High-pressure,high-temperature single-crystal study of Bi-IV

Bi-IV is the stable high-pressure, high-temperature phase of bismuth at ～4 GPa and ～500 K. It was first identified in 1958, but its structure has remained uncertain. An X-ray powder-diffraction study of Bi-IV reported the structure as monoclinic, but a subsequent reinterpretation of the same data concluded that the structure was C-centred orthorhombic (oC16), with the same atomic arrangement as in Cs-V and Si-VI. To resolve the uncertainty over the structure of Bi-IV, we investigated this phase at 3.2 GPa and 465 K by single-crystal synchrotron X-ray diffraction. All of the observed reflections could be indexed on the orthorhombic oC16 structure proposed by Degtyareva, with a=11.191(5) Å, b=6.622(1) Å and c=6.608(1) Å. The spacegroup was confirmed as Cmce. Refinement of the data resulted in an excellent fit (R=2.8% ), and gave atomic coordinates very similar to those of the oC16 structures in Cs-V and Si-VI.     

High pressure X-ray diffraction study on BaWO4-II

BaWO₄-II has been synthesized at 5 GPa and 610°C. Its high pressure behavior was studied by in situ synchrotron X-ray diffraction measurements at room temperature up to 17 GPa. BaWO₄-II retains its monoclinic structure. Bulk and axial moduli determined by fitting a third-order Birch–Murnaghan equation of state to lattice parameters are: K ₀=86.2±1.9 GPa, K ₀(a)=56.0±0.9 GPa, K ₀(b)=85.3±2.4 GPa, and K ₀(c)=146.1±3.2 GPa with a fixed K′=4. Analysis of axial compressible modulus shows that the a-axis is 2.61 times more compressible than the c-axis and 1.71 times more compressible than the b-axis. The beta angle decreases smoothly between room pressure and 17 GPa from 93.78° to 90.90°.     

A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti₂O₃ on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti₂O₃ was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti₂O₃ can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å³ at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å³ at ambient conditions. The Birch–Murnaghan equation of state yields K ₀=206 (3) GPa and K′₀=4 (fixed) for corundum phase, and K ₀=296 (4) GPa and K′₀=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti₂O₃ can be identified as Th₂S₃-type rather than U₂S₃-type structure. The transition from corundum-type to Th₂S₃-type structure accompanies the drastic change of the form of polyhedron: from TiO₆ octahedron in the corundum-type to TiO₇ polyhedron in the Th₂S₃-type structures.     

High-pressure effect on inverse spinel LiCuVO4:X-ray diffraction and Raman scattering

The effect of external quasi-hydrostatic pressure on the inverse spinel structure of LiCuVO 4 was studied in this paper. High-pressure synchrotron X-ray diffraction and Raman spectroscopy measurements were carried out at room temperature up to 35.7 and 40.3 GPa, respectively. At a pressure of about 20 GPa, both Raman spectra and X-ray diffraction results indicate that LiCuVO4 was transformed into a monoclinic phase, which remained stable up to at least 35.7 GPa. Upon release of pressure, the high-pressure phase returned to the initial phase. The pressure dependence of the volume of low pressure orthorhombic phase and high-pressure monoclinic phase were described by a second-order Birch-Murnaghan equation of state, which yielded bulk modulus values of B 0 = 197(5) and 232(8) GPa, respectively. The results support the empirical suggestion that the oxide spinels have similar bulk modulus around 200 GPa.     

Phase transitions in PbTe under quasi-hydrostatic pressure up to 50 GPa

PbTe has been investigated using synchrotron X-ray diffraction (XRD) in a diamond anvil cell under quasi-hydrostatic pressures up to 50 GPa. Upon compression to 6.6 GPa, the initial NaCl phase transforms to an intermediate phase, which is confirmed to be an orthorhombic structure with a space group Pnma. At 18.4 GPa, the intermediate Pnma phase undergoes a phase transition to the CsCl structure. The systemic analysis of the crystal structures between the NaCl and intermediate phases indicates that the structure of the Pnma phase could be derived from the distortion of the NaCl structure. The bulk modulus of the CsCl phase is B₀=52(2) GPa with V₀=60.8(4) ų and B′₀=4.0 (fixed), slightly larger than the NaCl phase (B₀=44(1) GPa) and the intermediate phase (B₀=49(3) GPa).     

