Site-occupancy Factors Research Articles

First-Principles Study of Order–Disorder Transitions in Pseudobinary Clathrates

It has been recently demonstrated that the pseudoternary Ba8AlxGayGe46–x–y clathrate undergoes an order–disorder transition with increasing temperature that can be observed via site occupation factors (SOFs) and manifests itself, e.g., in electrical transport properties. Here, we generalize this result and analyze the characteristics of this order–disorder transition in the pseudobinary clathrates Ba8GaxGe46–x, Ba8GaxSi46–x, Ba8AlxGe46–x, and Ba8AlxSi46–x. To this end, we employ atomistic simulations that combine alloy cluster expansions trained against density functional theory calculations with Wang–Landau and ensemble Monte Carlo simulations. The simulations show that all four systems studied here display order–disorder transitions for at least some composition range. Based on an extensive literature survey, we also provide evidence for signatures of the transition in earlier experimental studies that to the best of our knowledge have hitherto not been related to such transitions. The predicted transition temperatures are lower for Ba8GaxGe46–x and Ba8GaxSi46–x than for Ba8AlxGe46–x and Ba8AlxSi46–x, although it appears that the simulations underestimate the transition temperatures for Ga-containing systems compared to the experiment. This nonetheless provides a sensible explanation for why the experimentally determined Al SOFs agree better with the simulated high-temperature disordered configurations, while the Ga SOFs more closely agree with the simulated ground-state configurations. As a result of stronger interactions, the SOFs vary substantially, especially near the stoichiometric 16:30 composition, providing an indication of why it has proved difficult to synthesize Ba8AlxGe46–x and Ba8AlxSi46–x samples at this ratio. The present study thereby yields detailed atomic-scale insights into the ordering in inorganic clathrates that, given the connection to transport properties established earlier, are not only useful from a fundamental perspective but also relevant for applications.

The structure of the aluminium-abundant γ-brass-type Al8.6Mn4.4.

An aluminium-abundant Al8Mn5/γ-brass-type inter-metallic with formula Al8.6Mn4.4, which is isotypic with γ-Al8Cr5 and γ-Al8V5, was discovered by high-temperature sinter-ing of an Al/Mn mixture with initial composition Al2Mn. Structure analysis revealed that one special position (Wyckoff site 18h in space group R m) is shared by Al and Mn, with refined site occupancy factors of 0.7 and 0.3, respectively. The present low-temperature Al8Mn5-type phase crystallizes in the centrosymmetric space group R m (No. 166), rather than R3m (No. 160) as previously reported for the same inter-metallic characterized by TEM measurements [Zeng et al. (2018 ▸). Acta Mater. 153, 364-376].

Crystallographic characterization of rare-earth cyano-tri-phenyl-borate complexes and the cyano-borates [NCBPh3]1-, [NCBPh2Me]1-, and [NCBPh2(μ-O)BPh2]1.

