Dimorphie von SrTa4O11 - ein Schritt von der tetragonalen Bronzestruktur zur Struktur von CaTa4O11

Dimorphism of SrTa₄O₁₁–a Step from the Tetragonal Structure of Bronzes to the Structure of CaTa₄O₁₁ Hexagonal SrTa₄O₁₁ is a new modification and isostructural with CaTa₄O₁₁. It was obtained by heating the already known SrTa₄O₁₁ in a chlorine atmosphere at 1000–1100°C. The Guinier powder pattern could be indexed with the following hexagonal unit cell: a = 6.25 Å; c = 12.33 Å. In air at 1180°C SrTa₄O₁₁(hex.) changes into the well-known TTB-modification (corresponding to the tetragonal tungsten bronzes). The transition of SrTa₄O₁₁(TTB) to SrTa₄O₁₁(hex.) was only observed in the presence of a transporting agent (Cl₂) or a mineralizer (melt of B₂O₃) at temperatures below 1100°C. This transition could not be achieved by means of a solid state reaction. In a (Ca, Sr) Ta₄O₁₁ solid solution with at least 78 At.-% Ca the hexagonal form could be stabilized even at temperatures where otherwise the TTB-modification occurred.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Protonic conductivity

Conductivity of the Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with the cryolite structure was studied in the atmosphere with a high content of water vapors under temperature and oxygen activity variation in the gas phase. Appearance of the protonic conductivity component was proved at the temperatures below 700°C. It was found that protonic conductivity increases at a decrease in parameter x in the composition series, which is due to an increase of both the concentration of protonic defects formed in the structure and of their mobility. In the case of compositions with x < 0.15 at the temperatures below 550°C, the protonic transport becomes predominant.     

Strontium tantalum oxides with perovskite-type structure: synthesis and dye photodecomposition properties

In this work the preparation and photocatalytic properties of strontium tantalum oxides with perovskite-type structures are presented. The perovskite-type oxides were prepared by the sol–gel method and annealed at 800, 900 and 1,000 °C for 36 h. Before and after annealing the solids were characterized by XRD, N₂ adsorption (BET), UV–visible (diffuse reflectance), FTIR, TGA–DTA, and SEM-EDS techniques. The X-ray diffraction patterns of the samples showed the coexistence of three strontium tantalate oxides, Sr₂Ta₂O₇, SrTa₄O₁₁, and Sr₅Ta₄O₁₅, the relative amounts of which were highly dependent on the annealing temperature. It has been proposed that the photoactivity of the oxides in the decomposition of crystal violet dye could be related to the proportion of the Sr₂Ta₂O₇ phase in the annealed samples.     

Über Neue Oxoruthenate vom Typ der 6 L-Perowskite: Ba3SrRu2−xTaxO9 (x = 0,8 und 1,4) mit einem Beitrag über Ba3CaRu2O9

On Novel Oxoruthenates of the 6 L-Perovskite Type: Ba₃SrRu_2?xTa_xO₉ (x = 0.8 and 1.4) with a Comment on Ba₃CaRu₂O₉ Single crystals of the phases Ba₃SrRu_2?xTa_xO₉ [(I): x = 0.8 and (II): x = 1.4] and the compound (III): Ba₃CaRu₂O₉ were prepared by a BaCl₂ flux and investigated by X-ray methods. (I)–(III) crystallizes with hexagonal symmetry space group P6 2c with lattice constants: (I) a = 6.003 Å; c = 15.227 Å; (II) a = 5.988 Å; c = 15.220 Å and (III) a = 5.891 Å; c = 14.571 Å. The crystal structures of these substances corresponds to the 6 layer perovskites with the stacking sequence (hcc)₂. All of them show a so far not described slightly distorted oxygen framework caused by the Sr²⁺ and Ca²⁺ ions.     

Weitere neue Nb2O5-Modifikationen,metastabile Oxidationsprodukte von NbOx-Phasen (2,4 <x< 2,5)

Some New Nb₂O₅ Modifications, Metastable Oxidation Products of NbO_x Phases (2.4 <x< 2.5) The blue-black NbO_x Phases (2.4 <x< 2.5) are oxidized metastable in air to colourless (partly yellowish) Nb₂O₅ modifications already at temperatures about 200°C. At further heating stepwise and monotropic phase transformations are noticed; as final product the well known stable H modification appears. Some of the 15 new metastable Nb₂O₅ modifications obtained in this way appear in series. Therewith Nb₁₂O₂₉ (orh. and mon.) changes into two each, Nb₂₂O₅₄ and Nb₄₇O₁₁₆ into four each, Nb₂₅O₆₂ into two and Nb₅₃O₁₃₂ into one Nb₂O₅ phase. During oxidation and the following phase transformations at higher temperatures both the character of single crystal and the principle of building (block structure) of the starting material are maintained. This is especially proven from the electronoptical and X-ray investigations of Nb₁₂O₂₉ (orh. and mon.) and its products obtained by oxidation and further heating.     

