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Persistence of the R3m Phase in Powder GeTe at High Pressure and High Temperature Hu Cheng, Junran Zhang, Chuanlong Lin, Xiaodong Li, Feng Peng, Gong Li, and Yanchun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06511 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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The Journal of Physical Chemistry

Persistence of the R3m Phase in Powder GeTe at High Pressure and High Temperature Hu Cheng,1,2 Junran zhang,1,5 Chuanlong Lin,3 Xiaodong Li,1 Feng Peng,4, £ Gong Li2, $ and Yanchun Li,1,§ 1Multidiscipline

Research Center, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, China

2State

Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

3Center

4College

for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing, 100094, China

of Physics and Electronic Information, Luoyang Normal University, Luoyang 471022, People’s Republic of China， 5University

of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT As a phase-change material, rhombohedral GeTe (space group R3m) was believed to transform to the cubic rock-salt phase (B1) at 3–4 GPa, associated with the disappearance of a Peierls distortion. However, using a combination of synchrotron Xray diffraction and theoretical calculations, we found that the R3m phase persists from ambient pressure up to pressures of about 15.8 GPa, in contrast to previous reports. Neither was the B1 phase observed in a heating X-ray powder diffraction experiment. The spurious transformation from R3m to B1 is caused by changes to the compression ratio of lattice parameters in the R3m phase under high pressure/temperature. These findings provide insight into transitions of phase-change materials, relevant to other materials undergoing displacive transitions under high pressure/temperature.

1. INTRODUCTION Pressure-induced structural transitions in matters are of great interest in many disciplines, such as in physics, materials science, and geophysics.1 Generally, chalcogenide compounds undergo one or two phase-transitions with increasing temperature or pressure. Of these, the transitions between NaCl-type (B1) and distorted-

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NaCl structures (cinnabar, Cmcm, Pnma and R3m phases) play a significant role (Figure 1), e.g., transformations from cinnabar to B1,2-4 cinnabar to Cmcm,5-6 B1 to Cmcm7-10 or Pnma, 11-15 and Pnma to Cmcm16, 17 are all observed under high pressure. In many cases, conclusions about these phase transitions and their nature are based on Braggdiffraction studies. In such studies, a phase transition is identified when several diffraction lines abruptly appear or disappear. However, because of the similarity of the B1 and distorted-NaCl structures, it is not so easy to establish the occurrence of this phase transition and ascertain its transition pressure. For example, in the case of HgS, Huang and Ruoff reported a transition pressure of 13 GPa,

4, 18

while Werner et al.

reported that no transitions were detected for pressures up to 24 GPa.2

Figure 1 Transitions between the NaCl-type (B1) structures and some distorted-NaCl structures (cinnabar, Cmcm, Pnma and R3m phases) under high pressure. GeTe, a representative chalcogenide compound, is known to crystallize in the distorted rock-salt rhombohedral structure (R3m phase), with a small sheer relaxation along the pseudo-cubic [111] direction at room temperature. The driving force for the formation of the R3m phase has been the subject of various studies. 19 Furthermore, because of the large difference in optical reflectivity and electrical conductivity between amorphous and crystalline states, GeTe, together with Ge2Sb2Te5, are important phase-change materials (PCMs). They are commercially used in rewritable


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optical memory (CD, DVD) and electronic non-volatile memory technologies, 20-22 and are considered to be the next-generation universal memory type.

23

Despite the rich

nature of the underlying physics and potential applications of such materials, the structural phase transition of GeTe is not comprehensively understood under high pressure or temperature. Previous experiments showed that the R3m phase will undergo structural evolution towards a rock-salt structure at low pressures.

24, 25

On the other

hand, transformation from the low-temperature ferroelectric R3m phase to the paraelectric B1 phase was observed at a Curie temperature of Tc = 705 K.

26

These

studies, based on Bragg-diffraction, suggest that this transition was displacive in nature. The same conclusion was reached using Raman scattering, based upon the observation of phonon-mode softening with temperature. 27 More recently, however, an analysis of the X-ray absorption fine structure (EXAFS) data as a function of temperature revealed that the short and long bonds persist up to and beyond Tc and that an order-disorder transition was more likely than a displacive one, because local distortions remained essentially unchanged with temperature.

28

Subsequently, this transition type was

confirmed using a radial distribution function analysis of X-ray based total scattering. 29

Clearly, the crystallographic polymorphisms of GeTe under high pressure or

temperature are still unclear, requiring further systematic investigation. In this paper, the pressure-induced structural phase transitions in GeTe were investigated using high-resolution synchrotron X-ray diffraction (XRD) under a hydrostatic condition. We found that the R3m phase directly evolves towards an orthorhombic phase at about 15.8 GPa, indicated by the appearance of new diffraction peaks, without the appearance of the B1 phase. Complementary to the high-pressure XRD analyses, high-temperature X-ray powder diffraction experiments were carried out. Similar changes of the R3m phase were observed at different temperatures. We believe that meaningful changes of the a/c ratio lead to pseudomorphism of the transformation from R3m to B1. Undoubtedly, our results suggest a unique way to understand the phase transitions of PCMs under high pressure/temperature, which can be applied to other materials having similar displacive structural evolutions under high pressure/temperature.



2. EXPERIMENTAL AND COMPUTATIONAL DETAILS Two high-pressure experimental runs were performed in a symmetric diamond anvil cell (DAC) with a pair of diamond anvils having a culet diameter of 300 μm. Commercially available GeTe powder (Alfa Aesar, 99.999%) was analyzed using Inductively Coupled Plasma-Atomic Emission Spectrometric (ICP-AES) method, yielding an atomic ratio of 1:0.99 (Ge:Te). The GeTe powder was pressed into a thin slice, and placed in a 120 μm-diameter hole in a pre-indented stainless-steel gasket. Ruby chips were used for the pressure calibration by measuring the shift in their fluorescence as a function of pressure.

30

Ne was used as the pressure-transmitting

medium to provide a hydrostatic environment in the first experimental run using a BSRF-4W2 compressed gas loading system (run 1). In the second run, silicone oil was used as the pressure-transmitting medium (run 2). In situ high-pressure angle dispersive XRD experiments were conducted at the 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF) with a wavelength of 0.6199 Å and a beam size of approximately 30×10 μm2 (full width at half maximum; FWHM). XRD patterns were collected using a PILATUS3X 2M detector, and were integrated using the Fit2D software package. 31 The high-temperature XRD data were collected with a Bruker D8 ADVANCE powder diffractometer. The heating system comprised wide range temperature chambers from an instrument company specialized in materials research. Density-functional theory (DFT) calculations were performed using the planewave code VASP at 0 K, 32 with projector augmented-wave (PAW) potentials. 33 In order to examine the effect of temperature factors on the calculated results, we also performed molecular dynamics calculations at 300 K with the canonical NPT (Nnumber of particles, P-Pressure, T-temperature) ensemble. Ge 3d 4s 4p, and Te 4d 5s 5p valence states were expanded into plane waves. The influence of different k-point sampling and plane-wave cut-off energy were explored in a series of test calculations, leading to the calculations being performed with 18×18×6 k-point sampling for the R3m phase, and 18×18×18 k-point sampling for the B1 phase. A cutoff energy (Ecut) of 700 eV was selected. The self-consistent convergence of the total energy was about 5.0×10−6 eV/atom; the maximum atomic force was less than 0.01 eV/Å; the maximum


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atomic displacement was less than 5.0×10−4Å, and the maximum stress was lower than 0.02 GPa. The exchange-correlation functional used was the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE). 34 3. RESULTS AND DISCUSSION The XRD patterns of GeTe were measured at various pressures up to approximately 15.8 GPa and 14.4 GPa at room temperature for run 1 and run 2, respectively, the selected XRD patterns are shown in Figure 2(a) and Figure S1. Under ambient conditions, all diffraction peaks of GeTe could be indexed to the ambient rhombohedral structure. The typical Rietveld refinement results are shown in Figure S2, from which we obtained the lattice parameters a = 4.1666(3) Å and c = 10.661 (1) Å with Ge and Te atoms at the 3(a) sites, yielding u(Ge) = 0.7661(3) and u(Te) = 0.2381(3). As pressure was increased, as shown in Figure 2 (a), we clearly observed four pairs of diffraction peaks gradually merging into a single peak (marked as a, c, d, and e), respectively, caused by the difference in shift rate of the diffraction peaks, and it seems that the R3m phase transformed towards the B1 phase at about 4.9 GPa. When the pressure was further increased to 15.8 GPa, new Bragg peaks emerged (marked with asterisks), which implied definitely the appearance of the orthorhombic phase. 24, 35 The similar behaviors of GeTe was observed in run 2, as shown in Figure S1.

