Infinitely Adaptive Transition-Metal Ordering in Ln2O2MSe2-Type Oxychalcogenides

Mixed Rare-Earth Chalcogenide Borate Eu9-xRExMgS2B20O41 (RE = Sm, Gd) Featuring a 3D {[B20O41]22-}∞ Framework Connected by [B6O9(O0.5)6]6- and [B7O13(O0.5)3]8- Clusters

Rare-earth (RE) chalcogenide borates are very rarely discovered in view of the difficulties in synthesis though they have demonstrated attractive physical performances. Here, the first mixed RE chalcogenide borates Eu_5.4Sm_3.6MgS₂B₂₀O₄₁ (1) and Eu₃Gd₆MgS₂B₂₀O₄₁ (2) are synthesized by combining RE, sulfur, and borate ions into one structure. They crystallize in the centrosymmetric hexagonal space group P6₃/m, and their 3D honeycomb-like {[B₂₀O₄₁]^22-}_∞ open frameworks are built by [B₆O₉(O_0.5)₆]^6- and [B₇O₁₃(O_0.5)₃]^8- polyanionic clusters and consolidated by Mg²⁺ ions; both of which are formed by BO₄ tetrahedra and BO₃ planar triangles. The coordination modes of RE ions are rare REO₆S₂ bicapped trigonal prisms and REO₈S irregular polyhedra, and their band gaps are determined to be 2.25 and 2.22 eV, respectively. They exhibit antiferromagnetic interactions and distinct photocurrent responses. The corresponding theoretical calculations are also performed. The study of 1 and 2 perhaps stimulates interest in exploring new functional RE chalcogenide borates.

Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr2CrO2Cr2OAs2 and Sr2CrO3CrAs

Two novel chromium oxide arsenide materials have been synthesized, Sr₂CrO₂Cr₂OAs₂ (i.e., Sr₂Cr₃As₂O₃) and Sr₂CrO₃CrAs (i.e., Sr₂Cr₂AsO₃), both of which contain chromium ions in two distinct layers. Sr₂CrO₂Cr₂OAs₂ was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO₄As₂ octahedra containing Cr²⁺ and distorted CrO₂As₄ octahedra containing Cr³⁺. In contrast, Sr₂CrO₃CrAs incorporates Cr³⁺ in CrO₅ square-pyramidal coordination in [Sr₂CrO₃]⁺ layers and Cr²⁺ ions in CrAs₄ tetrahedra in [CrAs]⁻ layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr₂CrO₃CrAs the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering, while the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering. However, in Sr₂CrO₂Cr₂OAs₂, both the Cr²⁺ moments in the CrO₄As₂ environments and the Cr³⁺ moments in the CrO₂As₄ polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr³⁺ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr₂CrO₂Cr₂OAs₂ below the magnetic ordering transitions.

Sr₂CrO₂Cr₂OAs₂ and Sr₂CrO₃CrAs are both mixed-anion materials containing chromium ions in two unique layers. In Sr₂CrO₂Cr₂OAs₂, Cr³⁺ ions in CrO₂As₄ environments order antiferromagnetically at around 600 K and Cr²⁺ ions in CrO₄As₂ environments also order antiferromagnetically at a lower temperature of 530(10) K. In contrast, only the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering in Sr₂CrO₃CrAs as the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering.

pdCIFplotter: visualizing powder diffraction data in pdCIF format

A program is described for visualizing powder diffraction data and models published in powder CIF format.

A description is given of the program pdCIFplotter. This program is used for visualizing powder diffraction data and models published in powder CIF format (pdCIF). In particular, support for the visualization of multi-pattern data sets, such as in situ diffraction experiments, is provided by means of stack and surface plots. pdCIFplotter is written in Python 3 and can run wherever a compatible runtime is available. TOPAS macros for the production of pdCIF files are also presented.

Synthesis, crystal structure, and magnetic properties of layered SmCrS2−xSexO solid solutions

Layered SmCrS_2−xSe_xO (x = 0–2) solid solutions were synthesized which show typical antiferromagnetic orderings.

Synthesis, Crystal Structure, and Physical Properties of Layered LnCrSe2O (Ln = Ce-Nd)

The layered oxyselenides with the formula LnCrSe₂O (Ln = Ce-Nd) were synthesized via molten salt methods. The isostructural compounds crystallize in the monoclinic space group of C2/m. The crystal structures feature _∞²[CrSe₂O]^3- motifs stacked along the a axis, which are separated by Ln³⁺ ions. The _∞²[CrSe₂O]^3- layers are composed of [Cr1Se₆]^9- and [Cr2Se₄O₂]^9- octahedra via corner and edge sharing. Powder X-ray diffraction results confirm the phase purities of the as-synthesized compounds. LnCrSe₂O (Ln = Ce-Nd) show typical antiferromagnetic ordering with T_N = 125, 120, and 118 K, respectively. Heat capacity measurement for NdCrSe₂O indicates that the Debye temperature is 278.4 K. Similar metal-to-semiconductor phase transitions were observed for LnCrSe₂O (Ln = Ce-Nd) plates with transition temperatures of 115, 109, and 95 K, respectively. NdCrSe₂O also possesses a magnetoresistance effect at low temperature (<25 K) with a significant positive magnetoresistance ∼ 16% at 2 K and 1 T.