The investigation of the coordination chemistry of rare-earth metal complexes with cyanide ligands led to the isolation and crystallographic characterization of the Ln III cyano-tri-phenyl-borate complexes di-chlorido-(cyano-tri-phenyl-borato-κN)tetra-kis-(tetra-hydro-furan-κO)lanthanide(III), [LnCl2(C19H15BN)(C4H8O)4] [lanthanide (Ln) = dysprosium (Dy) and yttrium Y)] from reactions of LnCl3, KCN, and NaBPh4. Attempts to independently synthesize the tetra-ethyl-ammonium salt of (NCBPh3)- from BPh3 and [NEt4][CN] in THF yielded crystals of the phenyl-substituted cyclic borate, tetra-ethyl-aza-nium 2,2,4,6-tetra-phenyl-1,3,5,2λ4,4,6-trioxatriborinan-2-ide, C8H20N+·C24H20B3O3 - or [NEt4][B3(μ-O)3(C6H5)4]. The mechanochemical reaction of BPh3 and [NEt4][CN] without solvent produced crystals of tetra-ethyl-aza-nium cyano-diphenyl-λ4-boranyl di-phenyl-borinate, C8H20N+·C25H20B2NO- or [NEt4][NCBPh2(μ-O)BPh2]. Reaction of BPh3 and KCN in THF in the presence of 2.2.2-cryptand (crypt) led to a crystal of bis-[(2.2.2-cryptand)potassium] 2,2,4,6-tetra-phenyl-1,3,5,2λ4,4,6-trioxatriborinan-2-ide cyano-methyl-diphenyl-borate tetra-hydro-furan disolvate, 2C18H36KN2O6 +·C24H20B3O3 -·C14H13BN-·2C4H8O or [K(crypt)]2[B3(μ-O)3(C6H5)4][NCBPh2Me]·2THF. The [NCBPh2(μ-O)BPh2]1- and (NCBPh2Me)1- anions have not been structurally characterized previously. The structure of 1-Y was refined as a two-component twin with occupancy factors 0.513 (1) and 0.487 (1). In 4, one solvent mol-ecule was disordered and included using multiple components with partial site-occupancy factors.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Crystal structure of a 1,1-dibutyl-1H,3H-naphtho[1,8-cd][1,2,6]oxastannaborinin-3-ol.

The title oxastannaborininol compound, [Sn(C4H9)2(C10H7BO2)], has been synthesized and crystallized. While heterocycles containing a C-O-B group are common, heterocycles containing an E-O-B unit, where E is an element of the carbon group except for carbon, are rare. In fact, while heterocycles containing Si-O-B units are occasionally reported (although without crystal structures), there are no reports for the corresponding germanium, tin or lead analogues. Herein, the first synthesis and crystal structure of a heterocycle containing an Sn-O-B unit is described. The asymmetric unit contains one mol-ecule showing a notable disorder of the tin atom and the butyl groups. They occupy two sets of positions with site-occupancy factors of 0.295 (6) and 0.705 (6).

A material catalogue with glass-like thermal conductivity mediated by crystallographic occupancy for thermoelectric application

We proposed a new crystallographic parameter, namely the site occupancy factor, as an effective indicator to discover a material catalogue with low lattice thermal conductivity for thermoelectric application.

Study on magnetite oxidation using synchrotron X–ray diffraction and X–ray absorption spectroscopy: Vacancy ordering transition in maghemite (γ–Fe<sub>2</sub>O<sub>3</sub>)

The oxidation process from magnetite to hematite through maghemite was investigated by X–ray diffraction (XRD) and X–ray absorption spectroscopic techniques. The XRD pattern of magnetite heated at 100 °C for 3 h showed small reflections of maghemite with partially ordered distribution of vacancy (space group P4132 or P4332). Thereafter, the XRD pattern of magnetite heated at 250 °C for 3 h exhibited extra reflections corresponding to the tetragonal maghemite with fully ordered distribution of vacancy (space group P41212 or P43212). Diffraction peaks of hematite occurred from the magnetite heated at 250 °C, in which maghemite and hematite coexisted with magnetite. Diffraction peaks of magnetite subsequently disappeared at 300 °C. Instead, maghemite and hematite dominated the XRD pattern, but the amount of maghemite reduced from 300 °C. The maghemite completely disappeared at 500 °C, and hematite finally dominated the XRD pattern. Rietveld fitting results clearly showed that the a lattice parameter and site occupancy factor of Fe at the octahedral site continuously decreased at the temperatures from 25 to 300 °C. The X–ray absorption near edge structure (XANES) result showed that the Fe3+/ΣFe increased up to 300 °C and remained constant until 500 °C, indicating that Fe2+ in oxidized magnetite was completely oxidized to Fe3+ at 300 °C. Furthermore, the intensities of radial structure function (RSF) peaks at 1.7 and 3.1 Å corresponding to the Fe–O bonds in octahedral site and the Fe–Fe interaction between the octahedral sites reduced continuously from 25 to 300 °C. The fitting results of the first shells indicated that the coordination number and site occupancy factor at the octahedral site continuously decreased at the temperature range from 25 to 300 °C, which were approximately consistent with those of Rietveld fitting analysis. The a lattice parameter of the oxidized magnetite displayed a linear trend between stoichiometric magnetite and stoichiometric maghemite with a relationship of a = 0.0985x + 8.3397 (x = Fe2+/Fe3+). It was clearly confirmed that during the magnetite oxidation, Fe was continuously removed from the octahedral sites, which resulted in the formation of maghemite with partially ordered distribution of vacancy. Just after magnetite oxidation was completed, the vacancy ordering further progressed by the diffusion of Fe3+ within the structure, leading to the formation of maghemite with fully ordered distribution of vacancy.

