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The local structure of Cs(I) and Cs(II) in a Cs2ZnCl4 single crystal studied by nuclear magnetic resonance
Journal of Physics and Chemistry of Solids 64 (2003) 933–937 www.elsevier.com/locate/jpcs
The local structure of Cs(I) and Cs(II) in a Cs2ZnCl4 single crystal studied by nuclear magnetic resonance Ae Ran Lima,*, Oc Hee Hanb, Se-Young Jeongc b
a Department of Physics, Jeonju University, Jeonju 560-759, South Korea Solid-State Analysis Team, Daegu Branch, Korea Basic Science Institute, Daegu 702-701, South Korea c Department of Physics, Pusan National University, Pusan 609-735, South Korea
Received 18 September 2002; accepted 12 November 2002
Abstract The rotation patterns of the 133Cs (I ¼ 7/2) nuclear magnetic resonance (NMR) in a Cs2ZnCl4 single crystal grown by using the slow evaporation method were measured in two mutually perpendicular crystal planes. Two different groups of Cs resonances were recorded; this result points to the existence of two types of crystallographically inequivalent Cs(I) and Cs(II). The angular dependences of the NMR spectra led to different values for the quadrupole coupling constants and asymmetry parameters: e2 qQ=h ¼ 148 kHz and h ¼ 0.11 for the Cs(I) ion, and e2 qQ=h ¼ 274 kHz and h ¼ 0.66 for the Cs(II) ion. The EFG tensors of Cs(I) and Cs(II) are asymmetric, and the orientations of the principal axes of the EFG tensors do not coincide. Only, the principal Y-axes of the EFG tensors coincide for the Cs(I) and Cs(II) sites. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: 61.50cj: Crystal growth; 76.60k: Nuclear magnetic resonance; 77.80e: Ferroelectrics
1. Introduction Since the detection of ferroelectricity in Rb2ZnCl4 [1], many investigations have been made on A2BX4-type halide substances, where A ¼ alkaline ion, B ¼ Zn, Co, and X ¼ halide, with particular attention to ferroelectricity and incommensurate phase transitions [2]. Many of the substances have been reported to exhibit one or more phase transitions, and some of them show structurally incommensurate phases [3– 7] while others show no phase transitions [7 – 9]. However, it seems that very few studies have been made for substances of the Cs2BX4 type, where a Cs ion is put on the A site. It is known that Cs2ZnCl4 does not show any structural phase transition [9,10]. Also, Cs2ZnCl4 without a structural phase transition was studied * Corresponding author. Tel.: þ82-63-220-2514; fax: þ 82-63220-2054. E-mail address: [email protected] (A.R. Lim).
with a view to obtain details on the dynamics of the ZnCl4 anion through 35Cl NQR frequency and relaxation time measurements [11,12]. Although the physical properties for this material have been reported [13– 19], enough research for nuclear magnetic resonance (NMR) have not yet been studied. The unit cell of the Cs2ZnCl4 crystal is orthorhombic and belongs to the space group Pnam (D16 2h ) with four formula units per unit cell. The cell parameters are ˚ , b ¼ 7.40 A ˚ , and c ¼ 12.98 A ˚ [20]. Half of a ¼ 9.76 A the unit cell is shown in Fig. 1(a). The unit cell contains four ZnCl22 complexes. The local symmetry of the 4 ZnCl22 ion is governed by the mirror plane (010) 4 passing through the central ion and two Cl2 ions. However, the local symmetry is close to that of a tetragonal sphenoid. There are two types of crystallographically different Csþ ions in the cell [21]. The Cs(I) and the Cs(II) ions are surrounded by 11 and 9 chlorine ions, respectively, as shown in Fig. 1(b).
0022-3697/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 4 5 1 - 1
934
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
field was 9.4 T, and the central frequency was set at v0 =2p ¼ 52:48 MHz for the 133Cs nucleus. A special probe head was constructed for the cryomagnet system, which rendered possible to rotate the sample around an axis perpendicular to the static magnetic field B0 and to incline the complete probe head by angles up to about 58 with respect to the axis of the cryomagnet. Thus, the crystal orientation could be adjusted very sensitively with respect to the static magnetic field.
