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Structural phase transitions in RbBH4 under compression
Journal of Alloys and Compounds 476 (2009) 5–8
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural phase transitions in RbBH4 under compression Ravhi S. Kumar ∗ , Andrew L. Cornelius Department of Physics & Astronomy and High Pressure Science and Engineering Center (HiPSEC), University of Nevada Las Vegas, Las Vegas, NV 89154, USA
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 22 August 2008 Accepted 2 September 2008 Available online 17 October 2008 Keywords: Hydrogen storage Crystal structure High pressure X-ray diffraction
a b s t r a c t In situ high pressure synchrotron powder X-ray diffraction and Raman techniques were used to study the structural stability of RbBH4 at room temperature up to 27 GPa. Two successive pressure induced structural transitions were observed in RbBH4 from the ambient cubic Fm3m phase to an orthorhombic Pnma phase around 2.5 GPa, and then to a monoclinic P2/C phase above 8 GPa. The ambient cubic phase is found to be reversible. The experiments show that the structural polymorphs of RbBH4 observed at high pressures are different from the similar cubic hydrogen storage compounds NaBH4 and KBH4 . Published by Elsevier B.V.
1. Introduction Considerable research efforts have been devoted in recent years to find a viable, safe, efficient and environmental friendly hydrogen storage material [1,2]. Complex hydrides possess the largest gravimetric and volumetric hydrogen density and are most studied materials for portable applications [3]. Even though several problems remain unsolved regarding the practical usage of hydrogen fuel cells, studies show that borohydride fuel cells are promising due to their high performance and energy densities [4,5]. NaBH4 is actively investigated for its potential use in polymer electrolyte cells (PEFCs) and direct borohydride fuel cells (DBFCs). The DBFC has many advantages over the conventional direct methanol fuel cells (DMFC) as the borohydride fuel material used is safe, non-toxic and stable [5,6]. As the performance of these cells is limited due to oxidation of anodic material during hydrogen release reactions, search for suitable electrode materials are under progress [7–9]. More recently hydrolysis studies on KBH4 in addition to sodium and lithium borohydrides were performed to investigate the H2 release properties [10]. Recent studies are now focused on catalytic addition to reduce the high dissociation temperature and also to boost the borohydride hydrolysis reactions [11,12]. Despite the technological importance of these systems only little is known about the structural behavior on application of external temperature and pressure. Vajeeston et al. used density functional calculations to study the structural stability of a series of alkali hydrides with a
∗ Corresponding author. Fax: +1 7028950804. E-mail address: [email protected] (R.S. Kumar). 0925-8388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.09.022
general formula ABH4 (A = Li, Na, K, Rb and Cs) and provided the compression details of their polymorphs [13]. Several high pressure experiments were later performed in the past few years on tetraborohydrides and alkali aluminum hydrides to understand the structural behavior [14–17]. The availability of intense synchrotron sources and neutron diffraction techniques have elucidated high pressure phases of these compounds, and recent experiments performed on NaBH4 have shown two successive phase transitions from cubic to tetragonal and then to orthorhombic [18–20]. In order to understand the pressure induced changes in this family and to validate the theoretical simulations reported so far, high pressure experiments are required for ABH4 type compounds. Our experimental work is a part of systematic investigation of the behavior of these borohydrides under high pressure conditions. Here, we present results of combined high pressure X-ray diffraction and Raman experiments up to 27 GPa and demonstrate the pressure induced cubic to orthorhombic and then to monoclinic transitions in RbBH4 . 2. Experimental procedures (techniques) RbBH4 powder (97% purity) was obtained from Sigma Aldrich. X-ray diffraction measurements at ambient conditions showed single phase with cell parameter a = 7.031(2) Å, which agrees well with the previous literature [21]. For high pressure diffraction experiments, the sample along with a few tiny ruby chips was loaded in a Merrill–Bassett type diamond anvil cell in a stainless steel gasket (135 m initial hole diameter). Pressure was generated with 325 m culet diamonds. No pressure medium was used to avoid any reaction between the sample and the medium. Pressure inside the diamond anvil cell was determined by the standard ruby fluorescence method [22]. Angle dispersive X-ray diffraction (ADXRD) experiments were performed with an incident X-ray wavelength = 0.3857 Å at the high resolution powder diffraction beam line at Sector 16-IDB, HPCAT of the Advanced
