Crystal Structure Refinement Research Articles

Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit.

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80-1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg1.70Fe2+0.29Mn0.01)Σ2.00(Fe3+0.90Al0.08Mg0.02)Σ1.00O2(BO3). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO6]n- octahedra and isolated [BO3]3- triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe2+, M4 by Fe3+, Mg and Al. There are also oxo-centered [O4M4]n+ and [O2M5]n+ polyhedra building up a framework with the [BO3]3- triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe2+ → Fe3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO6]n- octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (αc = 9.1 × 10-6 K-1) and minimum expansion is in the ab plane (αa = 8.6 × 10-6, αb = 7.6 × 10-6 K-1), which is due to the preferred orientation of the [BO3]3- triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3), vonsenite (Fe2+,Mg)2(Fe3+,Mn2+,Sn,Al)O2(BO3) and ludwigite (Mg,Fe2+,Mn)2(Fe3+,Al,Mg)O2(BO3), is provided.

Fast response 4f15d1→4f2 emission and host-to-impurity energy transfer in Pr3+ doped Na3LuSi3O9 double salt silicate

Understanding of host-to-impurity energy transfer process is crucial for the development of efficient and fast response scintillator materials. In this work, Pr3+ doped Na3LuSi3O9 (NLS) double salt silicate has been successfully synthesized using a high temperature solid-state reaction method and further comprehensively characterized by means of XRD measurements, crystal structure refinement, luminescence spectroscopic and time-domain study upon VUV-UV and X-ray synchrotron excitation, and thermally stimulated luminescence (TSL) study. Based on these results a model of host to Pr3+ 4f15d1 and 4f2 has been proposed. It has been demonstrated that NLS:Pr3+ has a good potential for fast response scintillator applications due to the presence of an efficient energy transfer from host electronic excitations to Pr3+ 4f15d1 states. The Pr3+ 4f15d1→4f2 emission reveals a good thermal stability and is characterized by a lifetime of about 16 ns. Upon pulse X-ray excitation the main fast decay component is accompanied by slower one with lifetime of about 64 ns that is ascribed to the presence of shallow traps which retrap carriers and cause appearance of delayed recombination. Parameters for the corresponding traps have been calculated based on the TSL glow curve analysis.

Microstructure and microwave dielectric properties of nonstoichiometric 1:1 ordered Nd(Zn1/2Ti1/2+x)O3+2x perovskite ceramics with Ti4+ self-doping

The grain size, B-site 1:1 cation ordering, chemical defects and microwave dielectric properties of the non-stoichiometric Nd(Zn1/2Ti1/2+x)O3+2x (x = −0.02–0.02) complex perovskite ceramics were significantly tuned by self-doping methods. The increase of Ti4+ self-doping significantly increases the grain size of Nd(Zn1/2Ti1/2+x)O3+2x ceramics which could still maintain a monoclinic structure with B-site 1:1 cation ordering (space group P21/n) in the case of deviation from stoichiometry. However, the degree of 1:1 B-site cation ordering is tuned to be negatively correlated with the mutual occupancy of Ti4+ and Zn2+ by Ti4+ self-doping according to the results of crystal structure refinement. With the increase of Ti4+ component, the degree of 1:1 cation ordering first increased and then decreased, reaching the maximum value (S = 0.81) at x = 0.01. Non-stoichiometry can tune cation ordering because it modulates the effective radius difference and charge difference of B-site ions. The increase Ti4+ component promotes the formation of ordered domains with uniformly distributed domain boundaries and increasing domain size up to x = 0.01. With further increase of Ti4+ component to x = 0.02, ferroelastic domains appear due to the oxygen octahedral torsion. The B-site 1:1 cation ordering significantly reduces the dielectric loss of the ceramics, which makes the Q × f value exhibit the same variation rule as the degree of B-site 1:1 cation ordering. High Q × f value of 58000 GHz is achieved in the non-stoichiometric samples (x = 0.01), and it is 45% higher than those in traditional stoichiometric samples (x = 0). In this work, the microstructure and properties of the ceramics are effectively regulated by non-stoichiometry, which provides an effective way to tune the properties of 1:1 ordered ceramics.

