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High-pressure phase transition in Ho2O3
Materials Chemistry and Physics 120 (2010) 65–67
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
High-pressure phase transition in Ho2 O3 Dayana Lonappan a,b , N.V. Chandra Shekar a,∗ , T.R. Ravindran a , P. Ch. Sahu a a b
Condensed Matter Physics Division, Material Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India Crystal Growth Centre, Anna University, Chennai 600025, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 21 May 2009 Received in revised form 24 September 2009 Accepted 17 October 2009 PACS: 61.50.Ah 61.10.N2 07.35.+K 81.40.Vw 81.30.Hd
a b s t r a c t High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho2 O3 ) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm−1 disappears around the similar transition pressure. The bulk modulus of the parent phase is reported. © 2009 Elsevier B.V. All rights reserved.
Keywords: Holmium oxide High-pressure XRD Raman Phase transition
1. Introduction Polymorphism and other structural properties of rare-earth (RE) sesquioxides (R2 O3 ; R = rare-earth elements) have been studied extensively owing to the fact that these compounds are quite important scientifically as well as technologically. When doped with other rare-earth elements and transition-metal ions, they are used as laser rods for various wavelengths [1] and as phosphors [2,3]. They are also used as refractory and abrasive materials and have a wide range of other applications [4]. It is important to study the structural changes that occur in these oxides when subjected to high pressures and temperatures. As a continuation of our work on the study of a series of rare-earth sesquioxides, experiments on Ho2 O3 under high-pressure are reported here. The RE sesquioxides are known to exist in five polymorphic forms, namely, A and H which are hexagonal, B which is monoclinic, and C and X cubic [5]. The range and existence of each phase depends on the ionic radius of the RE and the temperature. Above 2000 ◦ C, the X and H phases are stable, while below this temperature, the A, B and C phases are commonly observed. Over many
∗ Corresponding author. Tel.: +91 44 27480347; fax: +91 44 27480081. E-mail address: [email protected] (N.V.C. Shekar). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.022
years, different research groups have studied various lanthanide sesquioxides under various conditions of pressure and temperature [6]. Recent studies on Gd2 O3 have revealed a phase transition from the cubic to a possible hexagonal phase around 12 GPa [7]. A Raman spectroscopic study [8] on cubic nanocrystalline Gd2 O3 revealed a phase transition at 12.6 GPa. Also, Terbium oxide shows unusually high structural stability up to a pressure of about 26 GPa and thereafter, transforms to a possible cotunnite structure [9]. Ho2 O3 has been investigated under high-pressure and temperature and found to stabilize in a monoclinic structure at 2.5 GPa and 1000 ◦ C [10]. The aim of the present investigation is to find out the stability of this oxide under high-pressure conditions and investigate its phase transition behaviour. 2. Experimental Holmium oxide, Cerac made, 99.99%, sample was characterized by X-ray diffraction technique using a high-resolution Guinier diffractometer with a scintillation detector having an overall resolution of ıd/d = 0.001 [11]. It was found to be of cubic structure having lattice parameter a = 10.624 ± 0.002 Å and space-group Ia-3 (No. 206) at an ambient pressure. In situ high-pressure X-ray diffraction was carried out using a Mao–Bell-type diamond-anvil cell (DAC) in an angle-dispersive mode. Microsamples were loaded into a 250 m hole drilled into a pre-indented stainless-steel gasket. A ruby chip was placed alongside the sample for pressure calibration by the ruby fluorescence method [12,13]. Ruby was excited with an argon ion laser of wavelength 514.5 nm and the fluorescence was recorded with a CCD-based spectrometer. A mixture of
66
D. Lonappan et al. / Materials Chemistry and Physics 120 (2010) 65–67
methanol, ethanol and water in the volume ratio 16:3:1 was used as the pressuretransmitting medium. An image plate based marResearch mar345dtb diffractometer was used for recording the high-pressure XRD patterns. The incident Mo X-ray beam obtained from a Rigaku ULTRAX-18 (18 kW) rotating anode X-ray generator was monochromatised with a graphite monochromator. The overall resolution of the diffractometer system is: ıd/d ∼ 0.001. The Mao–Bell-type DAC was fitted to the diffractometer and the sample to detector distance calibrated using a standard specimen like LaB6 . Raman spectra were recorded at different pressures in the backscattering geometry using the 488 nm line of an argon ion laser. Scattered light from the sample was collected by a SPEX double monochromator with a cooled photomultiplier tube operated in the photon counting mode as the detector. Scanning of the spectra and data acquisition were carried out using a programmable system on chip hardware controlled by LABVIEW® 7.1 program. The spectral range covered was 100–700 cm−1 [14].
