Temperature-dependent X-ray Diffraction Studies Research Articles

Studies of the Temperature Dependence of the Structure and Magnetism of a Hexagonal-Bipyramidal Dysprosium(III) Single-Molecule Magnet.

The hexagonal-bipyramidal lanthanide(III) complex [Dy(OtBu)Cl(18-C-6)][BPh4] (1; 18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane ether) displays an energy barrier for magnetization reversal (Ueff) of ca. 1000 K in a zero direct-current field. Temperature-dependent X-ray diffraction studies of 1 down to 30 K reveal bending of the Cl-Ln-OtBu angle at low temperature. Using ab initio calculations, we show that significant bending of the O-Dy-Cl angle upon cooling from 273 to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature and field dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, quantum tunnelling of magnetization, and Raman-II, in addition to the observation of a phonon bottleneck effect.

Nanometer Confinement-Driven Promotion and Stabilization of a Hexatic Phase Intervening between Ordered Rotator Phases.

Bulk phase binary mixture of two rotator phase forming alkanes, n-tricosane (C23H48) and n-octacosane (C28H58), has been previously studied. C23H48 exists in the RII and RI phases, whereas C28H58 exists in the RIII and RIV phases. Over a certain range of composition, this binary mixture was found to exist in RII, RI and an intervening mesophase was reported to be the hexatic phase, wherein the long-range two-dimensional in-plane hexagonal lattice order of the RII is lost and what remains is molecules present in hexagonal geometry without long-range positional correlation between individual hexagons. Upon confinement in cylindrical anodized alumina pores 200 nm wide, on the one hand, the temperature range of the hexatic phase was found to extend, and on the other hand, it underwent increased molecular ordering compared to the hexatic phase in bulk, exhibiting two counter-reacting behaviors in confinement. We provide here a temperature-dependent X-ray diffraction study and a theoretical approach combining the Landau and Flory-Huggins theories to, first, understand the underlying mechanism leading to emergence of the hexatic phase and then to explain the effect of confinement on it in the light of finite size and interfacial interaction between the alkanes and alumina pores.

Structural, vibrational and thermal studies on bis(l-glutaminato)copper(II)

Copper(II) complexes of amino acids have been widely studied as potentials medicines and dietary supplementation, so the knowledge about the metal-ligand sites, thermal stability and behavior of these complexes is an important subject of study. Although the Raman spectroscopy could help to elucidate the nature of the interactions into crystal there are only few information about vibrational study of this compound in the literature and no data depending on the temperature. In addition, there is no temperature-dependent X-ray diffraction study of this material. We report here Raman Spectroscopy and Powder X-ray Diffraction measurements, both as a function of temperature and as a way of studying the thermal stability of the material. After the synthesis of the sample and confirmation of its crystal structure by Powder X-ray Diffraction, Raman measurements were performed in the 70–3600 cm−1 spectral region as a function of temperature from 10 up to 300 K. Some peaks become more evident during the cooling, due to a decrease in width and an increase in intensity. There is a discontinuity in the wavenumbers evolution around 110 K, that should be associated with a conformation of the structure. Optimized geometry and vibrational frequencies were obtained by means of Density Functional Theory and for the first time the analysis of the vibrational modes was done in terms of the Potential Energy Distribution. X-ray diffraction measurements as a function of temperature and Rietveld refinement showed discontinuities in the lattice parameters and degradation around 493 K (at air atmosphere) and 513 K (under vacuum). These results were corroborated by the thermal analysis which indicates that the compound is stable up to 493 K.

Interlayer strain effects on the structural behavior of BiFeO3/LaFeO3 superlattices

Artificial (BiFeO3)0.5Λ/(LaFeO3)0.5Λ superlattices have been grown by pulsed laser deposition. The periodicity Λ was varied from 150 Å to 25 Å and the relative ratio between BiFeO3 (BFO) and LaFeO3 (LFO) is kept constant in each period. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy investigations indicate antiferroelectric-like structures for large periodicity (Λ ≥ 76 Å), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 Å. Room temperature magnetic measurements were obtained by vibrating sample magnetometry and suggest antiferromagnetic ordering with weak ferromagnetism. Temperature dependent x-ray diffraction studies show an important shift of paraelectric-antiferroelectric phase transition scaling with BFO thickness. Strain and size effects explain this behavior and discussion is also made on the possible role of the oxygen octahedral rotation/tilt degree of freedom.

