x = 0, 0.15 and 0.30) have thus been investigated in order to modify the thermal properties by the introduction of disorder on the Co site. We show here that the thermal conductivity can indeed be reduced by such a substitution, but that this substitution predominantly induces a reduction of the electronic part of the thermal conductivity and not of the lattice part. Interestingly, the magnonic contribution to S below TC disappears as x increases, while at high T, S tends to a very similar value (close to -42 µV K-1) for all the samples investigated. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Deciphering local complex order by HAADF in a disordered mixed polyanion iron oxide: Sr4Fe2[Fe0.5(SO4)0.25(CO3)0.25]O7.25.

A layered iron compound involving two divalent polyanions (carbonate and sulfate anions) was synthesized by solid state chemistry in a closed ampule in the form of a ceramic. The Sr4Fe2[Fe0.5(SO4)0.25(CO3)0.25]O7.25 compound derives from the third term of the Ruddlesden and Popper family, Sr4Fe3O10. A multiscale approach through transmission electron microscopy (TEM) and powder neutron diffraction (PND) studies shows that SO4 and CO3 groups substitute for the iron polyhedron of the central layer of the perovskite blocks and the absence of long range ordering between sulfate and carbonates and FeO5 groups. Nevertheless, waves disturbing the expected periodicity of the atomic layer are evidenced in HAADF images. An accurate analysis of these images provides a view of the local cationic order correlated with SO4 and CO3 for FeO5 substitution.

Combining Multiscale Approaches for the Structure Determination of an Iron Layered Oxysulfate: Sr4Fe2.5O7.25(SO4)0.5.

The new iron layered oxysulfate Sr4Fe2.5O7.25(SO4)0.5 has been prepared by a solid-state reaction in closed ampules into the form of ceramics and single crystals. Its atomic structure has been solved by means of spectroscopy, diffraction techniques, and high-resolution electron microscopy. Sr4Fe2.5O7.25(SO4)0.5 is a layered structure that derives from the Ruddelsden-Popper (RP) phases with the layer stacking sequence SrO/SrFeO2.5/SrFe0.5(SO4)0.5O1.25/SrFeO2.5. Within the mixed Fe3+/SO42- layer, the sulfur atoms are slightly shifted from the B site of the perovskite and each sulfate group shares two corners with iron pyramids in the basal plan without any order phenomenon. The electronic conductivity is thermally activated, while no ionic conductivity is detected.

Linear, Hypervalent Se34- Units and Unprecedented Cu4Se9 Building Blocks in the Copper(I) Selenide Ba4Cu8Se13.

Single-crystal and polycrystalline Ba4Cu8Se13 were synthesized; the average crystal structure was solved by single-crystal X-ray diffraction, and the structural model was confirmed by a detailed electron microscopy study of polycrystalline Ba4Cu8Se13. The title compound can be rationalized as (Ba2+)4(Cu+)8(Se2-)2(Se22-)4(Se34-) and crystallizes in a new structure type (space group C2/c with a = 9.171(8) Å, b = 9.146(8) Å, c = 27.35(3) Å, ß = 93.21(3)°, and V = 2291 Å3). It contains unprecedented Cu4Se9 fragments with planar Cu rectangles. These fragments form two-dimensional layers via regular (2c-2e) Se-Se bonds. Two of these layers are then connected in the third dimension via linear, hypervalent Se34- units, resulting in "sandwichlike", layered building blocks, which are stacked along c and separated by Ba. Ba4Cu8Se13 is the first example where Se22- and Se34- groups coexist. We were able to visualize the crystal structure by recording HAADF images, which clearly reveal the Cu4Se9 fragments and linear Se34- units. The title compound is a charge-balanced semiconductor and possesses a large Seebeck coefficient (380 µV K-1 at 200 K) and a low thermal conductivity (0.77 W m-1 K-1 at 200 K)-two requirements for efficient thermoelectric materials.

