Ordering of (Cr,V) Layers in Nanolamellar (Cr_0.5V_0.5)_<i>n</i>+1AlC_<i>n</i> Compounds

<div><p>Nanolamellar MAX phase compounds (Cr_0.5V_0.5)_<i>n</i>+1AlC_<i>n</i> are formed with <i>n</i> = 1, 2 and 3, and their 300 K structure is studied in detail by high-resolution neutron diffraction. While the <i>n</i> = 1 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the <i>n</i> = 2 and 3 compounds show strong tendency for these elements' ordering, with the layer in the 2a(0,0,0) site of (Cr_0.5V_0.5)₃AlC₂ fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2 < <i>T</i> < 550 K, revealing a negligible thermal expansion below 100 K for all of the compounds.</p></div

Structural Properties and Reversible Deuterium Loading of MgD₂–TiD₂ Nanocomposites

Structural properties and reversible deuterium uptake of MgD₂–TiD₂ nanocomposites have been studied by joint X-ray and neutron diffraction analyses to shed light on the extremely fast hydrogenation kinetics of these materials. (1 – <i>x</i>)­MgD₂–<i>x</i>TiD₂ nanocomposites with compositions ranging between <i>x</i> = 0 and 0.5 have been prepared by reactive ball milling of Mg and Ti powders under deuterium pressure. They consist of mixtures of MgD₂ (β-and γ-polymorphs) and ε-TiD₂ phases homogenously distributed at the nanoscale with crystallite sizes below 15 nm. Minor phase miscibility is detected with Mg solubility in the TiD₂ phase up to 8 at.% and Ti solubility in the β-MgD₂ up to 7 at.% Ti. At moderate temperatures and pressures (<i>T</i> < 600 K, <i>P</i>_D2 < 1 MPa) reversible deuterium loading in MgD₂–TiD₂ nanocomposites only occurs through the β-MgD₂ to Mg transformation. Mg/MgD₂ thermodynamics is not modified as γ-MgD₂ and Ti solubility in β-MgD₂ are metastable and do not operate during reversible deuterium loading. However, the TiD₂ phase allows for outstanding D-sorption kinetics in the Mg/MgD₂ system. This paper demonstrates that TiD₂ inclusions limit the grain growth of Mg and MgD₂ phases allowing for short D-diffusion paths. Furthermore, we provide evidence that the TiD₂ phase also favors H-mobility through the existence of coherent coupling between TiD₂ and Mg/MgD₂ phases and the presence of sub-stoichiometric MgD_2‑η and TiD_2‑η phases

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Mixed Metallic Ba(Co,Fe)X_0.2O_3−δ (X = F, Cl) Hexagonal Perovskites: Drastic Effect of Fe-Incorporation on Structural and Electronic Features

Starting from the parent 10H–Ba₅Co₅X_1–<i>x</i>O_13−δ (trimeric strings of face-sharing CoO₆ octahedra with terminal CoO₄ tetrahedra, stacking sequence (chhch′)₂) and 6H–Ba₆Co₆X_1–<i>x</i>O_16‑δ (similar with tetrameric strings, stacking sequence chhhch′) hexagonal perovskites forms (X = F, Cl; <i>c</i>, <i>h</i> = [BaO₃] layers ; <i>h</i>′ = [BaOX_1–<i>y</i>] layers), we show here that the Fe incorporation leads to large domains of solid solutions for both X = F and Cl but exclusively stabilizes the 10H-form independently of the synthesis method. In this form, the lowest concentration of h-layers is stabilized by a sensitive metal reduction with increasing the Fe ratio. In a more general context of competition between several hexagonal perovskite polymorphs available for most of the transition metals, this redox change is most probably the key factor driving 1D (face-sharing chains) to 3D (corner-sharing) connectivities. Strikingly, ND data evidence the location of oxygen deficiencies in the tetrahedral (Co/Fe) coordination. This effect is exaggerated at high temperature, while (Co/Fe)­O_4‑δ coordinations are completed by the displacement of X^– anions toward the (Co/Fe) sphere of coordination following a “<i>push-and-pull</i>” mechanism within <i>h</i>′–[BaOX_1–<i>y</i>] layers. The Fe-incorporation is also accompanied by increasing conduction gaps with predominant 1D variable range hopping. The full series show antiferromagnetic behavior with increasing <i>T</i>_<i>N</i> as [Fe] increases. For Fe-rich compounds <i>T</i>_<i>N</i> is estimated about 600 K, as rarely observed for hexagonal perovskite compounds. Finally, magnetic structures of all iron-doped compounds show a site-to-site AFM ordering, different of the magnetic structure of Co-only parent compounds. Here, DFT calculations predict low-spin octahedral Co configurations, but high-spin Fe species in the same sites

Incorporation of Jahn–Teller Cu²⁺ Ions into Magnetoelectric Multiferroic MnWO₄: Structural, Magnetic, and Dielectric Permittivity Properties of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.25)

Polycrystalline samples of Mn_1–<i>x</i>Cu_<i>x</i>WO₄ (<i>x</i> ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO₄ caused by replacing Jahn–Teller-inactive Mn²⁺ (d⁵, <i>S</i> = 5/2) ions with Jahn–Teller-active Cu²⁺ (d⁹, <i>S</i> = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ <i>x</i> ≤ 0.25 are solid solution that forms in the monoclinic <i>P</i>2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO₄ structure and suggest random distributions of Jahn–Teller-distorted CuO₆ octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO₄. Type-II multiferroicity survives a weak Cu substitution rate (<i>x</i> < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for <i>x</i> = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO₄ strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration

Localization of Oxygen Interstitials in CeSrGa₃O_7+δ Melilite

The solubility of Ce in the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ and La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce³⁺ into Ce⁴⁺. Under CO reducing atmosphere, the La in LaSrGa₃O₇ may be completely substituted by Ce to form the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ solid solution, which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ compositions displayed much lower Ce solubility (<i>x</i> ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce³⁺ cations. At 600 °C, CeSrGa₃O_7+δ showed a conductivity of ∼10^–4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa₃O₇. The oxidation of Ce³⁺ into Ce⁴⁺ in CeSrGa₃O_7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa₃O_7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La_1.54Sr_0.46Ga₃O_7.27. Neutron powder diffraction study on CeSrGa₃O_7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO₄ polyhedral environment, leading to distorted GaO₅ trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa₃O_7+δ, in contrast with those in La_1+<i>x</i>Sr_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>}, may be correlated with the cationic size contraction from the oxidation of Ce³⁺ to Ce⁴⁺. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure

Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa₂O₄

The red long-lasting luminescence properties of the ZnGa₂O₄:Cr³⁺ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄ (0 ≤ <i>x</i> ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to ⁷¹Ga solid state NMR studies. In contrast to ZnGa₂O₄:Cr³⁺ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps

Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa₂O₄

The red long-lasting luminescence properties of the ZnGa₂O₄:Cr³⁺ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄ (0 ≤ <i>x</i> ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to ⁷¹Ga solid state NMR studies. In contrast to ZnGa₂O₄:Cr³⁺ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps

Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa₂O₄

The red long-lasting luminescence properties of the ZnGa₂O₄:Cr³⁺ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄ (0 ≤ <i>x</i> ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to ⁷¹Ga solid state NMR studies. In contrast to ZnGa₂O₄:Cr³⁺ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps