Tridymite Structure Research Articles

Structural and Luminescent Characteristics of Divalent Europium Activated Barium Aluminate with the Tridymite Structure Synthesized by the Sol-Gel Technique

Abstract: In this study, alkaline earth aluminates were investigated as potential hosts for new blue luminescent materials with nanoparticles. Ba1-xEuxAl2O4 with the stuffed tridymite hexagonal structure was successfully synthesized at 900 °C under controlled sol-gel conditions. Divalent europium was effectively used to activate this host material, producing a brilliant blue-emitting phosphor. Its spectral photoluminescence at 408 nm appears to be in a region close to the laser blue region, with chromaticity coordinate values suitable for the standard blue of the PAL TV system. Keywords: Blue phosphors, BaAl2O4, Divalent Europium (Eu2+), Sol-Gel synthesis, Critical energy transfer.

Boosted Activity of Cobalt Catalysts for Ammonia Synthesis with BaAl2O4-xHy Electrides.

Electrides are promising support materials to promote transition metal catalysts for ammonia synthesis due to their strong electron-donating ability. Cobalt (Co) is an alternative non-noble metal catalyst to ruthenium in ammonia synthesis; however, it is difficult to achieve acceptable activity at low temperatures due to the weak Co-N interaction. Here, we report a novel oxyhydride electride, BaAl2O4-xHy, that can significantly promote ammonia synthesis over Co (500 mmol gCo-1 h-1 at 340 °C and 0.90 MPa) with a very low activation energy (49.6 kJ mol-1; 260-360 °C), which outperforms the state-of-the-art Co-based catalysts, being comparable to the latest Ru catalyst at 300 °C. BaAl2O4-xHy with a stuffed tridymite structure has interstitial cage sites where anionic electrons are accommodated. The surface of BaAl2O4-xHy with very low work functions (1.7-2.6 eV) can donate electrons strongly to Co, which largely facilitates N2 reduction into ammonia with the aid of the lattice H- ions. The stuffed tridymite structure of BaAl2O4-xHy with a three-dimensional AlO4-based tetrahedral framework has great chemical stability and protects the accommodated electrons and H- ions from oxidation, leading to robustness toward the ambient atmosphere and good reusability, which is a significant advantage over the reported hydride-based catalysts.

Synthesis and luminescence properties of Er3+-Yb3+ doped glass ceramics with tridymite structure

By melt annealing technique, Er3+-Yb3+ co-doped precursor glass (PG) was prepared. Glass ceramics (GCs) containing KZnPO4 crystal phase with tridymite structure were obtained by heat treatment of PG. The optimum heat treatment conditions of GCs are 670 °C for 60 min. Under these conditions, the average grain size of GC is about 205 nm, and the light transmittance in the visible region is about 80%. Through the analysis of the Rietveld refinement results of XRD, the lattice constants a and b of KZnPO4 crystal decreased from 18.155 Å to 18.109 Å, and c decreased from 8.504 Å to 8.451 Å. Therefore, it is inferred that Er3+ and Yb3+ replace the Zn2+ in the lattice. The green up-conversion luminescence of GC is about 10 times that of PG excited by 980 nm laser. The optimal doping concentrations of Er3+ and Yb3+ in GCs are 0.5% and 1.0%, respectively. The calculation results of chromaticity coordinates show that Er3+-Yb3+ co-doped GCs with KZnPO4 crystal phase is a promising green up-conversion luminescent material.

Phase and structural transformations of water during ice melting

The processes of phase transformation of water during ice melting are considered. Possible structures of liquid water are analyzed, corresponding to the results of experimental measurements and computer simulation. It is shown that when ice melts, the tridymite structure breaks down into individual clusters, the lifetime of which ranges from 2 to 8 ps and increases with an increase in their molecular weight. A sharp increase in the density of liquid water at 00C is explained by the formation of clathrates during the introduction of water molecules into the cavity of the structure and an increase in the degree of coordination from 4.0 to 4.34. The most stable are water clusters, consisting of 12–20 molecules, the mixture of which determines the density of the liquid phase. Molecules in the middle of clusters can form additional hydrogen bonds with their nearest neighbors in the cluster framework. Thus, one or two framework molecules can have three acceptor and two donor bonds of A3D2 type. The central molecule is most likely of type A1D2 or A1D1. An example of calculating the density of water when taking into account clathrates and vacancies is given.

