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Structural phase transition in d-benzil characterised by capacitance measurements and neutron powder diffraction
Solid State Communications 136 (2005) 543–545 www.elsevier.com/locate/ssc
Structural phase transition in d-benzil characterised by capacitance measurements and neutron powder diffraction D.J. Goossensa,*, Xiaodong Wub, M. Priorc a
Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia School of Physics and Materials Engineering, Monash University, Clayton, Vic. 3800, Australia c The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia b
Received 20 July 2005; received in revised form 14 September 2005; accepted 14 September 2005 by F. De la Cruz Available online 3 October 2005
Abstract The ferroelectric phase transition in deuterated benzil, C14H10O2, has been studied using capacitance measurements and neutron powder diffraction. Hydrogenous benzil shows a phase transition at 83.5 K from a high temperature P3121 phase to a cell-doubled P21 phase. The phase transition in d-benzil occurs at 88.1 K, a small isotope effect. Neutron powder diffraction was consistent with a low temperature phase of space group P21. Upon deuteration the transition remained first-order and the dynamics of the phenyl ring dominated the behaviour. The isotope effect can be attributed to the difference in mass and moment of inertia between C6H5 and C6D5. q 2005 Elsevier Ltd. All rights reserved. PACS: 61.12.Kq; 61.66.Hq Keywords: A. Benzil; A. Deuterated ferroelectric; D. Structural phase transition
1. Introduction Benzil, C14H10O2 (Fig. 1), is an archetypal molecular system. Its dynamic behaviour combines long-range modes with intra-molecular motions and local displacement correlations. Above 83.5 K its space group is P3121 ˚ , cZ13.655 A ˚ , hexagonal setting) [1]. On (aZ8.401 A cooling benzil undergoes a first-order phase transition to a ferroelectric ferroelastic phase. It is believed that this phase transition is driven by phonon softening at the M-point on the zone boundary which is triggered by a G point instability [2,3]. The effect of isotopic substitution has been studied extensively for the ferroelectric phase transition in several materials, including the molecular ferroelectric potassium * Corresponding author. Tel.: C61 2 6125 0750. E-mail address: [email protected] (D.J. Goossens).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.017
dihydrogen phosphate, KH2PO4 or KDP. In h-KDP the transition temperature, Tc, is 122 K; deuteration moves this to 229 K. This large shift has been related to a range of possible causes including proton tunnelling and geometric effects [4]. The low temperature phase of benzil is of space ˚, group P21 with lattice parameters aZ14.380 A ˚ , cZ13.359 A ˚ and bZ88.828 at 70 K [1]. bZ8.373 A
2. Experimental Crystals (5!5!5 mm3) of h- and d-benzil were grown by controlled evaporation of solvent from a saturated acetone solution into which a seed crystal was placed. The crystals were used for capacitance measurements at Monash University, using a variable-temperature, capacitance dilatometer facility designed for thermal expansion measurements [5]. For this experiment the parallel-sided
544
D.J. Goossens et al. / Solid State Communications 136 (2005) 543–545 H4
H5 O1
C6 C7 H3
C5
C2
C1
C4 C3 H2
H1
H5
H4 C7 C6
C1
C2
C5
H3
C3 C4
O1 H1
H2
Fig. 1. Schematic diagram of benzil; atoms with identical labels are equivalent in P3121.
crystal was the capacitor dielectric so that the capacitance measured (using an Andeen Hagerling 2000A Capacitance Bridge) was proportional to the dielectric constant of the crystal. Neutron powder diffraction (NPD) was performed using the MRPD at the Bragg Institute; as-purchased d-benzil was crushed, loaded into a vanadium can and attached to a closed cycle helium refrigerator. Powder diffraction rather than single crystal was used to access more temperatures in a given time and because the benzil crystals do not pass intact through the first-order phase transition. The d-benzil was sourced from CDN Isotopes. All H sites nominally w99.3% D.
3. Results 3.1. Dielectric measurements Tc for h-benzil from capacitance measurements agreed with the published figure and confirmed that the technique gave a strong signal. Tc for d-benzil was 88.1G0.5 K
˚ ). The upper Fig. 3. NPD pattern of d-benzil at 100 K (lZ1.664 A solid line gives the fit (RpZ3.4, RBraggZ1.3), the crosses the data and the lower line the difference. Inset: peak at 37.5–40.08 (boxed in the main figure) as it evolves with T.