Scandio-winchite, ideally ☐(NaCa)(Mg4Sc)(Si8O22)(OH)2: The first Sc-dominant amphibole-supergroup mineral from Jordanów Śląski, Lower Silesia, southwestern Poland

Abstract Scandio-winchite, the first natural Sc-dominant amphibole-supergroup mineral, has been discovered in a granitic pegmatite that crops out in close association with rodingite-like calc-silicate rocks and metasomatized granitic bodies in a serpentinite quarry at a Jordanów Śląski village near Sobótka, ~30 km south of Wrocław, Lower Silesia, SW Poland. It occurs as an isolated subhedral crystal, with the size of ~20 × 8 μm in planar section, and as three polycrystalline aggregates, up to 50 μm across, composed of needle-shaped crystals dominated by {110}. It is present within chlorite aggregates that supposedly represent remnants of partly recrystallized xenoliths of the blackwall chlorite schists and is in quartz-feldspar portions of the pegmatite adjoining such xenolithic assemblages. Owing to the scarcity of the material and the exceptionally small size of the crystals, the color, streak, and optical properties could not be measured. By analogy with other amphiboles, scandio-winchite has a vitreous luster, brittle tenacity, and a Mohs hardness of ~5½. The mineral shows an uneven fracture and {110} perfect cleavage, with an angle of ~56° between cleavage planes. The density calculated from the empirical formula and refined unit-cell parameters is 3.026 g/cm3. The holotype crystal is composed of (in wt%): 55.88 SiO2, 0.11 TiO2, 0.53 Al2O3, 9.22 Sc2O3, 0.44 MnO, 8.89 FeO, 12.77 MgO, 5.71 CaO, 4.12 Na2O, 0.17 K2O, and 2.09 H2Ocalc(+); total 99.93. The composition normalized on the basis of 22 O2– + 2 (OH)– ions corresponds to the empirical formula A(☐0.966K0.031Na0.003)Σ1B(Na1.132 Ca0.868)Σ2C(Mg2.704Fe1.0552+Mn0.053Sc1.140Al0.023Ti0.012)Σ4.987T(Si7.935Al0.065)Σ8.000O22(OH)2, simplified formula (☐,K)(Na,Ca)2[(Mg,Fe)4Sc](Si8O22)(OH)2, and the ideal formula ☐(NaCa)(Mg4Sc)(Si8O22)(OH)2. The crystal structure was refined in the monoclinic system, space-group symmetry C2/m, with R1 index of 6.57%. Its unit-cell parameters are: a = 9.864(2) Å, b = 18.163(3) Å, c = 5.3053(16) Å, β = 104.41(3)°, V = 920.6(4) Å3; Z = 2, and the a:b:c ratio is 0.5431:1:0.2921. The crystal-structure refinement indicates almost exclusively Si-occupied T sites, the M4 sites occupied by nearly equal amounts of Na and Ca, M1 and M3 sites by divalent Mg + Fe cations, and M2 filled in equal proportions by divalent cations and Sc. These results, along with the dominant vacancy at the A site, univocally indicate that the mineral corresponds to a M2Sc-analog of winchite. Scandio-winchite is most likely a secondary phase of metasomatic origin related to the evolution of the country rocks and partial alteration of the blackwall chlorite schists xenolith induced by the pegmatitic melt and associated fluids.

Crystal structure investigation of spinel‐type LiAlON with multiple atomic disorders

AbstractSpinel‐type lithium aluminum oxynitride (LiAlON) contains atomic disorders at every crystallographic position, including vacancies, cation, and anion disorders. In this work, the crystal structure of spinel‐type LiAlON with multiple atomic disorders was systematically studied by first‐principles calculation, solid‐state NMR, and the Rietveld crystal structure refinement. The theoretical simulations demonstrate that the Li+ occupies octahedral positions and prefers to be far away from the N3− and cation vacancies in the crystal structure of LiAlON. A spinel‐type LiAlON compound with chemical formula of Li0.06Al2.72O3.77N0.23 was prepared by solid‐state reaction. The local structure of Li+ and Al3+ was probed by 7Li and 27Al solid‐state NMR technology. Combined with theoretical calculations and solid‐state NMR, the crystal structure of Li0.06Al2.72O3.77N0.23 was fully resolved by the Rietveld refinement. This work provides a deep understanding on the disordered crystal structure of the spinel‐type LiAlON solid solution.