Figure 2 (a) Selected angle dispersive X-ray diffraction patterns of GeTe at room temperature under various pressures from ambient up to 15.8 GPa in run 1; (b)



Calculated total energy versus volume for the R3m and B1 phases and (c) Enthalpy differences (relative to the R3m phase) of two structures as functions of pressure at 0 K. The total energy and enthalpies are given per formula unit; (d) Relationships of the FWHM of peaks a―e versus pressure for GeTe; To compare the stabilities of the R3m and B1 phases for GeTe under compression, the energy-volume (E-V) and enthalpy-pressure (H-P) relationships were calculated for the two structures at 0 K, as shown in Figure 2(b)–(c) and Figure S3, respectively. The initially more stable phase of GeTe is the R3m phase, as shown in Figure 2(b) and Figure S3 (a). With decreasing volume, the difference in the total energy of the two structures reduced gradually and their energy are almost equal within the volume range 39.37–46.25 Å3. However, over the whole pressure range, the enthalpy of the B1 phase was always higher than that of the R3m phase, with the B1 phase becoming unstable above 8 GPa, as shown in Figure 2(c) and Figure S3 (b). Hence, the phase transition from R3m to B1 did not occur. To better understand the high-pressure behavior of GeTe at low pressure, we investigated the pressure dependence of the FWHM of peaks a―e over the pressure range of 2.2–15.8 GPa in run 1 (4.6–14.4 GPa in run 2), as shown in Figure 2(d) and the inset of Figure S1. The FWHM of peak b gradually increased as pressure increased up to 15.8 GPa (run 1) and 14.4 GPa (run 2) in the first and second run, respectively. Meanwhile, the FWHM of peaks a, c, d, and e decreased steeply between 2.2 and 7.0 GPa in run 1 (4.6 and 8.1 GPa in run 2), reaching minimal values, but then increased slowly up to 15.8 GPa (14.4 GPa in run 2). Surprisingly, the FWHM of peaks a, c, and d were much larger than that of peak b for the same pressure conditions. Typically, pressure-induced Bragg peak broadening occurs in XRD patterns upon compression related to the effects of structural disorder, amorphization, nonhydrostatic conditions, grain size effects, or other structural defects. 36, 37 However, the FWHM of the peaks of a stable structure underwent similar changes of size with increasing pressure. Therefore, given the difference in shift rate of the diffraction peaks under high pressure, we believe that peak b was the only real stable single diffraction peak, while peaks a, c, and d were overlapping peaks, indicating there was no transformation from R3m to B1.


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To obtain further insight into the mechanisms of the unique behavior of the R3m phase of GeTe under high pressure, relationships among lattice parameters, and the a/c ratio versus pressure of the R3m phase were studied using Rietveld refinement and calculations, as shown in Figure 3(a) and (b), respectively. The refined unit cell parameters obtained in two experimental runs at various pressures are given in the Table S1 and Table S2. Obviously, the c axis was more compressible than a axis (Figure 3(a)). Correspondingly, the a/c ratio increased monotonically (Figure 3(b)). However, the increase in a/c ratio flattened out with increasing pressure in experiments and calculations. Based on Bragg diffraction, we believe that the difference in compressibility of the lattice parameters is a reasonable explanation for the difference in shift rate of the diffraction peaks under high pressure. Considering the smooth change of a/c ratios above 4.9 GPa in run 1 (8 GPa in run 2) and the effect of pressure dependence of the FWHM, we hardly observed the process that the overlapping two peaks separate again. In Figure 3, we can observe that the values of the lattice parameters and a/c ratios in two experimental runs are inconsistent. We may attribute these differences to the differential stress in the different sample chambers. Nevertheless, the changing trend of the lattice parameters and a/c ratios as a function of pressures are consistent. In addition, we can also find that the temperature has little effect on the calculated results.

Figure 3 Relationships of the experimental and calculated lattice parameters a and c (a); and a to c ratio (a/c) versus pressure for GeTe (b). The process of the formation of overlapping peaks occurred not only during the



high-pressure experiments, but also during the high-temperature experiments. Figure 4 shows a set of XRD patterns for the R3m phase of GeTe at different temperatures. The Pt peaks are associated with the heater strip. Generally, the XRD lines shifted to smaller Bragg angles with increasing temperature. However, some unusual changes were observed. With increasing temperature, With increasing temperature, six pairs of splitting peaks gradually merged into one, respectively. Clearly, peak (101) shifted to the left, while peak (003) shifted to the right with increasing temperature. The two separate peaks almost merged into one at 910 K. The other five pairs of splitting peaks showed similar behavior. We found no evidence for the phase transition from R3m to B1, proposed by Rabe and Joannopoulos. 19 At about 850K, several new peaks were observed, representing the reaction product of the heater strip and R3m phase at high temperature. Temperature dependence of the lattice parameter and c/a ratio of R3m phase is shown in Figure 4(b) and (c). With increasing temperature, parameter a increased, while c decreased, leading to a marked increase in the c/a ratio. These results are consistent with our high-pressure experimental data. Clearly, the effects of high temperature and high pressure on the R3m phase were similar. PCMs, such as Ge2Sb2Te5 and GeTe, undergo rapid and reversible transformations between their crystalline and amorphous states, when a laser beam or electric pulse is applied. Compared with amorphization by melt-quenching, the crystallization process of the PCMs is slow, and is the data-rate limiting factor for phase change random access memory (PCRAM) devices. 38 The lack of a B1 phase in GeTe observed in this study will improve our understanding of phase-change mechanisms in PCMs under high pressure/temperature.