3. Experimental results and analysis The NMR spectra of the using the usual Hamiltonian
133
Cs nucleus were analyzed
H ¼ HZ þ HQ ;
ð1Þ
where HZ is the nuclear Zeeman term and HQ describes the nuclear quadrupole interaction of the 133Cs nucleus, which has nuclear spin I ¼ 7/2. The Zeeman interaction is given by [22,23] HZ ¼ 2gn bn B·I;
ð2Þ
where gn is the nuclear g-factor, bn the nuclear magneton, B the magnetic field, and I is the nuclear spin. The quadrupole Hamiltonian in the principal-axis system of the EFG tensor is described by Fig. 1. (a) Projection of the orthorhombic structure of Cs2ZnCl4 along the [001] direction. (b) Surroundings of the Cs(I) and the Cs(II) ions.
In this research, we studied the local structure around the cesium atoms by investigating the quadrupole interactions of the 133Cs nucleus in a Cs2ZnCl4 single crystal. The angular dependence of the 133Cs NMR spectra was measured at room temperature. From these results, the nuclear quadrupole coupling constant, e2 qQ=h; the asymmetry parameter, h, and the direction of the principal axes of the electric field gradient (EFG) tensor of 133Cs were determined. The 133Cs (I ¼ 7/2) NMR observations for the Cs2ZnCl4 single crystals are new, and these results show that the Cs(I) and the Cs(II) sites can be clearly distinguished by using 133Cs NMR.
2. Experimental procedure The Cs2ZnCl4 crystal was grown by slow evaporation of an aqueous solution of CsCl and ZnCl2. The crystallographic axis of the crystal was confirmed by X-ray Laue diffraction. In order to obtain the rotation pattern of 133Cs NMR, we mounted the sample perpendicular to the crystal faces (ab- and ca-planes). NMR signals of 133Cs in a Cs2ZnCl4 single crystal were measured using the Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute. The static magnetic
HQ ¼ e2 qQ=4Ið2I 2 1Þ½3I2Z 2 IðI þ 1Þ þ 12 hðI2þ þ I22 Þ; 2
ð3Þ
where e qQ=h is the quadrupole coupling constant and h is the asymmetry parameter. The conventional X, Y, and Z axes are such that lVXX l # lVYY l # lVZZ l ¼ eq; then, 0 # h # 1. The NMR spectrum of 133Cs (I ¼ 7/2) along the crystallographic c-axis at room temperature is shown in Fig. 2. Two different groups of Cs resonance were recorded. The spectra of the two groups are associated with two crystallographically inequivalent positions of the cesium atoms in the unit cell. The intensity of the central line is stronger than those of the other lines. The zero point of the x-axis shows the resonance frequency, 52.48 MHz, of the 133Cs nucleus. The Cs spectrum is displaced by a chemical shift to the higherfrequency side relative to the reference signal obtained for the 133 Cs line from an aqueous solution of CsNO3. The shifts of these two groups, one smaller and the other larger, represent the transitions of 133Cs NMR lines due to Cs(I) and Cs(II), respectively. Cs(I) and Cs(II) are represented by dark and open circles, respectively, in Fig. 2. The rotation patterns of 133 Cs measured in the crystallographic ab- and ca-planes at room temperature are shown in Figs. 3 and 4. The central transition is almost constant due to the quadrupolar interaction, but the satellite transitions are well resolved from the central spectrum. The satellite resonance lines show an angular dependence of 3cos2u 2 1 whereas the central line is angle independent. The rotation pattern of Cs(I) is different from that of Cs(II), as shown in Figs. 3 and 4. In case
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
935
Fig. 2. NMR spectrum of 133Cs in a Cs2ZnCl4 crystal. The static magnetic field B is parallel to the c-axis.
of Cs(I) and Cs(II), the frequency differences between the maximum and minimum separation at the b-axis were very large as about 40,197 and 97,144 Hz, respectively. Because the resonance frequency of the central line is almost constant and the spacings between adjacent lines are equal, the first-order perturbation of HQ with respect to HZ
should be sufficient for analysis. Two different Cs resonance groups with different magnitudes of the quadrupole coupling constant and the asymmetry parameter were analyzed. The quadrupole parameters were determined by using the experimental data of Figs. 3 and 4; e2 qQ=h ¼ 148 kHz and h ¼ 0.11 for Cs(I), and e2 qQ=h ¼ 274 kHz and h ¼ 0.66
Fig. 3. Rotation pattern of 133Cs NMR measured in the ab-plane at room temperature.