Phase transitions in rubidium hydrogen sulfate: crystal structures at 293 and 200 K

The crystal structure of the paraelectric phase of rubidium hydrogen sulfate has been redetermined at room temperature to be monoclinic with a = 14.3503(14), b = 4.6187(4), c = 14.3933(14)?Å, β = 118.03(1)° (space group P2₁/n). Both the sulfate groups are found to be ordered, unlike in previous reports. The crystal structure of the ferroelectric phase at 200?K belongs to the noncentrosymmetric space group Pn with a = 14.2667(12), b = 4.5878(4), c = 14.2924(12)?Å, β = 118.01(1)°, with distorted sulfate groups. The change in the Rb coordination is discussed in terms of bond-valence-sum calculations. Variable-temperature powder X-ray diffraction patterns at temperatures above 393?K indicate a possible reduction in symmetry, suggesting a phase transition.     

Stabilisation of Ce-Cu-Fe amorphous alloys by addition of Al

The present work describes the formation of amorphous alloys in the (Al_1?xCe_x)₆₂Cu₂₅Fe₁₃ quaternary system (0 ≤ x ≤ 1). When the amount of Ce falls in the range 0.67 ≤ x ≤ 0.83, the alloys obtained exhibit a completely amorphous structure confirmed by powder X-ray diffraction. Otherwise, at compositions x = 0.5, 0.58, 0.92 and 1, a primary crystalline phase forms together with an amorphous matrix. The crystallisation temperature (T_x) decreases with increasing Ce content, varying from 593 K for x = 0.5–383 K for x = 1. Composition x = 0.75 is considered as the best glass former, exhibiting a large supercooled liquid region of 40 K width that precedes crystallisation. In order to form bulk amorphous alloys, ribbons with this later composition were consolidated into few millimetre thick discs using pulsed electric current sintering at different temperatures, yet preserving the amorphous structure. Meanwhile, increasing temperature above 483 K triggers crystallisation of a primary phase isostructural to AlCe₃. Further increase in the temperature up to 573 K yields a higher fraction of the crystalline phase. Testing mechanical properties, using nanoindentation, revealed that both elastic modulus (E) and hardness (H) depend on the Al content, ranging from E = 85.6 ± 3.7 GPa and H = 6.2 ± 0.7 GPa for x = 0.5 down to E = 39.8 ± 1.0 GPa and H = 3.1 ± 0.2 GPa for x = 0.92.     

Pressure-induced phase transformation of CsPbI3 by X-ray diffraction and Raman spectroscopy

The structural transformation of cesium lead iodine (CsPbI₃) has been investigated in diamond anvil cells up to ～15 GPa at room temperature by employing synchrotron radiation X-ray diffraction and Raman spectroscopy. One reversible transformation from orthorhombic (Pnma) to monoclinic (P2₁/m) phase has been observed at 3.9 GPa. Isothermal pressure–volume relationship of orthorhombic CsPbI₃ is well fitted by the third-order Birch–Murnaghan equation of state with K₀ = 14(3) GPa, K′₀ = 6(2) and V₀ = 891(7) ų. The ultralow value of bulk modulus K₀ demonstrates the high compressible nature of CsPbI₃, similar to those of organic–inorganic metal halide perovskites. The present results provide essential information on the intrinsic properties and stability of CsPbI₃, which may be applied in photovoltaic devices.     

