Refined crystal structures of the scandium garnets Ca3Sc2Si3O12, Ca3Sc2Ge3O12, and Cd3Sc2Ge3O12

Journal of Structural Chemistry -     

Lattice enthalpies and magnetic loops in lanthanide iron garnets,Ln3Fe5O12

Experimental lattice enthalpies Δ_LH^θ of lanthanide iron garnets Ln₃Fe₅O₁₂ (LnIGs) with Ln = Ce − Lu, have been determined from the Born–Haber thermochemical cycle. The results have been compared with those reported by other authors using numerical simulation or an empirical equation. The lattice enthalpy of PmIG has been predicted by linear interpolation between Δ_LH^θ of the adjacent NdIG and SmIG. It has been found that the partial derivative (∂Δ_LH^θ/∂V_m) for the series of LnIGs corresponds by magnitude and dimension to an upper limit of their rigidity. New effect of magnetic loops has been noted in the same series in the dependence of the lattice magnetic entropies on the effective magnetic moments (theoretical and experimental values).     

Yttrium pyrogermanate,Y2Ge2O7

The structure of diyttrium digermanate, Y₂Ge₂O₇, has been determined in the tetragonal space group P4₃2₁2. It contains one Y, one Ge (both site symmetry 1 on general position 8b) and four O atoms [one on special position 4a (site symmetry ..2) and the remaining three on general positions 8b]. The basic units of the structure are isolated Ge₂O₇ groups, sharing one common O atom and displaying a Ge—O—Ge angle of 134.9 (3)°, and infinite helical chains of pentagonal YO₇ dipyramids, parallel to the 4₃ screw axis. The crystal investigated here represents the left‐handed form of the tetragonal R₂Ge₂O₇ compounds (R = Eu³⁺, Tb³⁺, Er³⁺, Tm³⁺ and Lu³⁺).     

Raman study of ACu3Fe4O12 (A = Ca,Sr, Y and Eu)

A vibrational study of ACu₃Fe₄O₁₂ (A = Ca, Sr, Y and Eu) compounds was carried out by means of micro-Raman scattering and lattice dynamics calculations. Polarized Raman scattering measurements were performed on rectangular microcrystals, with sizes close to 7 μm, and six Raman active modes were observed among the eight expected. It was then possible to assign the observed modes to the correct symmetry. Moreover, lattice dynamics calculations led to determine the main atomic displacements and a good agreement was found between experimental and theoretical wavenumbers. Furthermore, wavenumbers evolution versus A cation showed two behaviors separating the samples into two groups.     

Vanadate garnet,Ca2NaMg2V3O12

The vanadate garnet Ca₂NaMg₂V₃O₁₂ (dicalcium sodium dimagnesium trivanadium dodecaoxide), synthesized by a floating zone method, has a notable structural feature in that the dodecahedral–dodecahedral shared edge length is longer than the unshared dodecahedral edge length. It is also noteworthy that the octahedral–dodecahedral shared edge length is as long as the unshared octahedral edge length. These unusual structural features are closely related to the weak repulsions between dodecahedral cations and between dodecahedral and octahedral cations.     

Ionic and electronic transport in the garnet-type vanadate Ca2.5Mg2V3O12

Journal of Solid State Electrochemistry - The Ca2.5Mg2V3O12 vanadate with cubic structure (space group Ia $$ overline{3} $$ d) has been prepared by solid-state reaction in air. The crystal...     

Incorporation of trivalent cations in synthetic garnets A3B5O12 (A = Y, Lu-La, B = Al, Fe, Ga)

Static-lattice atomistic calculations have been used to study the solution energy for the incorporation of 13 foreign cations at 3 different lattice positions of 12 synthetic garnets. Trends have been obtained as a function of the ionic radius of the dopant cation, and the predictions about site preference have been compared with both literature and experimental data. The preferred substitution site is mainly determined by the ionic size and has been correctly predicted in all cases. Moreover, the energy difference between the preferred substitution site and the next favored site is relatively small in several cases, and hence the foreign ions can be inserted at two different positions by using the correct stoichiometry. A remarkably different behavior has been encountered for Al garnets, due to the smaller size of the unit cell. In particular, some cations, such as Fe3+ and Ga3+, can be inserted at the dodecahedral position usually occupied by the rare-earth ion. Despite the limitations of the static-lattice approach, the results of the present simulations help in the understanding of the defect chemistry of garnets, which is strongly responsible for the physicochemical properties (such as luminescence and ferrimagnetism) that make garnets interesting for technological applications. Such results lead to the possibility of tuning the optical and luminescence properties of garnets by the formation of different types of solid solutions.     