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A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
Fig. 4. Rotation pattern of 133Cs NMR measured in the ca-plane at room temperature.
for Cs(II) at room temperature. Therefore, the EFG tensors of Cs(I) and Cs(II) are asymmetric. The quadrupole coupling constant and the asymmetry parameter obtained for Cs(I) are smaller than those for Cs(II). The maximum separation of the resonance lines due to the quadrupole interaction was observed when the magnetic field was applied along the aaxis in case of Cs(I) and along c-axis of the crystal in case of Cs(II) as shown in Figs. 3 and 4. From this result, the principal axes of the EFG tensor are determined in the Cs2ZnCl4 crystal; X along c, Y along b, Z along a for Cs(I) and X along a, Y along b, Z along c for Cs(II), as shown in Fig. 5. The principal Y-axes of the EFG tensors coincide for the Cs(I) and the Cs(II) sites. In a previous report, based on the dielectric constants, Onodera et al. [18] reported that Cs2ZnCl4 had at least two transition points ( ¼ 326, 572 K) above room temperature. The rotation patterns of 133Cs NMR were measured at 333 K, above the phase transition temperature ( ¼ 326 K) suggested by the above authors. Within the error range,
Fig. 5. Principal axes of the EFG tensor for Cs(I) and Cs(II).
the NMR rotation patterns at 333 K are the same as those at room temperature. Therefore, this material does not exhibit a structural phase transition at 326 K.
4. Discussion and conclusion The NMR method makes it possible to study the local properties of the lattice, and it is particularly convenient in those cases in which information on the behavior of individual structural groups is required. The EFG tensor obtained from the NMR result gives a good approximate description of the degree of distortion in the groups of the Cs– Cl bonds. The main contribution to the 133Cs EFG originates from the nearest-neighbor Cl. The 133Cs NMR in a Cs2ZnCl4 single crystal grown by using the slow evaporation method was investigated by employing a Bruker FT NMR spectrometer. There are two sets of crystallographically inequivalent Cs(I) and Cs(II) ions. From the experimental results, all the parameters were determined, and they led to different values for the quadrupole coupling constant and asymmetry parameter. The quadrupole coupling constant e2 qQ=h ¼ eQð›Ez =›zÞ=h and asymmetry parameter h ¼ ð›Ex =›xÞ 2 ð›Ey =›yÞ=ð›Ez =›zÞ for Cs(I) and Cs(II) can be calculated directly from the experimental data, but in order to obtain eq ¼ ð›Ez =›zÞ; one needs to know Q for the Cs nucleus. From these results, the components of the asymmetry parameter were found; the values for lVXX l; lVYY l; lVZZ l are 0.909 £ 1021, 1.134 £ 1021, 2.043 £ 1021 V/m2 for Cs(I) and 0.643 £ 1021, 3.138 £ 1021, 3.781 £ 1021 V/m2 for Cs(II), respectively. We know that the Cs(II) ions surrounded by nine chlorine ions have more
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
largely charge distribution than the Cs(I) ions surrounded by 11 chlorine ions. Thus, the quadrupole coupling constants and the asymmetry parameters of Cs(I) and Cs(II) reveal the configuration of ionic charges around Csþ. The EFG tensors of Cs(I) and Cs(II) are both asymmetric, and the orientations of the principal axes of the EFG tensor do not coincide. In the polyhedron of the Cs(I) ion, the mean Cs– Cl distance is ˚ , and in that of the Cs(II) ion, 3.494 A ˚ , considerably 3.563 A smaller. Because Cs(II) is surrounded by fewer and nearer ligands than Cs(I), the quadrupole coupling constant of Cs(II) are larger than those of Cs(I). This is qualitatively consistent with the longer bond length of Cs(I) – Cl and the shorter Cs(II) – Cl bond. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion. The NMR results clearly distinguished the Cs(I) and Cs(II) sites.
Acknowledgements This work was supported by a Korea Research Foundation grant (KRF-2001-015-DP0181).
References [1] S. Sawada, Y. Shiroshi, A. Yamamoto, M. Takashige, M. Matsuo, J. Phys. Soc. Jpn 43 (1977) 2099.
