Temperature Diffraction Data Research Articles

A comprehensive variable temperature study of the layered oxide, Ca2Mn3O8

Abstract Ca2Mn3O8 forms a delafossite-related layered structure, which crystallises with monoclinic C2/m symmetry. Compared with the delafossite-structure, the MnO6 layers in Ca2Mn3O8 exhibit an ordered cation void which forms a magnetic ‘bow-tie’ like connectivity of Mn4+ ion layers separated by Ca2+ ions. In-situ variable temperature diffraction data demonstrates that the structure is robust up to a temperature of approximately 1173 K before the material decomposes into the perovskite, CaMnO3 and marokite, CaMn2O4 phases. Simultaneous thermal analysis suggests that a very small amount of water remains within the layers post synthesis. Impedance spectroscopy indicates that Ca2Mn3O8 is an electronic conductor in the range ∼400–700 K with an activation energy of 0.50 ± 0.01 eV.

Phase behaviour, thermal expansion and compressibility of SnMo2O8

The phase behaviour and thermoelastic properties of SnMo2O8, derived from variable temperature and pressure synchrotron powder diffraction data, are reported. SnMo2O8 is a member of the AM2O8 family of negative thermal expansion (NTE) materials, but unexpectedly, has positive thermal expansion. Over the P-T space explored (298–513K, ambient to 310MPa) four different forms of SnMo2O8 are observed: α, β, γ and γ′. The γ to β transition is temperature-, pressure-, and time-dependent. SnMo2O8 is a much softer material (α and γ form have BT = 29 and 26GPa at 298K) than other members of the AM2O8 family. Counter-intuitively, its high temperature β phase becomes stiffer with increasing temperature (BT ∼36GPa at 490K). The pressure dependence of the thermal expansion for each phase is reported.

Structural and magnetic properties of Nd2NiGe3

Structural analysis of the room temperature diffraction data of Nd2NiGe3 shows that it exhibits AlB2 type structure with space group P6/mmm. The crystal structure of Nd2NiGe3 consists of two dimensional Ni/Ge hexagonal units sandwiching the neodymium atoms between them. Temperature dependent magnetic susceptibility data follows Curie–Weiss law in the temperature range 20–300K yielding an effective magnetic moment of 3.83μB/Nd and paramagnetic Curie temperature −2.1K indicating trivalent Nd and weak antiferromagnetic interactions, respectively. Low field magnetic susceptibility measurement show a weak antiferromagnetic like ordering around 3K. Temperature dependent neutron diffraction measurements rules out long range antiferromagnetic ordering in this compound. The electrical resistivity measurement shows metallic nature of Nd2NiGe3, which exhibits sudden drop in resistivity below 3K.

Studies of electrical conductivity and magnetic properties of Bi1-xGdxFeO3 multiferroics

The polycrystalline samples of Bi 1-x Gd x FeO 3 (x = 0, 0.1, and 0.2) multiferroic oxides have been synthesized by a solid-state reaction/mixed oxide technique. The preliminary X-ray structural analysis with room temperature diffraction data confirmed the formation of single-phase systems. Study of room temperature scanning electron micrograph (SEM) of the surface of the above samples exhibits a uniform distribution of plate- and rod-shaped grains throughout the sample surface with less porosity. The dielectric behavior of the materials was studied in a wide range of frequency (1 kHz–1 MHz) and temperature (30–400°C). The nature of temperature dependence of dc conductivity confirms the Arrhenius behavior of the materials. The frequency–temperature dependence of ac conductivity suggests that the material obeys Jonscher's universal power law. An increase in Gd -content results in the enhancement of spontaneous magnetization BiFeO 3 (BFO) due to the collapse of spin cycloid structure. The magnetoelectric coupling coefficient of BFO has been enhanced on Gd -substitution.