3. Results and discussion The high-pressure XRD pattern (Fig. 1) of Ho2 O3 revealed that the sample remains stable in its parent phase, which is the cubic or bixbyite structure, up to a pressure of ∼9 GPa. The pattern taken under ambient condition has been indexed to the cubic phase, taking into account five prominent peaks (2 1 1), (2 2 2), (4 0 0), (4 4 0) and (6 2 2). At around 9.5 GPa new peaks emerge, which is a clear indication of initiation of a structural phase transition. The most prominent new peak appears at 2 = 14.7◦ which emerges between the parent (2 2 2) and the (4 0 0) peaks. Another prominent new peak appears at 2 = 19.9◦ . The high-pressure phase and the parent cubic phase coexist up to about 16 GPa, above which the parent phase completely disappears. There was no indication of reversal of the phase as the pressure was lowered to ambient conditions. The pressure–volume P–V data for Ho2 O3 is shown in Fig. 2. The P–V data up to 9.5 GPa of the parent phase was fitted to the Murnaghan equation of state [15] and the values of the bulk modulus and its pressure derivative obtained from the above fit are: Bo = 178 GPa and B0 = 4. Out of the five different polymorphic forms of the rare-earth sesquioxides, the hexagonal structure is the likely candidate for the high-pressure phase, with regard to the decreasing order of symmetry. The high-pressure structure at 16 GPa was fitted to a monoclinic phase with lattice parameters a = 13.8 Å, b = 3.5 Å and c = 8.2 Å; ˇ = 99.1◦ (Fig. 3). The high-pressure XRD pattern was not of very good quality for lending itself for a very detailed fitting and refinement. The phase stability of cubic Ho2 O3 extends up to ∼9 GPa at room temperature as compared to 2.5 GPa at 1000 ◦ C reported by Hoekstra and Gingerich [10]. Lowering of transition
Fig. 1. Angle-dispersive XRD patterns for Ho2 O3 for various pressures, indicating the structural transition at ∼9.5 GPa.
Fig. 2. The pressure versus V/Vo values of the parent cubic phase fitted to the Murnaghan equation of state.
pressures at elevated temperatures is known in many other systems as well. High-pressure Raman investigations were undertaken on Ho2 O3 to look for signatures of the phase transition indicated by X-ray diffraction. Raman spectra at several different pressures are shown in Fig. 4. The spectra at 0.1 MPa do not have sharp peaks. There is only one clear band at 372 cm−1 . This peak is typical of C-type rareearth sesquioxides and arises due to Ho–O stretching vibrations. Even though 22 Raman lines are expected from symmetry considerations, the spectra do not show the expected number of bands due to accidental degeneracies or low Raman scattering cross-sections of the corresponding vibrations [16]. The band centered around 370 cm−1 moves towards higher wave number as the lattice constant reduces. This indicates that the C-structure becomes more rigid as the volume is reduced. As the pressure is increased, the band becomes very broad and finally disappears above 7 GPa signifying a phase transition. In this experiment also, the transition is found to be irreversible as can be seen from the Raman spectra of the sample quenched to room pressure. The transition pressure
Fig. 3. XRD pattern of the high-pressure phase at ∼16 GPa and the monoclinic fitting with lattice parameters, a = 13.8 Å, b = 3.5 Å and c = 8.2 Å; ˇ = 99.1◦ .