Beiträge zum thermischen Verhalten wasserfreier Phosphate. IX. Darstellung und Kristallstruktur von Cr6(P2O7)4. Ein gemischtvalentes Pyrophosphat mit zwei- und dreiwertigem Chrom

Contributions on the Thermal Behaviour of Anhydrous Phosphates. IX. Synthesis and Crystal Structure of Cr₆(P₂O₇)₄. A Pyrophosphate Containing Di- and Trivalent Chromium Cr₆(P₂O₇)₄ (Cr₂²⁺Cr₄³⁺(P₂O₇)₄) can be obtained reducing CrPO₄ by phosphorus (950°C, 48 h, 100 mg iodine as mineralizer). By means of chemical transport reactions (transport agent iodine; 1050 → 950°C) the compound has been separated from its neighbour phases (Cr₂P₂O₇, CrP₃O₉) and crystallized (greenish, transparent crystals; edge length up to 0.3 mm). The crystal structure of Cr₆(P₂O₇)₄ (Spcgrp.: P-1; z = 1; a = 4.7128(8) Å, b = 12.667(3) Å, c = 7.843(2) Å, α = 89.65(2)°, β = 92.02(2)°, γ = 90.37(2) has been solved and refined from single crystal data (2713 unique reflections, 194 parameter, R = 0.035). Cr²⁺ is surrounded by six oxygen atoms which occupy the corners of an elongated octahedron (4 × d^{Cr? O} ≈? 2.04 Å; 2 × d^{Cr? O} ≈? 2.62 Å). The Cr³⁺ ions are also coordinated octahedraly (1.930 Å ≤ d_{Cr? O} ≤ 2.061 Å). The crystallographically independent pyrophosphate groups show nearly eclipsed conformation. The bridging angles (P? O? P) are 136.5° and 138.9° respectively.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Oxygen-ionic conductivity

Total conductivity of Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with a cryolite structure is studied in the atmosphere with a low water vapor content under variation of the temperature (500 < T < 1000°C) and oxygen activity in the gas phase (10⁻¹⁸ < aO₂ < 0.21). Conductivity is divided into components. It is found that both the oxygen-ionic conductivity and the mobility of oxygen ions increase and the percentage of p-type electronic conductivity decreases at an increase in the concentration of oxygen vacancies. It is shown that compositions with a high strontium oxide content (0 ≤ x < 0.15) and accordingly high concentration of oxygen vacancies correspond to the maximum values of the oxygen-ionic conductivity and low activation energies. The Sr₆Ta₂O₁₁ and Sr_5.92Ta_2.08O_11.12 compositions in a wide range of aO₂ are characterized by negligibly low fractions of electronic conductivity component. For the compositions of x > 0.15, a transition occurs from the electrolytic region to mixed oxygen-hole conductivity character (at aO₂ = 0.21) at an increase in the oxygen activity.     

An X-ray and neutron powder diffraction study of the Ca2+xNd8−x(SiO4)6O2−0.5x system

The variation in lattice parameters with bulk composition and preparation temperature has been determined from X-ray powder diffraction data for the system Ca_2+xNd_8?x(SiO₄)₆O_2?0.5x. The structures of two members of this system have been further refined from time-of-flight neutron powder diffraction data using the Rietveld method. Both structures belong to the $\text{P6}{}_3\text{m}$ space group and are isomorphous with natural apatite, Ca₁₀(PO₄)₆(F, OH)₂. Samples prepared at 1250°C exhibit an ordered distribution of Ca and Nd cations between two nonequivalent sites. The room temperature lattice parameters of Ca₂Nd₈(SiO₄)₆O₂ are a = 9.5291(5)Å and c = 7.0222(1) Å, while those of Ca_2.2Nd_7.8(SiO₄)₆O_1.9 are a = 9.5303(4) Å and c = 7.0147(1) Å. The composition of the latter member is believed to represent the upper limit of solid solution for this system at 1250°C.     