Strongly Modified Mechanical Properties and Phase Transition in AlPO4-17 Due to Insertion of Guest Species at High Pressure.

The porous aluminophosphate AlPO4-17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressure-induced amorphization, was studied at high pressure by single-crystal and powder X-ray diffraction in the penetrating pressure transmitting media N2, O2, and Ar. Under pressure, these guest species were confirmed to enter the pores of AlPO4-17, thus completely modifying its behavior. Pressure-induced collapse in the xy plane of AlPO4-17 no longer occurred, and this plane exhibited close to zero area compressibility. Pressure-induced amorphization was also suppressed as the elastic instability in the xy plane was removed. Crystal structure refinements at a pressure of 5.5 GPa indicate that up to 28 guest molecules are inserted per unit cell and that this insertion is responsible for the reduced compressibility observed at high pressure. A phase transition to a new hexagonal structure with cell doubling along the a direction was observed above 4.4 GPa in fluid O2.

Refinement of crystal structures at ultralow resolution with assistance from AlphaFold modeling and Rosetta optimization

Refinement of crystal structures at ultralow resolution with assistance from AlphaFold modeling and Rosetta optimization

Crystal structure refinement, optical, magnetic, and X-band microwave shielding properties of Dy3+ doped Ni2Y-type strontium hexagonal ferrites

Ni2Y-type strontium hexaferrites having chemical dddof Sr1·8Ba0·2Ni2Fe12-xDyxO22 (x = 0.0, 0.1, 0.2) were made utilizing the sol-gel combustion technique. All samples' XRD patterns demonstrate the production of NiFe2O4 and Fe3O4 which are a phase that is intermediate in the synthesis of the required Ni2Y-type strontium hexaferrites. The FTIR indicates the presence of absorption peaks at about 557 and 432 cm−1, this shows the possibility of the formation of Y-type hexaferrites. UV-vis NIR spectroscopy analysis indicates that the energy band-gap of Y-type hexaferrites is affected by the increase in Dy3+ concentration. The values of the band-gap were found to be between 1.66 and 1.78 eV. According to the FESEM analysis, platelet-structure is formed with particles that range in size from 3.665 to 3.927 μm. The observed coercivity, saturation, and remnant magnetization first decreased with an increase in the concentration of Dy3+ for the sample having x = 0.1 and then increases for the sample with x = 0.2. The sample with the greatest amount of Dy3+ (i.e.x = 0.2) has a value of total shielding effectiveness of 27.5 dB. The prepared Ni2Y-type hexaferrites sample could be use for efficient shielding of microwaves in X-band.

Synthesis, Hirshfeld atom refinement of crystal structure, and Hirshfeld surface analysis of 5-ethoxycarbonyl-6-methyl-4-(2-(trifluoromethyl)phenyl)-3,4-dihydropyrimidin-2-(1H)-one formic acid monosolvate

We report the cyanamide-based multicomponent synthesis and crystal structure of the title compound, 5-ethoxycarbonyl-6-methyl-4-(2-(trifluoromethyl)phenyl)-3,4-dihydropyrimidin-2-(1H)-one which crystallized as a binary cocrystal with formic acid, C15H15F3N2O3·HCO2H (DHPM···FA). The molecular structure of the DHPM···FA heterosynthon was unequivocally confirmed using X-ray single crystal diffraction techniques and spectroscopic methods. X-ray diffraction analysis shows that this cocrystalline material crystallizes in the monoclinic space group P21/n, Z = 4 with the 3,4-dihydropyrimidin-2-(1H)-one ring in a twist-boat form attached to the ester and phenyl group in an s−cis/s−cis and sp−conformation, respectively. The molecular structure and crystalline packing are stabilized by intermolecular N2–H2∙∙∙O1, N1–H1∙∙∙O5, O4–H4A∙∙∙O1, C4–H4∙∙∙O5, C6–H6∙∙∙O3 and C16–H16∙∙∙O3 hydrogen bonds into a 3D supramolecular network R22(8). Furthermore, the Hirshfeld surface analysis confirms that the most important contributions for the crystal packing are the H∙∙∙O/O∙∙∙H (58.7%), C − H···C (17.8%), H∙∙∙H (30.4%) and H∙∙∙F/F∙∙∙H (18.3%) surface contacts.