(Fig. 2), an increase of w5.5%, very different to the 88% increase on deuterating KDP. 3.2. Neutron powder diffraction NPD patterns were measured for 10%x%150 K. Fig. 3 shows a pattern at 100 K fitted using Rietica [6]. The inset shows a feature (the (1 2 2) and (0 1 5) reflections in P3121) as it evolves with T. At 85 K the peak centre-of-mass has shifted due to a splitting which becomes apparent at low T, marking the onset of the phase transition. The low temperature phase data were well-fitted in P21. In P21 there are three benzil molecules in the asymmetric unit, so powder diffraction data at this resolution do not allow determination of all parameters; the molecules were allowed to move only as rigid units while atoms which were symmetry related in P3121 were constrained to have the same atomic displacement parameters (adps). Only isotropic adps (BisoZ(BxxCByyCBzz)/3) could be used. Refinements 2
90 -β (deg.)
1.5
90−β Fit 1
0.5
0 20
Fig. 2. Capacitance, a measure proportional to the dielectric constant, versus temperature for d-benzil in an arbitrary orientation.
40
60 T (K)
80
100
Fig. 4. Extrapolation of the monoclinic angle to 908 suggests TcZ 98 K.
D.J. Goossens et al. / Solid State Communications 136 (2005) 543–545 8.0
90 Κ 95 Κ 150 Κ
6.0
H2 H1
B iso
H4 C3
4.0
C4
O1 C1
H3
C5
545
the most significant modes is well below 100 K, but also indicating the limitations of the isotropic adps. Even so, it can be said that the relevant structural unit in the dynamics is the entire phenyl ring, whose fractional increase in mass on deuteration is 6.5%, comparable to the fractional increase in Tc.
C6
2.0
C2
C7 H5
4. Conclusions 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Distance of atom from C1 (Angstrom) ˚ 2) as a function of distance from C1 at three Fig. 5. Biso (A temperatures in the high temperature P3121 phase.
showed that the two phases co-exist at 89 K, indicative of a first-order transition in d-benzil. Fig. 4 plots the monoclinic angle of P21, b, against T; it extrapolates to Tcw98 K, yet the capacitance measurement gives 88.1 K, suggesting that at Tc b goes discontinuously to zero, again indicative of a first-order transition. Refinements above Tc show an increase in Bisos of the C and D atoms in the phenyl rings with distance from the centre of the molecule and from the torsional axis through the phenyl group. Fig. 5 shows the Biso for the atoms in the asymmetric unit of the high temperature phase as a function of distance from C1. H1 exhibits an unexpectedly large adp. It has been suggested [7] that a key interaction between molecules in benzil is the H-bond from the O of one molecule to the para H (H3) of a neighbouring molecule. ˚, However, within a molecule the O1–H1 distance is 2.456 A ˚ ); less than this important intermolecular contact (2.554 A hence the motion of H1 is strongly coupled to that of the OaC–CaO bridge and to that of its intermolecular environment, which may explain its exceptional adp which does not follow the phenyl ring as closely as the other H (D) atoms. The adps of C3, the C attached to H1, and O1 are unexceptional and it is unlikely that the large adp of H1 apparent in d-benzil has a significant impact on the phase transition. The temperature dependence of the Biso is small over the range considered, indicating that the energy scale of
The ferroelectric phase transition in d-benzil occurs at 88.1G0.5 K compared to 83.5 K for h-benzil. The order of the phase transition is unaltered by deuteration; Tc is modified slightly through the change in mass of the phenyl rings. The situation is simpler than in some other molecular ferroelectrics such as KDP. Further neutron scattering studies using d-benzil can be conducted with confidence in their relevance to h-benzil.
Acknowledgements We thank AINSE for support through grant AINGRA04200. DJG thanks A. Prof T.R. Finlayson of Monash University for helpful discussions.
References [1] M. More, G. Odou, J. Lefebvre, Acta Crystallogr. B43 (1987) 398. [2] J.C. Tole´dano, Phys. Rev. B 20 (1979) 1147. [3] H. Terauchi, T. Kojima, K. Sakaue, F. Tajiri, H. Maeda, J. Chem. Phys. 76 (1982) 612. [4] S. Koval, J. Kohanoff, R. Migoni, E. Tosatti, Phys. Rev. Lett. 89 (2002) 187602. [5] M. Liu, T. Finlayson, T. Smith, Mater. Sci. Forum 56–58 (1990) 311. [6] B.A. Hunter, IUCr Commision on Powder Diffraction Newsletter, vol. 20, 1998 p. 21. [7] T.R. Welberry, D.J. Goossens, A.J. Edwards, W.I.F. David, Acta Cryst. A57 (2001) 101.