Structure and electrical properties of Sb2Te3 and Bi0.4Sb1.6Te3 metastable phases obtained by HPHT treatment

We have obtained the metastable phase of the thermoelectric alloy Bi_0.4Sb_1.6Te₃ with electron type conductivity for the first time using the method of quenching under pressure after treatment at P=4.0 GPa and T=400–850 °C. We have consequently performed comparative studies with the similar phase of Sb₂Te₃. The polycrystalline X-ray diffraction patterns of these phases are similar to the known monoclinic structure α-As₂Te₃ (C2/m) with less monoclinic distortion, β ≈ 92°. We have measured the electrical resistivity and the Hall coefficient in the temperature range of T=77?450 K and we have evaluated the Hall mobility and density of charge carriers. The negative Hall coefficient indicates the dominant electron type of carriers at temperatures up to 380 K in the metastable phase of Sb₂Te₃ and up to 440 K in the metastable state of Bi_0.4Sb_1.6Te₃. Above these temperatures, the p-type conductivity proper to the initial phases dominates.     

Synthesis and optical studies of GdCrO<Subscript>3</Subscript> nanoparticles

Here we report for the first time, a detailed synthesis mechanism of GdCrO₃ nanoparticles using a surfactant-less hydrothermal method. We also report a detailed study of their structural and optical properties. The selected area electron diffraction and powder X-ray diffraction studies of GdCrO₃ nanoparticles show pure phase and excellent crystallinity with average particle size around 50–60 nm. The structural analysis indicates a distorted perovskite crystal structure with lattice parameter values of a = 5.3099 ± 0.0022, b = 5.51793 ± 0.00226, and c = 7.60426 ± 0.00315 Å. For the first time, we report the room temperature UV–vis, photoluminescence, FTIR, and X-ray photoelectron spectroscopy results and their detailed analysis for GdCrO₃. These results provide optical signatures of the formation of GdCrO₃ where both Gd³⁺ and Cr³⁺ play dominant role in different frequency regions.     

High-pressure phase transitions in UP1-xSx

Abstract

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP_1-x -S_x (X=0.1; 0.25; 0.4) up to 53 GPa UP_1-x S_x is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP_1-x S_x with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP^1-x S_x 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.     

Conformational and Structural Analysis of 6‐(4‐Methoxy‐phenyl)‐1,5,7a‐triphenyl‐tetrahydro‐imidazo[1,5‐b][1,2,4]oxadiazol‐2‐one

Abstract

The cis stereochemistry of 6‐(4‐methoxy‐phenyl)‐1,5,7a‐triphenyl‐tetrahydro‐imidazo[1,5‐b][1,2,4]oxadiazol‐2‐one was studied by use of a PM3 semi‐empirical quantum mechanical model, and x‐ray crystallographic analysis. It crystallizes in the monoclinic space group P2 ₁ /n with a = 10.812(1) Å, b = 16.464(2) Å, c = 13.379(1) Å, α = 90.00°, β = 98.39(1)°, γ = 90.00°, V = 2356.07(4) Å³, Z = 4, D _calc = 1.3067 g cm^?3, F(0 0 0) = 976.41, and μ = 0.086 mm^?1. The structure was solved by direct methods and refined to R = 0.066 for 1257 independent reflections [I > 4σ (I)]. The results from x‐ray diffraction were seen to be generally consistent with the results from previously reported spectroscopic investigations, beside theoretical calculations, except for conformations of five‐membered fused heterocycles. Two inter‐ and intramolecular weak interactions in addition to carbon atoms (C1 and C3) with different chiralities were found in the structure. The conformational study was performed by randomly scanning the potential energy surface belonging to the title compound with respect to selected torsion angles.     