Structural studies and magnetism of Dy6Ni2.43Sn0.5 stannide

Intermetallic compound Dy6Ni2.43Sn0.5 was prepared by arc melting and annealing at 873 K. It was characterized by X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis. The crystal structure of low temperature Dy6Ni2.43Sn0.5 phase belongs to the orthorhombic Ho6Co2Ga structure type (space group Immm, a = 0.93116(1) nm, b = 0.94993(1) nm, c = 0.98947(1) nm). Crystal structure refinements showed the deviation from the ideal 6:2:1 stoichiometry corresponding to the formula Dy6Ni2.43Sn0.5. It exhibits a sequence of magnetic phase transitions; antiferromagnetic ordering sets in at 60 K, while further order-order magnetic phase transitions take place at lower temperatures.

Orthorhombic Crystal Structure of Grossular Garnet (Suva Česma, Western Serbia): Evidence from the Rietveld Refinement

The grossular garnet from rodingite-type rock from the Suva Česma area in western Serbia is characterized with its weak anisotropic nature. Because its anisotropy could indicate a non-cubic lower symmetry, SEM-EDS and Rietveld powder refinement methods were used. The SEM-EDS results have shown that the garnet has a (Ca3.00Mn0.01)3.01(Al1.82Fe0.15Ti0.02)1.99(Si2.97Al0.03)3.00O12 chemical composition (i.e., Grs91Adr08), which can be more specifically explained as ferric iron containing grossular. The next step further used Rietveld powder refinements of the various crystal structures in the Ia‾3d, R‾3c, R‾3, I41/a, Fddd, C2/c, and I‾1 space groups as well as a single mixture, which was followed by a comparative analysis of the R-values, site occupancy factors, and bond lengths and angles. The synthesis of these results showed both that the studied grossular garnet is not cubic and that it crystallized in the disordered Fddd space group with the final RB = 5.29% and RF = 1.75%. It was presumed that the grossular formed at temperatures between 150 and ~600 °C.

Manganrockbridgeite, Mn2+2Fe3+3(PO4)3(OH)4(H2O), a new member of the rockbridgeite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria

Abstract. Manganrockbridgeite, Mn22+Fe33+(PO4)3(OH)4(H2O), is a new member of the rockbridgeite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria. It occurs in association with frondelite, kenngottite, hureaulite and hematite. It forms compact intergrowths and clusters of shiny greenish black blades up to 200 µm long and 20 µm wide but only a few micrometres thick. The crystals are elongated on [100] and flattened on {001}, with perfect cleavage parallel to {001}. Individual thin blades are green in transmitted light and red under crossed polars. The calculated density is 3.40 g cm−3. Manganrockbridgeite is biaxial (+/-), with α= 1.795(5), β= 1.805(calc), γ=1.815(5) (white light) and 2V(meas.) = 90(2)∘. The empirical formula from electron microprobe analyses, Mössbauer spectroscopy and crystal structure refinement is (Mn1.072+Fe0.692+Fe0.163+)Σ1.92(Fe3+)2.88(PO4)3(OH)3.64(H2O)1.44. Manganrockbridgeite has monoclinic symmetry with space group P21/m and unit-cell parameters a=5.198(2), b=16.944(5), c=7.451(3) Å, β=110.170(9)∘, V=616.0(4) Å3 and Z=2. The crystal structure was refined using both laboratory and synchrotron single-crystal diffraction data. Whereas other rockbridgeite-group minerals have orthorhombic symmetry with a statistical distribution of 50 % Fe3+ / 50 % vacancies in M3-site octahedra forming face-shared chains along the 5.2 Å axis, monoclinic manganrockbridgeite has full ordering of Fe3+ and vacancies in alternate M3 sites along the 5.2 Å axis.

