Low-temperature Powder X-ray Diffraction Research Articles

Metal to Semimetal Transition in CaMgSi Crystals Grown from Mg−Al Flux

Single crystals of CaMgSi were produced using the metal flux synthesis method in a Mg/Al 1:1 mixture. The large rod-shaped crystals measure up to 7 mm in length. This phase crystallizes with the orthorhombic TiNiSi structure type (space group Pnma; a = 7.4752(2) Å, b = 4.42720(10) Å, c = 8.3149(2) Å; R1 = 0.021). Despite its relationship to semiconducting Zintl phases Mg2Si and Ca2Si, CaMgSi is metallic at room temperature; this produces a positive (∼160 ppm) 29Si MAS NMR chemical shift and is supported by DOS calculations. A metal to semimetal electronic transition at around 50 K is evident in the resistivity, magnetic susceptibility, and electron paramagnetic resonance measurements. Low temperature powder X-ray diffraction data indicates that a structural distortion accompanies this transition. The electronic heat capacity coefficient (0.4695 mJ/mol·K2) determined from low temperature heat capacity data supports the designation of CaMgSi as a semimetal at low temperature. The hydrogen storage capacity of this phase is negligible (≤0.5 wt % hydrogen), although exposure to hydrogen does destabilize the structure, inducing decomposition at 500 °C.

Effect of hole doping and antiferromagnetic coupling on the itinerant ferromagnetism of [formula omitted] through Cu substitution at Ru site

Systematic studies on the structural, transport and magnetic properties of SrRu 1− x Cu x O 3 ( x = 0.0 – 0.2 ) compounds have been performed. SrRu 0.8Cu 0.2O 3 shows a tetragonal structure unlike the other compositions which exhibit a pseudo-cubic structure. Low temperature powder X-ray diffraction data of SrRu 0.8Cu 0.2O 3 collected at a synchrotron beam line reveals that the tetragonal structure is stable down to 8 K. Ferromagnetic transition temperatures ( T c ) are significantly reduced from 160 to 34 K with Cu doping. All the compositions exhibit irreversibilities in M ZFC( T) and M FC( T) curves ascribable to the presence of domain structures. Magnetic susceptibility measurements show that the copper ions are anti-ferromagnetically coupled for concentrations higher than x = 0.16 . The antiparallel arrangement of Ru 5+ ions with its neighboring cations also contributes to the large reduction in the observed magnetic moment. X-ray photoemission spectroscopy measurements show evidence for both tetravalent and pentavalent Ru ions while copper is in a divalent state. We conclude from our resistivity data that Cu 2+ substitution promotes a polaronic type of conductivity.

Structural and magnetic phase transitions of theV4-cluster compoundGeV4S8

The phase transitions of the strongly correlated tetrahedral ${\text{V}}_{4}$-cluster compound ${\text{GeV}}_{4}{\text{S}}_{8}$ have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature (space group $F\overline{4}3m$) and transforms to orthorhombic (space group $Imm2$) at ${T}_{S}=30\text{ }\text{K}$. A Jahn--Teller distortion reduces the symmetry of the ${\text{V}}_{4}$-cluster from $\overline{4}3m$ to $mm2$. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at ${T}_{N}$. Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

Toward the Computational Design of Diastereomeric Resolving Agents: An Experimental and Computational Study of 1-Phenylethylammonium-2-phenylacetate Derivatives

The crystal structures, including two new polymorphs, of three diastereomerically related salt pairs formed by (R)-1-phenylethylammonium (1) with (S&R)-2-phenylpropanoate (2), (S&R)-2-phenylbutyrate (3), and (S&R)-mandelate (4) ions were characterized by low-temperature single crystal or powder X-ray diffraction. Thermal, solubility, and solution calorimetry measurements were used to determine the relative stabilities of the salt pairs and polymorphs. These were qualitatively predicted by lattice energy calculations combining realistic models for the dominant intermolecular electrostatic interactions and ab initio calculations for the ions' conformational energies due to the distortion of their geometries by the crystal packing forces. Crystal structure prediction studies were also performed for the highly polymorphic diastereomeric salt pair (R)-1-phenylethylammonium-(S&R)-2-phenylbutyrate (1-3) in an attempt to predict the separation efficiency without relying on experimental information. This joint experimental and computational investigation provides a stringent test for the reliability of lattice modeling approaches to explain the origins of chiral resolution via diastereomer formation (Pasteurian resolution). The further developments required for the computational screening of single-enantiomer resolving agents to achieve optimal chiral separation are discussed.