Investigation of Structural and Thermal Evolution in Novel Layered Perovskite NdSrMn2O5+δ via Neutron Powder Diffraction and Thermogravimetric Analysis

Neutron diffraction is one of the best methods for structural analysis of a complex, layered perovskite material with low symmetry by accurately detecting the oxygen positions through octahedral tilting. In this research, the crystal structure of NdSrMn2O5+δ was identified through X-ray diffraction (XRD) and neutron powder diffraction (NPD) at room temperature (RT), which indicated the formation of a layered structure in orthorhombic symmetry in the Pmmm (no. 47) space group. Rietveld refinement of the neutron diffraction data has confirmed the orthorhombic symmetry with unit cell parameters (a = 3.8367 (1) Å, b = 3.8643 (2) Å, and c = 7.7126 (1) Å), atomic positions, and oxygen occupancy. Thermogravimetric analysis revealed the total weight loss of about 0.10% for 20–950°C temperature, which occurred mainly to create oxygen vacancies at high temperatures. Rietveld analyses concurred with the XRD and neutron data allowing correlation of occupancy factors of the oxygen sites.

Limousinite, BaCa[Be4P4O16]·6H2O, a new beryllophosphate mineral with a phillipsite-type framework

ABSTRACT Limousinite, ideally BaCa[Be4P4O16]·6H2O, is a new beryllophosphate mineral discovered in the Vilatte-Haute pegmatite, Chanteloube near Razès, Limousin, Haute-Vienne, France. The new mineral is intimately associated with microcrystalline pale brown greifensteinite, black amorphous vitreous Mn-oxyhydroxide, triplite, and quartz. It forms isolated, partly corroded, colorless to snow-white crystals up to 0.9 mm long, showing rhombic cross sections. Limousinite is transparent with a vitreous luster, non-fluorescent, without cleavage planes; its calculated density is 2.58 g/cm3. Optically, the mineral is biaxial negative, α = 1.532(2), β = 1.553(3), γ = 1.558(2) (measured under 589 nm wavelength light), 2Vcalc. = 18°, non-dispersive, with Z parallel to the elongation of the prismatic crystals. Electron-microprobe analyses indicate an empirical formula of (Ba0.91K0.07)Σ0.98(Ca0.87Na0.05)Σ0.92[(Be3.87Al0.13)Σ4P4O16]·5.56H2O, calculated on the basis of 4 P atoms per formula unit, assuming 4 (Be + Al) pfu and a water content calculated from refined site-occupancy factors. A single-crystal structure refinement was performed to R1 = 4.90%, in the P21/c space group, with a = 9.4958(4), b = 13.6758(4), c = 13.4696(4) Å, β = 90.398(3)°, V = 1749.15(10) Å3, Z = 4. The crystal structure is characterized by a beryllophosphate framework similar to that of phillipsite-group zeolites, based on corner-sharing BeO4 and PO4 tetrahedra forming interconnected four- and eight-membered rings. Large cages within this zeolite framework contain Ba, Ca, and water molecules. Limousinite is the third known natural zeolite-type beryllophosphate, together with pahasapaite and wilancookite; it is also the first phosphate with a framework identical to that of a natural zeolite silicate.

