Space Group Pmcn Research Articles

Nomenclature of the ancylite supergroup

Abstract The ancylite supergroup has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, with the general crystal chemical formula (M3+ x M2+2–x)(CO3)2[(OH) x ⋅(2–x)H2O] (1 ≤ x ≤ 2, Z = 2). The ancylite supergroup can be divided into two groups defined by different proportions of the M cation and hydroxyl anion and/or water molecule: the ancylite group is defined for 1 ≤ x ≤ 1.5; the kozoite group is defined for 1.5 < x ≤ 2. The ancylite supergroup minerals are orthorhombic with space group Pmcn, or monoclinic with space group Pm11, and have a crystal structure with species-defining trivalent and divalent M cations (M = La3+, Ce3+, Nd3+, Ca2+, Sr2+ and Pb2+) which centre ten-vertex polyhedra formed by oxygen atoms at three independent O sites. Two vertices of the triangular (CO3)2– anion are oxygen atoms, whereas the third one, O(3), is statistically filled with (OH)– groups and H2O molecules. The triangular faces of three oxygen atoms of MO10 coordination polyhedra join the chains of this ten-vertex polyhedron, which is extended along the c axis. The (CO3) triangles connect chains in three dimensions. To date, eight valid mineral species with M2+ = Sr2+, Ca2+ and Pb2+ belong to the ancylite group [ancylite-(La), ancylite-(Ce), calcioancylite-(La), calcioancylite-(Ce), calcioancylite-(Nd), gysinite-(La), gysinite-(Ce) and gysinite-(Nd)]. Two hydroxyl carbonates with only rare earth elements as species-defining cations, kozoite-(La) and kozoite-(Nd) are members of the kozoite group.

Calcioancylite-(La), (La,Ca)2(CO3)2(OH,H2O)2, a new member of the ancylite group from Gejiu nepheline syenite, Yunnan Province, China

AbstractCalcioancylite-(La), ideally (La,Ca)2(CO3)2(OH,H2O)2, has been discovered from nepheline syenite of the Gejiu alkaline complex in the Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China. The mineral occurs as aggregates of subhedral grains, and the size of single crystals varies between 5–20 μm. Calcioancylite-(La) is colourless to pale pinkish grey and has transparent to translucent lustre. It is brittle with a Mohs hardness of 4. The calculated density is 4.324 g/cm3. The mineral is biaxial (−), with α =1.662, β = 1.730, γ = 1.771, 2Vmeas. = 70°(1) and 2Vcalc. = 73°. Electron microprobe analysis for holotype material yielded an empirical formula of (La0.58Ce0.55Pr0.14Nd0.10Ca0.39Sr0.20K0.04)Σ2.00(CO3)2[(OH)1.25F0.06⋅0.69H2O]Σ2.00. Calcioancylite-(La) is orthorhombic, with space group Pmcn, a = 5.0253(3) Å, b = 8.5152(6) Å, c = 7.2717(6) Å, V = 311.17(4) Å3 and Z = 2. By using single-crystal X-ray diffraction, the crystal structure has been determined and refined to a final R1 = 0.0652 on the basis of 347 independent reflections (I > 2σ). The seven strongest powder X-ray diffraction lines [d in Å (I) (hkl)] are: 2.334 (100) (013), 2.970 (80) (121), 4.334 (75) (110), 3.678 (68) (111), 2.517 (55) (200), 2.647 (47) (031) and 2.077 (44) (221). Calcioancylite-(La) is the La-analogue of calcioancylite-(Ce) and is a new member of ancylite-group minerals. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021-090).

The effect of long-range interactions on the infrared and Raman spectra of aragonite (CaCO3, Pmcn) up to 25 GPa

Long-range interactions are relevant in the physical description of materials, even for those where other stronger bonds give the leading contributions. In this work, we demonstrate this assertion by simulating the infrared and Raman spectra of aragonite, an important calcium carbonate polymorph (space group Pmcn) in geological, biological and materials science fields. To this aim, we used Density Functional Theory methods and two corrections to include long-range interactions (DFT-D2 and DFT-D3). The results were correlated to IR spectroscopy and confocal Raman spectrometry data, finding a very good agreement between theory and experiments. Furthermore, the evolution of the IR/Raman modes up to 25 GPa was described in terms of mode-Grüneisen’s parameters, which are useful for geological and materials science applications of aragonite. Our findings clearly show that weak interactions are of utmost importance when modelling minerals and materials, even when they are not the predominant forces.