Structural phase transition in d-benzil characterised by capacitance measurements and neutron powder diffraction D.J. Goossensa,*, Xiaodong Wub, M. Priorc a
Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia School of Physics and Materials Engineering, Monash University, Clayton, Vic. 3800, Australia c The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia b
Received 20 July 2005; received in revised form 14 September 2005; accepted 14 September 2005 by F. De la Cruz Available online 3 October 2005
Abstract The ferroelectric phase transition in deuterated benzil, C14H10O2, has been studied using capacitance measurements and neutron powder diffraction. Hydrogenous benzil shows a phase transition at 83.5 K from a high temperature P3121 phase to a cell-doubled P21 phase. The phase transition in d-benzil occurs at 88.1 K, a small isotope effect. Neutron powder diffraction was consistent with a low temperature phase of space group P21. Upon deuteration the transition remained first-order and the dynamics of the phenyl ring dominated the behaviour. The isotope effect can be attributed to the difference in mass and moment of inertia between C6H5 and C6D5. q 2005 Elsevier Ltd. All rights reserved. PACS: 61.12.Kq; 61.66.Hq Keywords: A. Benzil; A. Deuterated ferroelectric; D. Structural phase transition
1. Introduction Benzil, C14H10O2 (Fig. 1), is an archetypal molecular system. Its dynamic behaviour combines long-range modes with intra-molecular motions and local displacement correlations. Above 83.5 K its space group is P3121 ˚ , cZ13.655 A ˚ , hexagonal setting) [1]. On (aZ8.401 A cooling benzil undergoes a first-order phase transition to a ferroelectric ferroelastic phase. It is believed that this phase transition is driven by phonon softening at the M-point on the zone boundary which is triggered by a G point instability [2,3]. The effect of isotopic substitution has been studied extensively for the ferroelectric phase transition in several materials, including the molecular ferroelectric potassium * Corresponding author. Tel.: C61 2 6125 0750. E-mail address: [email protected] (D.J. Goossens).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.017
dihydrogen phosphate, KH2PO4 or KDP. In h-KDP the transition temperature, Tc, is 122 K; deuteration moves this to 229 K. This large shift has been related to a range of possible causes including proton tunnelling and geometric effects [4]. The low temperature phase of benzil is of space ˚, group P21 with lattice parameters aZ14.380 A ˚ , cZ13.359 A ˚ and bZ88.828 at 70 K [1]. bZ8.373 A
2. Experimental Crystals (5!5!5 mm3) of h- and d-benzil were grown by controlled evaporation of solvent from a saturated acetone solution into which a seed crystal was placed. The crystals were used for capacitance measurements at Monash University, using a variable-temperature, capacitance dilatometer facility designed for thermal expansion measurements [5]. For this experiment the parallel-sided
544
D.J. Goossens et al. / Solid State Communications 136 (2005) 543–545 H4
H5 O1
C6 C7 H3
C5
C2
C1
C4 C3 H2
H1
H5
H4 C7 C6
C1
C2
C5
H3
C3 C4
O1 H1
H2
Fig. 1. Schematic diagram of benzil; atoms with identical labels are equivalent in P3121.
crystal was the capacitor dielectric so that the capacitance measured (using an Andeen Hagerling 2000A Capacitance Bridge) was proportional to the dielectric constant of the crystal. Neutron powder diffraction (NPD) was performed using the MRPD at the Bragg Institute; as-purchased d-benzil was crushed, loaded into a vanadium can and attached to a closed cycle helium refrigerator. Powder diffraction rather than single crystal was used to access more temperatures in a given time and because the benzil crystals do not pass intact through the first-order phase transition. The d-benzil was sourced from CDN Isotopes. All H sites nominally w99.3% D.
3. Results 3.1. Dielectric measurements Tc for h-benzil from capacitance measurements agreed with the published figure and confirmed that the technique gave a strong signal. Tc for d-benzil was 88.1G0.5 K
˚ ). The upper Fig. 3. NPD pattern of d-benzil at 100 K (lZ1.664 A solid line gives the fit (RpZ3.4, RBraggZ1.3), the crosses the data and the lower line the difference. Inset: peak at 37.5–40.08 (boxed in the main figure) as it evolves with T.
