CHAPTER 5. HIGH PRESSURE MICRO RAMAN STUDIES ON SINGLE CRYSTAL Yb 2 O 3

Transcription

1 68 CHAPTER 5 HIGH PRESSURE MICRO RAMAN STUDIES ON SINGLE CRYSTAL Yb 2 O INTRODUCTION Yb 2 O 3 belongs to a group of heavy rare earth oxides and crystallizes in the bixbyite structure. Ytterbium oxide (Yb 2 O 3 ) film is attractive as a gate dielectric in electronic devices because of its dielectric constant of ~15 and an energy band gap greater than 5 ev (Tung and Wei 2009). The unit cell of the C-type Yb 2 O 3 has 16 formula units containing 32 Yb and 48 O ions. The Yb ions occupy two cation positions as in the case of all the cubic type rare earth sesquioxides. The two cation positions are the 8b position and the 24d position. ¼ of the cations are in the 8b position with three fold inversion symmetry C 3i and ¾ of the cations are in the 24d position with two fold symmetry C 2. These two sites are the centers for two polyhedrons with 6 coordinating oxygen atoms. For the C 3i site, two oxygens are missing across the body diagonal while for the C 2 site they are missing across the face diagonal. Raman spectra of ytterbium and erbium oxides were first reported by Schaack and Koningstein (1970) and the vibrational spectra of other rare earth sesquioxides are also reported. Meyer et al (1995) had performed high pressure Mossbaeur and energy dispersive XRD studies on C-type Yb 2 O 3. They observed a C to B transition above 13 GPa. Marek W. Urban et al

2 69 (1987) had also reported Raman and infra red studies on Yb 2 O 3 along with other rare earth sesquioxides. 5.2 EXPERIMENTAL DETAILS The single crystal of Yb 2 O 3 was supplied by Prof. Joseph R Smyth of the University of Colorado. The sample was initially transparent and a small part of the crystal was used for the experiment. The sample was excited using the nm green line of a Spectra Physics Ar ion laser, and the scattered light were detected by Dilor XY spectrometer equipped with a liquid nitrogen-cooled charge-coupled-device detector. A 50x objective was used to focus the incident laser light of 15 mw on the sample and also to collect the scattered light. No laser heating effects were observed as there was no color change after exposure to the laser. The spectrometer was calibrated using single-crystal silicon as a reference. The micro Raman spectra were recorded in the backscattering geometry. The single crystal of Yb 2 O 3 was loaded into a symmetric type diamond anvil cell with a piece of ruby as the pressure calibrant. Rhenium gasket was used with a hole of size 150 m was drilled to contain the sample. The pressure transmitting medium was liquid Argon which was loaded cryogenically. 5.3 RESULTS AND DISCUSSIONS Figure 5.1 displays the Raman spectra recorded for single crystal Yb 2 O 3 with increasing pressure values of up to ~ 44 GPa. An increase in phonon frequencies were observed with increasing pressure. At ambient

3 70 pressure 7 lines at 96.8, 120.4, , 308.3, 340.3, and cm -1 were observed. According to factor group theory analysis 22 Raman modes have been predicted for the C-type bixbyite structure in rare earth sesquioxides. White and Keramidas (1972) and Replin et al (1995) had showed that the irreducible representations obtained using the Bhagvantam method for the optical and acoustical modes were op = 4A g +4E g + 14F g +5A 2u +5E u +16F u and op = F u where A g, E g and F g are Raman active modes, F u is IR active and A 2u and E u are Raman inactive. The strongest peak is found between 330 and 380 cm -1 for all the rare earth sesquioxides and in this study it is at cm -1. This is assigned as the F g +A g mode and the high intensity suggests a large variation in the polarizability during vibrations. The low frequency modes are assigned to the bending modes or the oscillations of the O-Yb-O or the Yb-O-Yb bond angles (Alberto Ubaldini et al 2008).