Zur Kristallchemie der Oxometallate des Thoriums

The crystal chemistry of the valence stable Thorium (Th⁴⁺) shows in oxovanadates, ‐niobates, ‐molybdates, ‐tantalates etc. typical cationic property. It depends on the big size of the Th⁴⁺ ion in the order of r(Th⁴⁺) = 0.92 Å (C.N. = 6) up to r(Th⁴⁺) = 1.34 Å (C.N. = 12), comparable with the radii of alkaline and alkaline earth metals: r(Na⁺) = 1.02 Å (C.N. = 6) or r(K⁺) = 1.5 Å (C.N. = 8); r(Ca²⁺/Sr²⁺) = 1.1 Å/1.26 Å (C.N. = 8). There exists only one typical compound that may be of the oxothorate type with the composition NaK₃Th₂O₆. Na⁺ and K⁺ can be declared to be components of the cationic part of the crystal structure. In contrary, thorium and oxygen are common components of the anionic part of the crystal structure. From the point of view of the shared polyhedra (exclusively octahedra), NaK₃Th₂O₆ belongs to the thorium‐oxometallates too. The report on oxometallates of thorium shows compounds with one, two, four and five Th⁴⁺ atoms in combination with V⁵⁺, Nb⁵⁺, Ta⁵⁺ and Mo⁶⁺. W⁶⁺ is completely missing. Some of the Tantalates belong to the Jahnberg compounds, showing planar nets of corner shared pentagonal bipyramids. ThMO₄ and cationic mixed compounds, Th_1–xA_xMO₄, crystallize with the Scheelite (respectively Huttonite) structure and at least Th⁴⁺ is part of ordered and disordered perowskites showing more or less deviations from the cubic symmetry.     

Local environments and dynamics of hydrogen atoms in protonated forms of ion-exchangeable layered perovskites estimated by solid-state H NMR

The local environments and dynamics of hydrogen atoms in five samples of protonated forms of ion-exchangeable layered perovskites, Dion-Jacobson-type H[LaNb₂O₇] and H[LaTa₂O₇], Ruddlesden-Popper-type H₂[SrTa₂O₇] and H₂[La₂Ti₃O₁₀], and H_1.8[(Sr_0.8Bi_0.2)Ta₂O₇] derived from an Aurivillius phase, Bi₂Sr₂Ta₂O₉, have been investigated by solid-state ¹H nuclear magnetic resonance spectroscopy (NMR). Solid-state ¹H NMR with a magic-angle spinning technique conducted at room temperature reveals that the mean electron densities around the ¹H nuclei in these protonated forms are relatively low, and that they decrease in the following order: H_1.8[(Sr_0.8Bi_0.2)Ta₂O₇]>H[LaNb₂O₇]>H₂[SrTa₂O₇]>H[LaTa₂O₇]>H₂[La₂Ti₃O₁₀]. The temperature-dependent solid-state ¹H broad-line NMR spectra measured at 140-400 K reveal a decrease in the signal width for all of these five samples upon heating due to motional narrowing. The NMR spectra of H[LaNb₂O₇] and H[LaTa₂O₇] are different from the other three protonated forms due to the weaker dipole-dipole interactions at low temperatures and lower mobility of the hydrogen atoms at high temperatures.     

Novel Alkaline Earth Metal Chalcogenoborates: Syntheses and Crystal Structures of Sr4.2Ba2.8(BS3)4S and Ba7(BSe3)4Se

Systematic studies on quaternary thio‐ and selenoborates containing heavier alkaline earth metal cations led to the two new isotypic crystalline phases Sr_4.2Ba_2.8(BS₃)₄S and Ba₇(BSe₃)₄Se. Both compounds consist of trigonal‐planar BQ₃ (Q = S, Se) units, isolated Q^2– anions and the corresponding counter‐ions. The two new chalcogenoborates were prepared in solid state reactions from the metal sulfides (selenides), amorphous boron and sulfur (selenium). Evacuated carbon coated silica tubes were used as reaction vessels since temperatures up to 870 K were applied. Sr_4.2Ba_2.8(BS₃)₄S and Ba₇(BSe₃)₄Se crystallize in the monoclinic space group C2/c (no. 15) with a = 9.902(3) Å, b = 23.504(9) Å, c = 9.884(3) Å, β = 90.01(3)° and Z = 4 in the case of the thioborate, while for the selenoborate the lattice parameters a = 10.513(2) Å, b = 25.021(5) Å, c = 10.513(2) Å, β = 90.10(3)° were determined. X‐ray powder patterns are compared to calculated diffraction data obtained from single crystal X‐ray structure determination.     

