Structural and magnetic phase transitions in gadolinium under high pressures and low temperatures

High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169?GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp?→?Sm-type?→?dhcp?→?fcc?→?dfcc?→?monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169?GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25?GPa and a temperature of 10?K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Our experiments show that the magnetic transition temperature decreases with increasing pressure to 19?GPa and then increases when gadolinium is subjected to higher pressures.     

十种稀土金属电阻的压力效应

 使用Bundy和Dunn发展起来的带有烧结金刚石砧的Drickamer型高压装置，用固定点测压法标定实验压力，在室温及0~43 GPa的压力范围内测量了稀土金属中Pr、Nd、Sm、Gd、Tb、Dy、Ho、Tm、Lu和Yb的电阻随压力的变化。在各稀土元素的电阻随压力变化的曲线上，观测到了若干“凸起”和斜率突变点，根据Jayaraman提出的三价稀土在压力作用下的相变顺序，得到了这些突（凸）变点分别对应着hcp→Sm-type→dhcp→fcc相变顺序中的某一类型的相变压力。此外还观测到了Pr、Gd、Tb的fcc相随着压力再增高而发生的相变，根据已报导的关于Pr的工作，推测Gd和Tb的这一相变应为fcc→dfcc相变，它们分别发生在22.0和24.5 GPa。在本工作所得结果基础上对Johansson的三价稀土总相图进行了修正。     

Phase transition of samarium under high pressure

Abstract

The DAC X-ray power photograph method was employed for studing the phase transition of samarium up to 26.3 GPa. The experimental results show that the dhcp and fcc high pressure phase of Sm appeared at about 4.0 and 12.5 GPa and room temperature respectively. The dhcp phase was kept until 19.6 GPa. A model for Sm-type -? dhcp -? fcc phase transition is provided in this paper.     

A structural study of promethium metal under pressure

Abstract

The structural behaviour of Pm metal has been investigated up to 60 GPa of pressure using a Diamond Anvil Cell (DAC) and the energy dispersive X-ray diffraction technique. The room temperature/pressure structural form of Pm is dhcp and it transforms to a fcc phase by 10 GPa. This cubic phase of the metal converts by 18 GPa to a third phase, which has frequently been referred to as representing a distorted fcc structure. This latter form of Pm was retained up to 60 GPa, the maximum pressure studied, but subtle changes in the X-ray spectra between 50 and 60 GPa hinted that an additional structural change could be forthcoming at higher pressures. From the experimental data a bulk modulus (B₀) of 38 GPa and a B₀′ constant of 1.5 were calculated using the Birch equation. This modulus for Pm is in accord with the moduli reported for the neighboring lanthanide metals.     

Review on distorted face-centered cubic phase in yttrium via genetic algorithm

We present two distorted face-centered cubic (dfcc) structures of yttrium under high pressure, which have been found by a first-principles genetic algorithm technique. The structures are a tetragonal P4₃ (dfcc-I) and a triclinic P1¯ (dfcc-II), formed by slight distortions from a trigonal R3¯m structure reported as the dfcc phase earlier. The enthalpy difference between the two dfcc structures is less than 0.2 mRy/atom, and dfcc-I is marginally more stable than dfcc-II in lower pressure region. The enthalpy comparison among candidate structures indicates the structural phase transitions into dfcc-I at 41 GPa, into dfcc-II at 81 GPa, and into an orthorhombic Fddd structure at 106 GPa.     

Lattice vibrations and electronic transitions in the rare-earth metals: praseodymium under pressure

Praseodymium was investigated by Raman spectroscopy under pressure. A negative pressure shift of the E(2g) mode is observed in the dhcp phase, which indicates that the initial structural sequence hcp-->Sm-type-->dhcp-->fcc as a whole in the regular lanthanides is associated with a softening of this mode. The pressure response of the phonon modes, observed in the monoclinic and alpha-uranium phases, where 4f bonding becomes important, is characteristic for anisotropic bonding properties.     

Lattice dynamics of solid xenon under pressure

We use density-functional perturbation theory to obtain the phonon spectrum of fcc xenon under pressure. Thermodynamic properties obtained within the quasiharmonic approximation are in fair to good agreement with experiment at zero pressure. The transition pressure from the fcc to hcp phase is predicted to occur at 5 GPa. The fcc structure is found to be dynamically stable up to a pressure of 100 GPa, beyond which the phonon modes at the X and L symmetry points soften. We attribute the observed sluggish kinetics of the fcc-hcp transition to the small energy difference between the phases as well as to the high dynamical stability of the fcc phase.     