Gysinite-(La), PbLa(CO3)2(OH)⋅H2O, a new rare earth mineral of the ancylite group from the Saima alkaline complex, Liaoning Province, China

AbstractThe new mineral, gysinite-(La), with the ideal formula PbLa(CO3)2(OH)⋅H2O, has been discovered in lujavrite from the Saima alkaline complex, Liaoning Province, China. It commonly occurs as subhedral to anhedral, granular and platy crystals of 5 to 50 μm in size, in interstices or enclosed in microcline, aegirine and nepheline. Associated minerals include nepheline, aegirine, microcline, natrolite, eudialyte, lamprophyllite, bastnäsite-(Ce), parasite-(Ce), ancylite-(La), ancylite-(Ce), bobtraillite, britholite-(Ce), thorite, calcite and galena. The crystallisation of gysinite-(La) may be related to the post-magmatic carbonation event. Gysinite-(La) crystals are generally transparent, colourless, or pale yellow, with a vitreous lustre and white streak. It is brittle with an uneven fracture, and the estimated Mohs hardness is 3½ to 4. The calculated density is 5.007 g/cm3. Optically, gysinite-(La) is biaxial (–), α= 1.832(2), β= 1.849(4), γ = 1.862(5) in white light and 2Vmeas = 81.6°. The empirical formula of gysinite-(La) is (La0.93Pb0.61Nd0.23Pr0.14Sr0.04Gd0.02Sm0.01Eu0.01Ca0.01)Σ2(CO3)2(OH)1.34⋅0.66H2O, which is calculated on the basis of general formula (REExM2+2–x)(CO3)2(OH)x⋅(2–x)H2O. The strongest eight lines of its powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 5.596 (21) (011), 4.349 (100) (110), 3.732 (68) (111), 2.984 (61) (121), 2.667 (21) (031), 2.363 (48) (131), 2.090 (29) (221) and 2.028 (21) (212). Gysinite-(La) is orthorhombic, in the space group Pmcn, and unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 5.0655(2) Å, b = 8.5990(3) Å, c = 7.3901(4) Å, V = 321.90(2) Å3 and Z = 2. It is a new member of the ancylite group and isostructural with gysinite-(Nd), but with La and Pb dominant in the metal cation sites in the structure.

Structural and elastic behaviour of aragonite at high-pressure: A contribution from first-principle simulations

Aragonite (CaCO3, space group Pmcn) is an important mineral for both geological and biological reasons, being one of the phases that recycles carbon in deep Earth conditions and the product of biomineralization of several terrestrial and marine organisms, respectively. Because of its ubiquity, aragonite has been the subject of several investigations to understand its elastic behaviour and stability at different P-T conditions, but the results reported in literature are still very scattered. Aiming at providing further details on this topic, in the present work we determined the structural and elastic properties of aragonite at absolute zero (0 K) within the Density Functional Theory framework, using a posteriori correction to include the weak long-range interactions. The equation of state parameters for this mineral phase, calculated between 0 GPa – 25 GPa, were K0 = 80.2(7) GPa, K’ = 4.37(10) and V0 = 223.00(6) Å3, in good agreement with the bulk modulus calculated from the elastic moduli (KR = 78.49 GPa). The results were compared to previous experimental and theoretical data, finding them in line with some specific studies, and show that some structural features (e.g., the carbonate ion aplanarity) could be related to the mechanism of phase transition to the post-aragonite phase at high pressure. The present work highlights the importance of including van der Waals interactions in the physical treatment of the structural and elastic properties of aragonite, and further extends the knowledge of the behaviour of this mineral as a function of pressure.