In situ dehydration behavior of zeolite-like pentagonite: A single-crystal X-ray study

The structural modifications upon heating of pentagonite, Ca(VO)(Si4O10)·4H2O (space group Ccm21, a=10.3708(2), b=14.0643(2), c=8.97810(10)Å, V=1309.53(3)Å3) were investigated by in situ temperature dependent single-crystal X-ray structure refinements. Diffraction data of a sample from Poona district (India) have been measured in steps of 25 up to 250°C and in steps of 50°C between 250 and 400°C. Pentagonite has a porous framework structure made up by layers of silicate tetrahedra connected by V4+O5 square pyramids. Ca and H2O molecules are extraframework occupants.Room temperature diffraction data allowed refinement of H positions. The hydrogen-bond system links the extraframework occupants to the silicate layers and also interconnects the H2O molecules located inside the channels. Ca is seven-fold coordinated forming four bonds to O of the tetrahedral framework and three bonds to extraframework H2O. The H2O molecule at O9 showing a high displacement parameter is not bonded to Ca. The dehydration in pentagonite proceeds in three steps.At 100°C the H2O molecule at O8 was released while O9 moved towards Ca. As a consequence the displacement parameter of H2O at O9 halved compared to that at room temperature. The unit-cell volume decreased to 1287.33(3)Å3 leading to a formula with 3H2O per formula unit (pfu). Ca remained seven-fold coordinated.At 175°C Ca(VO)(Si4O10)·3H2O transformed into a new phase with 1H2O molecule pfu characterized by doubling of the c axis and the monoclinic space group Pn. Severe bending of specific TOT angles led to contraction of the porous three-dimensional framework. In addition, H2O at O9 was expelled while H2O at O7 approached a position in the center of the channel. The normalized volume decreased to 1069.44(9)Å3. The Ca coordination reduced from seven- to six-fold.At 225°C a new anhydrous phase with space group Pna21 but without doubling of c had formed. Release of H2O at O7 caused additional contraction of TOT angles and volume reduction (V=1036.31(9)Å3). Ca adopted five-fold coordination. During heating excursion up to 400°C this anhydrous phase remained preserved. Between room temperature and 225°C the unit-cell volume decreased by 21% due to dehydration. The dehydration steps compare well with the thermo-gravimetric data reported in the literature.

A new ferroelectric oxide Li2Pb2Pr2W2Ti4Nb4O30: Synthesis and characterization

A new ferroelectric oxide (Li2Pb2Pr2W2Ti4Nb4O30) of tungsten bronze structural family has been synthesized by a solid-state reaction (mixed-oxide) route at high temperature (∼1100°C). X-ray structural analysis with room temperature diffraction data confirms the formation of a single phase compound. The scanning electron microscopic (SEM) texture of the surface of material sample exhibits a uniform grain distribution with a few small voids suggesting the formation of high-density pellet sample. Detailed studies of dielectric constant, tangent loss and polarization with temperature and frequency confirmed the existence of ferroelectric properties in the material with transition temperature much above room temperature. Study of electrical properties (impedance, modulus, conductivity, etc.) of the material exhibits a strong correlation between its micro-structure (i.e., bulk, grain boundary, etc.) and electrical parameters. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The presence of ionic conductivity in the material was observed in its ac conductivity spectrum. Study of frequency dependence of ac conductivity suggests that the material obeys Jonscher's universal power law. The experimental electrical transport properties of the material clearly exhibit the existence of non-exponential-type of conductivity relaxation.

A closer look at the phosphorus–phosphorus double bond lengths in meta-terphenyl substituted diphosphenes

The single crystal X-ray structure of DmpP PDmp ( 1, Dmp = 2,6-Mes 2C 6H 3), which was previously reported to have a relatively short P P bond distance of 1.985(2) Å at room temperature, has been reexamined at variable temperatures. Crystallographic analyses of 1 at 100 K allow for resolution of disorder of the two phosphorus atoms (which is unresolvable from room temperature diffraction data), and for determination of a more conventional P P bond length of 2.029(1) Å. Single crystals of the closely related diphosphene DxpP PDxp ( 2, Dxp = 2,6-(2,6-Me 2C 6H 3) 2C 6H 3) show similar disorder in one of two crystallographically independent molecules in the unit cell. A value of 2.0276(4) Å is found for the non-disordered P P bonds at 100 K for 2. A new diphosphene Ar′P PAr′ ( 3, Ar′ = 2,6-Mes 2-4-OMe–C 6H 3) has been prepared and its structure has also been examined. The P P bond length for 3 was determined to be 2.0326(9) Å and relatively free of the effects of disorder.

