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°.     

Structural phase transitions in Cu2–x Te crystals (x = 0.00, 0.10, 0.15, 0.20, 0.25)

Structural phase transitions in Cu_2–x Te crystals have been investigated by high-temperature X-ray diffraction.

At room temperature Cu₂Te and Cu_1.90Te specimens are two-phased, i.e. they consist of an orthohombic phase with a = 7.319, b = 22.236, c = 36.458 Å and a hexagonal phase with a = 4.150, c = 7.188 Å. The changes in both compounds take place generally in the hexagonal phase with increasing temperature. At 821, 873 K they transform to a FCC phase. Monocrystals of the other compounds at room temperature crystallize in the hexagonal system and at 673, 773, 723 K, respectively, they transform to a FCC phase. It is determined that as the cation deficiency increases the crystal quality becomes better.     

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.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Superconductivity in the hole-doped oxy-arsenide RE1−xSrxFeAsO (RE = La, Pr)

Bulk superconductivity was observed in the FeAs-based RE_1−xSr_xFeAsO (RE = La, Pr) when the di-valence element Sr was substituted to the site of the tri-valence element La and Pr. The maximum superconducting transition temperatures T_c for the two systems are 26 K and 16.3 K, respectively. The doping dependence of the electrical properties and structure of these two systems were investigated systematically. A roughly monotonic increase of T_c and the lattice constants (a-axis and c-axis) with Sr concentration and a saturation behavior in the high doping levels were found. We confirmed that conduction in this type of materials is dominated by hole-like charge carriers by the Hall effect measurements. Also the resistive measurements revealed possible higher upper critical field in these systems comparing with the electron-doped ones.     

X-ray diffraction study of a sequence of phase transitions in Cs2HgCl4 crystals

The temperature behavior of lattice parameters and diffraction patterns of the reciprocal lattice in Cs₂HgCl₄ crystals is studied by x-ray diffraction in the temperature range from 4.2 to 300 K. A sequence of phase transitions is observed and attributed to the evolution of incommensurate and commensurate modulations along the crystallographic a and c axes of a unit cell in the initial Pnma structure.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

New compounds with incommensurate phases: Cs2CdI4 and Cs2ZnI4

Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs₂CdI₄ and Cs₂ZnI₄. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P2₁/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs₂ZnI₄ crystal in a broad pre-transition region (T - T _i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).     

Incommensurate cooperative tilting modes of MnF6 octahedra relatedto the structural phasetransitions in BaMnF4

The structural phase transitions of the layer compound BaMnF₄ were studied by high-resolution X-ray diffraction using synchrotron radiation. The intensities and profiles of two kinds of superlattice reflections having incommensurate reduced wave vectors q ₁= (～ ± 1/5,0,0) _p and q ₂ =(～ ± 2/5,1/2,1/2)_p, respectively, were measured as a function of temperature from 25 K to 280 K. These temperature dependencies show that incommensurate structural phase transitions of second order occur at 234 K and 244 K. These structural phase transitions are interpreted as successive condensations of a folding-screen-like incommensurate plane-distortion mode and a commensurate anti-ferro-distortive tilting mode of the MnF₆ octahedra around the primitive a₀ - and b₀ -axes, when cooled down. It is also found that there is another structural phase transition at about 45 K related to a precursor structural distortion for the antiferromagnetic transition occurring at about 26 K.     

Structure determination of Cs0.864Rb1.136SeO4

The present structure determination of Cs _x Rb_2-x SeO₄ shows that the crystal belongs to the β - K₂SO₄ family, like the related compounds Cs₂SeO₄ and Rb₂SeO₄. In β - K₂SO₄ compounds the cations occupy two different sites; one of these sites, which is surrounded by 11 anionic atoms, is occupied preferentially by Cs, while the other one with 9 neighbours by Rb. A slight excess of Rb has been determined by X-ray structure analysis in a single crystal prepared at 292 K; its formula is Cs_0.864(9)Rb_1.136(9)SeO₄.