Characterization and luminescence properties of Eu3+ doped BaCO3 nanoparticles synthesized by autocombustion method

Nanostructured powders of undoped and europium (Eu3+) doped barium carbonate (BaCO3) were prepared by the autocombustion method. We investigated the influence of Eu3+ doping on the optical properties of barium carbonate. The samples nanostructures were characterized by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) and photoluminescence (PL). The results obtained by XRD revealed that the samples consist of the BaCO3 phase with orthorhombic structure having the space group Pmcn, retaining this crystalline phase after doping with Eu3+. These products were found for undoped and BaCO3: Eu3+ nanoparticles with 56–71 nm and 49–60 nm ranges, respectively. SEM reveals that the barium carbonate nanoparticles had a spherical morphology. Symmetric stretches, asymmetric stretches, and in plane bending vibrations, and out of plane of CO32− complexes have detected. The photoluminescence of the obtained BaCO3:Eu3+ were studied. The CIE co-ordinates for the Eu3+ doped BaCO3 were x = 0.580 and y = 0.330. From the CIE chromaticity diagram for the emissions of Eu3+ doped BaCO3. The material has such color purity and reaches 76%. Thus, having color tenability and has been considerate as a candidate of the laser emission.

Chemical Preparation, Thermal Behavior and IR Studies of the New Chromium Diphosphate Hydrate and Crystal Structure of its Corresponding Anhydrous

Chemical preparation, thermal behavior, and IR studies are given for the diphosphate Cr4(P2O7)3.28H2O and its anhydrous form Cr4(P2O7)3. Cr4(P2O7)3.28H2O, is monoclinic P2/m with the following unit-cell dimensions : a = 16.169(1)Å, b = 9.336(5)Å, c = 9.446(4)Å, β = 124.796(5)°, and Z = 4. The total dehydration of Cr4(P2O7)3.28H2O, between 90°C and 450°C, leads to its anhydrous form, Cr4(P2O7)3. Cr4(P2O7)3 is isotopic to V4(P2O7)3, crystallizing in the orthorhombic system, space group Pmcn, Z = 4 with the following unit-cell dimensions: a = 7.25(2), b = 9.38(1)Å and c = 21.00(4)Å. Cr4(P2O7)3 is stable until its melting point at 1050°C. The thermal behavior of Cr4(P2O7)3.28H2O has been investigated and interpreted by comparison with IR absorption spectrometry and X-ray diffraction experiments.

Characterization and Rietveld refinements of new dense ceramics Ba3−xSrxTb3−xCexO9 (x = 1 and 1.5) perovskites

Ba3−xSrxTb3−xCexO9 (x = 1 and 1.5) ceramics (BSTC) with a relative density of 93% and a grain size distribution of 0.2–3 µm were prepared by the mixed-oxides reaction route. The crystalline structures, microstructures, valence states, and electrical properties of two ceramics were analyzed using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrical measurements. Rietveld analyses of XRPD patterns show that BSTC1 is indexed as a trigonal structure with the space group R-3c, and BSTC3/2 is indexed as an orthorhombic perovskite structure with the space group Pmcn. The EPR, XPS, and electrical conductivity results confirm that Ce and Tb ions in BSTC exist as Ce4+ and mixed-valence states of Tb4+/Tb3+, respectively. At room temperature, the two BSTC ceramics exhibit a similar semiconducting behavior. The relationships between electrical conductivity and temperature/frequency are provided. The defect chemistry is discussed.

Tunable blue-green emission and energy transfer properties in Ba2SiO4:Tb3+ obtained from sol-gel method

Herein, we introduce an adapted sol-gel synthesis of blue-green-emitting phosphors based on Ba2SiO4:Tb3+, which were obtained in a quite-soft calcination condition (1100 °C/2 h) as a monophasic crystalline phase, space group Pmcn (62), characterized by high-agglomerated particles. The optical band gap value decreases as the amount of Tb3+ increases, from 5.65 eV (undoped) to 4.40 eV (5 %-doped). All doped samples are excited by UV radiation (250 nm), emitting in the blue and green spectral region as a result of the Tb3+ transitions arising from the 5D3 and 5D4 excited levels, respectively. Yet, the overall emitted color of the phosphor is tuned by the Tb3+ concentration since the increase of the activator amount dislocates the emission color from blue toward green due to a dipole-dipole energy transfer between two close Tb3+ ions. The emission quantum efficiency of the 5D4 state is a function of the emission intensity and it enhances up to the 4 %-doped sample. A new approach to calculate the 5D3 state quantum efficiency of Tb3+ is also proposed and these values decrease from 95.3% to 42.4% as the doping amount enhances from 0.1 up to 5%. Thus, both the green-blue color tunability and the high quantum efficiency qualify this material for future photonic applications.