Rietveld and pair distribution function study of Hägg carbide using synchrotron X‐ray diffraction

Fischer–Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe₅C₂ Hägg carbide (χ‐HC) during FT synthesis. Arguably, χ‐HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of χ‐HC to iron oxide, a step‐wise decarburization to cementite (θ‐Fe₃C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of χ‐HC for the understanding of the synthesis process and for comparison with the first‐principles ab initio modelling. Here the results of high‐resolution synchrotron X‐ray powder diffraction data are reported. The atomic arrangement of χ‐HC was confirmed by Rietveld refinement and subsequent real‐space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of χ‐HC correspond well with that of a pseudo‐monoclinic phase of space group Pī [a = 11.5661 (6) Å, b = 4.5709 (1) Å, c = 5.0611 (2) Å, α = 89.990 (5)°, β = 97.753 (4)°, γ = 90.195 (4)°], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure.     

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract

High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD₃ND₃)₂MnCl₄. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) Å; b = 7.409 (8) Å c = 17.126 (12) Å and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (?10%) of the inter-layer distance. Space groups P2/c or P2₁/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.     

Phase transition in a chloro-elpasolite,Cs2KInCl6

Using high-purity starting materials, we synthesized a new room-temperature Cs₂KInCl₆ phase (I): monoclinic (C2/c), a = 25.484(11), b = 7.699(2) and c = 13.225(3) Å, β = 100.69(3)°. A ferroelasticparaelastic phase transition is noted at T_c = 100°C (by Thermal Analysis and X-ray, Raman-scattering, electrical permittivity versus temperature) leading to the prototype Fm3m phase(II) with a = 10.870(5) Å, quenchable when very slightly spoiled by impurities.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

An in situ high-pressure X-ray diffraction experiment of hydroxyapophyllite

The compression behavior of a natural hydroxyapophyllite is investigated up to about 10.01 GPa at 300 K using in situ angle-dispersive X-ray diffraction and a diamond anvil cell at High Pressure Experiment Station, Beijing Synchrotron Radiation Facility (BSRF). Over this pressure range, no phase change or disproportionation is observed. The isothermal equation of state is determined for the first time. The values of zero-pressure volume V₀, isothermal bulk modulus K₀, and K₀' refined with a third-order Birch-Murnaghan equation of state are V₀=1276.3±0.9 ų, K₀=71±3 GPa, and K₀'=8±1. Furthermore, we confirm that the values of linear compressibility β along a and c directions of hydroxyapophyllite are elastically anisotropic.     

Structural characterization,spectroscopic, thermal,AC conductivity and dielectric properties and antimicrobial studies of (C8H12N)2[SnCl6]

A new inorganic-organic hybrid material produced from 2,6-dimethylanilinium cations and tin halide (SnCl₆)^2? has been synthesized and structurally determined by X-ray diffraction method. The title compound crystallizes in the monoclinic system, space group C2/m with a = 19.8772(4), b = 6.9879(1), c = 8.3001(2) Å, β = 98.487(2)° and V = 1140.26(4) ų. The crystal structure is built up of sheets of (SnCl₆)^2? octahedral anions and 2,6-xylidinium cations. The optical band gap was calculated and found to be 4.11 eV. At high temperature this compound exhibits a structural phase transition at 338 K. This has been characterized by differential scanning calorimetric and dielectric studies. Measurements of AC conductivity as a function of frequency at different temperatures indicated the hopping conduction mechanism. The bioassay results showed that the structure exhibits significant antibacterial activity.     

铌酸钾钠基无铅压电陶瓷的X射线衍射与相变分析

分析了斜方相、四方相铌酸钾钠基无铅压电陶瓷材料的结构和X射线衍射图谱的特点. 对于铌酸钾钠基压电材料斜方相结构, 从构成晶胞的一个单斜原胞进行分析, 计算出X射线衍射谱上每个衍射角附近的衍射峰数目和相对强度. 提出了2θ在20°—60°范围内根据(1 0 2)衍射峰(52°附近)和(1 2 1)衍射峰(57°附近)劈裂的数目区分斜方和四方相的新方法. 对于多晶陶瓷粉末, 可以更简便的由22°(或45°)附近前后峰的相对高低来判断斜方、四方相. 关键词： 铌酸钾钠 无铅压电陶瓷 X射线衍射 相变