6
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
we could not find a good fit representing all lines in the diffraction pattern for these space groups. We have indexed the diffraction pattern collected above 8 GPa to a monoclinic P2/C phase. The cell parameters obtained at 26 GPa are a = 2.695(2) Å, b = 3.763(8) Å and c = 7.615(3) Å with ˇ = 112.08(6)◦ (Fig. 1(c)). In order to confirm the high pressure behavior we have further performed high pressure Raman experiments. The bending modes were observed from 1050 to 1300 cm−1 while the stretching modes were observed in the region 2100–2500 cm−1 . The assignment of all the Raman frequencies observed for the sample as loaded in the DAC are listed in Table 1. The frequencies of bending modes and stretching modes for the cubic phase of the sample (nearly at ambient conditions in the DAC) match well with previous literature. The variation of stretching modes as a function of pressure is shown in Fig. 2. Upon increasing pressure we have observed changes both in bending and stretching modes around 2.5 GPa. We have noticed the appearance of few additional lines as listed in Table 1 showing a pressure induced phase change. This transition pressure (∼2.5 GPa) matches very well with the transition pressure obtained in diffraction due to a cubic to orthorhombic transition. Around 9.4 GPa, we have further noticed changes in the Raman modes which is close to the orthorhombic monoclinic transition discussed earlier. Except peak broadening, the spectra above 9.4 GPa looked similar up to 20 GPa which again is consistent with the diffraction experiments. In both diffraction and Raman experiments the ambient
Fig. 1. (a) X-ray diffraction patterns of RbBH4 at different pressures. (b and c) Indexing of the high pressure phases of RbBH4 at 3.9 and 26 GPa. The markers represent the observed spectrum; continuous line is the calculated pattern. The phase markers and the difference line are shown below.
Photon Source. The XRD patterns were recorded on a MAR imaging plate using an exposure time of 300 s, with an incident beam size of 30 m × 30 m. The patterns were then integrated using the Fit2D software program and indexing was carried out either with the MDI Jade package. The refinements were carried out with Rietica (LHPM) Rietveld program. High pressure Raman experiments were conducted at room temperature at HPCAT using a He-Ne laser (632.8 nm). The sample was loaded in a Merrill–Bassett type diamond anvil cell with a culet diameter of 300 m and the spectra were collected using a Jobin-Yvon LabRAM laser microscope Raman system and a CCD detector. The typical exposure time for each spectrum is 20 s.
3. Results and discussion 3.1. High pressure X-ray diffraction and Raman spectra of RbBH4 The diffraction patterns collected at various pressures are shown in Fig. 1(a). The cubic structure is found to be stable up to 2.1 GPa. Around 2.7 GPa we noticed splitting of the (1 1 1) peak of the cubic phase (2 = 5.6◦ ). Further, new peaks started to show up around 2 = 7.88◦ , 10.35◦ and 12.45◦ . We have also observed a change in the intensity of (2 0 0) line of cubic phase above 2.7 GPa. These features clearly indicated a pressure induced phase transition. We have performed full profile LeBail fitting and the diffraction pattern at 3.9 GPa was well indexed to the Pnma orthorhombic phase with cell parameters a = 4.138(2) Å, b = 5.668(5) Å, and c = 5.673(1) Å as shown in Fig. 1(b). Around 8 GPa we have observed similar changes in the diffraction pattern. In addition to merging of (1 1 1) and (0 0 2) lines, splitting of the (0 0 1) peak corresponding to the orthorhombic phase has been observed indicating a second phase transition. The orthorhombic phase was found to transform completely around 11 GPa. The diffraction patterns above 11 GPa showed no further changes up to 26 GPa. We have considered high pressure phases such as P421 C, P42 /nmc as possible candidates for the second high pressure phase as discussed previously. However
Fig. 2. Raman stretching modes as a function of pressure for RbBH4 .