D. Lonappan et al. / Materials Chemistry and Physics 120 (2010) 65–67
67
technique. An irreversible structural transition initiated at around 9.5 GPa and the transition was completed at about 16 GPa. The high-pressure phase seems to be a possible monoclinic structure. Band structure calculations as a function of pressure would help in understanding the nature and mechanism of this transition. Acknowledgements The authors thank Dr. N. Subramanian, Shri N.R. Sanjay Kumar and Shri M. Sekar for encouragement. They thank Mr. V. Kathirvel and Mr. Y.A. Sorb for their help. The authors also thank Shri L. M. Sundaram for his help at various stages of work. They also thank Dr. A.K. Arora, Dr. C.S. Sundar and Dr. Baldev Raj for their encouragement and support. D.L expresses her gratitude to the IGCAR authorities for granting permission to carry out her Ph.D. research work at the Condensed matter Physics Division of IGCAR. References
Fig. 4. Raman spectra of Ho2 O3 showing the transition above 9 GPa and the irreversible nature of the transformation.
from cubic to possible monoclinic matches well with the conclusion reached from high-pressure X-ray diffraction experiments. The cubic-monoclinic transition in Ho2 O3 is understood to be of reconstructine type, involving extensive breakage of bonds, with its typical signatures like—coexistence of parent and daughter phases, sluggishness of the transition etc. The transition occurs with increase in coordination number, decrease in the density and symmetry. Such transitions sometimes end up with persistent metastable phase [17]. In the case of Gd2 O3 , for example, it was seen that heating the pressure quenched sample even up to 500 ◦ C did not make it revert back to parent structure. This was understandable because in these systems metal ion sublattice is very rigid and appreciable ion movement occurs only after about 1400 ◦ C [18]. 4. Concluding remarks We have studied the pressure-induced structural stability of bulk Ho2 O3 up to 17 GPa by both X-ray diffraction and laser-Raman
[1] [2] [3] [4] [5] [6] [7]
[8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
K. Petermann, L. Fornasiaro, E. Mix, V. Peters, Opt. Mater. 19 (2002) 67. E. Antic-Fidancev, J. Holsa, M. Lastusaari, J. Alloys Compd. 341 (2002) 82. T. Justel, J.C. Krupa, D.U. Wiechert, J. Lumin. 93 (2001) 179. L. Laversenne, Y. Guyot, C. Goutaudier, M.Th. Cohen-Adad, G. Boulon, Opt. Mater. 16 (2001) 475. O. Greis, J. Solid State Chem. 34 (1980) 39. G. Chen, J.R. Peterson, K.E. Brister, J. Solid State Chem. 111 (1994) 437. L. Dayana, N.V. Chandra Shekar, P.Ch. Sahu, B.V. Kumaraswamy, A.K. Bandhyopadhyay, M. Rajagopalan, Phil. Mag. Lett. 88 (2008) 473; F.X. Zhang, M. Lang, J.W. Wang, U. Becker, R.W. Ewing, Phys. Rev. B 78 (2008) 064114. D. Nita, M. Shalini, V. Deepak, S.K. Haldar, P. Himanshu, S.M. Sharma, B.V. Kumaraswamy, A.K. Bandopadhyay, Discussion meeting on materials under high pressures (MRSI-MHP), in: 18th Annual General Meeting (MRSI), National Physical Laboratory, New Delhi, 2007. L. Dayana, N.V. Chandra Shekar, P. Ch. Sahu, R. Paul, P. Paul, J. Alloys and Compds, (2008) doi:10.1016/j.jallcom.2009.10.068. H.R. Hoekstra, K.A. Gingerich, Science 146 (1964) 1163. P. Ch Sahu, N.V. Chandra Shekar, N. Subramanian, M. Yousuf, K. Govinda Rajan, Rev. Sci. Instrum. 66 (1995) 2599. R.A. Forman, G.J. Piermarini, J.D. Barnett, S. Block, Science 176 (1972) 284. G.J. Piermarini, S. Block, J.D. Barnett, R.A. Forman, J. Appl. Phys. 46 (1975) 2774. J. Jayapandian, R. Kesavamoorthy, A.K. Arora, J. Instrum. Soc. India 38 (2008) 55. F. Birch, Phys. Rev. 71 (1947) 809. A. Ubaldini, M.M. Carnasciali, J. Alloys Compd. 454 (2007) 374. N.V. Chandra Shekar, K. Govinda Rajan, Bull. Mater. Sci. 24 (2001) 1. L. Eyring, The binary rare earth oxides, in: K.A. Gschneidner (Ed.), Hand Book on the Physics and Chemistry of Rare Earths, vol. 3, Amsterdam, North Holland, 1978, p. 337.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
High-pressure phase transition in Ho2 O3 Dayana Lonappan a,b , N.V. Chandra Shekar a,∗ , T.R. Ravindran a , P. Ch. Sahu a a b
Condensed Matter Physics Division, Material Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India Crystal Growth Centre, Anna University, Chennai 600025, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 21 May 2009 Received in revised form 24 September 2009 Accepted 17 October 2009 PACS: 61.50.Ah 61.10.N2 07.35.+K 81.40.Vw 81.30.Hd
a b s t r a c t High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho2 O3 ) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm−1 disappears around the similar transition pressure. The bulk modulus of the parent phase is reported. © 2009 Elsevier B.V. All rights reserved.