Analysis of Silica Activity Index from Geothermal Silica Scaling

Silica scaling causes problems in geothermal powerplants piping system such as clogging and reduction of pipe diameter. A suspended bulk sample was taken from Dieng powerplant Indonesia, with more than 80 % silica content. Before further utilization, the properties of the silica powder should be well understood to ensure for an example the correct usage of the application or another example choosing the efficent procedure in clogging prevention. The measure of the activity of silica powder can be expressed in the term of silica activity index (SAI). This index is the level of strength for the silica reactivity, it also indicated the increasing amount of amorphous phase relative to the amount of the crystal phase of silica. This study aims to determine SAI in variation with temperature of silica sample taken from suspended powder from geothermal brine. Experimental treatment on SAI determination consists of samples without preheating and with preheating at 400 °C, 500 °C, 650 °C, and 900 °C. The index was then calculated from the ratio of dissolved silica in the solution of 1 and 2 M 100 ml NaOH for 10 minutes. The experimental results show that the maximum value of SAI is 79% in the sample without heating at dissolving temperature 99 °C. The minimum value of SAI was 8% in samples with 900 °C preheated, treated with the same solution at dissolving temperature 80 °C. The XRD characterization result indicated that the sample without heating has an amorphous structure and tends to crystalize versus increasing heating temperature. At 900°C heating, the sample has a cristobalite and tridymite structure. The XRF analysis in the soluble fractions showed that the silica content in samples without heating is 0.52 wt% and the concentration also decreasing versus increasing temperature to 0.05 wt% for samples with 900°C heating. Hence, the silica activity index is influenced by sample crystallization during treatment with a strong base and heated dissolution.

Hydrothermal single crystal growth and structural investigation of the stuffed tridymite family as NLO materials

The structures, crystal growth and preliminary NLO properties of a broad class of compounds known as the stuffed tridymites are investigated. The stuffed tridymites of the formula ABC O 4 , where A are the alkali metal ions and B and C are Group 13 and 14 metal ions, respectively, form an extensive series of structures, all of which are based on symmetry breaking of the parent tridymite (SiO 2 ) structure. In all cases the alkali ion resides in channels running parallel to the c- axis formed from six-membered rings of tetrahedra of the metal ions. A large array of structures can be formed though combinations of ordering of the B / C sites, staggered and eclipsed tetrahedra, up/down orientations of the tetrahedra relative to the c -axis, and distortions of the six-membered rings. These symmetry breaking steps can be mixed and matched to form many different structures, and nearly all are in polar acentric space groups. The primary cause of centrosymmetric crystal formation is disorder of the B / C sites. When the sites are well ordered most of the structures are acentric with polar axes. High quality single crystals of most products can be grown using a high pressure hydrothermal method. Preliminary NLO experiments indicate that the compounds are acentric, but have low NLO conversions, which is expected due to the low polarizability of the building blocks. The extremely high percentage of polar acentric structure types, and the ability to correlate the structures to systematic variations of the very large and flexible tridymite structural class, suggests that this can serve as a starting point for designing crystals with many other useful physical properties. • Stuffed tridymites of various compositions were synthesized hydrothermally. • Structural factors affecting symmetry reduction from parent tridymite were identified. • The family exhibits a high percentage of noncentrosymmetric structures. • Powder NLO data was measured on stuffed tridymites.

Frontispiece: Stuffed Tridymite Structures: Synthesis, Structure, Second Harmonic Generation, Optical, and Multiferroic Properties

Frontispiece: Stuffed Tridymite Structures: Synthesis, Structure, Second Harmonic Generation, Optical, and Multiferroic Properties

Stuffed Tridymite Structures: Synthesis, Structure, Second Harmonic Generation, Optical, and Multiferroic Properties.

The stuffed tridymite structure Ba(Zn/Co)1-x Si1-x M2x O4 (M=Al3+ and Fe3+ ) is explored for the possible multiferroic behavior and to develop new inorganic colored materials. The compounds were synthesized by employing conventional solid-state chemistry methods in the temperature range 1100-1175 °C for 24 h. The powder X-ray diffraction (PXRD) and Rietveld refinement studies indicate that the compounds stabilize in the P63 space group (no. 173). The refinement results were also rationalized by employing Raman spectroscopic studies. The compounds were found to be second harmonic generation (SHG) active and show weak ferroelectric behavior. The co-substitution of Co2+ and Fe3+ in the structure gives rise to a weak ferromagnetic behavior to the compound, BaCo0.75 Si0.75 Fe0.5 O4 , making it a multiferroic material. The optical studies on the prepared compounds exhibited blue color (Co2+ in Td geometry), purple color (Ni2+ in Td geometry), and simultaneous substitution of Co2+ and Fe3+ gives rise to blue-green color owing to metal-to-metal charge transfer (MMCT) effect.