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
7
Table 1 Raman frequencies of cubic and high pressure orthorhombic and monoclinic phases of RbBH4 DAC (P = 0.3 GPa)
Peak intensity
Raman mode assignment (cubic)
Orthorhombic (P = 4.2 GPa)
Monoclinic (P = 11 GPa)
1084 1122 1245
w
4 10
1126 (v-w) 1299 (w)
m
2
1100 (w) 1225 (m) 1245 (m)
2180
m
24 (A1)
2194 (w) 2213 (w)
2217 2230 2286 2309 2373
m w-sh m-sh s w
24 (F2) 210 4 (F2) 3 1 2 + 4
2229 (w)
2485
w
22
4
cubic phase is found to be completely reversible. The variation of cell parameters with pressure and the pressure–volume data for all three phases are shown in Fig. 3. The transition from the cubic to orthorhombic phase is accompanied by large volume collapse of 11%. However, the orthorhombic to monoclinic transition is followed by a calculated volume collapse of 3%. The pressure–volume data for the cubic phase has been fitted using the Birch–Murnaghan equation of state [23], P =
3 B0 2
−7/3 V
×
1+
V0
−
3 (B − 4) 4 0
V −5/3 V0
−2/3 V V0
−1
,
(1)
2342 (m-sh) 2380 (s)
2327 (w) 2359 (m) 2428 (m-sh) 2473 (s)
2481 (w) 2689 (w)
2523 (s) 2666 (w)
where B0 is the bulk modulus and B0 its pressure derivative. Least squares fitting resulted in B0 = 14.5(2) GPa with B0 = 4 for the cubic phase. The bulk modulus obtained in our experiments compare well with the bulk modulus of the cubic phase of NaBH4 and KBH4 . 3.2. Pressure induced transitions in RbBH4 The pressure induced phase transition path observed in RbBH4 is different from the cubic to tetragonal and orthorhombic transition sequence exhibited by similar cubic compounds NaBH4 and KBH4 [17,18,23]. On comparing the transition pressure in these systems, we find that the increase in the size of the alkali cation leads to a decrease in the transition pressure. The reason behind a direct cubic to orthorhombic transition in RbBH4 can be further understood by comparing the P–V curves of NaBH4 , RbBH4 and the bulk modulus of the cubic phase. In our previous works on NaBH4 , we have shown that the cubic to tetragonal transition is due to a slight distortion of the ambient cubic phase and also continuous with no significant volume collapse [18,19]. The lower bulk modulus observed for RbBH4 and a large volume collapse during the first transition imply that both the Rb–H bonds and the [BH4 ] units are more easily compressed and rearranged than NaBH4 , which may favor a direct transition from the cubic phase to the orthorhombic phase. The results of the Raman spectra as a function of temperature measured for these cubic borohydrides by Hageman et al., complement a similar scenario where the energy barrier of the reorientation of the [BH4]− anions decreases as a function of cation size [25]. Our high pressure results are consistent with this observation [24]. 4. Conclusions
Fig. 3. Pressure versus volume plot for RbBH4 . The top panel shows the change of cell parameters with pressure for various phases of RbBH4 .
From the experiments and theoretical calculations we have revealed a phase transition from the ambient Fm-3m cubic phase to a high pressure tetragonal (P421 c) phase at 4 GPa and subsequently to an orthorhombic (Pnma) phase at 11 GPa, which is very similar to the pressure behavior of the iso-structural compound NaBH4 . We speculate that the continuous cubic-tetragonal phase transition observed in the pressure–volume plots without a large volume collapse in KBH4 and NaBH4 is a consequence of a slight distortion of the cubic symmetry. We have recently carried out high pressure X-ray diffraction measurements on RbBH4 where we found pressure induces a direct transition to the orthorhombic (Pnma) phase. The reduction in the cubic to tetragonal transition pressure and the absence of tetragonal high pressure phase in RbBH4 suggest that the transition pressure is influenced by the size of the alkali atom during the structural rearrangement of [BH4 ]− anions and alkali cations. Better insight of the pressure induced changes may be obtained if the intermolecular bonding in the [BH4 ]− units for
8
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
these high pressure phases is studied with suitable experiments. In order to shed more light, high pressure in-elastic X-ray scattering experiments on boron K-edge are planned on NaBH4 and KBH4 . Acknowledgements The authors thank Stanislav Sinogeikin for technical help at HPCAT in X-ray and Raman measurements. Work at UNLV is supported by DOE award DE-FG36-05GO08502. HPCAT is collaboration among the UNLV High Pressure Science and Engineering Center, The Lawrence Livermore National Laboratory, the Geophysical Laboratory of the Carnegie Institution of Washington, and the University of Hawaii at Manoa. The UNLV High Pressure Science and Engineering Center was supported by the U.S. Department of Energy, National Nuclear Security Administration, under Co-operative agreement number DE-FC52-06NA26274. Use of APS was supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under contract number DE-AC02-06CH11357. References [1] L. Schlapbach, A. Zuttel, Nature 414 (2001) 353. [2] A. Zuttel, Mater. Today 6 (2003) 24–33. [3] A. Zuttel, A. Borgschulte, S.-I. Orimo, Scripta Mater. 56 (2007) 823.