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Figure 4 (a) X-ray diffraction patterns patterns of the R3m phase of GeTe measured at different temperatures. The lattice parameters a and c (b), and c/a ratio (c) of the R3m phase plotted against temperatures. The analysis of the pressure/temperature-induced structural evolutions of GeTe encouraged us to reinvestigate other displacive transitions under high pressure. Generally, the emergence of the splitting peaks has been regarded as a new phase transition. However, in one case, Li et al. believed that the intermediate phase of CdSe actually possesses Pnma symmetry, 13 rather than a Cmcm structure, 39-41 and proposed the appearance of the new reflections and reflection splitting with increasing pressure is caused by the changes of atomic relative locations in crystal lattice and the difference in the compression ratio of lattice parameters for the Pnma structure. Both the formation of overlapping diffraction peaks in GeTe and the splitting peaks in orthorhombic CdSe are usually accompanied by a marked change in the compression ratio of lattice parameters. Surprisingly, similar behavior of the c/a (or c/b) ratio were observed in other displacive transitions of chalcogenide compounds, including HgTe, 2, 42, 43 ZnS, 44 CdTe 10, 45-46 and HgS. 2 Hence, our conclusion may be extended to other compounds, especially those that undergo a displacive transition under high pressure. In particular, given the intrinsic limitation of Bragg-diffraction, multiple probing techniques combined with DFT calculations may be required to understand high-pressure behavior of PCMs. 4. CONCLUSIONS



Pressure-induced structural transitions of GeTe were revealed by synchrotron XRD and DFT calculations. We found no evidence of any phase transition in GeTe over the whole pressure range in two experimental runs. Similar behavior was observed in high-temperature experiments. We believe that marked changes in the compressibility of various lattice parameters of the R3m phase have led to the misinterpretation of the nature of the transformation from R3m to B1 in previous work. Our results have far-reaching implications for phase-change mechanisms of PCMs under extreme conditions, especially in those materials exhibiting similar phase transitions under high pressure.

ASSOCIATED CONTENT Supporting Information X-ray diﬀraction patterns of GeTe plotted against pressure in the first experimental run with silicone oil as the pressure transmitting medium; Typical Rietveld refinement results of the R3m phase under ambient conditions; The calculated total energy versus volume and Enthalpy difference (relative to R3m phase) as functions of pressure for the R3m and B1 phases of GeTe; The full unit cell results versus pressure for the R3m phase of GeTe. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author § Yanchun $

Li: Email: [email protected]

Gong Li: Email: [email protected]

£Feng

Peng: Email: [email protected]

Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS This work was supported by the NSCF (Grant No. 11474280).


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Figure 1 Transitions between the NaCl-type (B1) structures and some distorted-NaCl structures (cinnabar, Cmcm, Pnma and R3m phases) under high pressure. 170x101mm (264 x 264 DPI)



Figure 4 (a) X-ray diffraction patterns patterns of the R3m phase of GeTe measured at different temperatures. The lattice parameters a and c (b), and c/a ratio (c) of the R3m phase plotted against temperatures. 81x39mm (300 x 300 DPI)


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Fig. S2 Typical Rietveld refinement results of -GeTe under ambient conditions. The black open circles and red solid line represent the experimental and simulated data, respectively, and the blue solid lines at the bottom are the residual intensities. The solid short vertical bars indicate the peak positions. 73x58mm (300 x 300 DPI)



Fig. S3 (a) Calculated total energy versus volume for the R3m and B1 phases of GeTe, the inset shows the total energy of the two structures in the volume range 45–50 Å3. (b) Enthalpy difference (relative to R3m phase) of two structures as functions of pressure in GeTe. The total energy and enthalpies are given per formula unit.


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Fig. S1 X-ray diﬀraction patterns of GeTe plotted against pressure in the first experimental run with silicone oil as the pressure transmitting medium. The inset shows the relationships of the FWHM of peaks a―e versus pressure for GeTe. 85x86mm (300 x 300 DPI)



Figure 2 (a) Selected angle dispersive X-ray diffraction patterns of GeTe at room temperature under various pressures from ambient up to 15.8 GPa in run 1; (b) Calculated total energy versus volume for the R3m and B1 phases and (c) Enthalpy differences (relative to the R3m phase) of two structures as functions of pressure at 0 K. The total energy and enthalpies are given per formula unit; (d) Relationships of the FWHM of peaks a―e versus pressure for GeTe;


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Figure 3 Relationships of the experimental and calculated lattice parameters a and c (a); and a to c ratio (a/c) versus pressure for GeTe (b). 80x60mm (300 x 300 DPI)


Persistence of the R3

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