Katsarosite Zn(C2O4)·2H2O, a new humboldtine-group mineral from the Lavrion Mining District, Greece

Katsarosite, ideally Zn(C2O4)·2H2O, named for Īraklīs Katsaros, is a new mineral found at the Esperanza Mine in the Kaminiza area of the Lavrion Mining District, Greece. Katsarosite usually occurs directly on sphalerite or embedded in jarosite and/or hydrozincite, often intimately intergrown with gypsum and overgrown by goslarite and/or epsomite. Crystal aggregates are mostly fine granular to earthy, with individual crystals being usually rounded with an average diameter of 30 µm, sometimes prismatic along [001] or platy, exhibiting the indistinct forms {100}, {001}, {110}, and {101}. Katsarosite is malleable with a Mohs hardness of 1½ – 2 and exhibits a perfect cleavage on {110}; the fracture is uneven in all other directions. The colour depends on the iron (Fe2+) content, ranging from pure white in almost Fe-free samples to yellow in Fe-rich specimens. It has a resinous luster and a white streak; no luminescence has been observed under either short- or long-wave ultraviolet radiation. Katsarosite is optically biaxial (+). Refractive indices measured at a wavelength of 589 nm are nα = 1.488(2), nβ = 1.550(2), nγ = 1.684(2), with 2Vobs = 71(3)°. Chemical analysis gave on average C2O3 38.32 wt%, ZnO 38.99 wt%, FeO 1.92 wt%, and H2O 19.04 wt% (the latter was deduced based on the crystal-structure refinement), with traces of MgO and MnO. The new mineral is readily soluble in dilute acids. Katsarosite is monoclinic, space group C2/c, with unit-cell parameters a = 11.768(3), b = 5.3882(12), c = 9.804(2) Å, β = 127.045(8)°, V = 496.2(2) Å3 (Z = 4). The strongest lines in the Gandolfi X-ray powder pattern [dobs in Å, Iobs/I100, (hkl)] are: 4.6745, 100, (200); 4.7678, 94, (20overline{2 }); 2.9533, 51, (40overline{2 }); 4.7030, 37, (1 overline{1 },overline{1 }); 3.9266, 33, (002); 3.5686, 27, (111); 2.6574, 22, (1 overline{1 },overline{3 }); 3.5992, 8, (1 overline{1 },overline{2 }); 2.7032, 4, (020). The crystal structure was refined based on single-crystal X-ray diffraction data to R(F) = 0.08. The observed mass density of 2.50(2) g cm−3 compares well with the calculated value (2.508 g cm−3). Katsarosite belongs to the humboldtine group, whose crystal-structure type is well described for both isotypic minerals and synthetic compounds in the literature. The atomic arrangement in Zn(C2O4)·2H2O is characterized by chains consisting of isolated ZnO6 octahedra which are alternately linked along [010] via oxalate anions. These chains are interconnected through hydrogen bonds only, with Ow···O (with Ow denoting the O atom of the H2O molecule) donor–acceptor distances of ~ 2.8 Å.

SYNTHESIS AND CHARACTERIZATION OF CESIUM SILICATE TO DETERMINE ITS DETAILED PROPERTIES AS CHEMISORBED ONTO STRUCTURAL MATERIALS OF LIGHT WATER REACTOR DURING SEVERE ACCIDENT CONDITIONS

Cesium chemisorption phenomenon strongly contributes to the source terms transport during light water nuclear reactor accidents. Large amounts of cesium silicates are identified to be chemisorbed onto structure material, reduce cesium volatility, and affect the late release and re-vaporization phenomena. Although it has been studied for a long time, several characteristics of these compounds are still under discussion. In this study, Cs2SiO3, Cs2Si2O5, and Cs2Si4O9 were synthesized through the solid-state method and the results have been confirmed using X-Ray Diffraction (XRD) measurement. Furthermore, their crystal structures have been refined based on the XRD analysis. The crystal structure refinement of these compounds proves the previous studies, but with minor distinctions in the lattice parameters. XRD patterns changing over time when measured in the open-air environment also show that Cs2Si4O9 is the most stable species among other cesium silicate species. This indicates that the chemisorbed Cs-Si-O compound onto the structural material as identified by previous studies is most likely Cs2Si4O9 rather than Cs2SiO3 or Cs2Si2O5. Therefore, detailed Cs2Si4O9 identification including its thermodynamic properties characterization could be very useful to enhance the database that is being built to improve current source terms transport codes.