Keywords: Holmium oxide High-pressure XRD Raman Phase transition
1. Introduction Polymorphism and other structural properties of rare-earth (RE) sesquioxides (R2 O3 ; R = rare-earth elements) have been studied extensively owing to the fact that these compounds are quite important scientifically as well as technologically. When doped with other rare-earth elements and transition-metal ions, they are used as laser rods for various wavelengths [1] and as phosphors [2,3]. They are also used as refractory and abrasive materials and have a wide range of other applications [4]. It is important to study the structural changes that occur in these oxides when subjected to high pressures and temperatures. As a continuation of our work on the study of a series of rare-earth sesquioxides, experiments on Ho2 O3 under high-pressure are reported here. The RE sesquioxides are known to exist in five polymorphic forms, namely, A and H which are hexagonal, B which is monoclinic, and C and X cubic [5]. The range and existence of each phase depends on the ionic radius of the RE and the temperature. Above 2000 ◦ C, the X and H phases are stable, while below this temperature, the A, B and C phases are commonly observed. Over many
∗ Corresponding author. Tel.: +91 44 27480347; fax: +91 44 27480081. E-mail address: [email protected] (N.V.C. Shekar). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.022
years, different research groups have studied various lanthanide sesquioxides under various conditions of pressure and temperature [6]. Recent studies on Gd2 O3 have revealed a phase transition from the cubic to a possible hexagonal phase around 12 GPa [7]. A Raman spectroscopic study [8] on cubic nanocrystalline Gd2 O3 revealed a phase transition at 12.6 GPa. Also, Terbium oxide shows unusually high structural stability up to a pressure of about 26 GPa and thereafter, transforms to a possible cotunnite structure [9]. Ho2 O3 has been investigated under high-pressure and temperature and found to stabilize in a monoclinic structure at 2.5 GPa and 1000 ◦ C [10]. The aim of the present investigation is to find out the stability of this oxide under high-pressure conditions and investigate its phase transition behaviour. 2. Experimental Holmium oxide, Cerac made, 99.99%, sample was characterized by X-ray diffraction technique using a high-resolution Guinier diffractometer with a scintillation detector having an overall resolution of ıd/d = 0.001 [11]. It was found to be of cubic structure having lattice parameter a = 10.624 ± 0.002 Å and space-group Ia-3 (No. 206) at an ambient pressure. In situ high-pressure X-ray diffraction was carried out using a Mao–Bell-type diamond-anvil cell (DAC) in an angle-dispersive mode. Microsamples were loaded into a 250 m hole drilled into a pre-indented stainless-steel gasket. A ruby chip was placed alongside the sample for pressure calibration by the ruby fluorescence method [12,13]. Ruby was excited with an argon ion laser of wavelength 514.5 nm and the fluorescence was recorded with a CCD-based spectrometer. A mixture of
66
D. Lonappan et al. / Materials Chemistry and Physics 120 (2010) 65–67
methanol, ethanol and water in the volume ratio 16:3:1 was used as the pressuretransmitting medium. An image plate based marResearch mar345dtb diffractometer was used for recording the high-pressure XRD patterns. The incident Mo X-ray beam obtained from a Rigaku ULTRAX-18 (18 kW) rotating anode X-ray generator was monochromatised with a graphite monochromator. The overall resolution of the diffractometer system is: ıd/d ∼ 0.001. The Mao–Bell-type DAC was fitted to the diffractometer and the sample to detector distance calibrated using a standard specimen like LaB6 . Raman spectra were recorded at different pressures in the backscattering geometry using the 488 nm line of an argon ion laser. Scattered light from the sample was collected by a SPEX double monochromator with a cooled photomultiplier tube operated in the photon counting mode as the detector. Scanning of the spectra and data acquisition were carried out using a programmable system on chip hardware controlled by LABVIEW® 7.1 program. The spectral range covered was 100–700 cm−1 [14].
