in a LiNH 4 SO 4 Single Crystal

Transcription

1 Ae Ran Lim et al.: 7 Li Spin±Lattice Relaxation Time in LiNH 4 SO 4 Single Crystal 375 phys. stat. sol. (b) 214, 375 (1999) Subject classification: Es; Kb; S Li Spin±Lattice Relaxation Time in a LiNH 4 SO 4 Single Crystal Ae Ran Lim 1 (a), Tae Jong Han (a), and Se-Young Jeong (b) (a) Department of Physics, Jeonju University, Jeonju , Korea (b) Department of Physics, Pusan National University, Pusan , Korea (Received December 29, 1998; in revised form May 3, 1999) The temperature dependence of 7 Li spin±lattice relaxation time in a LiNH 4 SO 4 single crystal has been investigated at MHz in the temperature range of 210 to 340 K. The time T 1 is relatively long in the whole temperature range, and increases with temperature from 121 s at 210 K to 646 s at 340 K. The discontinuity in the curve of the spin±lattice relaxation time near 285 K corresponds to the phase transition in the crystal. This confirms that the phase-iii- to phase-ii-transition is a first-order phase transition. The activation energies in phases III and II are and 3.68 kj/mol, respectively. The large change of activation energy for phases III and II at 285 K indicates that the LiO 4 groups are significantly affected during this transition. 1. Introduction Many investigations have been performed on LiNH 4 SO 4, including dielectric [1 to 3], optical [4, 5], thermal [3, 6, 7], NMR [2, 4, 5, 8], elastic constants [9], Brillouin scattering [10], and pressure studies [3, 4, 11, 12]. Three phases of LiNH 4 SO 4 crystal can be distinguished. Phase I, for T > 459 K, has an orthorhombic structure. Phase II exists in the temperature range of 283 K < T < 459 K with almost the same parameters of the unit cell as phase I. Finally, phase III for temperature T < 283 K has a monoclinic structure [13 to 16]. There are four molecules per unit cell in phases I and II, and eight in phase III. It has been established that LiNH 4 SO 4 single crystals have a phase transition from phase II to phase III of first order, while the phase transition from phase I to phase II is of second order [17]. Also, Hildman et al. [18] showed that phase III is ferroelastic. Therefore, LiNH 4 SO 4, was the first example of a ferroelastic crystal that is not simultaneously ferroelectric. Recently, Lim and Jeong [19] confirmed, from examination of the stress±strain hysteresis loops that the low-temperature phase is ferroelastic. In this paper, the temperature dependence of the spin±lattice relaxation time, T 1, for 7 Li in a LiNH 4 SO 4 single crystal grown by the slow evaporation method was investigated using a pulse NMR spectrometer. The relaxation time of 7 Li in a LiNH 4 SO 4 single crystal at phase II and III is a new observation. 2. Crystal Structure Dollase [13] determined the room temperature crystal structure of LiNH 4 SO 4. The space group is Pc2 1 n with a ˆ 5:28 A, b ˆ 9:140 A and c ˆ 8:786 A. As seen in 1 Corresponding author, aeranlim@hanmail.net

2 376 Ae Ran Lim, Tae Jong Han, and Se-Young Jeong Fig. 1. LiNH 4 SO 4 polyhedral linkage along the c-axis. Only one tetrahedral layer is shown Fig. 1, the structure consists of the six-membered rings typical for a tridymite framework made up of tetrahedral LiO 4 and SO 4 groups. In each ring, three adjacent tetrahedra have their vertices pointing up and the other three pointing down, where the up and down pointing alternate around the ring. The six-membered rings are connected along the c-axis by an almost linear Li±O±S bond. The layers of six-membered rings form channels along this axis, in which the NH 4 groups are located. The NH 4 groups lie approximately at the centers of the large cavities in the tetrahedral framework. The group is eightfold coordinated, with six oxygen atoms forming the corners of a truncated trigonal prism and two equatorial oxygen atoms lying outside of the two prism faces. Each oxygen atom in the structure is coordinated to one Li and one S atom and forms a hydrogen bond with two NH 4 groups. 3. Experimental Procedure Good optical quality single crystals of LiNH 4 SO 4 were grown by slow evaporation of an aqueous solution. This solution was comprised by a stoichiometric mixture of Li 2 SO 4 H 2 O and NH 4 2 SO 4 at about 300 K. The resulting crystals were transparent and colorless. The size of the sample, parallelepiped in shape, was 0:5 0:5 8 mm 3. Its long axis was parallel to the [100] direction. These crystals were prismatically elongated along the a-axis, and were cleaved along the (001) plane [20]. Untwinned parts of the crystals were selected as samples for the measurements: The orientation of the crystal was identified by using an optical polarizing microscope and the X-ray Laue method. Nuclear magnetic resonance signals of 7 Li in the LiNH 4 SO 4 single crystal were measured by using a Varian 400 FT NMR spectrometer. The static magnetic field was 9.4 T and the central rf frequency was set at w 0 =2p ˆ 155:51 MHz. The temperature-dependent NMR measurements were taken in the temperature range of 210 to 340 K by the previously described method [21].