Crystal structure of Al2.95Cr0.59, a phase closely related to the η-phase in the binary Al-Cr system.

The monoclinic η-phase in the binary Al-Cr system was initially named by Bradley & Lu [Bradley & Lu (1937 ▸). J. Inst. Met. 60, 319-337] as having the composition Al11Cr2 (Al:Cr ratio = 5.5:1). Its crystal structure was later refined [Cao & Kuo (2008). J. Alloys Compd. 458, 30, 319-337] to have a slightly lower Al content (Al:Cr ratio = 5.16:1). In the present work, a monoclinic phase with composition Al2.95Cr0.59 (Al:Cr ratio = 5.04:1) was obtained by high-pressure sinter-ing (HPS) of a stoichiometric Al11Cr2 mixture. Structure analysis of this phase, hereafter named η', revealed a close relationship to the previously reported η-Al11Cr2 structure, but with different mixed-occupied sites. Five fully occupied sites exhibit refined site occupation factors of 0.899 (5), 0.984 (4), 0.977 (5), 0.946 (4) and 0.945 (4) for the corresponding Al atoms. Moreover, there are no split Al sites in the η'-structure as reported for the η-structure. The refined chemical composition of the η'-phase revealed that it comprises two Al atoms fewer and two Cr atoms more than the previously reported η-Al11Cr2 phase.

In situhigh-temperature X-ray diffraction study of Sc-doped titanium oxide nanocrystallites

Titanium dioxide is an inexpensive wide-gap highly ionic semiconductor with striking photocatalytic capabilities in several heterogeneous photoredox reactions. A small crystal size is desirable to maximize the surface area, since photocatalytic reactions occur at the surface of a photocatalyst. Presented here are the synthesis and microstructural characterization of 4 at.% Sc-doped TiO2(4SDT) prepared by water-based co-precipitation. The crystal structure of 4SDT was examined viain situhigh-temperature powder X-ray diffraction experiments from 25 to 1200°C. Rietveld analysis revealed single-phase anatase up to 875°C, while at 900°C the anatase-to-rutile phase transformation occurred and at higher temperatures additional reflections of Sc-rich phases (Sc2TiO5from 975°C and Ti3Sc4O12or Sc2O3at 1200°C) were observed. Debye function analysis (DFA) was applied to model the total scattering pattern directly in reciprocal space, allowing the reconstruction of Ti vacancies. Both Rietveld and DFA methods were applied to estimate the nanocrystallite size and shape with consistent growth in crystallite size with temperature: an ellipsoid shape with equatorial ∼4.7 nm / axial (001) ∼6.9 nm at 25°C to equatorial ∼27.9 nm / axial (001) ∼39.6 nm at 900°C refined by Rietveld analysis,versusa cylinder shape withDa,b= 4.3 nm and size dispersion σab= 1.5 nm,Lc= 4.9 nm and σc= 2.3 nm at 25°C toDa,b= 21.4 nm, σab= 8.3 nm,Lc= 23.9 and σc= 10.9 nm at 900°C estimated by DFA. The microstructural changes obtained by Rietveld and DFA methods were supported by high-resolution transmission electron microscopy image analysis, as well as by the less direct nitrogen sorption techniques that provide information on the size of non-agglomerated and dense particles. The Ti site-occupancy factor showed a linear increase from 0.6–0.8 at 25°C to unity at 900°C for anatase, and from ∼0.7 at 900°C to unity at 1200°C for rutile, via Rietveld analysis and DFA.