937
[2] F. Shimizu, T. Anzai, S. Sawada, M. Takashige, Ferroelectrics 185 (1996) 301. [3] R. Blinic, Phys. Rep. 79 (1981) 331. [4] H.Z. Cummins, Phys. Rep. 185 (1990) 211. [5] F. Milia, G. Papavassiliou, Phys. Rev. B 39 (1989) 4467. [6] S. Chen, D.C. Ailion, Solid State Commun. 69 (1989) 1041. [7] S. Plesko, R. Kind, H. Arend, Phys. Status Solidi A 61 (1980) 87. [8] H. Nakayama, N. Nakamura, H. Chihara, Bull. Chem. Soc. Jpn 60 (1987) 99. [9] O.P. Lamba, S.K. Sinha, Solid State Commun. 57 (1986) 365. [10] P.T.T. Wong, J. Chem. Phys. 64 (1976) 2186. [11] R.K. Subramanian, K. Venu, V.S.S. Sastry, J. Phys.: Condens. Matter 6 (1994) 137. [12] D.E. Scaife, Aust. J. Chem. 24 (1971) 1315. [13] H.A. Weakliem, J. Chem. Phys. 36 (1962) 2117. [14] P.T.T. Wong, J. Chem. Phys. 66 (1977) 2347. [15] R.P.J. Merks, P. van der Heide, R. de Beer, J. Phys. C: Solid State Phys. 12 (1979) 3097. [16] A.G.K. Moorthy, P.A.K. Sekhar, Curr. Sci. 53 (1984) 857. [17] R. Puget, M. Jannin, R. Perret, L. Godefroy, G. Godefroy, Ferroelectrics 107 (1990) 229. [18] A. Onodera, O. Watanabe, H. Haga, T. Kikuta, E. Suzuki, H. Yamashita, Y. Shiozaki, Ferroelectrics 125 (1992) 141. [19] P.J. Deren, J. Derouet, P. Porcher, D. Svoronos, J. Mol. Struct. 404 (1997) 167. [20] J.A. McGinnety, Inorg. Chem. 13 (1974) 1057. [21] R.P. Michel, J.R. Perret, L. Godefroy, G. Godefroy, Ferroelectrics 107 (1990) 229. [22] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961. [23] B. Cowan, Nuclear Magnetic Resonance and Relaxation, Cambridge University Press, Cambridge, 1997.
The local structure of Cs(I) and Cs(II) in a Cs2ZnCl4 single crystal studied by nuclear magnetic resonance Ae Ran Lima,*, Oc Hee Hanb, Se-Young Jeongc b
a Department of Physics, Jeonju University, Jeonju 560-759, South Korea Solid-State Analysis Team, Daegu Branch, Korea Basic Science Institute, Daegu 702-701, South Korea c Department of Physics, Pusan National University, Pusan 609-735, South Korea
Received 18 September 2002; accepted 12 November 2002
Abstract The rotation patterns of the 133Cs (I ¼ 7/2) nuclear magnetic resonance (NMR) in a Cs2ZnCl4 single crystal grown by using the slow evaporation method were measured in two mutually perpendicular crystal planes. Two different groups of Cs resonances were recorded; this result points to the existence of two types of crystallographically inequivalent Cs(I) and Cs(II). The angular dependences of the NMR spectra led to different values for the quadrupole coupling constants and asymmetry parameters: e2 qQ=h ¼ 148 kHz and h ¼ 0.11 for the Cs(I) ion, and e2 qQ=h ¼ 274 kHz and h ¼ 0.66 for the Cs(II) ion. The EFG tensors of Cs(I) and Cs(II) are asymmetric, and the orientations of the principal axes of the EFG tensors do not coincide. Only, the principal Y-axes of the EFG tensors coincide for the Cs(I) and Cs(II) sites. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: 61.50cj: Crystal growth; 76.60k: Nuclear magnetic resonance; 77.80e: Ferroelectrics
1. Introduction Since the detection of ferroelectricity in Rb2ZnCl4 [1], many investigations have been made on A2BX4-type halide substances, where A ¼ alkaline ion, B ¼ Zn, Co, and X ¼ halide, with particular attention to ferroelectricity and incommensurate phase transitions [2]. Many of the substances have been reported to exhibit one or more phase transitions, and some of them show structurally incommensurate phases [3– 7] while others show no phase transitions [7 – 9]. However, it seems that very few studies have been made for substances of the Cs2BX4 type, where a Cs ion is put on the A site. It is known that Cs2ZnCl4 does not show any structural phase transition [9,10]. Also, Cs2ZnCl4 without a structural phase transition was studied * Corresponding author. Tel.: þ82-63-220-2514; fax: þ 82-63220-2054. E-mail address: [email protected] (A.R. Lim).
