Energy Dispersive X-ray Powder Diffraction Research Articles

High pressure and high temperature investigations on intermetallic compounds using energy-dispersive X-ray powder diffraction

The intermetallic compounds Mg2Si, MoSi2, WSi2, CrSi2, NbSi2, Ti5Si3, TiSi2, ZrSi2 and Al3Nb were investigated in a pressure range up to 70 kbar and a temperature range up to 1500°C in order to determine previously unknown elastic constants and possible phase transformations. The experiments were carried out using the multi-anvil-X-ray apparatus MAX-80 and synchrotron radiation at HASYLAB. A complete phase transition of cubic Mg2Si to a hexagonal phase was observed at 70 kbar and 450°C. The high-pressure/high-temperature phase was quenched and recovered for further investigations. All the other compounds remained stable in the mentioned pressure and temperature range. The bulk moduli covered the range from 436 (10) (Mg2Si) to 3350 (190) (WSi2) kbar.

Kinetic studies of the crystallisation of coesite using synchrotron radiation

For energy-dispersive X-ray powder diffraction under high pressure and high temperature a multi-anvil-X-ray apparatus (MAX 80) has been installed at HASYLAB. This instrument allows to measure diffraction data at very short intervals and thus study time dependent phenomena. As an example we have investigated the kinetics of the α-quartz-coesite phase transformation. The kinetics was determined by in situ experiments between temperatures of 870 and 1170 K and in a pressure range of 30–50 kbar. The structural changes were directly imaged using synchrotron radiation. The experimental results yield the rate constant Kc for the growth of coesite from quartz as a starting material under dry conditions. For the transition three Kc values from 6.0 × 104to 2.2 × 104 s−1 for different p/t-ranges were determined.

Phase relationships in mercury telluride under high temperature and pressure

The p, T phase diagram od mercury telluride has been investigated up to 5.4 GPa and 1200K for the first time, by the energy dispersive powder X-ray diffraction method. Our results and analysis reconcile the apparent contradictions which appear in previous reports using differential thermal analysis. The proposed p, T diagram shows a liquid-liquid phase transition and a negative slope for the cinnabar-liquid melting curve.

Ferroelastic phase transition in SrAl2Si2O8 feldspar at elevated pressure

AbstractSynthetic end-member strontium feldspar (SrAl2Si2O8) has been studied in a diamond-anvil cell at elevated pressures up to 10.3 GPa using energy-dispersive X-ray powder diffraction. Cell parameters have been refined between ambient pressure and 6 GPa. SrAl2Si2O8 undergoes a ferroelastic phase transition from the ambient pressure monoclinic space group I2/c to the high-pressure triclinic space group I at 3.2 ± 0.4 GPa. The transition appears to be first-order and the ferroelastic and co-elastic components of the spontaneous strain tensor have been calculated.

X-Ray Diffraction Study of BaNd2CuO5 at High Pressures

Abstract The volume compression of BaNd2CuO5 has been measured to 8.7 GPa utilizing a diamond anvil high pressure eel and energy dispersive x-ray powder diffraction. The pressure dependence of the volume of the tetragonal unit cell is linear and follows the equation V = -1.6I5P + 260.622 determined by a least-squares fit of the data. Compression of the unit cell parameters of BaNd2CuO5 exhibit isotropic behavior within our experimental error. From the observed data, the bulk modulus of BaNd2CuO5 is determined to be 161 ± 6 GPa and the Young's modulus is estimated to be 193 ± 6 GPa. No evidence of a pressure-induced phase transformation in BaNd2CuO5 was found in the pressure range studied.

Real-time in situ X-ray diffraction of intercalation reactions

Real-time in situ energy dispersive X-ray powder diffraction techniques have been developed to monitor the intercalation of a guest species into a solid host; the time dependence of the extent of reaction (α) for the intercalation of cobaltocene CoCp2(Cp =η5-C5H5) by SnS2 has been experimentally determined and is consistent with a model involving nucleation followed by two-dimensional diffusion of the metallocene molecules into the host lattice.

Second-order phase transition in PbO and SnO at high pressure: Implications for the litharge-massicot phase transformation.