Re-investigation of ‘minasgeraisite-(Y)’ from the Jaguaraçu pegmatite, Brazil and high-temperature crystal chemistry of gadolinite-supergroup minerals

AbstractThe chemical composition (including B, Be and Li), the Raman spectrum and the crystal-structure evolution (at the temperature range 27–1000°C) of a Mn-bearing, Bi-rich gadolinite-subgroup mineral from the Jaguaraçu Pegmatite, Brazil (type-locality of minasgeraisite-(Y)) was studied. Elemental mapping revealed that the crystal investigated has complex chemical zonation with various Bi (~8–24 wt.% Bi2O3), Ca (~8–10 wt.% CaO) and Y (~11–17 wt.% Y2O3) content. The sample investigated has all the specific features of the chemical composition of minasgeraisite-(Y), except Ca excess and, thus, should be considered as hingganite-(Y). The Raman spectrum of the sample under study has bands at 140, 179, 243, 350, 446, 519, 559, 625, 902, 973, 3224, 3353, 3532 and 3763 cm–1, and is similar to that of hingganite-(Y) / -(Nd). Crystal-structure refinement confirmed that the crystal in question should be considered as hingganite-(Y) and is in line with the previously obtained data on gadolinite-subgroup minerals from the Jaguaraçu Pegmatite. High-temperature single-crystal X-ray diffraction studies revealed that the mineral starts to decompose above 800°C. We can conclude that beryllosilicates are most stable at high-temperature conditions within the gadolinite supergroup and that species with a higher M-site occupancy have higher stability upon heating.

Single Crystal Growth of Ilmenite-Type MnVO3 by Solid-State Recrystallization

We successfully obtained high-quality single-domain crystals of ilmenite-type MnVO3 under high-pressure and high-temperature conditions (4 GPa and 1173 K for 1–8 h). The maximum crystal size was ∼0.5 mm. Single crystals were grown by solid-state recrystallization near the perovskite–ilmenite phase boundary under high-pressure conditions. Crystal growth occurred without fluxes or water, unlike conventional single crystal growth by high-pressure synthesis methods. Crystal structure refinement revealed that V–V dimers, produced by direct chemical bonding between vanadium ions, existed below 550 K at ambient pressure. The V–V dimerization was accompanied by a structural phase transition from rhombohedral to triclinic ilmenite-type structures. The obtained single crystals can facilitate precise analyses using resonant inelastic X-ray scattering and angle-resolved photoemission spectroscopy, which can reveal the mechanism underlying the phenomenon of V–V dimerization.

Crystal-chemical characterisation and spectroscopy of fluorcarletonite and carletonite

AbstractThe minerals of carletonite group, fluorcarletonite, KNa4Ca4[Si8O18](CO3)4(F,OH)·H2O and carletonite, Na4Ca4[Si8O18](CO3)4(OH,F)·H2O, were investigated using a multi-method approach. A detailed comparative chemical study of the minerals was carried out using electron probe microanalysis and Fourier transform infrared spectroscopy. Using X-ray techniques and the results obtained, geometrical and distortion characteristics of the mineral structures are calculated and the successful crystal-structure refinement of these two natural compounds are given. Using spectroscopic and luminescence methods and ab initio calculations, it is shown that hole defects (CO3)•– are responsible for the colouration of the samples studied. Luminescence due to 5d–4f transition in Ce3+ ions is observed in both investigated compounds. Moreover, luminescence attributed to intrinsic luminescence, corresponding to the decay of electronic excitations of (CO3)2– complexes in the carletonite sample, is registered for the first time in phyllosilicates. An analysis of the optical absorption spectra and g-tensor values suggests that (CO3)•– defects in the crystal structure are localised in the C1 positions. Identification of these specific properties for these sheet silicates, with a two-dimensional infinite tetrahedral polymerisation, indicates that carletonites could be prospective materials for novel phosphors and luminophores.