Strain induced coexistence of monoclinic and charge ordered phases inLa1−xCaxMnO3

Coexistence of charge-ordered (CO) and monoclinic (M) phases has been investigated in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0.6$, 0.67, and 0.7). For this purpose, pellets and their corresponding powdered samples have been studied employing low-temperature powder x-ray diffraction down to $87\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and transmission electron microscopy down to $109\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Diffraction studies have revealed that the coexistence of M and CO phases occurs only in pellet samples and not in the powdered versions of the same samples. The powders, which have exactly the same composition as their corresponding pellets, show only CO phases. The observed coexistence of M and CO phases in pellets is not due to compositional inhomogeneity within the grain but due to the presence of intrinsic strain in the pellets that is being imposed by the grain boundary network on the grains of the Pnma phase. The occurrence of monoclinic phase has been attributed to the possible softening of ${\mathrm{C}}_{44}$ elastic modulus of the pseudocubic phase.

Physicohemical characterization of the freezing behavior of mannitol-human serum albumin formulations

The goal of the study was to analyze the impact of human serum albumin (HSA) quality (stabilized or nonstabilized HSA), the addition of NaCl, and the HSA stabilizers Na-octanoate and Na-N-acetyltryptophanate on the freezing behavior of mannitol-HSA formulations. The focus was on crystallization, Tg' (glass transition temperature of the maximally freeze-concentrated phase), and Tc (collapse temperature). Differential scanning calorimetry (DSC), cryomicroscopy, and low-temperature x-ray powder diffraction (LTXRD) were used to study the frozen state. In mannitol-HSA formulations, mannitol crystallization was inhibited and Tg' lowered to a greater extent by stabilized HSA (containing Na-octanoate, Na-N-acetyltryptophanate, and NaCl) than by unstabilized HSA. Detailed DSC and LTXRD studies showed that in the concentrations used for stabilizing HSA, NaCl led to changes in the freezing behavior, an effect that was less pronounced for the other stabilizers. NaCl further lowered the Tc, which was determined by cryomicroscopy. As the freezing behavior governs the lyophilization process, the changes have to be taken into consideration for the development of a lyophilization cycle, to avoid collapse and instabilities.

Impact of freezing procedure and annealing on the physico-chemical properties and the formation of mannitol hydrate in mannitol–sucrose–NaCl formulations

The goal was to investigate the impact of NaCl on the physico-chemical properties of mannitol–sucrose formulations during freezing and drying, with special focus on mannitol hydrate formation. Differential scanning calorimetry (DSC) and low-temperature X-ray powder diffraction (LTXRD) were used to study the frozen solutions. After lyophilization the products were analyzed with DSC, temperature-modulated DSC (TMDSC), X-ray powder diffraction (XRD) and Karl-Fischer titration. DSC showed an inhibition of mannitol crystallization by sucrose and NaCl during freezing. The glass transitions of the maximally freeze-concentrated solutions ( Tg′) were lowered by both mannitol and NaCl. By the application of an annealing step during lyophilization mannitol crystallinity could be increased. However, lyophilization with an annealing step promoted the formation of mannitol hydrate, which is known to undergo conversion into the anhydrous polymorphs of mannitol upon storage. LTXRD revealed that mannitol hydrate was formed at temperatures below −30 °C, but not at −27 °C. The tendency that mannitol hydrate is predominantly formed at lower temperature was confirmed by XRD of lyophilized products, produced at different annealing temperatures. For the development of lyophilization cycles the lowered Tg′, as well as the tendency to mannitol hydrate formation predominantly at lower temperature needs to be considered.