[4] C.P. Leon, F.C. Walsh, D. Pletcher, D.J. Browning, J.B. Lakerman, J. Power Sources 155 (2006) 172. [5] J.H. Wee, J. Power Sources 161 (2006) 1. [6] B.H. Liu, Z.P. Li, S. Suda, J. Power Sources 175 (2008) 226. [7] K.T. Park, U.H. Jung, S.U. Jeong, S.H. Kim, J. Power Sources 162 (2006) 192. [8] U.B. Demirci, J. Power Sources 172 (2007) 676. [9] J. Ma, Y. Liu, P. Zhang, J. Wang, Electrochem. Commun. 10 (2008) 100. [10] O. Sahin, H. Dolas, M. Ozdemir, Int. J. Hydrogen Energy 32 (2007) 2330. [11] P. Krishnana, K.L. Hsueh, S.D. Yim, Appl. Catal. B 77 (2007) 206. [12] S. Srinivasan, D. Escobar, M. Jurczyk, Y. Goswami, E. Stefanskos, J. Alloys Compd. 462 (2008) 294. [13] P. Vajeeston, P. Ravidndran, A. Kjekshus, H. Fjellvag, J. Alloys Compd. 387 (2005) 97. [14] V. Talyzin, B. Sundqvist, Phys. Rev. B 70 (2004) 180101. [15] C.M. Aruajo, R. Ahuja, A.V. Talyzin, B. Sundqvist, Phys. Rev. B 72 (2005) 054125. [16] R.S. Chellappa, D. Chandra, S.A. Gramsch, R.J. Humley, J.F. Lin, Y. Song, J. Phys. Chem. B 110 (2006) 11088. [17] R.S. Kumar, E. Kim, O. Tschauner, A.L. Cornelius, M.P. Sulic, C.M. Jensen, Phys. Rev. B 75 (2007) 174110. [18] R.S. Kumar, A.L. Cornelius, Appl. Phys. Lett. 87 (2005) 261916. [19] E. Kim, R.S. Kumar, P.F. Weck, A.L. Cornelius, M. Nicol, S.C. Vogel, J. Zhang, M. Hartl, A.C. Stowe, L. Daemen, Y. Zhao, J. Phys. Chem. B 111 (2007) 13873. [20] Y. Filinchuk, A.V. Talyzin, D. Chernyshov, V. Dmitriev, Phys. Rev. B 76 (2007) 092104. [21] G. Renaudin, S. Gomes, H. Hagemann, L. Keller, K. Yvon, J. Alloys Compd. 375 (2004) 98. [22] H.K. Mao, J. Xu, P.M. Bell, J. Geophys. Res. B91 (1986) 4673. [23] F. Birch, J. Geophys. Res. 83 (1978) 1257. [24] R.S. Kumar, E. Kim, A.L. Cornelius, J. Phys. Chem. B 112 (2008) 8452. [25] H. Hagemann, S. Gomes, G. Renaudin, K. Yvon, J. Alloys Compd. 363 (2004) 129.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural phase transitions in RbBH4 under compression Ravhi S. Kumar ∗ , Andrew L. Cornelius Department of Physics & Astronomy and High Pressure Science and Engineering Center (HiPSEC), University of Nevada Las Vegas, Las Vegas, NV 89154, USA
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 22 August 2008 Accepted 2 September 2008 Available online 17 October 2008 Keywords: Hydrogen storage Crystal structure High pressure X-ray diffraction
a b s t r a c t In situ high pressure synchrotron powder X-ray diffraction and Raman techniques were used to study the structural stability of RbBH4 at room temperature up to 27 GPa. Two successive pressure induced structural transitions were observed in RbBH4 from the ambient cubic Fm3m phase to an orthorhombic Pnma phase around 2.5 GPa, and then to a monoclinic P2/C phase above 8 GPa. The ambient cubic phase is found to be reversible. The experiments show that the structural polymorphs of RbBH4 observed at high pressures are different from the similar cubic hydrogen storage compounds NaBH4 and KBH4 . Published by Elsevier B.V.
