Performance of different types of detectors in a high-pressure X-ray study of phase transition

Abstract

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.     

X-ray Diffraction Study of Cd 0.8 Zn 0.2 Te Under High Pressure and Temperature

Energy dispersive X-ray diffraction using synchrotron radiation has been used to study phase transformations of Cd 0.8 Zn 0.2 Te under high pressure and temperature. We confirm the presence of a cinnabar phase between the zinc-blende and rock-salt phases. But contrary to the results in CdTe, this intermediate phase is found to be stable only on pressure decrease and in a narrower pressure and temperature range. Single-phase cinnabar patterns are obtained only at 300 and 373 K. At 673 K and above, even on pressure decrease, no evidence of the cinnabar phase is found. In this temperature range, a phase segregation phenomenon is observed in the zinc-blende phase during the zinc-blende transition in both upstroke and downstroke, and the retrieved sample at ambient conditions presents a double pattern corresponding to two different Zn contents.     

A system for doing low temperature-high pressure single crystal X-ray diffraction with a synchrotron source

Abstract

A new diamond anvil cell and a helium flow cryostat have been developed for X-ray diffraction on single crystals at low temperatures and high pressures using white radiation of a synchrotron beam. This novel instrument especially enables continuous change of temperature and pressure of the sample without any adjustment of alignment. Automatic search for diffraction peaks can be performed since less than 30 pm eccentricity can be maintained during the rotation of the cell in the cryostat and the rotation of the cryostat on the goniometer head. The minimum temperature reached is 46 K. Measurements of solid 4He at 11.8 GPa are presented which confirm the stability of the hcp phase on this isobar.     

Single crystal x-ray diffraction with a synchrotron source in a mdac at low temperature

Abstract

A new diamond anvil cell and a helium flow cryostat have been developed for x-ray diffraction on single crystals at low temperatures and high pressures using the white radiation of a synchrotron beam. This novel instrument especially enables continuous change of sample temperature and pressure without any adjustment of alignment. A minimum temperature of 46 K has been reached.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

P- V- T Data of hexagonal boron nitride h BN and determination of pressure and temperature using thermoelastic equations of state of multiple phases

Abstract

A synchrotron x-ray diffraction study on hexagonal boron-nitride (hBN) was conducted at simultaneous high pressures and temperatures. The pressure applied to the sample is pseudo-hydrostatic up to 9.0 GPa and the temperature was homogeneous in the range of 300 K to 1280 K. A modified Rietveld profile refinement has been applied to these diffraction spectra of low symmetry and multiple phases observed in the energy-dispersive mode. Thermoelastic parameters of hBN were derived by fitting a modified high temperature Birch-Murnaghan equation of state. The results are: bulk modulus K=17.6 GPa, pressure derivative K′=?K/?P=19.5, temperature derivative [kdot]=?K/?T=?0.69 × 10^?2 Gpa/K, volumetric thermal expansivity α=a+bT with values of a=4.38 × 10^?5K^?1 and b=1.75 × 10^?8K^?2, respectively. It is observed that the thermal expansion and compression along different crystal axes are significantly different. The crystal c-axis is much more expandable and compressible than the a-axis. This is attributed to the layered structure of the hBN. Because the thermoelastic equations of state of hBN and NaCl are quite different, the unit cell volumes of these two materials, derived from the same diffraction pattern, can be used to derive the experimental P-T conditions. The large intersection angle of isochoric lines of these phases in P-T space ensures a determination of P-T with satisfactory precision. The application and limitations of this method in obtaining experimental pressure and temperature using diffraction data and thermoelastic equations of state of multiple phases are discussed.     

Determination of the phase boundary of GaP using in situ high pressure and high-temperature X-ray diffraction

The high pressure behavior of gallium phosphide, GaP, has been examined using the synchrotron X-ray diffraction technique in a diamond anvil cell up to 27?GPa and 900?K. The transition from a semiconducting to a metallic phase was observed. This transition occurred at 22.2?GPa and room temperature, and a negative dependence of temperature of this transition was found. The transition boundary was determined to be P (GPa)?=?22.6???0.0014?×?T (K).     

Equation of state of boron subarsenide B12As2 to 47 GPa

Compressibility of boron subarsenide B₁₂As₂ has been studied by synchrotron X-ray diffraction up to 47?GPa at room temperature in a diamond anvil cell using Ne pressure transmitting medium. A fit of experimental p–V data by Vinet equation of state yielded the bulk modulus of 150(4) GPa and its first pressure derivative of 6.4(3). No pressure-induced phase transitions have been observed.     

Pressure Response of the Higher Fullerene C 84 Studied by Raman and X-ray Scattering

The pressure behavior of the intramolecular phonon modes of the fullerene C 84 and its structural stability have been studied for the first time by means of Raman spectroscopy and synchrotron X-ray diffraction at high pressure up to ～10 GPa. The volume of the cubic unit cell has been measured as a function of pressure. The experimental data fitted by the Murnaghan equation-of-state (EOS) gave K_{0}=19.5\pm 0.9\,\hbox{GPa} and K_{0}^{\prime}=16.4\pm 0.6 . The pressure coefficients and Grüneisen parameters of the intramolecular phonon modes of C 84 have been determined and compared with those of other fullerenes. The data obtained do not show any phase transition and the pressure behavior of the material is entirely reversible in the pressure region investigated.     

Pressure effect on the electromotive force of the type R thermocouple

ABSTRACT

The pressure effect on the electromotive force (EMF) of a Pt13Rh–Pt (type R) thermocouple was examined to determine the temperature measurement accuracy of solid pressure medium apparatuses in high-pressure experiments. Single-wire EMFs were measured up to pressure of 13?GPa and temperature of 1173?K with a Kawai-type multi-anvil apparatus for Pt13Rh and Pt based on the single-wire method. The pressure conditions along the wires were evaluated by in situ X-ray diffraction using synchrotron X-ray radiation. The pressure effect of the Seebeck coefficients of Pt13Rh and Pt were determined by the analysis of the single-wire EMFs and pressure–temperature profiles along the wires and was virtually consistent with those determined in previous studies at lower pressures and temperatures. For type R thermocouple, the difference between the nominal and real temperatures was determined to be as large as –75?K at 13?GPa and 873?K.     

Compression behavior of NiO in a diamond cell

Abstract

The compressibility of synthetic polycrystalline NiO was studied in a diamond anvil cell at room temperature utilizing two different X-ray sources. A standard film with a conventional X-ray source and the energy dispersive X-ray diffraction (EDXRD) method with synchrotron radiation were used for data acquisition. In the film method, the sample was compressed in a 4:1 methanol to ethanol solution up to 7 GPa with ruby fluorescence as a pressure calibrant. In the energy dispersive method, NiO powder was mixed with gold and compressed in two different conditions: gasketed and ungasketed up to 30 GPa. In the gasketed run, water was used as the pressure transmitting medium while gold was used as pressure calibrant in both runs.

Hydrostatic compression of NiO in both diffraction methods yields a bulk modulus (K _o) of 187 ± 7 GPa assuming K′ = 4. The compression of gasketed NiO of the synchrotron experiment, however, showed an obvious break at pressure exceeding 4 GPa due to the loss of hydrostaticity. NiO in a nonhydrostatic condition behaves with less compressibility than the hydrostatic results with a nominal K _o of 238 ± 10 GPa. The lower compressibility of NiO in synchrotron runs is attributed to the uniaxial loading effect which was more easily detected by the EDXRD geometry. The discrepancy in the bulk modulus can be attributed to the contrast in the shear strength between the sample and pressure medium and the Poisson effect of the sample under uniaxial loading.     

用同步辐射和高温DAC装置研究Fe2SiO4的γ→α相变动力学过程

 通过对γ-Fe₂SiO₄样品在常压和温度分别为894、926、948和993 K时转变为α相的实验的具体描述，介绍了采用同步辐射和高温DAC装置及高压高温原位X光衍射技术进行高温高压相变动力学过程研究的一般方法。     

Structure changes in vacancy-rich titanium monoxide at high pressures and high temperatures

Abstract

Powdered samples of TiO_0.82, TiO_1.04 and TiO_1.25 having the cubic rocksalt-type structure with high concentration of vacancies randomly distributed were held at combined conditions of high pressure, ranging from 1 GPa to 8 GPa, and high temperature, ranging from 973 K to 1173 K, and structural changes occurring were investigated by synchrotron radiation diffraction at the conditions and by conventional X-ray diffraction after the samples were brought back to ambient condition. Pressure has been shown to suppress formation of ordered arrangements of vacancies in all the samples and lead to precipitation of a hexagonal δ-Ti₃O₂ in TiO_0.82 and TiO_1.04 and precipitation of a corundum-type Ti₂O₃ in TiO_1.25. Irreversible change in the lattice parameter of the remaining rocksalt-type structure has been observed which is due to partial annihilation of vacancies under pressure.     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.     

In situ X-ray observation of phase transformation in Fe2O3 at high pressures and high temperatures

A laser-heated sample in a diamond anvil cell and synchrotron X-ray radiation was used to carry out structural characterization of the phase transformation of Fe₂O₃ at high pressures (30-96 GPa) and high temperature. The Rh₂O₃(II) (or orthorhombic perovskite) structure transforms to a new phase, which exhibits X-ray diffraction data that are indicative of a CaIrO₃-type structure. The CaIrO₃-type structure exhibited an orthorhombic symmetry (space group: Cmcm) that was stable at temperatures of 1200-2800 K and pressure of 96 GPa (the highest pressure used). Unambiguous assignment of such a structure requires experimental evidence for the presence of two Fe species. Based on the equation of state of gold, the phase boundary of the CaIrO₃-type phase transformation was P (GPa)=59+0.0022×(T−1200) (K).     

Equation of state and thermal stability of Al3BC

The lattice parameters of Al₃BC have been measured up to 5 GPa at ambient temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit to the experimental p-V data using Birch-Murnaghan equation of state gives values of the Al₃BC bulk modulus 116(4) GPa and its first pressure derivative 9(2). In the 1.6-4.8 GPa range at temperatures above 1700 K Al₃BC undergoes incongruent melting that results in the formation of Al₃BC₃, AlB₂ and liquid aluminum.     

高压下LiB3O5晶体-非晶体相变的研究

 采用X射线粉末衍射和同步辐射能散X射线衍射技术，对LiB₃O₅（LBO）晶体进行了压致非晶化相变的研究，压力达45.1 GPa。实验结果表明，LBO存在两个中间相变，压力分别为5 GPa和15.8 GPa。而当压力增加到25~31.6 GPa时，LBO出现非晶态，这个非晶化相变是不可逆的。     

Equation of state of turbostratic boron nitride

Abstract

The lattice parameters of turbostratic boron nitride (tBN) have been measured to 6.1 GPa at room temperature using energy-dispersive powder diffraction with synchrotron radiation. A fit to the experimental p-V data using Birch-Murnaghan equation of state gives values of the tBN bulk modulus 17.5(8) GPa and its pressure derivative ll.4(5). These values point to significantly higher compressibility of turbostratic BN as compared to three-dimensionally ordered graphite-like hexagonal and rhombohedral boron nitride.     

Thermoelastic behaviour of hexagonal graphite-like boron nitride

Abstract

A synchrotron X-ray diffraction study on hexagonal graphite-like boron nitride (h-BN) was performed under high pressures and temperatures. From the measured P-V-Trelation for h-BN (with a three-dimensional ordering parameter P₃ = 0.9) in the temperature range from 298 to 1273 K and up to 6.7 GPa, the thermoelastic parameters are derived by fitting a modified high temperature Birch-Murnaghan equation of state. The results are: bulk modulus B₀[GPa] = 27.6-0.0081(T[K]-298) and its pressure derivative B¹ = 10.5 + 0.0016(T [K] - 298). These values are for samples with P₃ = 0.9 and are quite different for samples with different values of the order parameter. This parameter is shown to have a leading role in the determination of the thermoelastic properties of h-BN, which explains and reconciles the differences between previous results.     

Equation of state of graphite-like BC

The compressibility of turbostratic boron-substituted graphite (t-BC) was measured up to 12 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit to the experimental p-V data using Birch-Murnaghan equation of state gives values of the t-BC bulk modulus 23(2) GPa and its pressure derivative 8.0(6). These values point to a higher compressibility of t-BC as compared to turbostratic graphite.