Enhancement of electrical properties of NASICON-type solid electrolytes (LiSn2P3O12) via aluminium substitution

Abstract NASICON-type electrolyte is one of the possible solid-state electrolytes to be employed in solid-state energy devices. The Li1+xAlxSn2-xP3O12 NASICON-based solid electrolyte materials were fabricated using a mechanochemical milling method at 650 °C for 8 h, where x is from 0 to 0.8. From the X-ray diffraction (XRD) analysis, the crystallographic phases of the parent material and the partial aluminium (Al) substituted one have been verified. Site occupancy factor (s.o.f) studies from the XRD data refinement illustrate that the material with x = 0.4 has a stoichiometry in good agreement with the nominal tin (Sn):aluminium (Al) ratio which is 8:2. Field emission scanning electron microscopy (FESEM) was conducted to study the surface morphological properties, revealing the homogenous single-crystalline structure in the sample with x = 0.4, which consists of particles of the smallest crystallite size of 3.77 nm among the samples investigated. The process of partial Al-substitution was further confirmed by the energy dispersive X-ray (EDX). The material with x = 0.4 possesses the maximum conductivity value of 7.57 × 10−6 S·cm−1, which was improved up to 3.79 × 10−5 S·cm−1 by a silver paint treatment. The correlation between the microstructure and the ionic conductivity has been explained via the EDX and impedance analysis results. Ions were found to be the main charge carriers from the transference number analysis (TNM). Variation of relaxation in the dielectric study verifies that Li1+xAlxSn2-xP3O12 obeys a non-Debye characteristic. Patterns of the dielectric results strengthened the trend of ionic conductivity.

Synthesis, crystal structure, DFT studies and biological evaluation of N-benzamido derivatives of oxahexacycloundecyl amines: A case of enantiomerism leading to molecular disorder

To explore the reactivity of the amine in oxahexacycloundecylamines and its effect on biological properties, two di-substituted N-benzamido-oxahexacycloundecylamine derivatives were synthesized. These compounds exhibited favourable biological properties in cytotoxicity, neuroprotection and calcium influx experiments. Steric and conformational factors are proposed to play an important role in the biological activities observed and X-ray crystallographic experiments were conducted in an attempt to gain insight into the structural features of these compounds. The propyl derivative, 3-(N-(8,11-oxahexacyclo[5.4.0.02,6.03,10.05,9]-undecyl)benzamido)propyl benzoate, crystallizes in the monoclinic space group P21/c with unit cell parameters: a = 6.2838(11) Å; b = 27.833(5) Å; c = 12.630(2) Å; β = 95.457(3) Å; V = 2199.0(7) Å3; Z = 4. The crystal structure revealed the existence of enantiomerism in the oxa-bridged polycycloundecylamine residue as a result of the nitrogen attached on either side of the oxa-bridge. This enantiomerism leads to molecular disorder of this residue in the crystal over two positions with refined site-occupancy factors (s.o.f.s) of x (= 0.774(2)) and 1-x (= 0.226(2)) corresponding to the respective enantiomeric residues with S- and R-configurations at partial atoms C18A and C18B in one molecule. For each two-fold disordered chiral atom, the presence of a centre of symmetry in the crystal generates the complementary pair of partial atoms within the crystal (s.o.f.s x and 1-x), thus rendering it racemic. DFT optimization studies on the two individual enantiomers corresponding to the two residues with S- and R-configurations at C18 within the disordered structure showed almost identical geometric parameters, indicating that the disorder observed is linked to the crystal packing. The ethyl derivative also showed similar geometric parameters to its propyl counterpart according to the DFT results. These findings correspond to earlier observations in that a number of steric and electronic factors could collectively be responsible for the biological activities observed for these compounds.

Crystal structure effect on the phase transition of (Ca0.28Ba0.72)x(Sr0.61Ba0.39)1−xNb2O6 ceramics

(Ca0.28Ba0.72)x(Sr0.61Ba0.39)1−xNb2O6 ceramics have been prepared by a solid-state reaction method. With the increase of x, the nature of the phase transition varies from the diffuse to the normal, and the phase transition temperature increases. The structural origins of these variations were studied by Rietveld refinement and Raman spectroscopy. The results indicate that the change of the phase transition type is related to the site-occupancy factors of cations at A sites and the increased phase transition temperature is because of the enhanced structural deformation of oxygen octahedral. This enhanced ferroelectricity makes the system have potential applications for high-temperature environments.