We have studied the structural behavior of PbO at high pressure by powder neturon diffraction in a McWhan cell, and by energy-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell. A phase (γ-PbO) occurs at room temperature between ∼0.7 and ∼2.5 GPa pressure, between the stability fields of litharge (<0.7 GPa) and massicot (2.5-10.1 GPa). There is presumably a triple point in the system at a few kbar and 0-200 °C. The phase is related to litharge by a reversible second-order transition. We infer that this is associated with the collapse of the eu acoustic mode. Unit-cell data at 1.6 GPa are Pm21n, a=4.027(3) , b=3.950(3) , c=4.767(4) , and Z=2. The pressure evolution of the spontaneous strain follows a simple Landau model. There are four distinct solid-state transformation paths between litharge and massicot that maintain the known topotactic relationship between the phases, maintain the translational symmetry common to both, and make use of continuous transitions between group-subgroup related structural intermediates. Both the γ phase and the modulated low-temperature phase of PbO are closely related to one step on one of these paths. Although there is evidence to suggest that the intermediate states do have a transient existence, several paths appear to be utilized. A transition to a γ-like phase also occurs in SnO, at 2.5 GPa, although there is no evidence of a massicotlike polymorph of this compound. The orthorhombic phase is stable to at least 7.5 GPa.

Hydrogen-bond formation in sodium and potassium hydrosulfides at high pressure.

Sodium and potassium hydrosulfide were investigated at high pressure using a combination of infrared and Raman spectroscopy and energy dispersive x-ray powder diffraction. NaSH was found to undergo an ordering transition below 9 kbar to yield a cell equivalent to that of the low-temperature phase, but reduced in size. A hydrogen-bonded phase was formed above 83 kbar, the diffraction pattern of which could be fitted using a TlI-type structural model. The refined cell parameters at 130 kbar are a=7.362(60) \AA{}, b=3.698(38) \AA{}, c=5.615(50) \AA{}, \ensuremath{\beta}=104.50(51)\ifmmode^\circ\else\textdegree\fi{}, Z=4. The space group would be P${2}_{1}$/a with bifurcated hydrogen-bonded chains running along b. KSH underwent two transitions on compression at 19 and 23 kbar, respectively. The first transition involved only a small volume change. The diffraction data for this phase could be indexed based on a TlI-type model structure, resulting in the following cell at 20 kbar: a=9.129(41) \AA{}, b=4.181(18) \AA{}, c=6.781(18) \AA{}, \ensuremath{\beta}=106.47(27)\ifmmode^\circ\else\textdegree\fi{}, Z=4. A large decrease in frequency, increase in bandwidth, and strongly negative d\ensuremath{\nu}/dp values for the infrared and Raman active S-H stretching vibrations indicated that strong hydrogen bonds were formed at the 23-kbar transition. A unit cell was obtained from the x-ray powder data with a=7.448(22) \AA{}, b=7.328(26) \AA{}, c=8.110(22) \AA{}, \ensuremath{\beta}=97.46(36)\ifmmode^\circ\else\textdegree\fi{}, Z=8 at 31 kbar. A structure model with a distorted CsCl structure, related to the hydrogen-bonded, low-temperature phase of CsSD containing four-membered, hydrogen-bonded ${\mathrm{S}}_{4}$${\mathrm{D}}_{4}^{4\mathrm{\ensuremath{-}}}$ units, is proposed. These and other recently reported results indicate that TlI-type structures are more common than previously believed and are, in fact, the favored intermediates between the NaCl- and CsCl-based structures for systems with nonspherical atoms or ions.

The phase stability of the alkali-metal vanadates at high pressures studied by synchrotron X-ray powder diffraction and infrared spectroscopy

The materials AVO3 (A identical to K, Rb, Cs) have been studied at high pressure by energy-dispersive X-ray powder diffraction (EDXRPD) using synchrotron radiation. KVO3 has also been investigated at high pressure by infrared spectroscopy. CsVO3 shows no phase change to 130 kbar. KVO3 exhibits a sluggish phase change from 27 to 48 kbar. A structural model for the high-pressure phase is proposed, in which the vanadate chains slip relative to each other along their axes, generating a monoclinic cell (space group P2/b11, in a non-standard setting). RbVO3 shows a phase change at 40 kbar, indicated by a break in slope for plots of each of the lattice parameters against pressure. No other changes were evident in the EDXRPD pattern at this discontinuity. This is accounted for by a model in which the chains move relative to each other in the a-direction, maintaining the orthorhombic symmetry of the cell.

The effect of high pressure upon the valence transition in EuPd2Si2

The so-called 'valence transition' in EuPd2Si2 has been observed using energy-dispersive synchrotron X-ray powder diffraction at elevated pressures and is centred at 8 kbar. The estimated increase in valence at the transition is 0.2, and is associated with crossover of 4f and 6d/s bands.

A high-pressure synchrotron X-ray powder diffraction study of K2SnBr6

Using synchrotron energy-dispersive X-ray powder diffraction (EDXRPD) the authors have shown that K2SnBr6 transforms at pressures of 14 kbar and above from the parent monoclinic cell to a cubic structure of symmetry Fm3m, which is stable to 120 kbar. A different phase appears to exist above 120 kbar.