Magnetodielectric Effect in Crystals of the Noncentrosymmetric CaOFeS at Low Temperature.

Single crystals of the stoichiometric iron calcium oxysulfide CaOFeS have been grown by a solid-state reaction. Structural analysis of CaOFeS at room temperature by combining single-crystal X-ray diffraction data and transmission electron microscopy leads to a stoichiometric hexagonal noncentrosymmetric P63mc layered structure isostructural to CaOZnS. It is built from alternating layers made of FeOS3 tetrahedra sharing sulfur apexes and stacked with Ca(2+) planes. All Fe-O bonds are parallel to the stacking axis; this breaks the centrosymmetry, leading to a polar structure. The dielectric measurements reveal the existence of a magnetodielectric effect near 33 K in good agreement with the Neel temperature, as evidenced near 35 K by specific heat measurements reported by a different group.

Sr21Bi8Cu2(CO3)2O41, a Bi5⁺ oxycarbonate with an original 10L structure.

The layered structure of Sr21Bi8Cu2(CO3)2O41 (Z = 2) was determined by transmission electron microscopy, infrared spectroscopy, and powder X-ray diffraction refinement in space group P63/mcm (No. 194), with a = 10.0966(3)Å and c = 26.3762(5)Å. This original 10L-type structure is built from two structural blocks, namely, [Sr15Bi6Cu2(CO3)O29] and [Sr6Bi2(CO3)O12]. The Bi(5+) cations form [Bi2O10] dimers, whereas the Cu(2+) and C atoms occupy infinite tunnels running along c⃗. The nature of the different blocks and layers is discussed with regard to the existing hexagonal layered compounds. Sr21Bi8Cu2(CO3)2O41 is insulating and paramagnetic.

Synthesis and structure determination of CaSi(1/3)B(2/3)O(8/3): a new calcium borosilicate.

This article reports on the identification, synthesis, and in-situ structure determination of a new crystalline calcium borosilicate compound of composition CaSi(1/3)B(2/3)O(8/3). Synthesis was carried out by complete crystallization on annealing from a corresponding glassy composition in the widely studied CaO-SiO2-B2O3 ternary system. The crystallographic structure was determined ab initio using electron diffraction information and the charge flipping algorithm performed on synchrotron and neutron powder diffraction data collected in situ at high temperature. CaSi(1/3)B(2/3)O(8/3) is found to crystallize in the Pna2(1) (no. 33) orthorhombic space group, with a = 12.1025(4) Å, b = 5.2676(1) Å, c = 3.7132(1) Å, and V = 236.71(1) Å(3) at 650 °C. Solid-state (29)Si and (11)B NMR experiments have confirmed the existence of finite chains along the c axis, formed by corner-sharing SiO4 tetrahedra and BO3 units. Silicon and boron species share a crystallographic site, and the Si/B distribution induces different possible arrangements of the chains which are discussed in light of DFT calculations. At room temperature, the existence of a superstructure, resulting from the ordering within nanoscale domains, was explored by transmission electron microscopy.

Frustration of magnetic and ferroelectric long-range order in Bi(2)Mn(4/3)Ni(2/3)O(6).

The slight incommensurate modulation of the structure of Bi(2)Mn(4/3)Ni(2/3)O(6) is sufficient to suppress the electrical polarization which arises in commensurate treatments of the structure, due to antiferroelectric coupling of local polar units of over 900 A(3). The incommensurate structure is produced by the competition between ferroelectric Bi lone pair-driven A site displacement, chemical order of Mn and Ni on the B site, and both charge and orbital order at these transition metals. The interplay between the frustrated polar Bi displacements and the frustrated spin order at the B site, induced by positional disorder, produces magnetodielectric coupling between the incommensurately modulated lattice and the spin-glass-like ground state with an unusual relationship between the magnetocapacitance and the applied field.

Three-coordinate metal centers in extended transition metal oxides.

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure.