Internal Pressure Standard Research Articles

Ultrapressure equation of state of cerium metal to 208 GPa

We report static pressure compression of cerium metal to 208 GPa (volume compression V/V0=0.37) in a diamond anvil cell at room temperature. Cerium is unique in the 4f elements because of proximity of the f shell to the Fermi energy and related phase transformations induced by pressure. The energy-dispersive x-ray diffraction studies were carried out on cerium metal to 208 GPa using a synchrotron x-ray source and an internal copper pressure standard. A collapsed body centered tetragonal phase is found to be stable to the highest pressure with axial ratio remarkably constant at 1.680±0.006 in the 90–208 GPa pressure range in excellent agreement with theory. Cerium is thus isostructural and isoelectronic with 5f-band metal thorium at ultrapressures. We present equation of state parameters, which describe the compression of cerium to ultrapressures.

高圧地球科学 Ｘ線その場観察法によるマントル鉱物相転移境界の決定

In Situ X-ray diffraction technique has been successfully used to determine the phase transition pressures of mantle minerals at high pressure and high temperature. This technique has an advantage in accurate estimation of pressure using internal pressure standards such as NaCl, Au, and MgO, as well as the simultaneous identification of the phases present at high temperature and high pressure. This paper reviews these studies which attempted to determine the phase boundaries based on in situ X-ray diffraction measurements, including our recent results obtained at the new synchrotron facility, Spring-8. Some technical problems inherent to this technique are discussed on the basis of these currently available experimental data.

Pressure-Induced Amorphous Transition of Li2Ge7O15 Crystal

Pressure-induced amorphous transition of Li2Ge7O15 (LGO) crystal has been investigated by using in situ high pressure energy dispersive x-ray diffraction with synchrotron radiation at the pressure up to 21.6 GPa. Diamond anvils were used to generate high pressure. Pressure was measured in terms of the diffraction of Pt as an internal pressure standard. The crystal structure of LGO is an orthorhombic lattice at atmospheric pressure. According to the high pressure x-ray diffraction results, new diffraction lines begin to appear and LGO undergoes an intermediate phase transition at the pressure of 12.9 GPa. But, the diffraction lines disappear wholly and LGO becomes an amorphous phase at the pressure of above 14.8 GPa. This amorphous transition is irreversible.

Structural Phase Transitions of Iodine at High Pressure

The compression and polymorphic phase transitions of iodine have been studied by using x-ray diffraction with a diamond anvil cell at high pressure up to 77 GPa. Considering that iodine is reactive with iron in air, a corrosion-resisting and high-strength gasket was put between the diamonds, thus the sample can be mounted at atmospheric pressure and room temperature. Pressure was measured in terms of the ruby fluorescence and the molybdenum diffraction as an internal pressure standard. According to the x-ray diffraction results, the iodine undergoes the following three structural phase transitions with pressure increasing: (1) At 21 GPa, from base centred orthorhombic phase (phase I) to body centred orthorhombic phase (phase II). A 2% change in volume occurs. (2) At 44 GPa, from phase II to body centred tetragonal phase (phase III). This is a second order phase transition. (3) At 71 GPa, from phase III to fcc phase (phase IV). The volume decreases by 3%. At this time, the volume is 45% of the original one at atmospheric pressure.

Experimental determination of thermal expansivity of several alkali halides at high pressures

Unit-cell volumes of four alkali halides, LiF, NaF, KF and CSCl, were measured to 90 kbar and 800°C using X-ray powder diffraction techniques. NaCl was used as an internal pressure standard. Experimental results were analyzed based on Decker's equation of state for NaCl and thermal expansívities of these four materials were determined as a function of pressure. Volume dependence of thermal expansivity is different for the NaCl and CsCl structures. Comparisons of the present results with theoretical calculations by Birch and Anderson are presented.

Direct determination of coesite- stishovite transition by in-situ X-ray measurements

The coesite—stishovite transition was determined over the temperature range 500–1100° C by means of in-situ X-ray measurements with NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power X-ray diffraction system were used for this study. Based on Decker's new pressure scale (1971), the coesite—stishovite transformation curve was represented by the linear equation P(kbar) = (80 ± 2) + (0.011 ± 0.003)(° C). Both the transition pressure at 1000° C and the slope dP/ dT obtained in the present investigation are a little smaller than the previous determinations by the quenching method. Comparison with the thermodynamic data suggests that some uncertainty is still involved in the determination of the entropy of stishovite.

Pressure fixed points between 100 and 200 kbar based on the compression of NaCl

Transformation pressures in three materials, Pb, ZnS, and GaAs, were measured by means of an x-ray diffraction technique using NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power x-ray diffraction system were used for this study. Based on Decker′s equation of state for NaCl, transition pressures in the increasing and decreasing pressure cycle were determined as follows: for Pb 142±3 and 111±2 kbar, for ZnS 162±4 and 110–100 kbar, and for GaAs 193±5 and 115–100 kbar. These values and the high Ba transformation pressure reported from this laboratory (126±2 and 111±2 kbar in the increasing and decreasing pressure cycle) form a set of pressure fixed points between 100 and 200 kbar at room temperature.

Isothermal compression of grossular garnets to 250 Kbar and the effect of calcium on the bulk modulus

The effect of pressure on the lattice parameter, and hence on the volume, of two samples of grossularite garnet (Ca3Al2Si3O12) has been measured at room temperature up to 250 kbar with the use of a diamond anvil highpressure X ray diffraction camera. The pressure to which the sample was subjected was measured by means of the NaCl internal pressure standard. A least squares fit of the pressure-volume to the Birch-Murnaghan equation yields a bulk modulus value of KO = 1.74 ± 0.04 Mbar when a (∂K/∂P)∣P=0 value of 4.25 (Halleck, 1973) is used. This KO value is in agreement with the value of 1.704 Mbar obtained by Halleck [1973], using ultrasonic techniques. A comparison of the bulk moduli of grossularite, pyrope, and almandite shows that KO∝ Vo−1 for Ca and Mg garnets as expected from static lattice models. On the other hand, KO∝ Vo−5 for the Mg-Fe garnet solid solutions. This behavior has not been explained by available theories.

Direct determination of the quartz-coesite transition by in situ X-ray measurements

The quartz-coesite transition has been determined over the temperature range 600–1100° C by in Situ X-ray measurements with NaCl as internal pressure standard. An internally heated high-pressure X-ray apparatus (“Belt”-type) was used which is based on the principle developed by Freud and Sclar (1969). The obtained quartz-coesite equilibrium line may be represented by the equation P=31±1+0.0075 T where P is in kb and T in ° C.

Compressibilities of some layer dichalcogenides

Mean compressibilities, perpendicular and parallel to the layer planes, have been measured for MoS2, NbSe2 and HfS2. Experiments were carried out in an opposed diamond anvil apparatus whereby changes of lattice spacing with pressure could be observed. NaCl was used as an internal pressure standard.