Structural Distortion in Perovskite Type KCaH3–xFx (0.54 ≤ x ≤ 3)

Representatives of the solid solution series KCaH3–xFx were synthesized by solid state reactions from binary metal hydrides and fluorides. Crystal structures were analyzed by Rietveld refinement based on X‐ray powder diffraction. The degree of substitution was determined by refinement of site occupancy factors as well as elemental analysis for hydrogen. Three sections of x in KCaH3–xFx can be distinguished. For x < 0.54 no hydride fluoride exists, i.e. there is no hydride of the composition KCaH3 and the solid solution starts only at x = 0.54. The tetragonal SrTiO3 type structure with partial ordering of hydrogen and fluorine atoms is found for 0.54 ≤ x ≤ 1.7. Both anion positions show mixed occupation with some preference of hydrogen atoms for 8h and fluorine atoms for 4a sites (I4/mcm, SrTiO3 type). For fluorine‐rich compounds a solid solution with orthorhombic GdFeO3 type structure (Pnma) and a perfectly statistical distribution of hydrogen and fluorine atoms is found (1.8 ≤ x ≤ 3). Interatomic distances resulting from the structure refinements are in the range of typical K–H, K–F, Ca–H, and Ca–F distances for mainly ionic compounds.

Structure and Stability of the Stoichiometric Al3Fe Phase

A disordered stoichiometric Al3Fe phase was obtained when synthesizing Al8Fe3 by the spark plasma sintering (SPS) approach and its crystal structure was determined by the single-crystal X-ray diffraction (SXRD) techniques. The refined structure is an isotype of the reported Al5Fe2 phase, only different in the site occupation factors (s. o. f.) of Al2 and Al3 atoms, which was refined to be 0.431 (13) and 0.569 (13), respectively. Stimulated by the structural refinement results, an ordered stoichiometric Al3Fe phase was established. First-principles calculations reveal that the ordered Al3Fe phase is mechanically and dynamically stable and has a much lower value of enthalpy of formation than any other proposed Al3Fe phases, although it is also metallic and metastable.

Structural dissymmetrization of optically anisotropic Grs64±1Adr36±1Sps2grandite from Meka Presedla (Kopaonik Mt., Serbia)

In this paper, grandite core with Grs64±1Adr36±1Sps2composition was crystallographically studied. This core represents zone A of the macroscopically visible five A–E zones of the optically anisotropic Grs58–64Adr36–42Sps2grandite. The applied procedure includes the detailed analysis of the powder diffraction patterns, and the Rietveld refinements of the crystal structures in a series of 18 space groups and two mixtures, which were followed by the comparative analysis of theR-values, site occupancy factors, and the bond lengths and angles. Synthesis of all of the presented results allows us to undoubtedly conclude that studied grandite is not cubic, neither as monophase nor as multiple phases in a mixture. Namely, it was established that structural dissymmetrization occurred and that it crystallized in the disordered rhombohedral$R\bar{3}c$or orthorhombicFdddspace groups, whereby the first one is more probable. Beside the established lower symmetry of the studied grandite, which could be treated as the primary cause, the residual strain is also not excluded as the second possible cause for its slight optical anisotropy.

Structural Trends and Solid-Solutions Based on the Crystal Chemistry of Two Hausmannite (Mn3O4) Samples from the Kalahari Manganese Field