Partial nitrogen loss in SrTaO2N and LaTiO2N oxynitride perovskites

SrTaO₂N heated in a helium atmosphere began to release nitrogen of approximately 30 at% at 950 °C while maintaining the perovskite structure and its color changed from orange to dark green. Then it decomposed above 1200 °C to a black mixture of Sr_1.4Ta_0.6O_2.73, Ta₂N, and Sr₅Ta₄O₁₅. The second decomposition was not clearly observed when SrTaO₂N was heated in a nitrogen atmosphere below 1550 °C. After heating at 1500 °C for 3 h under a 0.2 MPa nitrogen atmosphere, the perovskite product became dark green and conductive. Structure refinement results suggested that the product was a mixture of tetragonal and cubic perovskites with a decreased ordering of N³⁻/O²⁻. The sintered body was changed to an n-type semiconductor after a partial loss of nitrogen to be reduced from the originally insulating SrTaO₂N perovskite lattice. LaTiO₂N was confirmed to have a similar cis-configuration of the TiO₄N₂ octahedron as that of TaO₄N₂ in SrTaO₂N. It also released some of its nitrogen at 800 °C changing its color from brown to black and then decomposed to a mixture of LaTiO₃, La₂O₃, and TiN at 1100 °C. These temperatures are lower than those in SrTaO₂N.     

Experimente zur Mischkristallbildung zwischen Zinktantalaten und -antimonaten: ZnTa2−xSbxO6 und Zn4Ta2−xSbxO9

Experiments about the Mixed Crystal Formation between Zincoxotantalates and -antimonates: ZnTa_2?xSb_xO₆ and Zn₄Ta_2?xSb_xO₉ In the area of substituted oxotantalates of zinc two new phases of the composition A: ZnTa_1·8Sb_0·2O₆ and B: Zn₄Ta_1·2Sb_0·8O₉ were prepared and investigated by X-ray single crystal technique. A crystallizes with tetragonal symmetry (space group D–P4₂/mnm, a = 4.7314; c = 9.2160 Å; Z = 2). B is monoclinic (space group C–C2/c; a = 15.103; b = 8.839; c = 10.378 Å; β = 93.81°; Z = 8). A crystallizes with trirutile structure, although there is a small replacement of Ta⁵⁺ by Sb⁵⁺. B maintains the Zn₄Ta₂O₉ structure. One of the point positions of the M⁵⁺ ions is occupied statistically by Ta⁵⁺/Sb⁵⁺ and Zn²⁺. B is a metastable compound.     

Eine kubische Hochtemperaturform von CaTa2O6

About a Cubic Modification of CaTa₂O₆ Single crystals of cub.-CaTa₂O₆ were prepared with a CO₂-Laser-technique. X-ray investigations lead to a cubic symmetry (space group Pm3? T_h¹, a = 7.78 Å, Z = 4). Ta⁵⁺ and O^2? occupy nearly the positions of this ions in the Perovskite structure. For this reason there is a similarity to Ca_0.5TaO₃. The decisive difference to the Ca_0.5TaO₃ is a ordered distribution of Ca²⁺.     

Crystal Structure and Property of Perovskite-Type Oxides Containing Ion Vacancy

Studies on the role of oxygen vacancy in structural change of nonstoichiometric perovskites and a property of oxygen-deficient perovskite-related K₂NiF₄ compounds are reviewed.The structural changes on which the authors focused are cation ordering and lattice distortion. The relationship between the distortion and oxygen vacancy was investigated by comparing the structures of Sr₂(Sr_1-xM_x)TaO_6-d (M = Ca²⁺ and Nd³⁺) solid solutions. It was found that distortion of a perovskite-type lattice decreased with an increasing amount of oxygen vacancies. In order to investigate the relationship between the cation ordering on octahedral sites and oxygen vacancy, structures of stoichiometric Sr_2-xLa_xCo_1-yTa_1+yO₆ and oxygen-deficient Sr_2-xLa_xMg_1-yTa_1+yO_6-d solid solutions were compared. The authors' work reveals that the cation ordering affects the amount of oxygen vacancies in addition to cation charge and size.     

Strontium Tantalates with a Perovskite Structure: Their Conductivity and High-Temperature Interaction with Water

The interaction of perovskite-like solid solutions Sr_{6 – 2x} Ta_{2 + 2x} O_{11 + 3x} (x= 0–0.28) with water is studied, along with dependences of the solutions' conductivity on their composition and the atmosphere's temperature and humidity. The Sr_{6 – 2x} Ta_{2 + 2x} O_{11 + 3x} phases with high concentrations of structural oxygen vacancies are high-temperature mixed oxygen–hydrogen ionic conductors whose conduction is sensitive to the presence of water vapor up to 900°C. According to a thermogravimetric study, the amount of water incorporated into the complex-oxide matrix is proportional to the concentration of structural oxygen vacancies. The process of water incorporation is considered in terms of crystalline and chemical properties of the structure. The oxygen-deficient perovskites containing coordination-unsaturated metalatoms can reconstruct their coordination polyhedron by adding water molecules, with subsequent partial dissociation of water to hydroxyl groups. The proposed mechanism explains different states of water in the oxide and a two-stage nature of its removal: water molecules coordinating the metal atom and those surrounding OH^–leave the core in the first and second stages, respectively.     