3. Results and discussion The high-pressure XRD pattern (Fig. 1) of Ho2 O3 revealed that the sample remains stable in its parent phase, which is the cubic or bixbyite structure, up to a pressure of ∼9 GPa. The pattern taken under ambient condition has been indexed to the cubic phase, taking into account five prominent peaks (2 1 1), (2 2 2), (4 0 0), (4 4 0) and (6 2 2). At around 9.5 GPa new peaks emerge, which is a clear indication of initiation of a structural phase transition. The most prominent new peak appears at 2 = 14.7◦ which emerges between the parent (2 2 2) and the (4 0 0) peaks. Another prominent new peak appears at 2 = 19.9◦ . The high-pressure phase and the parent cubic phase coexist up to about 16 GPa, above which the parent phase completely disappears. There was no indication of reversal of the phase as the pressure was lowered to ambient conditions. The pressure–volume P–V data for Ho2 O3 is shown in Fig. 2. The P–V data up to 9.5 GPa of the parent phase was fitted to the Murnaghan equation of state [15] and the values of the bulk modulus and its pressure derivative obtained from the above fit are: Bo = 178 GPa and B0 = 4. Out of the five different polymorphic forms of the rare-earth sesquioxides, the hexagonal structure is the likely candidate for the high-pressure phase, with regard to the decreasing order of symmetry. The high-pressure structure at 16 GPa was fitted to a monoclinic phase with lattice parameters a = 13.8 Å, b = 3.5 Å and c = 8.2 Å; ˇ = 99.1◦ (Fig. 3). The high-pressure XRD pattern was not of very good quality for lending itself for a very detailed fitting and refinement. The phase stability of cubic Ho2 O3 extends up to ∼9 GPa at room temperature as compared to 2.5 GPa at 1000 ◦ C reported by Hoekstra and Gingerich [10]. Lowering of transition
Fig. 1. Angle-dispersive XRD patterns for Ho2 O3 for various pressures, indicating the structural transition at ∼9.5 GPa.
Fig. 2. The pressure versus V/Vo values of the parent cubic phase fitted to the Murnaghan equation of state.
pressures at elevated temperatures is known in many other systems as well. High-pressure Raman investigations were undertaken on Ho2 O3 to look for signatures of the phase transition indicated by X-ray diffraction. Raman spectra at several different pressures are shown in Fig. 4. The spectra at 0.1 MPa do not have sharp peaks. There is only one clear band at 372 cm−1 . This peak is typical of C-type rareearth sesquioxides and arises due to Ho–O stretching vibrations. Even though 22 Raman lines are expected from symmetry considerations, the spectra do not show the expected number of bands due to accidental degeneracies or low Raman scattering cross-sections of the corresponding vibrations [16]. The band centered around 370 cm−1 moves towards higher wave number as the lattice constant reduces. This indicates that the C-structure becomes more rigid as the volume is reduced. As the pressure is increased, the band becomes very broad and finally disappears above 7 GPa signifying a phase transition. In this experiment also, the transition is found to be irreversible as can be seen from the Raman spectra of the sample quenched to room pressure. The transition pressure
Fig. 3. XRD pattern of the high-pressure phase at ∼16 GPa and the monoclinic fitting with lattice parameters, a = 13.8 Å, b = 3.5 Å and c = 8.2 Å; ˇ = 99.1◦ .
