High-pressure diffraction

TABLE3.3: Masses of the Sb2Te3−xSex(x=0, 0.6, 1.2, 1.8, 3) samples

used for low-temperature heat capacity measurements.

Sample Sb2Te3 Sb2Se0.6Te2.4 Sb2Se1.2Te1.8 Sb2Se1.8Te1.2 Sb2Se3

Mass / mg 9.0(1) 9.7(1) 9.1(1) 10.5(1) 11.0(1)

sample and addenda measurements, were performed with two repetitions per tem-perature step. A linear temtem-perature increase and a heating rate of 2 % based on the actual temperature were chosen. In order to determine the addenda heat capacity at the temperatures of the sample, a polynomial interpolation between two neighbor-ing data points was performed usneighbor-ing the automatic procedure implemented in the PPMS software.

3.4 High-pressure diffraction

3.4.1 Diamond anvil cell

Mechanical pressure, p = F/A, is defined as the ratio between a force, F, applied to an area, A. Thus, pressure can be exerted by minimization of the area and/or a maximization of the applied force.

Figure 3.2 shows a schematic construction of a diamond anvil cell (denoted as "DAC"

in the following) [76]. Diamonds are suitable anvil materials due to their extreme

FIGURE 3.2: Right: Scheme of diamond anvil cell [76]. Left: En-largement of the sample chamber. The sample chamber contains the sample, a pressure transmitting medium and a pressure standard, e.

g. ruby.

hardness [77, 78, 79, 80, 81]. Moreover, diamonds are transparent for a part of the electromagnetic spectrum and hence, in-situ studies by diffraction or spectroscopic techniques are feasible [82, 76]. In order to exert pressures up to the megabar regime

36 Chapter 3. Experimental part the diamond anvils are equipped with a culet. The size of the culet determines the reachable pressure. The pressure is increased or released by applying a moderate force via moving the anvils either by screws [83, 76] or by a membrane [83, 84]. A metal foil called gasket is placed between the opposing diamonds [82, 76]. A hole in the gasket serves as a sample chamber [82, 76]. The resulting sample volumes are typically in the order of 10⁴-10⁶µm³. The sample chamber contains the sample, a pressure transmitting medium and a ruby chip [82, 76]. The pressure transmitting medium is used to transmit the applied unaxial pressure either quasi-hydrostatically or non-hydrostatically to the sample. For a quasi-hydrostatic pressure transmission usually noble gases e. g. Ne, He, .. or liquids, e. g. a methanol/ethanol mixture are used [82, 76]. For a non-hydrostatic pressure transmission e. g. salts like NaCl are used [82, 76]. In this case, the pressure is not transmitted homogeneously to the sample and hence, a strain is applied [82, 76]. The ruby chip deals as a pressure standard and allows the estimation of the pressure on the sample [82, 76, 85, 86].

3.4.2 Ruby fluorescence method

The estimation of pressure in a DAC is possible either from diffraction [87, 88, 89, 90]

or through spectroscopic methods [85, 86]. In the spectroscopic estimation, pressure-induced shifts of either Raman active modes (e. g. cubic BN [91],¹²C [91]) or fluo-rescence lines (e. g. ruby [85, 86], SrB₄O₇:Sm²⁺ [92]) are considered. Based on the pressure-induced shift of the R1 fluorescence line of ruby a method for the pressure estimation in DACs was developed [85] and later improved [86]. Equation (3.1) de-scribes the relation between the pressure and the wavelength of the R1 fluorescence line of ruby, whereλ₀is the wavelength determined under ambient conditions andλrepresents the wavelength at an elevated pressure. The empirical parameters A and B are A=19.04 MPa andB=7.665 [86].

3.4.3 The Extreme Conditions Beamline P02.2 at PETRA III, DESY

All high-pressure diffraction experiments reported in this study have been performed at the extreme conditions beamline P02.2 at PETRA III, DESY. Figure 3.3 shows a sketch of the beamline. A detailed description of the technical setup and the in-frastructure of the beamline is given in [93]. Therefore, in this chapter only a brief overview of the beamline is given.

After the incident beam passes the undulator and a series of slits, it is monochrom-atized by a silicon double crystal monochromator. The monochrommonochrom-atized beam is focused by Al and Be compound refractive lenses (CRL) to a spot size of 8×3µm². High-pressure experiments were performed at the general purpose sample stage (figure 3.3).

3.4. High-pressure diffraction 37

FIGURE3.3: Setup of the Extreme Conditions Beamline P02.2 at PETRA III (adapted from [93]). Red circle: Sample stage.

An online ruby system allows an in-situ determination of the pressure. Prior to the measurement the DAC was carefully centered [94] and during exposure the DAC was rotated along theω-axis by a few degrees to gain better statistics.

3.4.4 High-pressure powder diffraction

All measurements were performed at a wavelength of 0.2905 Å (beam energy of 42.7 keV). The powdered samples were loosely compressed between two glass plates and then loaded together with small ruby chips in BX90 diamond anvil cells equipped with a membrane to drive the pressure. Rhenium gaskets, pre-indented to a thick-ness of 80µm and with a hole of 150µm drilled by electro-corrosion, were used.

For the high-pressure experiments on the GeSe_xTe₁−x (x=0, 0.2, 0.5, 0.75, 1) mixed crystals Neon was used as a pressure-transmitting medium. The measurements on Sb₂Te₃ were performed at non-hydrostratic conditions. The cells were placed at a distance of 500 mm (exception GeTe: 450 mm) to the Perkin Elmer detector (Type XRD1621). The distance was calibrated using a CeO₂standard. Pressure in the mea-surements on the GeSexTe₁−x mixed crystals and Sb2Te3 was increased in steps of approximately 1 GPa. The pressure in the cell was determined before and after each measurement using the ruby luminescence method. Datasets for GeTe and GeSe_0.2Te_0.8were collected up to maximum pressures of about 20-21 GPa, while for GeSe_0.5Te_0.5 and GeSe_0.75Te_0.25 the maximum pressure was approximately 25 GPa.

The measurement on Sb2Te3 was performed up to a maximum pressure of about

38 Chapter 3. Experimental part 18 GPa. At all pressure points the diffraction patterns were collected using an expo-sure time of 12 sec and an angular rotation of 3^◦. In order to confirm the homogeneity of pressure-transmission in the cells, 3×3 grids with setoff of 5µm steps inxandy direction were measured. The data were integrated to yield 1-dimensional powder diffraction diagrams using the program Dioptas [95]. Reflections from the diamonds of the cell were masked prior to integration.