1. Introduction Considerable research efforts have been devoted in recent years to find a viable, safe, efficient and environmental friendly hydrogen storage material [1,2]. Complex hydrides possess the largest gravimetric and volumetric hydrogen density and are most studied materials for portable applications [3]. Even though several problems remain unsolved regarding the practical usage of hydrogen fuel cells, studies show that borohydride fuel cells are promising due to their high performance and energy densities [4,5]. NaBH4 is actively investigated for its potential use in polymer electrolyte cells (PEFCs) and direct borohydride fuel cells (DBFCs). The DBFC has many advantages over the conventional direct methanol fuel cells (DMFC) as the borohydride fuel material used is safe, non-toxic and stable [5,6]. As the performance of these cells is limited due to oxidation of anodic material during hydrogen release reactions, search for suitable electrode materials are under progress [7–9]. More recently hydrolysis studies on KBH4 in addition to sodium and lithium borohydrides were performed to investigate the H2 release properties [10]. Recent studies are now focused on catalytic addition to reduce the high dissociation temperature and also to boost the borohydride hydrolysis reactions [11,12]. Despite the technological importance of these systems only little is known about the structural behavior on application of external temperature and pressure. Vajeeston et al. used density functional calculations to study the structural stability of a series of alkali hydrides with a
∗ Corresponding author. Fax: +1 7028950804. E-mail address: [email protected] (R.S. Kumar). 0925-8388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.09.022
general formula ABH4 (A = Li, Na, K, Rb and Cs) and provided the compression details of their polymorphs [13]. Several high pressure experiments were later performed in the past few years on tetraborohydrides and alkali aluminum hydrides to understand the structural behavior [14–17]. The availability of intense synchrotron sources and neutron diffraction techniques have elucidated high pressure phases of these compounds, and recent experiments performed on NaBH4 have shown two successive phase transitions from cubic to tetragonal and then to orthorhombic [18–20]. In order to understand the pressure induced changes in this family and to validate the theoretical simulations reported so far, high pressure experiments are required for ABH4 type compounds. Our experimental work is a part of systematic investigation of the behavior of these borohydrides under high pressure conditions. Here, we present results of combined high pressure X-ray diffraction and Raman experiments up to 27 GPa and demonstrate the pressure induced cubic to orthorhombic and then to monoclinic transitions in RbBH4 . 2. Experimental procedures (techniques) RbBH4 powder (97% purity) was obtained from Sigma Aldrich. X-ray diffraction measurements at ambient conditions showed single phase with cell parameter a = 7.031(2) Å, which agrees well with the previous literature [21]. For high pressure diffraction experiments, the sample along with a few tiny ruby chips was loaded in a Merrill–Bassett type diamond anvil cell in a stainless steel gasket (135 m initial hole diameter). Pressure was generated with 325 m culet diamonds. No pressure medium was used to avoid any reaction between the sample and the medium. Pressure inside the diamond anvil cell was determined by the standard ruby fluorescence method [22]. Angle dispersive X-ray diffraction (ADXRD) experiments were performed with an incident X-ray wavelength = 0.3857 Å at the high resolution powder diffraction beam line at Sector 16-IDB, HPCAT of the Advanced
6
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
we could not find a good fit representing all lines in the diffraction pattern for these space groups. We have indexed the diffraction pattern collected above 8 GPa to a monoclinic P2/C phase. The cell parameters obtained at 26 GPa are a = 2.695(2) Å, b = 3.763(8) Å and c = 7.615(3) Å with ˇ = 112.08(6)◦ (Fig. 1(c)). In order to confirm the high pressure behavior we have further performed high pressure Raman experiments. The bending modes were observed from 1050 to 1300 cm−1 while the stretching modes were observed in the region 2100–2500 cm−1 . The assignment of all the Raman frequencies observed for the sample as loaded in the DAC are listed in Table 1. The frequencies of bending modes and stretching modes for the cubic phase of the sample (nearly at ambient conditions in the DAC) match well with previous literature. The variation of stretching modes as a function of pressure is shown in Fig. 2. Upon increasing pressure we have observed changes both in bending and stretching modes around 2.5 GPa. We have noticed the appearance of few additional lines as listed in Table 1 showing a pressure induced phase change. This transition pressure (∼2.5 GPa) matches very well with the transition pressure obtained in diffraction due to a cubic to orthorhombic transition. Around 9.4 GPa, we have further noticed changes in the Raman modes which is close to the orthorhombic monoclinic transition discussed earlier. Except peak broadening, the spectra above 9.4 GPa looked similar up to 20 GPa which again is consistent with the diffraction experiments. In both diffraction and Raman experiments the ambient
Fig. 1. (a) X-ray diffraction patterns of RbBH4 at different pressures. (b and c) Indexing of the high pressure phases of RbBH4 at 3.9 and 26 GPa. The markers represent the observed spectrum; continuous line is the calculated pattern. The phase markers and the difference line are shown below.