Flux synthesis, crystal structures, and physical properties of new lanthanum vanadium oxyselenides

The new lanthanum vanadium oxyselenides LaVSe₂O, La₅V₃Se₆O₇, La₅V₃Se₇O₅, La₇VSe₅O₇, and La₁₃V₇Se₁₆O₁₅ were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined using single-crystal and powder X-ray diffraction experiments. LaVSe₂O and La₅V₃Se₆O₇ adopt known structure types, whereas La₅V₃Se₇O₅, La₇VSe₅O₇, and La₁₃V₇Se₁₆O₁₅ crystallize in hitherto unknown structure types. The main building blocks of these compounds are chains of edge-sharing VSe₆, VSe₅O, and/or VSe₄O₂ octahedra, linked together by edge-sharing OLa₄ and/or OLa₃V tetrahedra forming fluorite-like ribbons. LaVSe₂O, La₅V₃Se₇O₅, and La₇VSe₅O₇ contain only V(iii) ions, whereby La₅V₃Se₆O₇ and La₁₃V₇Se₁₆O₁₅ contain mixtures of either V(iii)/V(iv) or V(iii)/V(v) cations. Magnetic measurements indicate Curie-Weiss paramagnetism and magnetic ordering of the vanadium moments at low temperatures. More precisely, we observe antiferromagnetism for La₅V₃Se₆O₇, metamagnetism for La₅V₃Se₇O₅, ferromagnetism for La₇VSe₅O₇ and a complex magnetic structure for La₁₃V₇Se₁₆O₁₅.

Flux Synthesis, Crystal Structures, and Magnetic Ordering of the Rare-Earth Chromium(II) Oxyselenides RE2CrSe2O2 (RE = La-Nd)

The rare-earth chromium(II) oxyselenides RE₂CrSe₂O₂ (RE = La-Nd) were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined by single-crystal and powder X-ray diffraction (Pb₂HgCl₂O₂-type, C2/m, Z = 2). The magnetic structure of La₂CrSe₂O₂ was solved and refined from neutron powder diffraction data. Main building blocks are chains of edge-sharing CrSe₄O₂ octahedra linked together by two edge-sharing ORE₃Cr tetrahedra forming infinite ribbons. The Jahn-Teller instability of divalent Cr²⁺ (d⁴) leads to structural phase transitions at 200 and 130 K in La₂CrSe₂O₂ and Ce₂CrSe₂O₂, respectively. RE₂CrSe₂O₂ are Curie-Weiss paramagnetic above T_N ≈ 14-17 K. Neutron powder diffraction reveals anti-ferromagnetic ordering of the Cr²⁺ moments in La₂CrSe₂O₂ below T_N = 12.7(3) K with an average ordered moment of 3.40(4) μ_B/Cr²⁺ at 4 K, which was confirmed by muon spin rotation experiments.

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O' anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations.

The modulated structures of La2–x Pr x O2MnSe2 (0 ≤ x ≤ 1) and La2–x Nd x O2MnSe2 (0 ≤ x ≤ 0.6)

The solid solutions La2–x Pr x O2MnSe2 (0≤x≤1) and La2–x Nd x O2MnSe2 (0≤x≤0.6) were synthesized in a NaI/KI flux between 800 and 900°C. The selenide oxides adopt a ZrCuSiAs-related structure with modulated [MnSe2]2–-layers which consist of a mixture of edge- and corner-sharing MnSe4/2-tetrahedra. The crystal structures are described with a (3+1)D model in superspace group Cmme(α0½)0s0. The modulation vector q can be controlled by partial substitution of La3+ for Pr3+ and Nd3+ via the unit cell volume leading to, amongst others, (La0.55Pr0.45)2O2MnSe2 with α=1/6, which allows the projection onto 3D space by using a simple sixfold a axis.

Narrowing the gap: from semiconductor to semimetal in the homologous series of rare-earth zinc arsenides RE2−yZn4As4·n(REAs) and Mn-substituted derivatives RE2−yMnxZn4−xAs4·n(REAs) (RE = La–Nd, Sm, Gd)

Condensing rocksalt-type [REAs] slabs into thicker blocks narrows the gap between valence and conduction bands within a homologous series of rare-earth-deficient ternary arsenides.