Hexagonal to cubic phase transition in YH3 under high pressure

X-ray diffraction studies on bulk yttrium trihydride, in a diamond anvil cell, have been carried out up to 25 GPa. Pressure induced hexagonal-to-cubic phase transformation in YH₃ has been found at pressure of about 8 GPa. The lattice parameter of the new cubic phase was determined as equal to 5.28 Å. This finding confirms the theoretical predictions based on first principle calculations of such a transformation. Equations of state have been determined for both the hexagonal hcp and cubic fcc YH₃ phases. As compared to the pure yttrium metal, bulk modulus for YH₃ is about four times bigger. The similarity of this transition to that observed in the other 4-f trivalent hydrides has been discussed.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Magnetic ordering in Fe<Subscript>1.087</Subscript>Te under pressure

The crystal and magnetic structures of Fe_1.087Te have been studied by neutron powder diffraction in the temperature range from 1.7 to 80 K at pressures of  ≈0.4 and ≈1.2 GPa. No symmetry change of the tetragonal paramagnetic ambient pressure phase (space group P4/nmm) was observed for temperatures above 60 K and pressures up to  ≈1.2 GPa. A novel pressure-induced phase of Fe_1.087Te having orthorhombic symmetry (space group Pmmn) and incommensurate antiferromagneticbicollinear order was observed in the temperature range from 50 to 60 K at  ≈1.2 GPa. The known monoclinic ambient pressure phase of Fe_1.087Te (space group P2 ₁/n) with commensurate antiferromagnetic order was found to be stable up to at least  ≈1.2 GPa at low temperature.     

压力高达27 GPa下天然奥长石的XRD分析

在室温高达27 GPa压力下对天然奥长石(Na_0.86K_0.02Ca_0.12Mg_0.01(Fe_0.01Al_1.12Si_2.87O₈))粉晶进行了原位同步辐射X光衍射(XRD)测量,获得了样品的状态方程。实验数据表明随着压力增大奥长石样品在大约3.5 GPa发生了三斜向单斜的相变(P1→C2)和在大约10.0 GPa发生了单斜对称相变(C2→C2/m)。样品三个相的体模量计算值分别为K₀=73.8 GPa (K′=10.98), K_(C2)=124 GPa (K′=1.05) 和K_(C2/m)=272 GPa (K′=0.625)。样品的元素组成影响其T-O-T 键角的刚度、M-O键的强度和Si-O-Al键角的弯曲,从而导致奥长石样品在高压行为的特殊变化。三斜相的奥长石晶胞压缩性具明显的各向异性。实验结果表明在冷俯冲带奥长石可能是碱金属和碱土金属深循环的载体。     

室温下Fe62Ni27Mn11（wt%）合金的压致fcc-hcp相变

 本文采用Mao-Bell型金刚石对顶砧（DAC）及高压在位（in situ）粉末X光衍射照相方法研究了Fe₆₂Ni₂₇Mn₁₁（wt%）合金在0~43.2 GPa压力范围内的压致结构相变和等温压缩行为，实验结果表明，该合金在低压时为fcc结构，在19.4 GPa压力附近出现压致fcc→hcp结构相变，直到43.2 GPa一直保持fcc、hcp二相共存；相变过程中，二相的molar体积相同；高压hcp相得晶格参数比值c/a基本上不随压力而变，可以表示为c/a=1.630±0.006；在卸压过程中，hcp相可保持到5.8 GPa，当卸压到常压时，该合金完全恢复到fcc结构；用Murnaghan等温固体状态方程对其压缩数据进行最小二乘法拟合，得到B₀=(166±12) GPa，B₀'=5.2±0.5；本文还给出了该合金的压致fcc→hcp结构相变模型，并对存在很宽的二相共存区间问题进行了初步探讨。     

Melting of the rare earth metals and f-electron delocalization

Melting curves for Pr, Nd, Sm, Gd, and Y were measured in a diamond-anvil-cell to nearly 100 GPa and 4000 K. f-electron volume collapses are observed as triple points for Pr (24 GPa and 1400 K) and Gd (65 GPa and 3100 K). These pressures coincide with the volume collapses observed at room temperature. For Nd and Sm, the f-electron volume collapse has not been observed at room temperature but appears at approximately 2000-2500 K as a broad minimum in the melting curve, similar to that of Ce, near 50 GPa (Nd) and 70 GPa (Sm). The melting curve of Y goes smoothly along the entire rare earth sequence.     