with a view to obtain details on the dynamics of the ZnCl4 anion through 35Cl NQR frequency and relaxation time measurements [11,12]. Although the physical properties for this material have been reported [13– 19], enough research for nuclear magnetic resonance (NMR) have not yet been studied. The unit cell of the Cs2ZnCl4 crystal is orthorhombic and belongs to the space group Pnam (D16 2h ) with four formula units per unit cell. The cell parameters are ˚ , b ¼ 7.40 A ˚ , and c ¼ 12.98 A ˚ [20]. Half of a ¼ 9.76 A the unit cell is shown in Fig. 1(a). The unit cell contains four ZnCl22 complexes. The local symmetry of the 4 ZnCl22 ion is governed by the mirror plane (010) 4 passing through the central ion and two Cl2 ions. However, the local symmetry is close to that of a tetragonal sphenoid. There are two types of crystallographically different Csþ ions in the cell [21]. The Cs(I) and the Cs(II) ions are surrounded by 11 and 9 chlorine ions, respectively, as shown in Fig. 1(b).
0022-3697/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 4 5 1 - 1
934
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
field was 9.4 T, and the central frequency was set at v0 =2p ¼ 52:48 MHz for the 133Cs nucleus. A special probe head was constructed for the cryomagnet system, which rendered possible to rotate the sample around an axis perpendicular to the static magnetic field B0 and to incline the complete probe head by angles up to about 58 with respect to the axis of the cryomagnet. Thus, the crystal orientation could be adjusted very sensitively with respect to the static magnetic field.
3. Experimental results and analysis The NMR spectra of the using the usual Hamiltonian
133
Cs nucleus were analyzed
H ¼ HZ þ HQ ;
ð1Þ
where HZ is the nuclear Zeeman term and HQ describes the nuclear quadrupole interaction of the 133Cs nucleus, which has nuclear spin I ¼ 7/2. The Zeeman interaction is given by [22,23] HZ ¼ 2gn bn B·I;
ð2Þ
where gn is the nuclear g-factor, bn the nuclear magneton, B the magnetic field, and I is the nuclear spin. The quadrupole Hamiltonian in the principal-axis system of the EFG tensor is described by Fig. 1. (a) Projection of the orthorhombic structure of Cs2ZnCl4 along the [001] direction. (b) Surroundings of the Cs(I) and the Cs(II) ions.
In this research, we studied the local structure around the cesium atoms by investigating the quadrupole interactions of the 133Cs nucleus in a Cs2ZnCl4 single crystal. The angular dependence of the 133Cs NMR spectra was measured at room temperature. From these results, the nuclear quadrupole coupling constant, e2 qQ=h; the asymmetry parameter, h, and the direction of the principal axes of the electric field gradient (EFG) tensor of 133Cs were determined. The 133Cs (I ¼ 7/2) NMR observations for the Cs2ZnCl4 single crystals are new, and these results show that the Cs(I) and the Cs(II) sites can be clearly distinguished by using 133Cs NMR.