D. Lonappan et al. / Materials Chemistry and Physics 120 (2010) 65–67
67
technique. An irreversible structural transition initiated at around 9.5 GPa and the transition was completed at about 16 GPa. The high-pressure phase seems to be a possible monoclinic structure. Band structure calculations as a function of pressure would help in understanding the nature and mechanism of this transition. Acknowledgements The authors thank Dr. N. Subramanian, Shri N.R. Sanjay Kumar and Shri M. Sekar for encouragement. They thank Mr. V. Kathirvel and Mr. Y.A. Sorb for their help. The authors also thank Shri L. M. Sundaram for his help at various stages of work. They also thank Dr. A.K. Arora, Dr. C.S. Sundar and Dr. Baldev Raj for their encouragement and support. D.L expresses her gratitude to the IGCAR authorities for granting permission to carry out her Ph.D. research work at the Condensed matter Physics Division of IGCAR. References
Fig. 4. Raman spectra of Ho2 O3 showing the transition above 9 GPa and the irreversible nature of the transformation.
from cubic to possible monoclinic matches well with the conclusion reached from high-pressure X-ray diffraction experiments. The cubic-monoclinic transition in Ho2 O3 is understood to be of reconstructine type, involving extensive breakage of bonds, with its typical signatures like—coexistence of parent and daughter phases, sluggishness of the transition etc. The transition occurs with increase in coordination number, decrease in the density and symmetry. Such transitions sometimes end up with persistent metastable phase [17]. In the case of Gd2 O3 , for example, it was seen that heating the pressure quenched sample even up to 500 ◦ C did not make it revert back to parent structure. This was understandable because in these systems metal ion sublattice is very rigid and appreciable ion movement occurs only after about 1400 ◦ C [18]. 4. Concluding remarks We have studied the pressure-induced structural stability of bulk Ho2 O3 up to 17 GPa by both X-ray diffraction and laser-Raman
[1] [2] [3] [4] [5] [6] [7]
[8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
K. Petermann, L. Fornasiaro, E. Mix, V. Peters, Opt. Mater. 19 (2002) 67. E. Antic-Fidancev, J. Holsa, M. Lastusaari, J. Alloys Compd. 341 (2002) 82. T. Justel, J.C. Krupa, D.U. Wiechert, J. Lumin. 93 (2001) 179. L. Laversenne, Y. Guyot, C. Goutaudier, M.Th. Cohen-Adad, G. Boulon, Opt. Mater. 16 (2001) 475. O. Greis, J. Solid State Chem. 34 (1980) 39. G. Chen, J.R. Peterson, K.E. Brister, J. Solid State Chem. 111 (1994) 437. L. Dayana, N.V. Chandra Shekar, P.Ch. Sahu, B.V. Kumaraswamy, A.K. Bandhyopadhyay, M. Rajagopalan, Phil. Mag. Lett. 88 (2008) 473; F.X. Zhang, M. Lang, J.W. Wang, U. Becker, R.W. Ewing, Phys. Rev. B 78 (2008) 064114. D. Nita, M. Shalini, V. Deepak, S.K. Haldar, P. Himanshu, S.M. Sharma, B.V. Kumaraswamy, A.K. Bandopadhyay, Discussion meeting on materials under high pressures (MRSI-MHP), in: 18th Annual General Meeting (MRSI), National Physical Laboratory, New Delhi, 2007. L. Dayana, N.V. Chandra Shekar, P. Ch. Sahu, R. Paul, P. Paul, J. Alloys and Compds, (2008) doi:10.1016/j.jallcom.2009.10.068. H.R. Hoekstra, K.A. Gingerich, Science 146 (1964) 1163. P. Ch Sahu, N.V. Chandra Shekar, N. Subramanian, M. Yousuf, K. Govinda Rajan, Rev. Sci. Instrum. 66 (1995) 2599. R.A. Forman, G.J. Piermarini, J.D. Barnett, S. Block, Science 176 (1972) 284. G.J. Piermarini, S. Block, J.D. Barnett, R.A. Forman, J. Appl. Phys. 46 (1975) 2774. J. Jayapandian, R. Kesavamoorthy, A.K. Arora, J. Instrum. Soc. India 38 (2008) 55. F. Birch, Phys. Rev. 71 (1947) 809. A. Ubaldini, M.M. Carnasciali, J. Alloys Compd. 454 (2007) 374. N.V. Chandra Shekar, K. Govinda Rajan, Bull. Mater. Sci. 24 (2001) 1. L. Eyring, The binary rare earth oxides, in: K.A. Gschneidner (Ed.), Hand Book on the Physics and Chemistry of Rare Earths, vol. 3, Amsterdam, North Holland, 1978, p. 337.