Pb0.8Sn0.2Te晶体高压相变的X射线衍射研究

 利用X射线粉末衍射方法，在室温高压下观察到了Pb_0.8Sn_0.2Te晶体的压致相变现象。实验是在DAC高压装置上完成的，压力从零逐步加至25.2 GPa。在常温常压下，Pb_0.8Sn_0.2Te具有fcc结构。从实验结果看到，有两个结构相变存在，分别在压力为6.1 GPa和14.9 GPa附近。我们认为，前一个结构相变与Pb_0.8Sn_0.2Te晶体的金属化有密切关系。实验分别从10.0 GPa和25.2 GPa降压至零，发现相变均是可逆的。     

Static compression of rare earth metal holmium to 282 GPa

ABSTRACT

The phase transitions and equation of state measurements were carried out on rare earth metal Holmium (Ho) to 282?GPa using toroidal diamond anvils thereby doubling the pressure range to which it has been studied previously. The first set of experiment employed standard beveled diamond anvils utilizing copper as an x-ray pressure standard to 217?GPa. The second set of experiment employed toroidal diamond anvils utilizing platinum as an x-ray pressure standard to 282?GPa. The recently proposed 16-atom orthorhombic structure (oF16) appeared to be stable between 103 and 282?GPa. The scaled axial ratio (c/a) shows a narrow range of variation of 1.58?±?0.05 for the five known crystalline phases of Ho to 282?GPa. The experimental equation of state of Ho is presented up to a threefold volume compression V/V_o?=?0.322.     

Search for polytipic structures of gadolinium

Abstract

The trivalent rare-earth element gadolinium has been studied up to 8 GPa and 700°C by energy-dispersive synchrotron x-ray diffraction. Except for the known crystal structures of hcp, Sm-type, and dhcp, no long-period polytypes have been observed. It is found that the hcp structure transforms directly to the dhcp structure with increasing pressure at high temperatures.     

Pressure-induced phase transition in an orthorhombic C60 crystal

X-ray diffraction studies of an orthorhombic C₆₀ single crystal grown from CS₂ solution have revealed a phase transition to a monoclinic phase between 1.1 and 2.2GPa. Compressibility of three principal axes is measured up to 3GPa and found to be nearly isotropic. Its bulk modulus is obtained as 10.5±1.9GPa, and this crystal is more compressible than an fcc one. We discuss the structural characteristic differences under pressure between the orthorhombic crystal and the fcc crystal.     

Phonon Spectrum and Structural Transformations at High Pressures in Vanadyl IV Phthalocyanine Crystals

The Raman scattering spectra and crystalline structure of vanadyl IV phthalocyanine (VOPc) at normal and high pressures has been studied. According to the X-ray diffraction data, the initial microcrystalline powder represented a mixture of the triclinic α phase (79%) and the monoclinic β phase (21%) possessing P$$\bar {1}$$ and P21/c symmetry, respectively. Raman spectra of the two phases are similar, but the phonon modes of the β phase are shifted toward higher frequencies (energies). The pressure dependence of the spectra of the α phase has been determined and it is established that the interval of 2.3–3.4 GPa reveals reversible pressure-dependent variations: above 3 GPa, some phonon modes exhibit splitting and the coefficients of pressure-induced (baric) shift for almost all modes show a decrease. A high-pressure feature observed in the Raman spectra can be related to changes in intermolecular interactions in crystalline structure of the α phase. The pressure dependence of the α phase unit cell volume measured at pressures increasing up to 4 GPa is a smooth monotonic function that can be well described by the Murnaghan equation of state. The obtained data were used to calculate the Grüneisen parameters of VOPc phonon modes.     

Reconstructive structural phase transitions in dense Mg

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.     

High-pressure phases of thorium and uranium compounds with the rocksalt structure

Abstract

The high-pressure crystal structures of the actinide compounds ThX and UX (X= C, N, P, S, As, Se, Sb, Te) have been studied by X-ray diffraction using synchrotron radiation, in the pressure range up to about 60 GPa Distorted fcc structures were observed for UC (27 GPa), UN (29 GPa), UP (10/28 GPa), US (10 GPa) and ThS (20 GPa). No phase transition has been observed for ThC and ThN. Compounds with As, Se, Sb all transform to the CsCl structure. ThP transform to the CsCl structure at 30 GPa. ThTe has the CsCl structure at ambient pressure and no further phase transition has been observed. UTe transforms to the CsCl structure at 9 GPa.