Crystal structure of η″-Fe3Al7+x determined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy

ABSTRACT The crystal structure of η″-Fe3Al7+x , the low-temperature phase of η-Fe2Al5 with a composition on the Fe-rich side of the solid solubility range, has been determined by synchrotron X-ray single-crystal diffraction combined with scanning transmission electron microscopy. The η″ phase possesses commensurate long-period-ordered superlattice structures (space group Pmcn) based on the parent orthorhombic unit cell of η-Fe2Al5, consisting of twin domains (orientation variants) alternately stacked along the long-periodicity axis. Each of the twin domains possesses a motif structure belonging to the base-centered monoclinic space group C2/m, with a cell volume twice that of the parent orthorhombic unit cell (space group Cmcm). One-fourth of the c-axis chain sites corresponding to Al2- and Al3-sites in the η phase are respectively occupied by both Fe and Al atoms and exclusively by Al atoms in a regular manner. This regularity is disturbed in the twin-boundary region, giving rise to structural/compositional modulation. Because of the different chemical compositions between the motif structure and twin-boundary region, the η″ phase with various compositions can be constructed only by changing the number of the parent orthorhombic unit cells to be stacked along the orthorhombic c-axis, without changing the atomic arrangements for the motif structure or the twin boundary to account for the observed solid solubility range. The chemical formula of the η″ phase can thus be expressed as Fe3Al7+x under a simple assumption on the occupancies for Al/Fe atoms in the c-axis chain sites.

High-pressure phase behavior of SrCO3: an experimental and computational Raman scattering study

The high-pressure phase behavior of strontianite (SrCO3) was both experimentally and theoretically investigated by Raman spectroscopy up to 78 GPa in a diamond anvil cell and density functional theory-based calculations. Our study shows a phase transition between 23.7 and 26.8 GPa during compression from space group Pmcn to post-aragonite SrCO3, which is accompanied by significant changes in the vibrational spectrum. The excellent agreement between the observed and computed Raman frequencies and intensities implies that the high-pressure polymorph has space group Pmmn and contributes to resolving an existing disagreement concerning the correct space group symmetry of this high-pressure polymorph. It is shown that the transition pressure from the aragonite to a post-aragonite phase increases linearly with decreasing cation radius for (Ca, Sr, Ba, Pb) carbonates.

Efeito da variação de pH na síntese e nas propriedades de BaCe0,2Pr0,8O3 obtido pelo método de complexação EDTA-citrato

Cerato de bário (BaCeO3) tem estrutura perovskita do tipo ABO3, na qual A e B são cátions metálicos. Estes materiais, dopados ou não, têm sido estudados por apresentarem características que os tornam promissores na aplicação em células a combustível de óxido sólido e permeação de hidrogênio e oxigênio. Porém, como os materiais cerâmicos de condutividade mista têm sido produzidos por métodos distintos de síntese, algumas condições influenciam diretamente as propriedades finais, sendo uma das mais importantes a dopagem do sítio B, que pode ter influência direta no tamanho de cristalito. Este trabalho teve como objetivo a substituição parcial do cério por 80% de praseodímio a partir do método de complexação, combinando EDTA-citrato, a fim de verificar a influência do pH da solução de síntese na obtenção da fase, nas propriedades cristalográficas e morfológicas. Foi verificado que o material apresenta estrutura cristalina ortorrômbica, com grupo espacial Pmcn e a fase foi obtida, independentemente da alteração no valor do pH da solução. Quanto ao tamanho de cristalito, com pH de síntese ajustado para 5, foi obtido maior valor, 352,7 nm, apesar de todos terem tido aumento em relação à amostra com pH corrigido para 11. Esse aumento se deu, provavelmente, pelo tempo insuficiente no qual a amostra foi submetida à ação da temperatura de síntese até a formação do gel final, ocasionando uma menor desagregação dos aglomerados formados na reação de complexação, resultado concordante com as análises de microscopia eletrônica de varredura.