Thermal analysis of some polynuclear coordination compounds as precursors of iron garnets (M3Fe5O12, M=Y3+ or Er3+)

The thermal behaviour of four coordination compounds (NH₄)₆[Y₃Fe₅(C₄O₅H₄)₆(C₄O₅H₃)₆]·12H₂O, (NH₄)₆[Y₃Fe₅(C₆O₇H₁₀)₆(C₆O₇H₉)₆]·8H₂O, (NH₄)₆[Er₃Fe₅(C₄O₅H₄)₆(C₄O₅H₃)₆]·10H₂O and (NH₄)₆[Er₃Fe₅(C₄O₆H₄)₆(C₄O₆H₃)₆]·22H₂O has been studied to evaluate their suitability for garnet synthesis. The thermal decomposition and the phase composition of the resulted decomposition compounds are influenced by the nature of metallic cations (yttrium-iron or erbium-iron) and ligand anions (malate or gluconate).     

High-Temperature Synthesis of the Mg2-x Ca xP4O12 and Cd2-x CaxP4O12

Abstract

The cyclo-tetraphosphates of the M^II _2-xCa_xP₄O₁₂ type (MI^II = Mg, Cd) have been synthetized as new binary compounds, and their existence for x ε (0; 1 > (Mg) or for x ε (0; 0.7 > (Cd) has been proved. The synthesis is based on a two-step thermal process. The first step starts from pure cyclo-tetraphosphates of the two divalent metals and Ca(PO₃)₂ which are melted in normal atmosphere and then abruptly cooled to give a vitreous amorphous product composed of higher linear phosphates of the summary formula (M^II _2-x Ca_x)_n/4H₂P_nO_3n+1 · In the second step, this product is repeatedly heated to a suitable temperature and recrystallized to give microcrystalline product M^II _2-x Ca_xP₄O₁₂ · The colourless (white) products crystallize in the monoclinic aystem, C2c group. Their structural parameters have the values for Mg_2-x Ca_xP₄O₁₂: a = 11.749(5) to 12.063(4) Å, b = 8.278(4) to 8.635(4) Å c = 9.905(4) to 9.875(3) Å and β = 118.92(2)° to 118.03(2)°; or for Cd_2-x Ca_xP₄O₁₂: a = 12.328(4) to 12.457(5) Å b = 8.639(3) to 8.732(4) Å c = 10.388(3) to 10.443(4) Å and β = 119.33(2)° to 119.45(2)°.     

Cationic Channels with Partial Anion Occupation in the Zintl Phases Ba2Mg12Ge7.33 and Ba6Mg17.4Li2.6Ge12O0.64

In the system Ba/(Mg, Li)/Ge, two new Zintl phases with the composition Ba₂Mg₁₂Ge_7.33 (P6₃/m, Z = 1, a = 1121.7(5) pm, c = 440.2(2) pm) and Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 (P6₃/m, Z = 1, a = 1537.8(8) pm, c = 454.6(2) pm) are found and structurally characterized. Their structures are described with respect to the Zintl‐Klemm concept, structure directing rules, and chemical twinning. These new compounds contain as a specific structural feature cationic channels with partial anion occupation which allows to adjust the electron count. In Ba₂Mg₁₂Ge_7.33, the channels are formed by Mg²⁺ cations and are partially filled with germanium dumb‐bells, while the channels in Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 are formed by Li⁺ and Mg²⁺ cations and host O^2— anions. The electronic structure of both compounds has been investigated using Extended‐Hückel calculations with special emphasis on the states of the cationic channels and their interstitial heteroatoms. The potentiality of using the electron localization function (ELF) to find missing atoms in structures has been tested and verified for both compounds.     

