guoyin shen- high pressure melting studies in diamond cells

8/3/2019 Guoyin Shen- High Pressure Melting Studies in Diamond Cells

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High Pressure Melting Studies in Diamond Cells

Guoyin Shen

Consortium for Advanced Radiation Sources, University of Chicago, Chicago, USA; [email protected]

Data from high pressure melting experiments provide two important pieces of information

to Earth’s convention models. One is the depth dependence of the viscosity, because it scales

with the homologous temperature (the ratio of the temperature to the melting temperature)

[Weertman and Weertman, 1975]. The other is the temperature profile of the Earth’s interior

[see reviews by Williams, 1998 and Boehler, 2000]. The high pressure melting of iron is

probably the most extensively studied subject, because it provides a first order constraint onthe temperature of the core. Estimates rely on the assumption that the boundary between the

solid inner core and the liquid outer core is at the melting temperature of the core material.The temperature at the core mantle boundary (CMB) may be constrained by three primary

parameters: the temperature at the 670 km discontinuity; the temperature at the inner core-

outer core boundary (IOB); and the adiabatic temperature gradients within the Earth’s lower

mantle and the outer core. Information of these primary parameters could be obtained

through studies on phase transitions responsible for the 670-km discontinuity, high pressure

melting of iron and iron alloys, and thermodynamic properties of major minerals in the lower

mantle and the outer core.

In recent years, the generation and measurement of simultaneous high pressures and hightemperatures has undergone rapid development with the diamond anvil cell (DAC) technique

[e.g., Mao and Hemley, 1996; Boehler, 2000; Shen et al., 2001a]. The amount of available

data on melting behavior at high pressure is increasing. However, considerable controversy

has surrounded the determination of the melting temperature, e.g., of iron at P > 30 GPa. The

melting determination in DAC experiments has been a subject of many articles [ Duba, 1994;

Jeanloz and Kavner , 1996; Lazor and Saxena, 1996]. Recently, Boehler [2000] pointed out

the emerging convergence of static data on high pressure melting of iron, and stated that “it

may be difficult to significantly expand the pressure range and accuracy of melting

experiments in the diamond anvil cell”. Here I argue that we are actually at a breakthrough of

a significant increase in accuracy of melting experiments, by examining a critical issue: themelting criteria. In fact, the recent data from synchrotron experiments [ Hemley and Mao,

2001; Ma et al., 2001] show that the melting temperature of iron is about 700 K higher than

that by Boehler [2000] at 105 GPa and the extrapolated melting curve yields 1000 K difference in melting temperature at the IOB (Fig. 1).

Melting is thermodynamically defined as equilibrium between a solid and a liquid. When

materials melt, their physical properties, such as density, viscosity, absorption properties, and

electrical resistance, involve a sudden change. Such property changes are characteristic for a

first order phase transition and are often used for recognition of melting. Different from other

first order phase transitions, melting is characterized by the loss of long-range order and

resistance to shear . To definitively identify melting, one or both of these two characteristicsshould be documented. Jeanloz and Kavner [1996] reviewed five types of melting criteria in



laser heated DAC experiments, namely fluid flow, glass formation, quench texture, change in

sample properties, and temperature-verses-laser-power correlation, and concluded that the

most reliable criteria for determining melting inside the laser heated DAC are fluid flow and

quenched glass formation. It is true that fluid flow observation is a good measure of the loss

of resistance to shear. Therefore it has been considered as one of the best criteria and widely

used by almost all groups in the world. [ Boehler , 2000; Jephcoat and Besedin, 1996; Saxenaet al., 1994; Shen et al., 1993; Sweeney and Heinz, 1998]. Shen et al. [1993] pointed out thatvisual observation (fluid flow) is less obvious as pressure increases; above 40 GPa (where the

lack of agreement starts) there exists a large temperature gap of a few hundreds degrees

between occasional small movement (not fluid flow) and fluid-like motion, making it difficult

to unambiguously define the onset of melting. The subjective nature of visual observation

may account for the inconsistent results in literature. Another way to document the loss of

rigidity is the use of Brillouin spectroscopy, which is still under development. Synchrotron x-

ray diffraction has been combined with laser heated DAC and used for melting studies to

document the loss of long range order upon melting [ Ma et al., 2001; Shen et al., 1998]. Theappearance of diffraction peaks at a certain pressure-temperature condition clearly indicates

the presence of a crystalline phase. However, a simple loss of diffraction peaks is notnecessarily indicative of melting due to insufficient statistics arising from possible crystal

growth and small x-ray beam size. Shen et al. [1998] cautioned that the conclusive

identification of melts is still limited by the energy dispersive x-ray diffraction technique.

Recently, an area detector with a monochromatic x-ray beam was successfully used for

melting determination in a diamond anvil cell. Melting at high pressure was identified by the

appearance of diffuse scattering from the melt with the simultaneous loss of crystalline

diffraction signals [Shen et al., 2001b]. The new method relies on positive signals (diffuse

scattering) together with a measure of the characteristic property of melting (loss of longrange order), providing an objective way of signifying melting and an important extension of

the visual observation method (Fig. 2). The observation of the diffuse scattering has also been

applied to studies of structure factors and molar volumes of melts at high pressures and high

temperatures [Shen et al., 2001c; Shen et al., 2001d]. For metals, melting is found to be

reversible (Fig. 2e), so that the same sample can be used again in a single run at different

pressures. For glass forming materials, cautions should be taken by paying attentions to the

experimental pressure-temperature paths.

Efforts are currently being made on melting studies of Earth materials utilizing an area

detector and micro-x-ray beam to expand the pressure with the double sided laser heated DAC.

Unambiguous phase identification requires the x-ray probe, heating laser beams, and samplesto be aligned to the same point. It is conceptually simple, but rather challenging in practice.

Under intense heating for melting studies, maintaining the alignment for the entire system

requires large efforts. The possible sample diffusion at high temperatures close to meltingfurther complicates the x-ray analysis. Diffuse wings of the x-ray beam may pick up

additional diffraction peaks from materials outside the area of interest. All these difficulties

need to be overcome for high accuracy of melting studies. The progress is going on and it is

believed that accurate data on high pressure melting with definite melting recognition will

emerge in the near future.

In recent years, nuclear resonant x-ray scattering techniques that utilize synchrotron

radiation have made great progresses in the study of vibrational and magnetic properties of condensed matter under extreme conditions [ Burkel , 2000; Mao et al., 2001]. In particular for



(e)

Fig. 2 (Color) X-ray scattering images and their integrated pattern at the onset of melting of

indium at about 2 GPa Two sharp rings are the diffraction of medium material (NaCl). (a)

Crystalline indium shows a spotty pattern at 519(1) K. (b) The indium diffraction spots fade out

and diffuse scattering starts to appear at 526(1). (c) A clear homogeneous diffuse ring of the

melt can be seen at 531(1) K. (d) Integrated patterns by FIT2D for images from (a) to (c). A broad diffuse band can be clearly seen in the vicinity of indium (101). (e) Intensity change of the

diffuse scattering band at the onset of melting. Solid circles are the data upon heating; whilecrosses are those on cooling. A sharp change in intensity occurs in the same temperature region

upon both heating and cooling, indicating the melting is reversible. Lines are freehand fits for

eyes. (after Shen et al., 2001b).

Acknowledgments: This work is supported by NSF-EAR 00011498. The GSECARS sector is

supported by the NSF, DOE and the W. M. Keck Foundation.

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