New Antiferroelectric Solid Solution of Pb(Mg1/2 W1/2 )O3 -Pb(Zn1/2 W1/2 )O3 as Dielectric Ceramics

A new solid solution of (1−x)Pb(Mg1/2W1/2)O3–xPb(Zn1/2W1/2)O3 has been prepared in the form of ceramics by solid-state reaction with composition x up to 30%. It is found that with the substitution of Zn2+ for Mg2+ on the B site of the of complex perovskite structure the antiferroelectric (AFE) Curie temperature TC of PMW increases from 40°C (x = 0) to 67°C (x = 30%), indicating an enhancement of antiferroelectric order, whereas, at the same time, the phase transition becomes more diffuse due to a higher degree of chemical inhomogeneity. X-ray diffraction analysis indicates that the crystal structure adopts an orthorhombic space group (Pmcn) with a decrease in lattice parameter a, but an increase in b and c as the Zn2+ concentration increases. The low dielectric constant (~ 102), low dielectric loss (tanδ ≈ 10−3), linear-field-induced polarization, and significantly high breakdown field (~ 125 kV/cm) at room temperature make this family of dielectric materials a promising candidate for ceramic insulators.

Synthesis and thermodynamic stability of new phase BaCe0.6Y0.3In0.1O2.8

• We synthesized new phase BaCe 0.6 Y 0.3 In 0.1 O 2.8 . • We measured formation enthalpy of BaCe 0.6 Y 0.3 In 0.1 O 2.8 . • The phase is thermodynamically stable with respect to decomposition into binary oxides at room temperature. • BaCe 0.6 Y 0.3 In 0.1 O 2.8 is more thermodynamically favored than BaCe 0.9 In 0.1 O 2.95 . The preparation of BaCeO 3 doped by yttrium and indium oxides (BaCe 0.6 Y 0.3 In 0.1 O 2.8 ) has been performed by solid-state reaction from BaCO 3 , CeO 2 , Y 2 O 3 , and In 2 O 3 . The compound BaCe 0.6 Y 0.3 In 0.1 O 2.8 has been synthesized for the first time. The X-ray measurements have shown that BaCe 0.6 Y 0.3 In 0.1 O 2.8 has an orthorhombic structure (space group Pmcn ). The standard molar enthalpy of formation of BaCe 0.6 Y 0.3 In 0.1 O 2.8 has been determined by solution calorimetry combining standard molar enthalpies of dissolution of BaCe 0.6 Y 0.3 In 0.1 O 2.8 and BaCl 2 + 0.6CeCl 3 + 0.3YCl 3 + 0.1InCl 3 mixtures in 1 M HCl with 0.1 M KI at 298.15 K and literature data. It has been found out that the mixed oxide mentioned above is thermodynamically stable as to its decomposition into binary oxides at room temperature. It has also been shown that BaCe 0.6 Y 0.3 In 0.1 O 2.8 was more thermodynamically favored than BaCe 0.9 In 0.1 O 2.95 .

Stability and equation of state of post-aragonite BaCO3

At ambient conditions, witherite is the stable form of BaCO3 and has the aragonite structure with space group Pmcn. Above *10 GPa, BaCO3 adopts a post- aragonite structure with space group Pmmn. High-pressure and high-temperature synchrotron X-ray diffraction exper- iments were used to study the stability and equation of state of post-aragonite BaCO3, which remained stable to the highest experimental P-T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K0 = 88(2) GPa with K 0 = 4.8(3) and V0 = 128.1(5) A u 3 using a third-order Birch- Murnaghan fit to the 300 K experimental data. We also carried out density functional theory (DFT) calculations of enthalpy (H) of two structures of BaCO3 relative to the enthalpy of the post-aragonite phase. In agreement with previous studies and the current experiments, the calcula- tions show aragonite to post-aragonite phase transitions at *8 GPa. We also tested a potential high-pressure post-post- aragonite structure (space group C2221) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, DH between the C2221 structure and post-aragonite (Pmmn) decreases with pressure, but the Pmmn structure remains energetically favorable to pressures greater than 200 GPa. We conclude that post-post-aragonite phase transformations of carbonates do not follow system- atic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.