Single-crystal analysis of La-doped pyromorphite [Pb5(PO4)3Cl

Abstract Rare earth elements (REE) in calcium apatite have been widely described in the literature. Based on the investigations of minerals and their synthetic analogs, the mechanism of substitution of REE3+ for Ca2+ and their structural positions are well established. Although the presence of REE in natural pyromorphite has been reported, the structural response of substitution of REE3+ for Pb2+ is not established. A better understanding of REE-rich Pb-apatite may facilitate the potential use of this mineral in industrial processes. Two La-doped pyromorphite analogs [Pb5(PO4)3Cl] and two control pyromorphite analogs (with the absence of La) were synthesized from aqueous solutions at 25 °C. Na+ and K+ were used as charge-compensating ions to facilitate the incorporation of trivalent REE cations (La3+ + Na+ ↔ 2Pb2+ and La3+ + K+ ↔ 2Pb2+). Microprobe analysis, scanning electron microscopy, and Raman spectroscopy were used to confirm the purity of obtained phases. High-precision crystal structure refinements (R1 = 0.0140–0.0225) of all four compounds were performed from single-crystal X-ray diffraction data. The La content varied from 0.12(1) to 0.19(1) atoms per formula unit with the counter ions of K+ and Na+, respectively. Both substituting ions were accommodated at the Pb1 site only. By comparing the La-doped pyromorphite analogs with their control samples, it was possible to detect small changes in bond distances and polyhedral volumes caused by the La substitution. Variations in individual and mean interatomic distances reflected the cumulative effect of both the amount of substitution and ionic radii of substituting ions (La3+, Na+, and K+).

Comparative study of red-emitting pyrophosphate phosphors: Site-selective replacement of Eu3+ and luminescent properties

Red phosphors play a key role in improving the optical quality of white light emitting diodes (WLEDs). In this work, Eu3+-activated red-emitting pyrophosphate phosphors of NaREP2O7:Eu3+ (RE = Sc, Lu) were prepared and the comparative investigations on crystal structures and luminescent properties were carried out. The X-ray diffraction (XRD), scanning electron microscope (SEM), and spectroscopy techniques were employed to characterize the materials. The features of crystal structures with regard to the Eu3+ doping were analyzed in detail on the basis of Rietveld crystal structure refinements. It is found that the Eu3+ tends to substitute Lu3+ in the NaLuP2O7:Eu3+, while in NaScP2O7:Eu3+ it tends to substitute Na+ due the principle of ionic size matching. The disparate selective occupation behaviour of Eu3+ ion in both crystals has resulted in a certain difference in fluorescence emission nature from each other. The doping concentration of Eu3+ was optimized for the strongest luminescence under near-ultraviolet (UV) excitation. The measurement of temperature-dependent luminescence showed that the emission of NaLuP2O7:Eu3+ at 150 °C maintained 77% of that at room temperature, and that for NaScP2O7:Eu3+ is 72%. Under the near-UV excitation, the fluorescence efficiency of the phosphors were assessed. The quantum efficiencies of NSPO:0.125Eu3+ and NLPO:0.125Eu3+ were measured to be 41.6% and 40.8%, respectively. The results imply that the Eu3+-activated pyrophosphate phosphors are promising red-emitting components in WLEDs.

Improving the Accuracy of Small‐Molecule Crystal Structures Solved from Powder X‐Ray Diffraction Data by Using External Sources

AbstractStructural analysis using powder X‐ray diffraction data has overcome many obstacles and nowadays is readily applicable for structural analysis of all types of compounds and materials. Being less straightforward than single crystal diffraction, it requires significant users’ input and consequently, implementation of standardized tools to assess the accuracy of crystal structures. This article discusses potential errors in crystal structure solution and refinement of small‐molecule structures obtained from PXRD data. Moreover, it proposes how accuracy of these structures can be improved by using high‐quality PXRD data, complementary external analytical techniques, knowledge stored in crystal structure databases, as well as an approach to search the parameter space to avoid local minima in testing different sets of geometry restraints.