The crystal chemistry of two hausmannite samples from the Kalahari manganese field (KMF), South Africa, was studied using electron-probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD) for sample-a, and high-resolution powder X-ray diffraction (HRPXRD) for sample-b, and a synthetic Mn3O4 (97% purity) sample-c as a reference point. Hausmannite samples from the KMF were reported to be either magnetic or non-magnetic with a general formula AB2O4. The EPMA composition for sample-a is [Mn2+0.88Mg2+0.11Fe2+0.01]Σ1.00Mn3+2.00O4 compared to Mn2+Mn3+2O4 obtained by refinement. The single-crystal structure refinement in the tetragonal space group I41/amd gave R1 = 0.0215 for 669 independently observed reflections. The unit-cell parameters are a = b = 5.7556(6), c = 9.443(1) Å, and V = 312.80(7) Å3. The Jahn–Teller elongated Mn3+O6 octahedron of the M site consists of M–O × 4 = 1.9272(5), M–O × 2 = 2.2843(7), and an average <M–O>[6] = 2.0462(2) Å, whereas the Mn2+O4 tetrahedron of the T site has T–O × 4 = 2.0367(8) Å. The site occupancy factors (sof) are M(sof) = 1.0 Mn (fixed, thereafter) and T(sof) = 1.0008(2) Mn. The EPMA composition for sample-b is [Mn0.99Mg0.01](Mn1.52Fe0.48)O4. The Rietveld refinement gave R (F2) = 0.0368. The unit-cell parameters are a = b = 5.78144(1), c = 9.38346(3) Å, and V = 313.642(1) Å3. The octahedron has M–O × 4 = 1.9364(3), M–O × 2 = 2.2595(6), and average <M–O>[6] = 2.0441(2) Å, whereas T–O × 4 = 2.0438(5) Å. The refinement gave T(sof) = 0.820(9) Mn2+ + 0.180(9) Fe2+ and M(sof) = 0.940(5) Mn3+ + 0.060(5) Fe3+. Samples-a and -b are normal spinels with different amounts of substitutions at the M and T sites. The Jahn–Teller elongation, Δ(M–O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn3O4.

Crystal Structure and Photoluminescence Properties of an Incommensurate Phase in EuO- and P2O5-Doped Ca2SiO4.

We have for the first time clarified the incommensurately modulated crystal structure as well as the photoluminescence properties of Eu2+-activated Ca2SiO4 solid solution, the chemical formula of which is (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4, where □ denotes vacancies in Ca sites with the replacement of Si4+ by P5+. The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a full width at half-maximum of ca. 80 nm. The crystal structure was made up of the four types of β-Ca2SiO4-like layers with one type of interlayer. The incommensurate modulation with superspace group Pnma(0 β 0)00 s was induced by the long-range stacking order of these layers. The modulation wavevector was 0.27404(2) × b*, with the basic unit-cell dimensions being a = 0.68355(2) nm, b = 0.54227(2) nm, and c = 0.93840(3) nm ( Z = 4). The basic structure contained two nonequivalent Ca sites. One site was fully occupied by Ca2+ and free from Eu2+ in the overall incommensurate structure. The occupational modulation at the other site was so significant that the sum of site occupation factors for Ca2+ and Eu2+ as low as 0.5 was seen at the interlayer. This site was too large for accommodation of Ca2+ but was suitable for Eu2+. Thus, the Eu2+ ions would exclusively concentrate at the relevant site, which would cause the emission peak of the incommensurate phase to be shifted to the shorter wavelength ranges as compared with those of the other commensurate phases such as β and α'L.

Size-controlled synthesis of Mn3O4 nanoparticles: characterization and defect chemistry

Identification of defect chemistry and variation of particle size are key factors for understanding many intrinsic properties of materials. Herein, size selective synthesis of Mn3O4 nanoparticles was performed by precipitation of Mn(OH)2 via NaOH or ammonia followed by air oxidation in ethanol/water solution. The amount of ethanol in water affects the average particle size that drops from ~ 25 nm in pure water to ~ 10 nm in 95% EtOH/H2O. TEM studies showed that the Mn3O4 samples have a sphere-cube like morphologies. Calculation of site occupancy factor and unit cell parameters via Rietveld analysis and the excess mass of Mn3O4 samples depending on size by thermogravimetric analysis and IR revealed that the cationic deficiency observed in the structure of the smallest size sample is compensated mainly by anionic vacancies. Ideal spinel structure was found for largest size sample. The degree of tetragonal distortion (c/ $$ \sqrt{2} $$ a) of Mn3O4 unit cell increases with particle size.