ELDRAGONITE, Cu6BiSe4(Se2), A NEW MINERAL SPECIES FROM THE EL DRAGON MINE, POTOSI, BOLIVIA, AND ITS CRYSTAL STRUCTURE

Eldragonite, with the simplified formula Cu 6 BiSe 4 (Se 2 ), is a new mineral species discovered in a telethermal vein-type deposit with selenides at the El Dragon mine, Province of Quijarro, Department of Potosi, Bolivia. It forms inclusions in krut’aite, and is associated with clausthalite, klockmannite, umangite and tiemannite, as well as with watkinsonite, petrovicite and two unnamed phases in the system Cu–Pb–Hg–Bi–Se. The unique vein of eldragonite-bearing krut’aite is hosted within sandstones and shales of Devonian age. Eldragonite occurs in anhedral grains and polycrystalline aggregates attaining a size of up to 100 × 80 μm. Megascopically, the mineral has a brownish to light-maroon color, is opaque and lacks internal reflections. It has a metallic luster and a brownish black streak, is brittle with an uneven to conchoidal fracture, without observable cleavage. The VHN 15 values range between 212 and 243 (mean 225) kg/mm 2 , corresponding to a Mohs hardness of ~3 ½. In plane-polarized light, eldragonite is distinctly bireflectant and pleochroic, from light grayish brown to cream; it is strongly anisotropic with rotation tints in shades of orange and blue-black. The reflectances (in air and oil, respectively) for the COM standard wavelengths are: 32.5–34.5, 17.7–19.7 (470 nm), 32.95–36.3, 18.0–21.4 (546 nm), 33.3–36.8, 18.3–21.6 (589 nm), 34.0–36.9, 19.1–21.7 (650 nm). Electron-microprobe analyses gave (mean of 24 analyses): Cu 35.9, Fe 1.25, Ni 0.35, Bi 20.3, Se 42.5, total 100.3 wt.%, corresponding to (Cu 5.98 Fe 0.24 Ni 0.06 ) ∑6.28 Bi 1.03 Se 5.70 . The ideal formula is Cu 6 BiSe 4 (Se 2 ), which requires Cu 35.84, Bi 19.64, Se 44.52 wt.%. Eldragonite has an orthorhombic cell, space group Pmcn , with a 4.0341(4), b 27.056(3), c 9.5559(9) A, V 1043.0(3) A 3 , and Z = 4. The calculated density is 6.76 g/cm 3 . The strongest X-ray powder-diffraction lines [ d in A ( I ) hkl ] are: 6.547(58)031, 3.579(100)052, 3.253(48)141, 3.180(77)081, 3.165(56)013, 3.075(84)102, 3.065(75)151,112, 2.011(53)200, 1.920(76)154, 1.846(52)1103. The crystal structure was solved from single-crystal data, and was refined to R 1 = 0.026 on the basis of 1731 unique reflections. There are one Bi and six Cu positions. Among the six Se positions, two Se atoms form a Se 2 pair [d(Se–Se) = 2.413 A]; eldragonite is thus a mixed selenide–diselenide compound. The crystal structure is organized according to two slabs alternating along a . The thin slab with formula Cu 6 Se 6 is a zigzag layer derived from the CaF 2 archetype; the thick slab, Cu 6 Bi 2 Se 6 , is similar to that of wittichenite, Cu 3 BiS 3 . The Se 2 pair is at the junction between these two slabs. This new mineral species is named after the location where it was discovered.