(Fig. 2), an increase of w5.5%, very different to the 88% increase on deuterating KDP. 3.2. Neutron powder diffraction NPD patterns were measured for 10%x%150 K. Fig. 3 shows a pattern at 100 K fitted using Rietica [6]. The inset shows a feature (the (1 2 2) and (0 1 5) reflections in P3121) as it evolves with T. At 85 K the peak centre-of-mass has shifted due to a splitting which becomes apparent at low T, marking the onset of the phase transition. The low temperature phase data were well-fitted in P21. In P21 there are three benzil molecules in the asymmetric unit, so powder diffraction data at this resolution do not allow determination of all parameters; the molecules were allowed to move only as rigid units while atoms which were symmetry related in P3121 were constrained to have the same atomic displacement parameters (adps). Only isotropic adps (BisoZ(BxxCByyCBzz)/3) could be used. Refinements 2
90 -β (deg.)
1.5
90−β Fit 1
0.5
0 20
Fig. 2. Capacitance, a measure proportional to the dielectric constant, versus temperature for d-benzil in an arbitrary orientation.
40
60 T (K)
80
100
Fig. 4. Extrapolation of the monoclinic angle to 908 suggests TcZ 98 K.
D.J. Goossens et al. / Solid State Communications 136 (2005) 543–545 8.0
90 Κ 95 Κ 150 Κ
6.0
H2 H1
B iso
H4 C3
4.0
C4
O1 C1
H3
C5
545
the most significant modes is well below 100 K, but also indicating the limitations of the isotropic adps. Even so, it can be said that the relevant structural unit in the dynamics is the entire phenyl ring, whose fractional increase in mass on deuteration is 6.5%, comparable to the fractional increase in Tc.
C6
2.0
C2
C7 H5
4. Conclusions 0.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Distance of atom from C1 (Angstrom) ˚ 2) as a function of distance from C1 at three Fig. 5. Biso (A temperatures in the high temperature P3121 phase.
showed that the two phases co-exist at 89 K, indicative of a first-order transition in d-benzil. Fig. 4 plots the monoclinic angle of P21, b, against T; it extrapolates to Tcw98 K, yet the capacitance measurement gives 88.1 K, suggesting that at Tc b goes discontinuously to zero, again indicative of a first-order transition. Refinements above Tc show an increase in Bisos of the C and D atoms in the phenyl rings with distance from the centre of the molecule and from the torsional axis through the phenyl group. Fig. 5 shows the Biso for the atoms in the asymmetric unit of the high temperature phase as a function of distance from C1. H1 exhibits an unexpectedly large adp. It has been suggested [7] that a key interaction between molecules in benzil is the H-bond from the O of one molecule to the para H (H3) of a neighbouring molecule. ˚, However, within a molecule the O1–H1 distance is 2.456 A ˚ ); less than this important intermolecular contact (2.554 A hence the motion of H1 is strongly coupled to that of the OaC–CaO bridge and to that of its intermolecular environment, which may explain its exceptional adp which does not follow the phenyl ring as closely as the other H (D) atoms. The adps of C3, the C attached to H1, and O1 are unexceptional and it is unlikely that the large adp of H1 apparent in d-benzil has a significant impact on the phase transition. The temperature dependence of the Biso is small over the range considered, indicating that the energy scale of
The ferroelectric phase transition in d-benzil occurs at 88.1G0.5 K compared to 83.5 K for h-benzil. The order of the phase transition is unaltered by deuteration; Tc is modified slightly through the change in mass of the phenyl rings. The situation is simpler than in some other molecular ferroelectrics such as KDP. Further neutron scattering studies using d-benzil can be conducted with confidence in their relevance to h-benzil.
Acknowledgements We thank AINSE for support through grant AINGRA04200. DJG thanks A. Prof T.R. Finlayson of Monash University for helpful discussions.
References [1] M. More, G. Odou, J. Lefebvre, Acta Crystallogr. B43 (1987) 398. [2] J.C. Tole´dano, Phys. Rev. B 20 (1979) 1147. [3] H. Terauchi, T. Kojima, K. Sakaue, F. Tajiri, H. Maeda, J. Chem. Phys. 76 (1982) 612. [4] S. Koval, J. Kohanoff, R. Migoni, E. Tosatti, Phys. Rev. Lett. 89 (2002) 187602. [5] M. Liu, T. Finlayson, T. Smith, Mater. Sci. Forum 56–58 (1990) 311. [6] B.A. Hunter, IUCr Commision on Powder Diffraction Newsletter, vol. 20, 1998 p. 21. [7] T.R. Welberry, D.J. Goossens, A.J. Edwards, W.I.F. David, Acta Cryst. A57 (2001) 101.