Synthesis and structure determination of copper perrhenate, CuReO 4

Copper(I) perrhenate, CuReO 4, has been synthesized as a single-phase sample in sealed silica tubes at 500–800 °C, and its structure was determined for the first time by single-crystal X-ray diffraction. CuReO 4 forms a new structure type derived from the diamond-structure and crystallizes in space group I4 1 cd with lattice parameters a=13.6965(5) Å and c=7.7729(5) Å, Z=16 (rotation method data acquisition using ω and ϕ scans, R 1=0.0191, w R 2=0.0403). Cu and Re atoms are tetrahedrally coordinated by O atoms, these tetrahedra are corner-shared, forming spirally twisted rings of 4-6-8-10- MeO 4 like Si x O y -rings in some silicon dioxide modifications or aluminosilicates. According to low-temperature powder X-ray diffraction and differential thermal analysis, CuReO 4 shows no phase transition down to 80 K and up to its melting point, 703 K.

Material preparation and superconducting properties of Li–Pd–B ternary boride

A superconducting material of Li 2Pd 3B with the superconducting transition temperature ( T c) at 8 K was recently discovered. This material has a cubic structure with a lattice parameter a = 0.675 nm and a P4 332 symmetry composed of distorted 6Pd octahedrons centered by B. We have tried several methods to prepare the samples by arc-melting, radio-frequency heating and heat treatment in Ta tube. The loss of Li was evaluated by chemical analysis to synthesize the stoichiometric Li 2Pd 3B. Low-temperature X-ray powder diffraction down to T c was performed. Shrinking of the lattice parameters with a decrease in the temperature was observed, but no structural transformation occurred between 7.6 K, just below T c, and room temperature. The upper critical field H c2(0) was 62 kOe determined by the measurement of electrical resistance under up to 50 kOe. The heat capacity and magnetization were also measured and their data proved the bulk superconductivity in Li 2Pd 3B.

Synchrotron X-ray wavelength calibration using a diamond internal standard: application to low-temperature thermal-expansion studies

Typically, wavelength instabilities at synchrotron beams are largely due to thermomechanical phenomena at the primary-beam monochromators and fluctuations of the synchrotron orbit. Although they are small, they may have a negative influence on some kinds of diffraction experiments, for example in thermal expansion studies. Using a wavelength calibration procedure is a natural way of solving or at least reducing this problem. Diamond can be used as a calibration material because of its known low-thermal expansion and low X-ray absorption. Low-temperature X-ray powder diffraction measurements were carried out at the powder diffractometer at the B2 beamline at Hasylab/DESY (Hamburg). The cryostat ensured a good temperature stability and accuracy. Unit-cell parameters for cubic (spinel-type) silicon nitride, c-Si3N4, and for copper indium selenide, CuInSe2, were determined in the temperature range from 14 up to 302K. An improvement of the data quality due to elimination of the wavelength fluctuation effect is demonstrated.

Powder X-ray diffraction study of the volume change of ice VIII under high pressure

Low-temperature powder X-ray diffraction measurements have been carried out on H 2O ice VIII in the pressure range from 5 to 50 GPa. The volume change associated with the ice VII–VIII phase transition and the compression curves of ice VIII have been obtained experimentally. The slope of the VII–VIII phase-boundary is explained qualitatively by the volume change, namely, an almost zero volume-change in a pressure-insensitive flat region below 10 GPa, and a negative volume-change in a negative slope region above 10 GPa.

Low temperature polymorphism in 3-amino-1-propanol

3-Amino-1-propanol ( 3AP) was investigated by differential scanning calorimetry, and low temperature powder X-ray diffraction and Raman spectroscopy. Within the range of temperatures studied (−150–25 °C), 3AP was found to be able to crystallize in two monotropic polymorphs. Fast cooling rates produce an amorphous state that, on heating, crystallizes into the metastable polymorph. At higher temperatures, this metastable crystalline phase converts into the stable crystal. Using intermediate cooling rates, 3AP crystallizes as the metastable polymorph, the solid⇛solid transition leading to conversion of this form into the stable polymorph occurring during the subsequent heating. Slower cooling rates enable formation of the stable crystal on cooling. The two crystalline polymorphs were structurally characterized by powder X-ray diffraction and Raman spectroscopy. It was concluded that different conformations are assumed by the individual molecules of 3AP in the two crystalline varieties, with the molecules assuming the all- trans configuration in the metastable crystalline state and having the heavy atom backbone trans but the NH 2 and OH groups gauche in the stable crystal.