2. Experimental procedure The Cs2ZnCl4 crystal was grown by slow evaporation of an aqueous solution of CsCl and ZnCl2. The crystallographic axis of the crystal was confirmed by X-ray Laue diffraction. In order to obtain the rotation pattern of 133Cs NMR, we mounted the sample perpendicular to the crystal faces (ab- and ca-planes). NMR signals of 133Cs in a Cs2ZnCl4 single crystal were measured using the Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute. The static magnetic
HQ ¼ e2 qQ=4Ið2I 2 1Þ½3I2Z 2 IðI þ 1Þ þ 12 hðI2þ þ I22 Þ; 2
ð3Þ
where e qQ=h is the quadrupole coupling constant and h is the asymmetry parameter. The conventional X, Y, and Z axes are such that lVXX l # lVYY l # lVZZ l ¼ eq; then, 0 # h # 1. The NMR spectrum of 133Cs (I ¼ 7/2) along the crystallographic c-axis at room temperature is shown in Fig. 2. Two different groups of Cs resonance were recorded. The spectra of the two groups are associated with two crystallographically inequivalent positions of the cesium atoms in the unit cell. The intensity of the central line is stronger than those of the other lines. The zero point of the x-axis shows the resonance frequency, 52.48 MHz, of the 133Cs nucleus. The Cs spectrum is displaced by a chemical shift to the higherfrequency side relative to the reference signal obtained for the 133 Cs line from an aqueous solution of CsNO3. The shifts of these two groups, one smaller and the other larger, represent the transitions of 133Cs NMR lines due to Cs(I) and Cs(II), respectively. Cs(I) and Cs(II) are represented by dark and open circles, respectively, in Fig. 2. The rotation patterns of 133 Cs measured in the crystallographic ab- and ca-planes at room temperature are shown in Figs. 3 and 4. The central transition is almost constant due to the quadrupolar interaction, but the satellite transitions are well resolved from the central spectrum. The satellite resonance lines show an angular dependence of 3cos2u 2 1 whereas the central line is angle independent. The rotation pattern of Cs(I) is different from that of Cs(II), as shown in Figs. 3 and 4. In case
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
935
Fig. 2. NMR spectrum of 133Cs in a Cs2ZnCl4 crystal. The static magnetic field B is parallel to the c-axis.
of Cs(I) and Cs(II), the frequency differences between the maximum and minimum separation at the b-axis were very large as about 40,197 and 97,144 Hz, respectively. Because the resonance frequency of the central line is almost constant and the spacings between adjacent lines are equal, the first-order perturbation of HQ with respect to HZ
should be sufficient for analysis. Two different Cs resonance groups with different magnitudes of the quadrupole coupling constant and the asymmetry parameter were analyzed. The quadrupole parameters were determined by using the experimental data of Figs. 3 and 4; e2 qQ=h ¼ 148 kHz and h ¼ 0.11 for Cs(I), and e2 qQ=h ¼ 274 kHz and h ¼ 0.66
Fig. 3. Rotation pattern of 133Cs NMR measured in the ab-plane at room temperature.
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A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
Fig. 4. Rotation pattern of 133Cs NMR measured in the ca-plane at room temperature.
for Cs(II) at room temperature. Therefore, the EFG tensors of Cs(I) and Cs(II) are asymmetric. The quadrupole coupling constant and the asymmetry parameter obtained for Cs(I) are smaller than those for Cs(II). The maximum separation of the resonance lines due to the quadrupole interaction was observed when the magnetic field was applied along the aaxis in case of Cs(I) and along c-axis of the crystal in case of Cs(II) as shown in Figs. 3 and 4. From this result, the principal axes of the EFG tensor are determined in the Cs2ZnCl4 crystal; X along c, Y along b, Z along a for Cs(I) and X along a, Y along b, Z along c for Cs(II), as shown in Fig. 5. The principal Y-axes of the EFG tensors coincide for the Cs(I) and the Cs(II) sites. In a previous report, based on the dielectric constants, Onodera et al. [18] reported that Cs2ZnCl4 had at least two transition points ( ¼ 326, 572 K) above room temperature. The rotation patterns of 133Cs NMR were measured at 333 K, above the phase transition temperature ( ¼ 326 K) suggested by the above authors. Within the error range,
Fig. 5. Principal axes of the EFG tensor for Cs(I) and Cs(II).
the NMR rotation patterns at 333 K are the same as those at room temperature. Therefore, this material does not exhibit a structural phase transition at 326 K.