The pressure-induced disordering of ammonium metavanadate

The structural behaviour of ammonium metavanadate, NH4VO3, under high pressures has been studied to 65 kbar using energy-dispersive synchrotron X-ray powder diffraction, and to 106 kbar by FTIR methods, in a diamond anvil cell. A reversible crystalline-to-disordered phase transition was found at 49 kbar. The transition is thought to be due to relative slipping of the vanadate chains parallel to their axes, the product being disordered, possibly amorphous, rather than crystalline because the process is hindered and the solid prevented from attaining periodicity by hydrogen bonding to ammonium ions.

Time-Resolved Observations of Solid Reactions and Structure Transitions using a PSD, An SSD and Computer Aided Measurement and Control

AbstractFor time-resolved diffraction studies under high temperature and/or high pressure, we designed a new diffractometer incorporating a curved position sensitive detector and tested the rapid intensity measurement and the resolution of the angular dispersive diffraction. The diffraction profiles were compared with those from energy dispersive diffraction. Rapid intensity collection has been carried out with a CAMAC module. A curved position sensitive detector with a 120° working angular region was installed on the off-centered four circle diffractometer. Only a few seconds were needed to obtain a whole powder diffraction because of the extremely high counting efficiency due to the streamer mode.Time-resolved observations of the dehydration reactions of Mg(OH)2 and Ca(OH)2, have been undertaken by angular dispersive and energy dispersive x-ray powder diffraction at high temperature. The in situ observations of the dehydration process provide kinetic information, such as the reaction rate, the apparent activation energy and the dehydration mechanism.

Effects of pressure on the thermal decomposition kinetics, chemical reactivity and phase behavior of RDX

The effedts of pressure on the thermal decomposition kinetics, chemical reactivity, and phase behavior of RDX have been studied by a combination of measurement techniques in conjunction with a high-pressure diamond anvil cell. These techniques include (1) Fourier transform infrared (FTIR) spectroscopy for kinetic measurements and phase identification, (2) energy dispersive x-ray powder diffraction for identification of the observed polymorphic forms and also compression measurements, and (3) optical polarizing microscopy for visual detection and confirmation of phase transformations and determinations of transition pressures. The ruby method of pressure measurement was used in all methods employed. Studies were generally limited to the region where decomposition rates could be measured within reasonable laboratory time, i.e., below 10 GPa and 573 K. The P-T phase diagram for RDX was determined to 573 K and 7.0 GPa, delineating the stability fields of three solid phases, α, β, and γ, and the liquids. The α and β phases of RDX were found to thermally decompose, while the γ phase transformed to either α or β before reaching decomposition temperatures. The decomposition rate of α phase was found to increase with increasing pressure, suggesting a bimolecular-type mechanism. An activation energy of 51 kcal/mol and activation volume of −5.6 cm 3/mol were calculated for the thermal decomposition reaction of α RDX.

X-ray diffraction study of Hafnia under high pressure using synchrotron radiation

Hafnia, HfO 2, has been studied to 20 GPa at ambient temperature using synchrotron energy-dispersive X-ray powder diffraction methods. Traces of the initial monoclinic (baddeleyite) phase could still be identified at 20 GPa, but a phase transition begins at 2.6 GPa to HfO 2 II, the lines of which can be indexed on an orthorhombic cell ( Pbcm, Z = 4). The relative intensities of the (111) and (−111) monoclinic reflections suggests that a further phase change may begin near 10 GPa but work at high temperatures is needed to establish this. The bulk compressibility of hafnia is 1.2 × 10 −4kbar −1. A Rietveld analysis of the baddeleyite phase gave results in close agreement with earlier work but showed the sample to be internally strained.

Effects of pressure on the structure and the thermal decomposition kinetics of RDX

Abstract High pressure and temperature structural changes for RDX were investigated to 7.0 GPa and 570 K in a diamond anvil cell apparatus using FTIR absorption, optical microscopy, and energy-dispersive powder x-ray diffraction techniques. Three distinct solid phases were observed. The effects of pressure on the thermal decomposition kinetics as a function of RDX pressure were investigated using an infrared absorption technique. Solid phase I was found to have a pressure enhanced reaction rate with an energy and volume of activation of 51 Kcal/mole and -5.6 cc/mole respectively. Solid II was not observed to react and the observed reaction rate of Solid III decreased with increasing pressure.

ENDIX – a program to simulate energy-dispersive X-ray powder diffraction diagrams

ENDIX – a program to simulate energy-dispersive X-ray powder diffraction diagrams