Photon Source. The XRD patterns were recorded on a MAR imaging plate using an exposure time of 300 s, with an incident beam size of 30 m × 30 m. The patterns were then integrated using the Fit2D software program and indexing was carried out either with the MDI Jade package. The refinements were carried out with Rietica (LHPM) Rietveld program. High pressure Raman experiments were conducted at room temperature at HPCAT using a He-Ne laser (632.8 nm). The sample was loaded in a Merrill–Bassett type diamond anvil cell with a culet diameter of 300 m and the spectra were collected using a Jobin-Yvon LabRAM laser microscope Raman system and a CCD detector. The typical exposure time for each spectrum is 20 s.
3. Results and discussion 3.1. High pressure X-ray diffraction and Raman spectra of RbBH4 The diffraction patterns collected at various pressures are shown in Fig. 1(a). The cubic structure is found to be stable up to 2.1 GPa. Around 2.7 GPa we noticed splitting of the (1 1 1) peak of the cubic phase (2 = 5.6◦ ). Further, new peaks started to show up around 2 = 7.88◦ , 10.35◦ and 12.45◦ . We have also observed a change in the intensity of (2 0 0) line of cubic phase above 2.7 GPa. These features clearly indicated a pressure induced phase transition. We have performed full profile LeBail fitting and the diffraction pattern at 3.9 GPa was well indexed to the Pnma orthorhombic phase with cell parameters a = 4.138(2) Å, b = 5.668(5) Å, and c = 5.673(1) Å as shown in Fig. 1(b). Around 8 GPa we have observed similar changes in the diffraction pattern. In addition to merging of (1 1 1) and (0 0 2) lines, splitting of the (0 0 1) peak corresponding to the orthorhombic phase has been observed indicating a second phase transition. The orthorhombic phase was found to transform completely around 11 GPa. The diffraction patterns above 11 GPa showed no further changes up to 26 GPa. We have considered high pressure phases such as P421 C, P42 /nmc as possible candidates for the second high pressure phase as discussed previously. However
Fig. 2. Raman stretching modes as a function of pressure for RbBH4 .