Synthesis of Fe3O4, Fe2O3, Ag/Fe3O4 and Ag/Fe2O3 nanoparticles and their electrocatalytic properties

Two important iron oxides:Fe3O4 and Fe2O3,as well as Fe3O4 and Fe2O3 nanoparticles mingling with Ag were successfully synthesized via a hydrothermal procedure.The samples were confirmed and characterized by X-ray diffraction(XRD),and X-ray photoelectron spectroscopy(XPS).The morphology of the samples was observed by transmission electron microscopy(TEM).The results indicated Fe3O4,Fe2O3,Ag/Fe3O4 and Ag/Fe2O3 samples all were nanoparticles with smaller sizes.The samples were modified on a glassy carbon electrode and their elctrocatalytic properties for p-nitrophenol in a basic solution were investigated.The results revealed all the samples showed enhanced catalytic performances by comparison with a bare glassy carbon electrode.Furthermore,p-nitrophenol could be reduced at a lower peak potential or a higher peak current on a glassy carbon electrode modified with Ag/Fe3O4 or Ag/Fe2O3 composite nanoparticles.     

Vibration frequencies of Ca3Fe2Si3O12 andradite: an ab initio study with the CRYSTAL code

The vibrational spectrum of Ca3Fe2Si3O12 andradite is calculated at the Gamma point by using the periodic ab initio CRYSTAL program that adopts an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The full set of frequencies (17 IR active, 25 Raman active, and 55 inactive modes) is calculated. The effect of the basis set on the calculated frequencies is discussed. The modes are characterized by direct inspection of the eigenvectors and isotopic substitution. The present calculations permit us to clarify some of the assignment problems raised by experiments. The mean absolute differences of the various modes with respect to the available experimental IR and Raman data are as small as 9 and 5 cm(-1), respectively.     

Ca3Al2Ge3O12:Cr3+的光谱性质及晶场参数计算

为了解Cr3 离子在钙铝锗酸盐Ca3Al2Ge3O12石榴石中的光谱性质,合成了Ca3Al2Ge3O12:Cr3 多晶材料;测量了其X射线衍射图,漫反射光谱,激发、发射光谱等;分析了Cr3 离子在钙铝锗酸盐中的发光特性;计算了其晶场强度(Dq/B),Stokes位移(ΔEs)及黄昆-里斯因子(S)等.在450nm激发下,Ca3Al2Ge3O12:Cr3 室温发射光谱主要由三个宽带及附加其上的弱R线构成,分别对应于Cr3 离子的4T1、4T2、2T2到4A2能级跃迁.低温时R线变得强而锐.通过计算,Dq/B=2.43,ΔEs=1884cm-1,S=5.21.表明在Ca3Al2Ge3O12中Cr3 离子处于较弱的晶场强度,电子-声子耦合较强,为发展可调谐激光材料提供重要线索.     

ZrO2、Y2O3对CH4燃烧催化剂Fe2O3/γ-Al2O3的改性作用

γ-Ａｌ2Ｏ3作为催化剂载体具有较大的比表面积,机械强度高,孔结构适宜,但不耐高温。近年来,氧化锆载体以其耐高温等[1]独特性质引起多方面的关注[2-4],它能与所负载的金属产生强烈的电子相互作用,影响催化剂的吸附、氧化和还原性能。但是ＺｒＯ2比表面积较小,且随焙烧温度的升高急剧下降,如单独作为催化剂载体,其应用受到很大限制。若将ＺｒＯ2分散到γ-Ａｌ2Ｏ3表面上,可制得兼备两者优点的复合载体。当ＺｒＯ2中加入Ｙ2Ｏ3,能产生特殊的氧空穴[5],具有氧离子传导功能和导电性;与活性组分相结合能在很大程度上提高反应速度。我们用Ｙ…     

Modeling of Al12N12, Mg12O12, Ca12O12, and C23N nanostructured as potential anode materials for sodium-ion battery

Journal of Solid State Electrochemistry - The rising costs of lithium and other versatile metals which are of electrochemical importance have sprouted concerns in the electrochemical world. Sodium...