Enhancement of magnetocaloric effect by external hydrostatic pressure in MnNi0.75Fe0.25Ge alloy

A systematic investigation on the structural and magnetic properties of an Fe-doped MnNiGe alloy with nominal composition MnNi0.75Fe0.25Ge has been performed. Temperature dependent x-ray diffraction studies indicate a clear structural phase transition (martensitic type) from the high temperature hexagonal austenite phase (space group P63/mmc) to the low temperature orthorhombic martensite phase (space group Pnma). Interestingly, about 1.4% of the high temperature hexagonal phase has been observed at 15 K, which is well below the martensitic phase transition (MPT) temperature. The studied alloy is found to be ferromagnetic in nature at the lowest temperature of measurement and the saturation moment increases in the presence of external hydrostatic pressure (P). In addition, it shows a significantly large conventional (negative) magnetocaloric effect with an adiabatic entropy change () of about −16.2 J kg−1 K−1 around the MPT for a magnetic field changing from 0 → 5 T. The most interesting observation is the ∼40.1% increase in the peak value of on application of 6 kbar of external P. A considerable increment in the refrigeration capacity has also been noted with the applied P.

Temperature Dependent Structure Stability Studies on Thermoelectric Yb0.025Fe0.3Co0.7Sb3

ABSTRACTDepending on their application temperature thermoelectric (TE) materials are classified in three main categories; as low (up to 250°C), intermediate (up to 550°C) and high (above 600°C) temperature. Currently, Skutterudites (CoSb3) based materials have shown promising results in the intermediate temperature range (300-500°C). This family of material is highly suitable for automotive, marine transportation and industrial power generation applications to recover the waste heat from the exhaust and generate electricity. Conventional TE modules need p- and n-type semiconductor materials and for the skutterudite family, iron (Fe) has proven to be among the best candidates for the substitution of cobalt sites. Additionally, rare earths are introduced as rattlers in the crystal cages of the skutterudite to decrease the thermal conductivity, thus improving the figure of merit ZT of the TE material. For practical application for device fabrication, stability of these materials is of great importance. Compositional stability is being addressed as the material decomposes above certain temperature. Temperature dependent x-ray diffraction study was performed on Fe substituted, Yb-filled skutterudites, using Beam Line I711 at MAX LAB, to observe the crystal structure as a function of temperature. Diffraction patterns were collected from room temperature up to 500°C by utilizing Huber furnace. The results show success in filling process showing almost 80% reduction of the thermal conductivity from bulk. Additionally the thermal expansion coefficient value was within the average value for skutterudites which proves practical application of this powder for industrial applications.

Two Reversible Phase Transitions in the Fluorophosphate (NH4)2PO3F·(NH4)NO3

Fluorophosphates have multiple applications, e.g. as wood preservatives, as corrosion inhibitors or as toothpaste additives [1]. Fluorophosphates contain the tetrahedral anion PO3F2- that is characterized by three P-O bonds of similar length (ca. 1.51 Å) and one considerably longer P-F bond (ca. 1.60 Å). In the course of systematic crystal growth experiments of new fluorophosphates, we have isolated the double salt (NH4)2PO3F·(NH4)NO3. Temperature-dependent X-ray diffraction studies on polycrystalline and single-crystalline samples revealed two reversible phase transitions. The room-temperature polymorph I (space group P21/n) transforms at ca. 170 K into the intermediate polymorph II (space group P-1) that in turn transforms at ca. 140 K into the low-temperature polymorph III (space group P21/n). The driving force of the phase transitions I->II and II->III is a tilting of the nitrate groups within the structure relative to the other building blocks. The two phase transitions are accompanied by an increase of the unit cell volume on cooling. The three-dimensional structural set-up in the three polymorphs of (NH4)2PO3F·(NH4)NO3 is dominated by an intricate N-H...O hydrogen-bonding system between ammonium donor groups and nitrate and fluorophosphates acceptor groups. It is interesting to note that hydrogen-bonding of the type N-H...F plays a minor role.