Deposition and dielectric characterization of strontium and tantalum-based oxide and oxynitride perovskite thin films

We have synthesized the composition x = 0.01 of the (Sr_1-xLa_x)₂(Ta_1-xTi_x)₂O₇ solid solution, mixing the ferroelectric perovskite phases Sr₂Ta₂O₇ and La₂Ti₂O₇. Related oxide and oxynitride materials have been produced as thin films by magnetron radio frequency sputtering. Reactive sputter deposition was conducted at 750 °C under a 75 vol.% (Ar) + 25 vol.% (N₂,O₂) mixture. An oxygen-free plasma leads to the deposition of an oxynitride film (Sr_0.99La_0.01) (Ta_0.99Ti_0.01)O₂N, characterized by a band gap E_g = 2.30 eV and a preferential (001) epitaxial growth on (001) SrTiO₃ substrate. Its dielectric constant and loss tangent are respectively Epsilon' = 60 (at 1 kHz) and tanDelta = 62.5 × 10⁻³. In oxygen-rich conditions (vol.%N₂ ≤ 15%), (110) epitaxial (Sr_0.99La_0.01)₂(Ta_0.99Ti_0.01)₂O₇ oxides films are deposited, associated to a larger band gap value (E_g = 4.55 eV). The oxide films permittivity varies from 45 to 25 (at 1 kHz) in correlation with the decrease in crystalline orientation; measured losses are lower than 5.10⁻³. For 20 ≤ vol.% N₂ ≤ 24.55, the films are poorly crystallized, leading to very low permittivities (minimum Epsilon' = 3). A correlation between the dielectric losses and the presence of an oxynitride phase in the samples is highlighted.     

Darstellung und Kristallstrukturen von Li4−2xSr2+xB10S19 (x ≈ 0,27) und Na6B10S18. Zwei neue Thioborate mit hochpolymeren Makrotetraeder-Netzwerken

Synthesis and Crystal Structures of Li_4?2xSr_2+xB₁₀S₁₉ (x ≈ 0.27) and Na₆B₁₀S₁₈. Two Novel Thioborates with Highly Polymeric Macro-tetrahedral Networks Li_4?2xSr_2+xB₁₀S₁₉ (x ≈ 0.27) and Na₆B₁₀S₁₈ were prepared from the reaction of strontium sulfide and lithium sulfide (sodium sulfide) with boron and sulfur at 700°C in graphitized silica tubes. Li_4?2xSr_2+xB₁₀S₁₉ (x ≈ 0.27) crystallizes in the monoclinic space group P2₁/c with a = 10.919(2) Å, b = 13.590(3) Å, c = 16.423(4) Å, and β = 90.48(2)°, Na₆B₁₀S₁₈ in the tetragonal space group I4₁/acd with a = 14.415(3) Å, c = 26.137(4) Å. Both structures contain supertetrahedral B₁₀S₂₀ units which are linked through tetrahedral corners to form a three-dimensional polymeric network in the case of Na₆B₁₀S₁₈ and one-dimensional chains in the case of Li_4?2xSr_2+xB₁₀S₁₉ (x ≈ 0.27). All boron atoms are in tetrahedral BS₄ coordination (B? S bond lengths vary from 1.879(5) to 1.951(5) Å (1.875(10) to 1.987(9) Å)). The strontium and lithium (sodium) cations are located within large channels formed by the anions.     

Luminescence in some tantalate host lattices

The luminescence of the following systems are reported: ScTa_1−xNb_xO₄, Li₃Ta_1−xNb_xO₄, CaTa_2−2xNb_2xO₆ and Mg₄Ta_2−2xNb_2xO₉. The dependence of the luminescence properties on x becomes more pronounced if the distance between the pentavalent ions becomes shorter. In these systems the NbO₆ group gives an efficient blue emission at 300°K. The compound Mg₄Ta₂O₉ gives an efficient ultraviolet emission at 300°K. At 77°K both Mg₄Nb₂O₉ and Mg₄Ta₂O₉ show in addition a weaker emission peaking at wavelengths longer than the wavelength of the room temperature emission. This low-temperature emission can be excited by radiation with wavelengths longer than that of the absorption edge. The Mn²⁺ ion in Mg₄Ta₂O₉Mn acts as an efficient activator with a deep-red emission.