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
7
Table 1 Raman frequencies of cubic and high pressure orthorhombic and monoclinic phases of RbBH4 DAC (P = 0.3 GPa)
Peak intensity
Raman mode assignment (cubic)
Orthorhombic (P = 4.2 GPa)
Monoclinic (P = 11 GPa)
1084 1122 1245
w
4 10
1126 (v-w) 1299 (w)
m
2
1100 (w) 1225 (m) 1245 (m)
2180
m
24 (A1)
2194 (w) 2213 (w)
2217 2230 2286 2309 2373
m w-sh m-sh s w
24 (F2) 210 4 (F2) 3 1 2 + 4
2229 (w)
2485
w
22
4
cubic phase is found to be completely reversible. The variation of cell parameters with pressure and the pressure–volume data for all three phases are shown in Fig. 3. The transition from the cubic to orthorhombic phase is accompanied by large volume collapse of 11%. However, the orthorhombic to monoclinic transition is followed by a calculated volume collapse of 3%. The pressure–volume data for the cubic phase has been fitted using the Birch–Murnaghan equation of state [23], P =
3 B0 2
−7/3 V
×
1+
V0
−
3 (B − 4) 4 0
V −5/3 V0
−2/3 V V0
−1
,
(1)
2342 (m-sh) 2380 (s)
2327 (w) 2359 (m) 2428 (m-sh) 2473 (s)
2481 (w) 2689 (w)
2523 (s) 2666 (w)
where B0 is the bulk modulus and B0 its pressure derivative. Least squares fitting resulted in B0 = 14.5(2) GPa with B0 = 4 for the cubic phase. The bulk modulus obtained in our experiments compare well with the bulk modulus of the cubic phase of NaBH4 and KBH4 . 3.2. Pressure induced transitions in RbBH4 The pressure induced phase transition path observed in RbBH4 is different from the cubic to tetragonal and orthorhombic transition sequence exhibited by similar cubic compounds NaBH4 and KBH4 [17,18,23]. On comparing the transition pressure in these systems, we find that the increase in the size of the alkali cation leads to a decrease in the transition pressure. The reason behind a direct cubic to orthorhombic transition in RbBH4 can be further understood by comparing the P–V curves of NaBH4 , RbBH4 and the bulk modulus of the cubic phase. In our previous works on NaBH4 , we have shown that the cubic to tetragonal transition is due to a slight distortion of the ambient cubic phase and also continuous with no significant volume collapse [18,19]. The lower bulk modulus observed for RbBH4 and a large volume collapse during the first transition imply that both the Rb–H bonds and the [BH4 ] units are more easily compressed and rearranged than NaBH4 , which may favor a direct transition from the cubic phase to the orthorhombic phase. The results of the Raman spectra as a function of temperature measured for these cubic borohydrides by Hageman et al., complement a similar scenario where the energy barrier of the reorientation of the [BH4]− anions decreases as a function of cation size [25]. Our high pressure results are consistent with this observation [24]. 4. Conclusions
Fig. 3. Pressure versus volume plot for RbBH4 . The top panel shows the change of cell parameters with pressure for various phases of RbBH4 .
From the experiments and theoretical calculations we have revealed a phase transition from the ambient Fm-3m cubic phase to a high pressure tetragonal (P421 c) phase at 4 GPa and subsequently to an orthorhombic (Pnma) phase at 11 GPa, which is very similar to the pressure behavior of the iso-structural compound NaBH4 . We speculate that the continuous cubic-tetragonal phase transition observed in the pressure–volume plots without a large volume collapse in KBH4 and NaBH4 is a consequence of a slight distortion of the cubic symmetry. We have recently carried out high pressure X-ray diffraction measurements on RbBH4 where we found pressure induces a direct transition to the orthorhombic (Pnma) phase. The reduction in the cubic to tetragonal transition pressure and the absence of tetragonal high pressure phase in RbBH4 suggest that the transition pressure is influenced by the size of the alkali atom during the structural rearrangement of [BH4 ]− anions and alkali cations. Better insight of the pressure induced changes may be obtained if the intermolecular bonding in the [BH4 ]− units for
8
R.S. Kumar, A.L. Cornelius / Journal of Alloys and Compounds 476 (2009) 5–8
these high pressure phases is studied with suitable experiments. In order to shed more light, high pressure in-elastic X-ray scattering experiments on boron K-edge are planned on NaBH4 and KBH4 . Acknowledgements The authors thank Stanislav Sinogeikin for technical help at HPCAT in X-ray and Raman measurements. Work at UNLV is supported by DOE award DE-FG36-05GO08502. HPCAT is collaboration among the UNLV High Pressure Science and Engineering Center, The Lawrence Livermore National Laboratory, the Geophysical Laboratory of the Carnegie Institution of Washington, and the University of Hawaii at Manoa. The UNLV High Pressure Science and Engineering Center was supported by the U.S. Department of Energy, National Nuclear Security Administration, under Co-operative agreement number DE-FC52-06NA26274. Use of APS was supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under contract number DE-AC02-06CH11357. References [1] L. Schlapbach, A. Zuttel, Nature 414 (2001) 353. [2] A. Zuttel, Mater. Today 6 (2003) 24–33. [3] A. Zuttel, A. Borgschulte, S.-I. Orimo, Scripta Mater. 56 (2007) 823.
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