Phase transformation and hysteresis behavior in Cs 1 − xRb xH 2PO 4

A new theory on the origin of hysteresis in first order phase transformations was evaluated for its applicability to the phase transformation behavior in the Cs 1 − x Rb x H 2PO 4 solid solution system. Specifically, the correlation between λ 2, the middle eigenvalue of the transformation matrix describing the cubic-to-monoclinic superprotonic transition, and the transformation hysteresis was examined. The value of λ 2 was estimated from a combination of room temperature diffraction data obtained for compositions in the solid solution system and high temperature diffraction data obtained for the CsH 2PO 4 end-member. The transformation hysteresis was determined for Cs 1 − x Rb x H 2PO 4 compositions ( x = 0, 0.25, 0.50 and 0.75) by single-frequency electrical impedance measurements. It was found that the transition temperature increases monotonically with increasing Rb content, from 227.6 ± 0.4 °C for the end-member CsH 2PO 4 to 256.1 ± 0.3 °C for Cs 25Rb 75H 2PO 4, as does the hysteresis in the phase transition, from 13.4 °C to 17.4 °C. Analysis of the transformation matrix reveals that, for this system, λ 2 depends only on the b lattice parameter of the paraelectric phase and the a 0 lattice parameter of the cubic phase. The computed values of λ 2, based on extrapolations accounting for chemical contraction with increasing Rb substitution and thermal expansion on heating, were far from 1, ranging from 0.9318 to 0.9354. The observation of λ 2 increasing with Rb content is attributed to the relatively large thermal expansion in the b-axis of the low temperature monoclinic phase in combination with an increase in transition temperature with increasing x. That the hysteresis does not decrease as λ 2 approaches 1, counter to the theoretical expectations, may reflect uncertainties in the method of estimating λ 2 for Rb substituted compositions, or the discovery of a system in which hysteresis is not dominated by considerations of crystallographic compatibility.

The α−γ−ε triple point of iron investigated by high pressure–high temperature neutron scattering

We present high pressure–high temperature diffraction data in the 0–10 GPa and 300–1000 K range of all three main iron phases. The refinements of the diffraction patterns give molar volumes to high precision and show that the α−γ−ε triple point is located at 8.2±0.1 GPa and 678 K. These values are significantly lower than currently admitted (10.4 GPa/740 K), but are consistent with reported in situ x-ray diffraction data obtained in multianvil presses. These measurements have been made possible by the use of a high P/T cell which uses metallic toroidal gaskets and which suffers from less absorption, gives clean diffraction patterns, and shows better pressure performance than other setups.

Over-loaded Cu-ZSM-5 upon heating treatment: A time resolved X-ray diffraction study

The over-loaded Cu-ZSM-5 under heating treatment (50–910 °C) in oxidative conditions exhibits the formation of copper aggregates, which show different oxidation states as a function of temperature (Cu 0, Cu 2O and CuO). At room temperature the excess of copper is not detectable by X-ray diffraction (XRD). It is likely present as divalent copper species (Cu x (OH) y ) dispersed on the zeolite surface that appear amorphous at XRD. This is confirmed by the UV–Vis diffuse reflectance spectroscopy data. From Rietveld refinement of room temperature and high temperature diffraction data, there are no indications of the presence of copper clusters inside the zeolite cavities. As the heating treatment proceeds, the dispersed copper species transform to metallic copper particles with increasing diffracting domains as a function of temperature. Above 510 °C the metallic copper starts to progressively oxidise to Cu 2O and finally CuO. During heating treatment the ZSM-5 framework does not show relevant structural modifications. The transition from monoclinic to orthorhombic form was observed in the early stages of the temperature ramp (ca. 100 °C).