4. Discussion and conclusion The NMR method makes it possible to study the local properties of the lattice, and it is particularly convenient in those cases in which information on the behavior of individual structural groups is required. The EFG tensor obtained from the NMR result gives a good approximate description of the degree of distortion in the groups of the Cs– Cl bonds. The main contribution to the 133Cs EFG originates from the nearest-neighbor Cl. The 133Cs NMR in a Cs2ZnCl4 single crystal grown by using the slow evaporation method was investigated by employing a Bruker FT NMR spectrometer. There are two sets of crystallographically inequivalent Cs(I) and Cs(II) ions. From the experimental results, all the parameters were determined, and they led to different values for the quadrupole coupling constant and asymmetry parameter. The quadrupole coupling constant e2 qQ=h ¼ eQð›Ez =›zÞ=h and asymmetry parameter h ¼ ð›Ex =›xÞ 2 ð›Ey =›yÞ=ð›Ez =›zÞ for Cs(I) and Cs(II) can be calculated directly from the experimental data, but in order to obtain eq ¼ ð›Ez =›zÞ; one needs to know Q for the Cs nucleus. From these results, the components of the asymmetry parameter were found; the values for lVXX l; lVYY l; lVZZ l are 0.909 £ 1021, 1.134 £ 1021, 2.043 £ 1021 V/m2 for Cs(I) and 0.643 £ 1021, 3.138 £ 1021, 3.781 £ 1021 V/m2 for Cs(II), respectively. We know that the Cs(II) ions surrounded by nine chlorine ions have more
A.R. Lim et al. / Journal of Physics and Chemistry of Solids 64 (2003) 933–937
largely charge distribution than the Cs(I) ions surrounded by 11 chlorine ions. Thus, the quadrupole coupling constants and the asymmetry parameters of Cs(I) and Cs(II) reveal the configuration of ionic charges around Csþ. The EFG tensors of Cs(I) and Cs(II) are both asymmetric, and the orientations of the principal axes of the EFG tensor do not coincide. In the polyhedron of the Cs(I) ion, the mean Cs– Cl distance is ˚ , and in that of the Cs(II) ion, 3.494 A ˚ , considerably 3.563 A smaller. Because Cs(II) is surrounded by fewer and nearer ligands than Cs(I), the quadrupole coupling constant of Cs(II) are larger than those of Cs(I). This is qualitatively consistent with the longer bond length of Cs(I) – Cl and the shorter Cs(II) – Cl bond. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion. The NMR results clearly distinguished the Cs(I) and Cs(II) sites.
Acknowledgements This work was supported by a Korea Research Foundation grant (KRF-2001-015-DP0181).
References [1] S. Sawada, Y. Shiroshi, A. Yamamoto, M. Takashige, M. Matsuo, J. Phys. Soc. Jpn 43 (1977) 2099.
937
[2] F. Shimizu, T. Anzai, S. Sawada, M. Takashige, Ferroelectrics 185 (1996) 301. [3] R. Blinic, Phys. Rep. 79 (1981) 331. [4] H.Z. Cummins, Phys. Rep. 185 (1990) 211. [5] F. Milia, G. Papavassiliou, Phys. Rev. B 39 (1989) 4467. [6] S. Chen, D.C. Ailion, Solid State Commun. 69 (1989) 1041. [7] S. Plesko, R. Kind, H. Arend, Phys. Status Solidi A 61 (1980) 87. [8] H. Nakayama, N. Nakamura, H. Chihara, Bull. Chem. Soc. Jpn 60 (1987) 99. [9] O.P. Lamba, S.K. Sinha, Solid State Commun. 57 (1986) 365. [10] P.T.T. Wong, J. Chem. Phys. 64 (1976) 2186. [11] R.K. Subramanian, K. Venu, V.S.S. Sastry, J. Phys.: Condens. Matter 6 (1994) 137. [12] D.E. Scaife, Aust. J. Chem. 24 (1971) 1315. [13] H.A. Weakliem, J. Chem. Phys. 36 (1962) 2117. [14] P.T.T. Wong, J. Chem. Phys. 66 (1977) 2347. [15] R.P.J. Merks, P. van der Heide, R. de Beer, J. Phys. C: Solid State Phys. 12 (1979) 3097. [16] A.G.K. Moorthy, P.A.K. Sekhar, Curr. Sci. 53 (1984) 857. [17] R. Puget, M. Jannin, R. Perret, L. Godefroy, G. Godefroy, Ferroelectrics 107 (1990) 229. [18] A. Onodera, O. Watanabe, H. Haga, T. Kikuta, E. Suzuki, H. Yamashita, Y. Shiozaki, Ferroelectrics 125 (1992) 141. [19] P.J. Deren, J. Derouet, P. Porcher, D. Svoronos, J. Mol. Struct. 404 (1997) 167. [20] J.A. McGinnety, Inorg. Chem. 13 (1974) 1057. [21] R.P. Michel, J.R. Perret, L. Godefroy, G. Godefroy, Ferroelectrics 107 (1990) 229. [22] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961. [23] B. Cowan, Nuclear Magnetic Resonance and Relaxation, Cambridge University Press, Cambridge, 1997.