ECLARITE: NEW DATA AND INTERPRETATIONS

The crystal structure of eclarite, first determined by Kupcik in 1984, has been refined on material from the scheelite deposit at Felbertal, Salzburg Province, Austria, to an R 1 value of 4.5% on the basis of 4141 unique reflections [ F o > 4σ( F o )]. The chemical formula of material examined, based on 20.85 large cations (Z = 4) is Cu 0.76 Fe 0.45 Ag 0.12 Pb 8.14 Bi 12.59 S 27.91 ; its unit-cell parameters are a 4.0307(1), b 22.7011(6), and c 54.615(1) A, space group Pmcn . All the principal features of Kupcik’s structure were confirmed. Three cation sites appear split into Pb and Bi subsites, and the octahedral site of the “cosalite-like” fragment is a partially occupied Bi site, flanked by three-fold coordinated, partly occupied Cu1 and Cu2 sites. The tetrahedral (Cu,Fe) site is split into a triangular position and a tetrahedral position. Replacement of Fe by Cu at the tetrahedral site and replacement of Bi by Cu at the octahedral site are positively correlated. They result in a general formula of eclarite equal to Cu 1.5 n Fe 1− n Pb 9–1.25 n Bi 12+ n S 28 in which n is the molar proportion of the copper-based fully substituted ideal end-member Cu 1.5 Pb 7.75 Bi 13.0 S 28 , and (1– n ), that of the Fe-based ideal end-member FePb 9 Bi 12 S 28 . We propose a new interpretation of the crystal structure of eclarite, as a patchwork of large galenobismutite-like regions joined by interstitial elements.

The crystal structure of a biogenic aragonite from the nacre of an ammonite shell

The crystal structure of a biogenic aragonite from the nacre of an ammonite shell was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. The well-preserved ammonite sample is from Alberta, Canada, and is from the Cretaceous period. The aragonite structure was refined in space groupPmcn, Z = 4, and the cell parameters obtained are a = 4.96265(2), b = 7.97016(4), c = 5.74474(3) A, and V = 227.222(2) A3. The chemical analyses indicate a formula of [Ca0.995Sr0.004Ba0.001]∑=1.0(CO3). The average and distances are 2.5281(3) and 1.2871(6) A, respectively, and the average angle is 119.94(8)°. The CO3 groups are non-planar. Based on crystal-structure data for biogenic and non-biogenic aragonite samples, aragonite from ammonite nacre has minimal structural distortions and is very similar to non-biogenic aragonite, in particular, a sample from Spain.

Low temperature phase transition studies on Pb(Mg 0.5W 0.5)O 3 ceramic

We present here the results of X-ray diffraction (XRD), dielectric and calorimetric studies on lead magnesium tungustate, Pb(Mg 0.5W 0.5)O 3 (PMW) ceramic. It is shown that the low temperature antiferroelectric phase of PMW having orthorhombic structure (space group Pmcn) transforms to paraelectric cubic (space group Fm3 m) phase at 281 K. The phase transition is of first order character as confirmed by coexistence of Pmcn and Fm3 m phases over wide temperature range ∼50 K. The first order character of phase transition is also revealed by the observation of thermal hysteresis in the real part of dielectric permittivity and calorimetric studies. We do not find any evidence for the additional intermediate phase between antiferroelectric ( Pmcn) and paraelectric ( Fm3 m) phases as reported in the literature. Anomalies in the heat flow and dielectric measurements support the finding of our XRD results and reveals that the phase transition temperature (T c) is 281 K instead of 312 K reported in the literature.

Structural and thermoelastic study of the protonic conducting perovskite SrCe0.95Yb0.05Oξ (ξ ∼ 3) between 373 K and 1273 K

The temperature dependence of the crystal structure of the protonic conducting perovskite SrCe0.95Yb0.05Oξ (ξ ∼ 3) has been determined from Rietveld refinement of neutron time-of-flight powder diffraction data in 25 K steps from 373 K to 1273 K. In contrast to most SrBIVO3 perovskites which show a sequence of structural phase transitions from Pmcn – Incn – I4/mcm - \( Pm\overline 3 m \) with increasing temperature, SrCe0.95Yb0.05Oξ remains orthorhombic with space group Pmcn from 4.2 K to the highest temperature measured. Crystallographic results are presented graphically in the form of the magnitude of the seven unique symmetry adapted basis vectors of a hypothetical aristotype perovskite phase. Thermoelastic properties of SrCe0.95Yb0.05Oξ deduced from the temperature variation of the unit cell volume and Debye-Waller factors are compared with literature values derived from independent calorimetric, Raman and elastic measurements. Thermodynamically, SrCe0.95Yb0.05Oξ behaves as a Debye-like solid with an average Gruneisen parameter in the range 1.4–1.5, but with two characteristic temperatures, ∼200 K and ∼600 K, the lower temperature being more related to the cation vibrations, the higher to the anion vibrations.