Time Induced Changes in Phase Transition Behavior and Stability of Zn4Sb3

ABSTRACTThe stability of high performance thermoelectric Zn4Sb3 has been studied, by using synchrotron powder diffraction to establish differences in phase transition temperatures of two samples. High resolution multi temperature diffraction data has been collected, with a time interval of 13 months, and the phase transition temperature was determined based on the results of Rietveld refinements. The refinements show a difference in transition temperature from data collected the first time till data collected the second time. Furthermore the samples showed impurity peaks after being exposed to air for 13 months, indicating that the sample decomposes over time.

The role of uranium (V) ion in the chemical composition of meta-autunite from pegmatites of Quintos de Baixo, Brazil

Meta-autunite from the pegmatite mineralization of Quintos de Baixo, Brazil was investigated. The total uranium content and elemental analysis yield the formula Ca(UVIO2)2-x(UVO2)x(H3O+)x(PO4)2.(6-x)H2O for the meta-autunite. The natural sample crystallizes tetragonally in the space group P4/nmm having unit cell parameters a=6.990(1) A ; ; ; , c=8.445 (1) A ; ; ; and V=412.6(1) A ; ; ; 3 calculated from X-ray diffraction data. Thermal analysis showed a weight loss of 16.03 wt% in four distinct processes up to 1500o ; ; ; C. Electron paramagnetic resonance (EPR) spectroscopy provides evidence that part of the uranium present in meta-autunite is in the pentavalent state. High temperature diffraction data show lowering of the symmetry, decreasing the unit cell parameters and finally, the breaking of the structure of the phosphate phase and formation of monoclinic uranium oxide and an amorphous phase. At 80°C meta-autunite I transforms into the meta-autunite II with orthorhombic symmetry, while further heating produces changes in the diffraction pattern due to dehydration, although all the heating products could be by analogy identified as meta-autunite II. The calculated unit cell parameters for this phase decrease from a=7.253(4) A ; ; ; , b=6.537(3) A ; ; ; , c=7.008(4) A ; ; ; at 82°C to a=6.38(2) A ; ; ; , b=5.82(1) A ; ; ; , c=6.68(1) A ; ; ; at 204°C. Uranium oxide phases originating from the breakdown of the phosphate structure show temperature dependant stability. At 1050°C hexagonal U2O3 analogous with  -Al2O3 is present with symmetry (R c) and unit cell parameters (a=4.806(1) A ; ; ; , c=13.138(4) A ; ; ; , V=262.9(1) A ; ; ; 3).

Structural investigation of Ca 2MnO 4 by neutron powder diffraction and electron microscopy

The first member of the Ruddlesden–Popper family, Ca 2MnO 4, has been revisited. Coexistence of two structures has been shown from electron microscopy at room temperature and neutron diffraction data have evidenced two antiferromagnetic structures at low temperature. Two forms, with an orthorhombic Aba2 ( a≈b≈a p 2 and c≈12 Å) and a tetragonal I4 1/ cad ( a≈a p 2 and c≈24 Å) symmetries, were found to coexist coherently within the same matrix.

Neutron scattering and thermal measurements on the superionic-conducting Ag 2S–AgPO 3 glass system

Superionic-conducting glasses (Ag 2S) x (AgPO 3) 1− x with x=0.1, 0.2, 0.3, 0.5, 0.7, 0.8 and 0.9 have been prepared by rapid quenching methods, although for x=0.8 and 0.9 they also contain crystalline Ag 2S. We report DTA measurements of the thermodynamic glass transition temperature, T g, and neutron diffraction data taken as a function of temperature through T g. The neutron measurements show no prepeak at any composition or temperature, indicating that there is no clustering, in contrast with the AgI–AgPO 3 system where clustering is found with a length scale of order 10 Å [1]. Both systems form superionic-conducting glasses, showing that clustering is not essential for such conductivity.