Nd3+-doped LiBaPO4 phosphors for optical temperature sensing within the first biological window: A new strategy to increase the sensitivity

In this work, powders of LiBaPO4:Nd3+ were synthesized by the sol–gel method using PVA as a chelating agent. X-ray diffraction indicates a tridymite structure with a hexagonal unit cell and space group P63. SEM images show that the samples have particles of different sizes with the majority varying between 5 and 20 μm. Under 532 nm excitation, four infrared emission bands were observed, with maxima at 749.9, 802.4, 873.2 and 953.4 nm assigned to the transitions from 4F7/2, 4F5/2, 4F3/2 to 4I9/2 and from 4F5/2 to 4I11/2. The performance of this material aiming optical thermometry based on the fluorescence intensity ratio methodology (FIR) was investigated within the range from 294 to 373 K, suitable for applications in biological systems. Under visible excitation, sensor performance is comparable to previously investigated Nd schemes. However, the sensing performance under infrared excitation was strongly enhanced by exploiting the FIR associated to the 4F5/2 → 4I11/2 and 4F3/2 → 4I9/2 transitions. The obtained relative sensitivity with this approach is one order of magnitude higher when compared with other Nd-doped materials excited in the infrared. These results suggest that the analyzed material can be used for application in temperature sensing of biological systems exploring the first biological window.

Using powder XRD and pair distribution function to determine anisotropic atomic displacement parameters of orthorhombic tridymite and tetragonal cristobalite.

Determination of the crystal structures of low-temperature tridymite and cristobalite using single-crystal XRD has been challenging because they generally occur as metastable fine-grained crystals in the geological environment. Therefore, using powder diffraction and scattering techniques is critical to study the low-temperature crystals. Synchrotron powder X-ray diffraction (XRD), pair distribution function (PDF) and transmission electron microscopy were used to investigate the structure of orthorhombic tridymite with C2221 symmetry and tetragonal cristobalite with P41212 symmetry, including their anisotropic atomic displacement parameters (ADPs). Rietveld refinement was used to determine the unit-cell parameters, fractional coordinates and isotropic atomic displacement parameters (Uiso) of the tridymite and cristobalite. The PDF method was used to determine ADPs for each atom. The results suggest that the crystal structure with high quality ADP values can be obtained using the combined methods of XRD and PDF analyses. The method can be used for characterizing crystals that are not suitable for single-crystal XRD.

Phase Transformation of Hydrothermally Stressed Adsorbents

Inorganic materials of zeolite type, silica gel or related materials are used as adsorbents in air conditioning machines. The cooling effect is obtained by evaporation of water thereby the adsorbent acts as pump system. After itôs saturation, the storage must be regenerated by heating. The hydrothermal stress may generate decomposition of their structure accompanied by a loss of sorption capacity. This work describes the hydrothermal stability of AlPO‐5, SAPO‐34, and silica gel in comparison with a newly developed dealuminated Y zeolite DAY. DAY (Si/Al = 3.1) and AlPO‐5 zeolites are hydrothermally stable over a wide temperature range even under harsh condition of maximum water loading of their pore system. DAY exhibits a higher water sorption capacity but becomes amorphous with increasing temperature, whereas AlPO‐5 transforms into a tridymite analogous crystalline phase with an intermediate state at 140 °C. But, both phase transformation processes are not relevant for the application of the compounds in low‐temperature driven heat pumps due to less hydrothermal stressing there. SAPO‐34 decomposes already under mild hydrothermal conditions. In analogy to AlPO‐5 it forms the tridymite analogous structure at high temperature. The silica gel looses the sorption capacity systematically because of the healing of its structure by condensation of free silanol groups, whereby Q2 and Q3 groups change into Q4 groups.