Crystals were grown by evaporation from equimolar water solutions of Cs₂SeO₄ and Rb₂SeO₄ at 292 and 301 K. No phase transition was detected by DSC in the range 103-323 K for either of the samples. Lattice parameters of different crystals isolated at various stages of crystallization indicate formation of crystals with a different proportion of Cs/Rb cations. The lattice parameters as well as their ratios differed significantly from those given by Endo et al., (1983).     

Temperature dependences of thermal expansion and exchange magnetostriction of holmium and dysprosium single crystals

Information on the character of the temperature relationships of linear expansion coefficients (α_a, α_c) and exchange magnetostrictions (λ_a, λ_a) of single crystal Ho and Dy, based on temperature dependence of thec- anda-axis lattice constants, were accurately determined by X-ray diffraction from 4,2 to 300 K. In the helical phases we observed a large anisotropy of expansion coefficients (α_a>0, α_c<0) and their variations at 24,5, 45 and 96 in Ho and 156 K in Dy. These anomalies were caused by the commensurate transitions. The Curie transition in Ho is found to be of the 1-st order.     

Hysteresis behavior in Raman intensities in YBa2Cu3O7-y -family superconductors between T c and T o = 234 K

The intensity of the 646 cm^-1 Raman line in YBa₂Cu_3-x V _x O_7-y has been measured for cooling and heating from 77 K to 234 K. The intensity exhibits intensity hysteresis of order 20% in the cooling versus heating values between T _c = 89 K and T _o = 234 K. This effect is unexplained but is extremely similar to that reported by Ledbetter et al. for the bulk modulus and C ₁₁ elastic coefficient. In each case the hysteresis vanishes below T _c , independent of the value of T _c , which may be adjusted from 60 K to 94 K by changing oxygen concentration y. Qualitative interpretation requires that the structural phase transition near 234 K be intimately related to the superconducting transition at T _c presumably through partial ordering of oxygens.     

Orthorhombic structure: a necessity in superconducting 1-2-3 compounds

X-ray and resistivity measurements on YBa₂Cu₃O_7−δ (1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase shows aT _c ∼ 31 K. The effect of oxygen concentration onT _c is isolated.T _c=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the superconducting mechanism to be operative.     

The experimental and theoretical investigation of vibration spectra in ferroelectric semiconductor SbSBrxI1−x crystals

The vapor grown SbSBr_xI_1−x (x=0.1; 0.5; 0.9) crystals with clear mirror surfaces have been used for infrared reflection measurements with Fourier spectrometer. The vibration frequencies along c(z)-axis have been derived from Kramers–Kroning and optical parameters fitting analysis of the experimental reflectivity spectra at T=300 K. The theoretical vibration spectra of SbSBr_xS_1−x (x=0.1; 0.5; 0.9) crystals in paraelectric phase (T=300 K) along c(z)-axis have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. The theoretical vibration spectra of these crystals in a–b(x−y) plane have been determined in harmonic approximation. In this work we discuss the nature of anharmonism in SbSBr_xI_1−x crystals along the c(z)-axis.     

Characterization of organic–inorganic hybrid layered perovskite and intercalated compound (n-C12H25NH3)2ZnCl4

We report on some electrical properties and solid–solid phase transitions of organic–inorganic hybrid layered halide perovskite and intercalated compound (n-C₁₂H₂₅NH₃)₂ZnCl₄ which is one member of the long-chain compounds of the series (n-C_nH_2n+1NH₃)_2,(n = 8–18). The complex dielectric permittivity ?*(ω,T) and the ac conductivity σ (ω,T) were measured as functions of temperature 100 K < T < 390 K and frequency 5 kHz < f < 100 kHz. Moreover, the differential scanning calorimetery and the differential thermal analysis thermograms were performed. The analysis of our data confirms the existence of a structural phase transition at T ≈ (362?±?2) K, where the compound changes its state from intercalation to non-intercalation with a drastic increase in the c-axis by about 16.4%.

The behavior of the frequency-dependent conductivity follows the Jonscher universal power law: σ (ω, T) α?^s(?,T). The mechanism of electrical conduction in the low-temperature phase (phase II) can be described as quantum mechanical tunneling model.     