Discotic liquid crystals of transition metal complexes 44: synthesis of hexaphenoxy-substituted phthalocyanine derivatives showing spontaneous perfect homeotropic alignment

We have synthesized novel hexaphenoxy-substituted phthalocyanine derivatives, 2-(12-hydroxydodecyloxy)-3-methoxy-9,10,16,17,23,24-hexakis(3,4-di-n-alkoxyphenoxy)phthalocyaninato copper(II) (abbreviated as Cn(OC12OH)PcCu : n = 10, 12, 14), and investigated their columnar mesomorphism and homeotropic alignment property. These hexaphenoxy-substituted Pc derivatives could be successfully isolated and purified from the mixture products by polarity difference. It was revealed by using polarizing optical microscopic observations, differential scanning calorimetry and temperature-dependent X-ray diffraction studies that each of the hexaphenoxy-substituted Pc derivatives has plural mesophases, and that the tetragonal columnar (Coltet) mesophase shows spontaneous perfect homeotropic alignment between two non-surface-treated glass plates without any defects and polydomain boundaries.

Superconducting properties and thermal expansion of YbGaxSi2−x

YbGaxSi2−x forms the hexagonal AlB2-type crystal structure in the chemical composition range of 1.12 ≤ x ≤ 1.49 and shows superconductivity with critical temperatures TC ≤ 2.5 K. In this paper, we investigate the electronic and thermal properties of the YbGaxSi2−x compounds by means of electrical resistivity (ρ) and specific heat (C) measurements under magnetic fields, as well as by the temperature dependent x-ray diffraction study. The onset temperature of superconductivity, TonsetC, decreases with the increase of the Ga content x, from 2.5 K for x = 1.15 to 1.9 K for x = 1.31. Estimated upper critical field at zero temperature, Hc2(0), concomitantly decreases from 5.9 kOe for x = 1.15 to 3.1 kOe for x = 1.31. The normal-state electronic specific-heat coefficient is found to be almost independent to x; γ = 8mJ/molK2 for x = 1.18, 1.31, and 1.41. The small value of γ indicates electron-electron correlation is weak in this compound. An irreversible behavior is observed in ρ(T). ρ(T) of YbGa1.2Si0.8 shows a hump-like increase below about 200 K on cooling while it shows a rather monotonic increase on heating. Powder x-ray diffraction of this sample taken at 12 K shows no evidence of structural phase transition. Temperature dependence of the lattice parameters in this sample show normal behavior: both a and c decrease with decreasing temperature with thermal expansion coefficients similar to those in YbGaGe. Possible mechanism for the irreversible behavior in ρ(T) is discussed.

Thermal expansion in 3 d-metal Prussian Blue Analogs—A survey study

We present a comprehensive study of the structural properties and the thermal expansion behavior of 17 different Prussian Blue Analogs (PBAs) with compositions M II 3[( M′) III(CN) 6] 2· nH 2O and M II 2[Fe II(CN) 6]· nH 2O, where M II=Mn, Fe, Co, Ni, Cu and Zn, ( M′) III=Co, Fe and n is the number of water molecules, which range from 5 to 18 for these compounds. The PBAs were synthesized via standard chemical precipitation methods, and temperature-dependent X-ray diffraction studies were performed in the temperature range between −150 °C (123 K) and room-temperature. The vast majority of the studied PBAs were found to crystallize in cubic structures of space groups Fm3̄ m, F4̄3 m and Pm3̄ m. The temperature dependence of the lattice parameters was taken to compute an average coefficient of linear thermal expansion in the studied temperature range. Of the 17 compounds, 9 display negative values for the average coefficient of linear thermal expansion, which can be as large as 39.7× 10 −6 K −1 for Co 3[Co(CN) 6] 2·12H 2O. All of the M II 3[Co III(CN) 6] 2· nH 2O compounds show negative thermal expansion behavior, which correlates with the Irving–Williams series for metal complex stability. The thermal expansion behavior for the PBAs of the M II 3[Fe III(CN) 6] 2· nH 2O family are found to switch between positive (for M=Mn, Co, Ni) and negative ( M=Cu, Zn) behavior, depending on the choice of the metal cation ( M). On the other hand, all of the M II 2[Fe II(CN) 6]· nH 2O compounds show positive thermal expansion behavior.