Anomalous heating rate response of beta irradiated Sm3+ and Tb3+ doped BaAl2O4 phosphors

The stuffed derivative of the tridymite structure, the oxide BaAl2O4 has been regarded as a long persistent phosphor. Thermoluminescence (TL) glow curve peaks of thin pellet samples of undoped and, Sm3+ and Tb3+ doped BaAl2O4 prepared by combustion method was reported. The structure of the compounds was monitored by X-ray diffraction and found a single-phase compound with a hexagonal structure. Performing a series of TL measurements for the temperature range from room temperature to 400 °C, the effect of beta radiation dose on TL response, various TL heating rate (HR) properties of undoped and Sm3+ and Tb3+ doped BaAl2O4 phosphors were investigated. Peak shape method was also utilized to assess the trap parameters. The activation energy of the main peak at 128 °C was found to be 0.97 eV. TL glow curves obtained from undoped and Tb3+ doped BaAl2O4 decrease with increasing HR value (varying from 0.5 to 15 °C s−1) in a way matching with thermal quenching effect. Contrary to expectations, anomalous HR dependence was observed on Sm3+ doped BaAl2O4 sample and a semi-localized transition model has been used in order to explain such behaviour.

Preparation, Thermal Diffusivity, and Thermal Conductivity of Phosphate Ceramics with the Tridymite Structure

We have synthesized powders of the CsMIIPO4 (MII = Mg, Mn, Co) phosphates with the β-tridymite structure. Phosphate ceramics were produced by conventional cold pressing with inorganic sintering aids, followed by programmed annealing, and by an innovative method: high-speed spark plasma sintering. The thermal diffusivity and thermal conductivity (0.3–0.4 W/(m K)) of the CsMIIPO4 ceramics indicate that they are heat insulators with a high working temperature.

Molecular dynamics study of tridymite

Structural changes in tridymite have been investigated by molecular dynamics simulation. Two thermal processes were carried out, one cooling from the high-temperature hexagonal structure of tridymite (HP-tridymite) and the other heating from the low-temperature monoclinic structure of tridymite (MX1-tridymite). The former process showed that HP, LHP (low-temperature hexagonal structure), OC (orthorhombic structure with C2221 symmetry) and OP (orthorhombic structure with P212121 symmetry)-like structures appeared in sequence. In contrast, the latter process showed that MX1, OP, OC, LHP and HP-like structures appeared in sequence. Detailed analysis of the calculated structures showed that the configuration underwent stepwise changes associated with several characteristic modes. First, the structure of HP-tridymite determined from diffraction experiments was identified as a time-averaged structure in a similar manner to β-cristobalite, thus indicating the important role of floppy modes of oxygen atoms at high temperature - one of the common features observed in silica crystals and glass. Secondly, the main structural changes were ascribed to a combination of distortion of the six-membered rings in the layers and misalignment between layers. We suggest that the slowing down of floppy oxygen movement invokes the multistage emergence of structures with lower symmetry on cooling. This study therefore not only reproduces the sequence of the main polymorphic transitions in tridymite, except for the appearance of the monoclinic phase, but also explains the microscopic dynamic structural changes in detail.

Synthesis and properties of novel turquoise-green pigments based on BaAl2-xMnxO4+y

BaAl2-xMnxO4+y (0 ≤ x ≤ 0.10) showing intense turquoise to green colors were prepared by the sol-gel method from metal nitrides as precursors and followed by a thermal treatment at 900 °C. The X-ray diffraction of all of the as-synthesized samples are indexed with BaAl2O4 stuffed tridymite structure. The oxidation state of Mn is confirmed to be +5 by X-ray photoelectron spectroscopy analysis. The colors of the samples are tuned to intense turquoise and green with the increase of Mn content. The optical properties are characterized by UV–Vis absorbance spectra and CIE L∗a∗b∗ coordinates. The reason for the intense colors is due to the Mn5+ doping in the [AlO4] tetrahedral sites. The pigments are applied to poly(methyl methacrylate) (PMMA) and the colours of the polymer composite were characterized.

Immobilization of cesium into mineral-like matrices of tridymite, kosnarite, and langbeinite structure

Cesium-containing phosphates CsMgPO4, CsZr2(PO4)3, and Cs2Mg0.5Zr1.5(PO4)3 of framework structure were synthesized. The chemical stability of the crystalline phosphates in neutral, acidic, and alkaline media was studied. Quantitative data on the chemical stability in a wide pH range and on the ion-exchange capacity of phosphates containing cesium, magnesium, and zirconium ions were obtained by potentiometric titration. The Cs leach rates from the ceramic samples upon long contact with water were from 3 × 10–4 to 4 × 10–6 g cm–2 day–1. The phosphates CsZr2(PO4)3 and Cs2Mg0.5Zr1.5(PO4)3 are thermally stable in the temperature range from–268 to 1000°C, and CsMgPO4 undergoes a low-temperature phase transition at–37°C without break of chemical bonds in the structure. The stability of ceramic and powdered samples of CsMgPO4, CsZr2(PO4)3, and Cs2Zr1.5Mg0.5(PO4)3 was studied in different aspects, and these materials proved to be promising for cesium consolidation. High cesium content of CsMgPO4 allows its use as a material of the active core of 137Cs/137mBa sources.