NdNi2Ge2化合物的结构和电磁输运性质

利用非自耗真空电弧熔炼法制备了NdNi₂Ge₂化合物样品,采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其晶体结构. 结果显示该化合物的空间群为I4/mmm,点阵参数为:a=4.120(1),c=9.835(0),Z=2,Nd原子占据2a晶位,Ni原子占据4d晶位,Ge原子占据4e晶位. NdNi₂Ge₂化合物呈现顺磁性,应用居里-外斯定律拟合计算得到居里-外斯常数为25.8,居里-外斯温度为6.24 K. 有效势磁矩为3.69μ_B,这与理论计算Nd³⁺的磁矩相符,表明磁矩主要源于Nd³⁺. 电阻率变化范围为0.3 Ω ·μm^-1-1 Ω ·μm,电阻曲线拟合显示NdNi₂Ge₂呈半金属性. 关键词： 2Ge₂')" href="#">NdNi₂Ge₂ Rietveld结构精修 电磁输运     

Synthesis of ZnFe2O4 nanomagnets by Fe-ion implantation into ZnO and post-annealing

Fe ions of dose 8 × 10¹⁶ cm^-2 are implanted into a ZnO single crystal at 180 keV. Annealing at 1073 K leads to the formation of zinc ferrite (ZnFe₂O₄), which is verified by synchrotron radiation X-ray diffraction (SR-XRD) and X-ray photoelectron spectroscopy (XPS). The crystallographically oriented ZnFe₂O₄ is formed inside the ZnO with the orientation relationship of ZnFe₂O₄ (111)//ZnO (0001). Superconducting quantum interference device (SQUID) measurements show that the as-implanted and post-annealing samples are both ferromagnetic at 5 K. The synthesized ZnFe₂O₄ is superparamagnetic, with a blocking temperature (T_B = 25 K), indicated by zero field cooling and field cooling (ZFC/FC) measurements.     

In situ high-pressure study of LiNbO3-type FeTiO3: X-ray diffraction and Mössbauer spectroscopy

The structure of LiNbO₃-type FeTiO₃ and the oxidation state of Fe have been investigated using X-ray diffraction and Mössbauer spectroscopy in the diamond anvil cell up to 18 GPa at room temperature. A structural phase transition is observed at 15.7 GPa from LiNbO₃-type to perovskite-type, accompanied by a volume collapse of 1.5%. LiNbO₃-type FeTiO₃, which is shown to contain only ferrous iron up to this pressure, and no charge transfer is observed. In addition to the c/a axial ratio that has been used to distinguish between ilmenite and LiNbO₃-type FeTiO₃, the hyperfine parameters (isomer shift and quadrupole splitting) provide an efficient way to discriminate between these two phases.     

硼(氮、氟)掺杂对TiO2纳米颗粒光学性能的影响

利用X射线衍射谱、拉曼光谱和紫外-可见光吸收光谱研究了硼(氮、氟)掺杂对TiO₂纳米颗粒光学性能的影响.X射线衍射谱和拉曼光谱结果表明,掺硼(氮、氟)对TiO₂纳米颗粒的锐钛矿相晶体结构无明显影响,而其锐钛矿晶格出现畸变(c/a值增大),这被归因于掺杂原子对TiO₂纳米颗粒表面氧原子缺位沿晶格c轴方向的占据.另外,掺硼(氮、氟)TiO₂纳米颗粒吸收带红移与TiO相似文献   

MgB2 thick films deposited on stainless steel substrate with Tc higher than 39K

Thick MgB₂ (magnesium diborate) films, ∼10 μm, with T _c (onset) = 39.4 K and T _c (zero) = 39.2 K have been successfully grown on a stainless steel substrate using a technique called hybrid physical-chemical deposition (HPCVD). The deposition rate is high, ∼6.7 nm/s. The X-ray diffraction (XRD) indicates that it is highly (101) and c-axis oriented. The scanning electron microscope (SEM) images demonstrate that the film grown is in “island-mode”. The uniform superconducting phase in the film is shown by the M-T measurement.