Temperature dependent x-ray diffraction study of lightly doped NaxWO3

Temperature dependent x-ray diffraction studies have been carried out on nonstoichiometric lightly doped sodium tungsten bronze (NaxWO3 for x=0.025). The investigation reveals a structural modification around 230 K. Although the high and low temperature phases are monoclinic but at low temperature the corner sharing WO6 octahedra get significantly distorted due to displacement of tungsten and oxygen atoms from its mean position. This structural modification induces polaron formation in Na0.025WO3 below 230 K.

Temperature-dependent X-ray diffraction studies of mullite-type (Bi1−xSrx)2M4O9phases

Temperature-dependent X-ray diffraction studies of mullite-type (Bi_1−<i>x</i>Sr_<i>x</i>)₂<i>M</i>₄O₉phases

P-Alkoxybiphenyls with guanidinium head groups displaying smectic mesophases

Novel calamitic guanidinium salts with p-alkoxybiphenyl or p-alkoxyphenyl core unit (7, 11, 12) have been synthesised, utilising the corresponding nitro compounds as synthetic intermediates. Differential scanning calorimetry (DSC), polarising optical microscopy (POM) and temperature-dependent X-ray diffraction studies reveal smectic A (SmA) mesophases for all series. Whereas the p-alkoxybiphenyl guanidinum chlorides 7 display SmA phases with a minimum chain lengths of C8 and broad mesophase width from 80°C (for C8 derivative 7b) up to 180°C (for C16 derivative 7g), the corresponding p-alkoxyphenyl guanidinium chlorides 11 display much smaller mesophase width from 4°C (for C10 derivative 11c) up to 72°C (for C16 derivative 11e). For the series 12 a large influence of the anion on the clearing point is observed, with clearing points ranging from 209°C for the guanidinium chloride 7f down to 127°C for guanidinium hexafluorophosphate 12d.

Phase coexistence, microstructure and magnetism in Ni–Mn–Sb alloys

We present here a detailed investigation on the structural and magnetic transformations in the Ni50Mn50−xSbx Heusler alloys with composition x = 12, 13 and 14. The evolution of martensitic to austenitic transformation has been investigated using temperature dependent x-ray diffraction study. A wide region of phase coexistence is observed across the martensitic transformation. The unit cell volume contracts by 0.82% and 0.74% for x = 12 and 14, respectively, at the martensitic transformation temperature. The martensitic transition temperature decreases with increase in Sb concentration. The magnetic interactions in the martensitic and austenitic phases are found to be ferromagnetic, however, with different strengths, giving rise to two separate magnetic transitions with . The magnetization studies reveal the presence of antiferromagnetic–ferromagnetic interactions in the alloys. Exchange bias blocking temperature has been observed around 100 K for the alloy x = 12. Magnetic field induced structural transition has been observed in x = 14 alloy, in which the structural transition temperature shifts by 25 K in a field of 3 T.

Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO3 with divergence between zero-field cooled and field cooled magnetizations below ∼100K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO3 one also observes finite coercivity at lower temperature, which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi∕Fe ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as electrical responses.

Pressure- and temperature-dependent X-ray diffraction studies of NdCrO 3

High-temperature and high-pressure X-ray diffraction experiments were conducted on the perovskite NdCrO 3. High-temperature measurements were used to investigate the temperature dependence of the crystal structure up to 823 K. The magnitude of the octahedral tilting distortion decreased with an increase in temperature. The pressure dependence of the lattice parameters up to 6.4(2) GPa were determined using a diamond anvil cell and monochromatic synchrotron X-ray diffraction. The a lattice parameters exhibit an unusual elongation with pressure, but there was no indication of a pressure-induced phase transition in the pressure range examined and a least squares fit to the volume–pressure data with a second-order Birch–Murnagahan equation of state yielded a large bulk modulus of 338(20) GPa, making it one of the least compressible perovskite oxides.