New structure, reconstruction, and behaviour of the coordination polymer framework [CuZn(CN)4]– towards hydrated alkali metal ions

A new framework structure of the coordination polymer [CuZn(CN)4]– has been discovered in the newly synthesized [Na(H2O)n][CuZn(CN)4] (n = 12.7; 1). X-ray crystal structure analysis revealed a 3D framework comprised of tetrahedral Cu(I) and Zn(II) centres with bridging cyanide ligands containing both octagonal and square channel cavities; these channels contain hydrated Na+ ions as guests. In open air, complex 1 is extremely unstable and loses water molecules to form [Na(H2O)n][CuZn(CN)4] (n = 5.2–6.8; 2) within 30 min. Powder X-ray diffraction patterns showed that this change results in the conversion of the framework of complex 1 to the same framework observed in [K(H2O)n][CuZn(CN)4]. The [CuZn(CN)4]– framework of [K(H2O)n][CuZn(CN)4] has a tridymite structure with hexagonal channel cavities occupied by hydrated K+ ions. Like 1, 2 is also unstable under atmospheric conditions, yielding [Na(H2O)n][CuZn(CN)4] (n = 3), which has an unknown structure. While the framework of [K(H2O)n][CuZn(CN)4] is structurally flexible and robust, the framework of 2 is unstable and fragile. This contrast comes from the difference between the nature of the hydrated K+ and Na+ ions present in the frameworks.

Preparation of phosphoric acid-based geopolymer foams using limestone as pore forming agent – Thermal properties by in situ XRPD and Rietveld refinements

In this work, geopolymer foams were obtained by reacting metakaolin with phosphoric acid and using natural calcite/dolomite as foaming agent. Total porosity and thermal conductivity were ca. 70% and 0.083±0.008W/mK, respectively. Rietveld refinements, using both ex- and in situ XRPD data, were performed in order to elucidate the phase stability of the formed binder up to 1200°C. The results showed that the amorphous matrix partially crystallized in tridymite and cristobalite type structures of AlPO4–SiO2 solid solutions at about 700°C. At 1000°C, 3:2 mullite started to crystallize, possibly from unreacted metakaolinite, resulting in co-crystallization of SiO2 cristobalite. At the same time, the amount of tridymite-type structure decreased, possibly due to selective phase transformation of AlPO4 tridymite to cristobalite, leaving behind the SiO2 isostructure. The geopolymer paste composition allows to tailor the mullite content in the refractory foam.

The Stability, Electronic Structure, and Optical Property of TiO2 Polymorphs

Phonon density of states calculation shows that a new TiO2 polymorph with tridymite structure is mechanically stable. Enthalpies of 9 TiO2 polymorphs under different pressure are presented to study the relative stability of the TiO2 polymorphs. Band structures for the TiO2 polymorphs are calculated by density functional theory with generalized gradient approximation and the band energies at high symmetry k-points are corrected using the GW method to accurately determine the band gap. The differences between direct band gap energies and indirect band gap energies are very small for rutile, columbite and baddeleyite TiO2, indicating a quasi-direct band gap character. The band gap energies of baddeleyite (quasi-direct) and brookite (direct) TiO2 are close to that of anatase (indirect) TiO2. The band gap of the newly predicted tridymite-structured TiO2 is wider than the other 8 polymorphs. For optical response calculations, two-particle effects have been included by solving the Bethe-Salpeter equation for Coulomb correlated electron-hole pairs. TiO2 with cotunnite, pyrite, and fluorite structures have optical transitions in the visible light region.

In-Situ Deposition of C-Axis Oriented Barium Ferrite Films for Microwave Applications

We report the deposition of M-type films by the alternating target laser ablation deposition using Fe2O3 and BaFe4O7 targets to simulate the unit cell of BaFe12O19 rather than a previous used tridymite structure, such as BaFe2O4 for R-block. In fact, the natural composition of the R-block is easier to be simulated by BaFe4O7. This target provides a flexible control over magnetic properties of the deposited films. Magnetic, electrical, and structural properties were measured using both types of targets in simulating barium ferrite. The films are consistent with bulk properties of barium ferrite. However, the use of BaFe4O7 targets improved resistivity of the films.