Internal friction and Jahn–Teller effect in the charge-ordered La1−xCaxMnO3 (0.5⩽x⩽0.87)

The Jahn–Teller effect in the charge-ordered (CO) state for La1−xCaxMnO3 (0.5⩽x⩽0.87) was studied by measuring the low-temperature powder x-ray diffraction, internal friction, and shear modulus. We find that the electron–lattice interaction with the static Jahn–Teller distortion is the strongest near x≈0.75 in the CO state. It was particularly observed that a crossover of the Jahn–Teller vibration mode from Q2 to Q3 near x=0.75 induces crossovers of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic, and of the magnetic structure from CE-type to C-type near x=0.75. The experimental results give strong evidence that the Jahn–Teller effect not only plays a key role in stabilizing the CO state, but also determines the magnetic and crystal structures in the CO state for La1−xCaxMnO3.

Magneto-crystalline anisotropy in TbPdIn, DyNiAl and GdNiAl studied by using X-ray powder diffraction at low temperatures

Temperature changes of lattice parameters in TbPdIn, DyNiAl and GdNiAl were investigated using the low-temperature X-ray powder diffraction, and related to the magnetic ordering in these compounds. All three compounds exhibit a strong magneto-crystalline anisotropy. Additional powder diffraction measurements were performed in external magnetic fields to assess the orientation of magnetic moments with respect to the crystal axes. No significant preferential orientation of crystallites due to the macroscopic ordering in magnetic field was observed below T c in TbPdIn. On the contrary, the other compounds became macroscopically well ordered below the magnetic-phase transition temperature. DyNiAl had the preferred orientation 00l parallel to the direction of the magnetic field; the preferred orientation of crystallites in GdNiAl exposed to external magnetic field was hk0 .

Dielectric and Structure Properties of Charge Competing System YFe 2 O 4

YFe 2 O 4 m i ( i < 0.005), which is a member of charge-frustrated system RFe 2 O 4 was synthesized and investigated by the low-temperature synchrotron powder X-ray diffraction. The structure between 225K and 250K is the same with the three-dimensional charge ordered state of LuFe 2 O 4 . Below the charge ordering temperature, at least five different phase transitions were detected. Since the balance of competing interaction between ionic charge essentially dominates the presence of multi phases in frustrated system, the low oxygen deficiency in YFe 2 O 4 m i is considered as the origin of the multi phases in low temperature.

Dielectric and Structure Properties of Charge Competing System YFe 2 O 4

YFe 2 O 4 m i ( i < 0.005), which is a member of charge-frustrated system RFe 2 O 4 was synthesized and investigated by the low-temperature synchrotron powder X-ray diffraction. The structure between 225K and 250K is the same with the three-dimensional charge ordered state of LuFe 2 O 4 . Below the charge ordering temperature, at least five different phase transitions were detected. Since the balance of competing interaction between ionic charge essentially dominates the presence of multi phases in frustrated system, the low oxygen deficiency in YFe 2 O 4 m i is considered as the origin of the multi phases in low temperature.

Magnetic and Crystallographic Properties of LnCrO4 (Ln=Nd, Sm, and Dy)

Zircon-type compounds LnCrO4 (Ln=Nd, Sm, and Dy) were prepared. Their precise crystal structures at room temperature were determined from X-ray diffraction measurements. These compounds have a tetragonal system with space group I41/amd. Magnetic susceptibility and specific heat measurements have been performed for all the compounds in the temperature range between 1.8 and 300 K. For NdCrO4, an antiferromagnetic transition was found at 25.2 K. SmCrO4 and DyCrO4 show magnetic transitions at 15.0 and 22.8 K, respectively. In addition, structural phase transitions were observed at 58.5 and 31.2 K, respectively. For DyCrO4, the crystal structure below the transition temperature was determined by low-temperature powder X-ray diffraction measurements to be orthorhombic with space group Imma.