Dielectric anomaly at the orbital order–disorder transition inLaMnO3+δ

We report a novel dielectric anomaly around the Jahn–Teller orbital order–disorder transition temperatureTJT inLaMnO3+δ. The transition has been characterized by resistivity(ρ) versustemperature (T), calorimetry, and temperature dependent x-ray diffraction studies. Measurements of complex dielectricpermittivity ε* (= ε′−iε′′) over a low-frequency range (1 Hz–10 MHz) acrossTJT reveal a distinct anomaly. This observation, and the reported relatively high static dielectric constantat T = 0 (ε0∼18–20),possibly indicate that the orbital order gives rise to intrinsic polarization that undergoes transitionat TJT. The frequency dispersion of the dielectric response at any given temperature,however, reveals that the dielectric response consists of Maxwell–Wagnercomponent, due to interfaces, within such a low frequency range. TheTJT and the natureof the anomaly in ε′(ω,T),ε′′(ω,T) at TJT, ofcourse, vary—from a sharp upward feature to a smeared plateau and then a downward trend—dependingon the Mn4+ concentration of the sample. The observation of an intrinsic dielectric response due to long-range orbitalorder in LaMnO3—where no ferroelectric order is possible due to the absence of off-centre distortion inMnO6 octahedra—may throw a new light onto these classes of materials vis-à-vis multiferroicmaterials.

Flying-seed-like liquid crystals

In 1911, Vorlarnder reported liquid-crystalline properties of unique “flying-seed-like” alkali metal carboxylates. They are neither “rod-like” nor “disk-like”. Moreover, they have neither a rigid core nor flexible long chains. The molecular shapes are totally different from about 93000 general liquid crystalline compounds. These flying-seed-like liquid crystals have been forgotten for about 100 years. We revealed and established for the first time from precise microscopic observations and temperature-dependent X-ray diffraction studies that sodium diphenylacetate exhibits a hexagonal columnar mesophase, and that potassium diethylacetate and potassium dimethylacetate exhibit a smectic A mesophase. These “old and new” flying-seed-like liquid crystals will become a novel field of science.

Large negative thermal expansion and phase transition in (Pb1−xCax)TiO3 (0.30 ≤ x ≤ 0.45) ceramics

High-temperature dilatometric studies on (Pb1−xCax)TiO3 (x = 0.35, 0.35, 0.40, 0.45) ferroelectric ceramics reveal negative thermal expansion for x ≤ 0.40. The negative thermal expansion coefficient for x = 0.30, as obtained by dilatometry and powder x-ray diffraction, were found to be −8.541 × 10−6 K−1 and −11 × 10−6 K−1, respectively, which are comparable to those of other well-known negative thermal expansion materials like ZrW2O8, NaZr2(PO4)3. Results of temperature-dependent x-ray diffraction studies are also presented to show that the large negative thermal expansion behavior for x = 0.30 persists in a very wide range of temperatures, 70–570 K. Ca2+ substitution reduces the value of the negative thermal expansion coefficient of pure PbTiO3 crystal, but it enables the preparation of strong sintered ceramic bodies. The negative thermal expansion behavior is shown to disappear above the ferroelectric Curie point and is restricted to only the tetragonal compositions of (Pb1−xCax)TiO3.

Temperature dependent X-ray diffraction study on Gd 5Sn 4 compound

The structure and the magnetic properties of the Gd 5Sn 4 compound have been investigated by means of X-ray diffraction (XRD) and magnetization measurements. Temperature dependent XRD investigation reveals that there is no structural transition around the temperature at which the first-order magnetic transition occurs in this compound, i.e. the compound retains the Sm 5Ge 4-type structure from 80 K(<T C) to 298 K (>T C), which is different from the Gd 5(Si, Ge) 4 system. Based on the Rietveld refinement of the XRD pattern of Gd 5Sn 4 compound, we show that the unit cell volume exhibits a dip near the Curie temperature, i.e. a negative coefficient of thermal expansion α≈−1.5×10 −4 K −1 from 82 to 90 K and a positive coefficient α≈2.2×10 −4 K −1 from 90 to 98 K. The coefficient of thermal expansion from 98 to 298 K is only 2.5×10 −5 K −1.