Observing metastable states using time resolved high temperature diffraction data

Observing metastable states using time resolved high temperature diffraction data

Ferromagnetism in 1d and 2d triangular nickel(II) -based compounds

We discuss in this paper the structure and magnetic properties of the one- and two-dimensional nickel(II) hydroxynitrates, Ni(OH)(NO3)H2O and Ni2(OH)2+x(NO3)2−x (x = 0.64, 0.74, 1.14), respectively. The former is described as a new double-chain system with two Heisenberg in-chain interactions between S = 1 spins, and a zero-field splitting contribution arising from single-ion anisotropy. From numerical computations up to 10 spin units, the exchange interactions of both systems are shown to be ferromagnetic, i.e. J1 = +26.9 K, and J2 = +6.9 K, with g = 2.22, and D = +1.76 K. At very low temperature, the magnetization and neutron diffraction data indicate the occurrence of long-range antiferromagnetic order with a metamagnetic transition for Hc = 0.25 T. The non-stoichiometric layered compounds Ni2(OH)2+x(NO3)2−x show the signature of 2d ferromagnetism, as demonstrated for the hydroxide parent compound Ni(OH)2. From a comparison of these lamellar derivatives, it appears that the increase in the interlayer distance when (OH)− is substituted by (NO3)−, associated with turbostratic disorder, strongly affects the 3d long-range order. Indeed, whereas Ni(OH)2 is a metamagnet with TN = 25.8 K, Ni2(OH)2+x(NO3)2−x exhibits a spin-glass-like behavior below 10 K.

Crystallographic investigations of biotin and carboxybiotin derivatives.

The structures of a family of biotin and carboxybiotin derivatives have provided information on the mechanism of biotin action. The ureido moiety of the uncarboxylated cofactor is polarized and able to interact with ions and polar molecules; intermolecular interactions in the biotin derivatives suggest biochemical mechanisms resulting in nucleophilic activation to the enol tautomer. N1' carboxylation of biotin is important not only as a chemical reaction to generate the carboxyl-transferring species, carboxybiotin, but also in acting as a switch to depolarize the ureido carbonyl oxygen, and thereby facilitating interactions with non-polar molecules. The structure of an N1' methoxycarbonyl biotin derivative reveals such an interaction between the carbonyl oxygen, O2', and a neighboring methyl group. A computer-generated space-filing model of the van der Waals contacts involved in this interaction reveals that the methyl group is locked with respect to rotation and thus suggests a structural basis for the stereospecificity observed in the carboxyl-transferring half-reaction. The flexibility of the valeryl side chains in this family of structures provides translocation models in line with magnetic resonance data which indicate that the translocation events involve motions of, at most, 7 A. Our models demonstrate that such motions may be accomplished by simple, observed conformational changes in bonds of the valeryl side chain which locally adjoin the bicyclic ring system. High resolution, low temperature diffraction data will allow visualization of the bonding and lone pair electrons in biotin. These studies will serve to extend and fine-tune our description of the electronic structure of biotin which is currently based on accurate measurements of bond distances and angles.

Differences between diffraction and vibrational spectroscopic results for AgNO3

Differences between diffraction and vibrational spectroscopic results for AgNO₃

Crystallographic and magnetic ordering studies of plutonium carbides using neutron diffraction

Neutron diffraction studies were carried out on polycrystalline samples of carbon-saturated PuC 1−x and Pu 22C 3. In both, the targets were prepared from material that was highly enriched in 240Pu to minimize the fast neutron background due to the thermal fission of 239Pu. A magnetic transition was observed to occur in PuC 1-x at approximately 100 °K. The ordered structure was found to be composed of ferromagnetic layers coupled antiferromagnetically. The planes of constant spin are the (001) planes and the spin vectors are aligned perpendicular to the (001) planes. No low temperature magnetic ordering was observed in Pu 2C 3. Room temperature diffraction data were used to compute the crystal positional parameters for this compound: X c = 0.2896 ± 0.0005 X Pu = 0.0492 ± 0.0005, and the thermal parameter: B = 0.69 ± 0.06 A ̊ 2 . The C-C bond length calculated from these data is 1.39 Å. Using room temperature data for both compounds, the coherent scattering amplitude of pure 240Pu was found to be 0.38 × 10 −12 cm.

High temperature diffraction data on selected reflections of an andesine and anorthite1

High temperature diffraction data on selected reflections of an andesine and anorthite¹