4 Yb 2 O GPa Intensity ( Arb. Units ) GPa 3.8 GPa 2.8 GPa 2.4 GPa 1.8 GPa 1.5 GPa 1.3 GPa 0.01 MPa Raman Shift cm Yb 2 O GPa (Arb. Units) Intensity GPa 37.2 GPa 34.0 GPa 32.6 GPa 29.0 GPa 26.5 GPa 25.5 GPa 21.7 GPa 17.8 GPa GPa 6.0 GPa Raman Shift cm -1 Figure 5.1 Raman spectra for Yb 2 O 3 showing the phase transitions occurring at 12 GPa and after 29 GPa under hydrostatic pressure

5 72 The Raman spectra before and after the phase transition is shown in Figure 5.1. The lines seems to be shifted slightly from what is reported earlier by Schaack and Koningstein (1970). In Figure 5.1 one can see that all the Raman modes shift to higher frequencies with increasing pressure and above 12 GPa one can see that there are a number of new peaks appearing and the band at cm -1 (F g +A g mode) vanishes completely after 6 GPa. This suggests that there is a change in the atomic arrangement of Yb 2 O 3. This transition has already been reported as the transition to the B-type structure (Meyer et al 1995). After 26 GPa, there is a further change in the spectra and a transition is observed probably to the A-type structure as the pattern is similar to the structure of A-type rare earth sesquioxides (Gouteron et al 1981). The structure of the A-type rare earth sesquioxide is described in detail in Chapter 1. The new strong intensity mode arises at around 565 cm -1. The B phase and the A phase seems to coexist in this part as some of the low frequency modes from the B phase still remain up to a pressure of 44 GPa (Figure 5.2). After the sample was quenched, the B phase seems to exist and the sample does not revert back to the parent phase. The frequency variation of these modes versus hydrostatic pressure is shown in Figure 5.3. The zero pressure frequencies are slightly shifted from the previous studies.

6 73 Yb 2 O 3 After return cycle(ambient) Return cycle Intensity (Arb. Units) GPa 32.7 GPa 17.7 GPa Ambient 0.01 MPa Raman Shift cm -1 Figure 5.2 Raman spectra for Yb 2 O 3 showing the full cycle including the return cycle C Type B Type B+A Type Raman shift cm P(GPa) Figure 5.3 The Raman shift vs. hydrostatic pressure for Yb 2 O 3

7 74 The mode Gruneisen parameters ( ) describe the effect that changing the volume of a crystal lattice has on its vibrational properties. The variation in frequencies with pressure for the C Type phase are linear and at such low pressures the logarithmic derivative of the Gruneisen parameter ln / lnv with volume can be considered as equal to zero. This is equivalent to assuming that the frequency versus volume is linear. Jiang et al (2010) had calculated for Lu 2 O 3. The F g + A g mode of Lu 2 O 3 falls at and for the same mode is found to be around 1.63 and in this study, the for the F g + A g mode is found to be at The value is definitely higher as the bulk modulus for Lu 2 O 3 is at 214 GPa and also the F g + A g mode for Yb 2 O 3 falls at cm -1. The bulk modulus used here for Yb 2 O 3 is B o = 181 GPa which is taken from the High pressure X-Ray diffraction studies by Meyer et al (1995). The mode Gruneisen parameters are determined from the following formulae (Shermen et al 1982) and shown in table 5.1. = (ln( ))/ ln(v) = (B 0 * K H )/ 0 K H = ( / P) P=0 = 0 + K H *P where K H = hydrostatic linear pressure coefficient 0 = the phonon frequency at zero pressure B 0 = Isothermal Bulk modulus