Structural change and charge ordering correlated ultrasonic anomalies inLa1−xCaxMnO3(x=0.5,0.83)perovskite

Ultrasonic sound velocity and attenuation have been measured in polycrystalline manganese oxide ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.5,0.83,1.0)$ at a frequency of 10 MHz. For $x=0.5,$ on cooling down from high temperature, a slight softening of the sound velocity above the charge ordering transition temperature ${T}_{\mathrm{CO}}$ and dramatic stiffening below ${T}_{\mathrm{CO}}$ coincided with big attenuation peaks for both longitudinal and transverse waves were observed. It was found that these ultrasonic anomalies near ${T}_{\mathrm{CO}}$ are correlated with the fine structure (i.e., the lattice parameters) change caused by the Jahn-Teller effect. For $x=0.83,$ the sound velocity starts to soften dramatically with decreasing temperature from higher temperature to ${T}_{S}$ (180 K), and stiffens dramatically below ${T}_{S}.$ The large softening and stiffening of the sound velocity accompanied by a big attenuation peak are strongly correlated with a cubic-to-tetragonal structural phase transition at ${T}_{S},$ which is confirmed by the low-temperature powder x-ray diffraction measurements. It is suggested that this structural phase transition be due to the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra and related to the charge ordering transition. For ${\mathrm{CaMnO}}_{3},$ the anomaly in sound velocity is small.

Crystallographic data of new superlattice phases for spinel LiMn2O4 at low temperatures

Extensive analyses of low-temperature powder x-ray diffraction data for spinel LiMn2O4 (Fd3¯m at room temperature) make it clear that two structural phase transitions occur: first around 285 K from cubic to orthorhombic, second around 65 K from orthorhombic to tetragonal. At temperatures under 285 K, superlattice peaks appear in the diffraction pattern that were successfully indexed by tripling the a and b axes of the spinel unit cell. At 250 K, the unit cell is face-centered orthorhombic, Fddd, F2dd, or Fd2d, with a=24.855(1), b=24.755(2), c=8.2014(3) Å, V=5046.1(4) Å3, Dx=4.284 g/cm3, Z=72. The unit cell at 30 K was confirmed to be body-centered tetragonal I41/amd or I41/a, with a=17.5176(3), c=8.1961(2) Å, V=2515.1(1) Å3, Dx=4.298 g/cm3, Z=36.

Polymorphism of methylchloromethane compounds: VIII. Low-temperature phases of t-butyl chloride

The experimentally determined crystal structure of ordered t-butyl chloride in its lowest temperature Phase III state has been found to agree with that predicted by the theoretical calculations of Chen and Bartell [J. Phys. Chem. 97 (1993) 10 645]. An analysis of the volume changes (ΔV) which occur during the phase transition, by resolving ΔV into components parallel and perpendicular to the threefold molecular axis, sheds light on the mechanism of the phase transition. However, the crystalline phase reported by Ohtani and Hasabe [Chem. Lett. (1986) 1283] to exist between 217.7 and 219.5K has been shown to actually be a manifestation of heterophase pretransition activity as the orientationally-disordered Phase I begins to form while the bulk of the sample still exists in the partially-disordered Phase II. Synchrotron radiation low-temperature X-ray powder diffraction data obtained approximately at every 0.3K between 215 and 220K show that the intensity of the Phase I (111) reflection (2θ∼13.29°, λ=1.15032Å) increases as the intensity of the Phase II (110) reflection (2θ∼13.25°) decreases in the temperature region 2–3K below the transition temperature. This is explained in terms of the transition being initiated simultaneously in several microregions of the sample in accordance with the description given by Ubbelohde [The Molten State of Matter, 1978]. A labeling scheme for uniquely identifying polymorphic phases is introduced so as to prevent the confusion (in the scientific literature and subsequent data bases) which was caused by the same label being assigned to two different crystalline phases. This proposed scheme includes the phase identifier, the temperature of formation, whether it is formed as the temperature is being raised or lowered, the pressure of formation, and the label of the immediately preceding phase.