8 75 Table 5.1 Mode Gruneisen parameters for the phonon modes of cubic structure Phonon mode 0 K H E g T u F g +A g F g Anomalies in bonding for Yb 2 O 3 and the relation to high pressure studies Raman spectra of Yb 2 O 3 had been studied by Schaack and Koningstein (1970) at 10, 80 and 300K.The value of the Raman shift are given in Table 5.1 for comparative study. In most of the RES studied earlier, the prominent line with the maximum intensity is between cm -1 and it is 20 times greater in intensity than all the other lines. In the case of Yb 2 O 3, this was not the case, even in our studies the line at is comparable in intensity with the line at cm -1.The broad gap in the Raman spectra between 200 cm -1 and 310 cm -1 suggests an assignment of the lines at higher frequencies to the inner vibrations of RE-O6 octahedron. Electronic Raman transitions were also observed by them in Yb 2 O 3.The line at 334 cm -1 is attributed to the electronic transitions. In our studies the line is observed as a shoulder peak at cm -1.When compared with the spectra of other oxides, the lines for Yb 2 O 3 and Eu 2 O 3 seems to be shifted to about 30 cm -1 to the lower side of the wave numbers and they attribute it to the internal stress due to the flame fusion assisted crystal growth.

9 76 Nita Dilwar et al (2008) had also studied the vibrational spectra of Yb 2 O 3 along with other rare earth sesquioxides. They also observed lower wave numbers for the F g mode for Yb 2 O 3 and Eu 2 O 3. Raman effect is a second order phenomenon involving an electron excitation and a phonon creation, These anomalies may indicate a significant electron-phonon interaction. Marek Urban et al (1987) suggests that electron phonon coupling is present in Eu 2 O 3 and Yb 2 O 3 and also report unique wave number shifts for Eu (f 6 ) and Yb (f 13 ) oxides. The F g mode is the most intense band in the Rare Earth Sesquioxide Raman spectra and it indicates large polarizability change during vibration and this band is also expected to be the most sensitive to changes in chemical bonding. They too observe anomalies with Europium and Ytterbium sesquioxides. Both the infra red and the Raman band [F u and the F g mode] drop in energy for these two compounds and it does not coincide with the half filled or filled electronic configurations but with f 6 and f 13 electronic configurations. The general trend of increasing wave numbers across the period can be explained by lanthanide contraction. Since the lattice parameter decreases, there is an increase in force constant and vibrational frequencies. Europium and ytterbium oxides display large vibrational energy decreases which represent significant variations in force constants and chemical bonding. There are no anomalies in lattice constants. They explain the anomalies by comparing it with the enthalpy of formation and free energy of formation values which show the same trend as the decrease in wave number changes. These properties also show uniquely smaller values for these two oxides. Therefore the decrease in wave number is associated with weaker bonding and is reflected in the vibrational energies for both IR and Raman but not in the lattice parameter, since the changes in lattice parameter with atomic

10 77 number are monotonic. The wave number decreases for Eu 2 O 3 and Yb 2 O 3 can only be caused by force constant reduction. The decrease in force constant can be caused by unique electronic structures which broaden the potential well 2 V/ q 2. The curvature change or broadening in potential function can result from the proximity of f-electron states to the Fermi level. Heden et al (1971) shows by XPS that the 4f level electrons in Eu and Yb should lay comparatively close to the Fermi level. This decrease or softening of the force constants indicates that the electron shells of Yb and Eu ions are deformable, less stiff than other lanthanides. These underlying f-level electrons can play a significant part in the chemical bonding of solids. These properties are related to the presence of f 7 and f 14 structures in the +2 oxidation state, but even the studies on trivalent oxides suggest the anomalous properties and are therefore attributed to the unique electronic structures. 5.4 CONCLUSIONS According to the above studies, the anomalies present in Yb 2 O 3 and Eu 2 O 3 and the fact that the electron shells of Yb and Eu are less stiff than other oxides, one can expect phase transitions to occur at lower pressure than other oxides. On the contrary Yb 2 O 3 shows the phase transition almost at the same pressure as other oxides. Structural transition for almost all of these oxides happens between 9 GPa and 12 GPa. There were no anomalies observed in the case of Yb 2 O 3.