3 7 Li Spin±Lattice Relaxation Time in LiNH 4 SO 4 Single Crystal Experimental Results and Analysis The spin±lattice relaxation is an energy transfer from the nuclear spin system to the surrounding environment [22]. This energy flow rate is proportional to the coupling strength of the nuclear spins with the neighborhood, and to the quantum excitation probability. In order to measure the spin±lattice relaxation time, T 1 a pulse sequence of 180 ±t±90 was applied at 54.7 from the c-axis in the ac-plane of the crystal. The resonance spectrum in this direction consists of a single resonance line instead of three resonance lines for Li. The nuclear magnetization M z t of 7 Li I ˆ 3=2 at time t after the 180 pulse was determined from the inversion recovery sequence following the pulse. The recovery traces measured at several different temperatures are defined by the following exponential function [23]: M z t ˆ M exp t=t 1 Š ; 1 where M z, is the projection of the spin magnetization towards the z-axis. The relaxation time, T 1, in equation (1) was determined directly from the slope of the log M 0 M z t =2M 0 Š versus time t plot. This trace is nicely explained by a single exponential function of the type in equation (1). The temperature dependence of T 1 for 7 Li in the single crystal is shown in Fig. 2. The spin±lattice relaxation time is very long with T 1 ˆ 550 s at 300 K. The discontinuities in the curve of T 1 near 285 K correspond to the phase transition. This means that the phase-iii- to phase-ii-transition is a firstorder type. The relaxation time increases with temperature in both phases, although the rate is slightly slower in phase II. Fig. 2. Temperature dependence of the spin±lattice relaxation time, T 1, for 7 Li in a LiNH 4 SO 4 single crystal

4 378 Ae Ran Lim, Tae Jong Han, and Se-Young Jeong Ta b l e 1 The activation energies (in kj/mol) for 7 Li and 1 H in LiNH 4 SO 4 single crystal phase II phase III ref. 7 Li present work 1 H Shenoy and Ramakrishna [8] The temperature dependence of T 1 follows a simple Arrhenius expression T 1 exp E a =kt ; 2 where E a is the activation energy for LiO 4 reorientations. The activation energy for LiNH 4 SO 4 crystals was determined by a curve fitting equation (2). The activation energies in phases III and II were determined to be and 3.68 kj/mol, respectively. Our observation of changes in the activation energy is consistent with the activation energy obtained from the relaxation times for 1 H by Shenoy and Ramakrishna [8]. These activation energies are shown in Table 1 for the phases II and III. The large change in the activation energy at 285 K indicates that the LiO 4 groups are significantly affected during this transition. Since the LiO 4 tetrahedron is distorted, the main effect on the Li nucleus must originate from its nearest neighbor oxygens. Further studies on the temperature dependence of the 7 Li nuclear magnetic resonance in this crystal are currently being conducted. Acknowledgements This work was supported by the Basic Science Research Institute Program, Ministry of Education, 1998, Project No. BSRI (2411) and by Korea Science and Engineering Foundation (KOSEF) through the Research Centre for Dielectric and Advanced Matter Physics (RCDAMP) at Pusan National University (1997 ±2000). It was also supported in part by the Faculty Research Grant Jeonju University (1999). References [1] T. Mitsui, T. Oka, Y. Shiroishi, M. Takashige, K. Iio, and S. Sawada, J. Phys. Soc. Jpn. 39, 845 (1975). [2] V. I. Yuzvak, I. Zherebtsova, V. B. Shkuryaeva, and I. P. Aleksandrova, Soviet Phys. ± Cryst. 19, 480 (1975). [3] H. Shimizu, A. Oguri, N. Yasuda, and S. Fujimoto, J. Phys. Soc. Jpn. 45, 565 (1978). [4] K. S. Aleksandrov, I. P. Aleksandrova, A. T. Anistratov, and V. E. Shabanov, Izv. Akad. Nauk. SSSR, Ser. Fiz. 41, 599 (1977). [5] I. P. Aleksandrova, I. S. Kabanov, S. V. Melnikova, T. I. Chekmasova, and V. I. Yuzvak, Soviet Phys. ± Solid State 19, 605 (1977). [6] I. M. Tomaszewski and I. N. Flerov, Soviet Phys. ± Solid State 9, 605 (1977). [7] P. E. Tomaszewski and A. Pietraszko, phys. stat. sol. (a) 56, 467 (1979). [8] R. K. Shenoy and J. Ramakrishna, J. Phys. C 13, 5429 (1980). [9] K. S. Aleksandrov, I. P. Aleksandrova, L. I. Zherebtsova, A. I. Kruglik, A. I. Krupnyi, S. V. Melnikova, V. I. Shneider, and L. A. Shuvalov, Izv. Akad. Nauk SSSR, Ser. Fiz. 39, 943 (1975). [10] S. Hirotsu, Y. Kunii, I. Yamamoto, M. Miyamoto, and T. Mitsui, J. Phys. Soc. Jpn. 50, 3392 (1981). [11] T. I. Chekmasova, I. S. Kabanov, and V. I. Yuzvak, phys. stat. sol. (a) 44, K155 (1977).

5 7 Li Spin±Lattice Relaxation Time in LiNH 4 SO 4 Single Crystal 379 [12] T. Nakamura, S. Kojima, M. Takashige, T. Mitsui, K. Asaumi, S. Itoh, and S. Minomura, Jpn. J. appl. Phys. 18, 711 (1979). [13] W. A. Dollase, Acta Cryst. B25, 2298 (1969). [14] T. Mitsui, J. Phys. Soc. Jpn. 39, 845 (1975). [15] A. I. Kruglik, Kristallografiya 23, 494 (1978). [16] A. I. Kruglik, M. A. Simonov, and K. S. Aleksandrov, Soviet Phys. ± Cryst. 23, 274 (1978). [17] V. I. Yuzvak, L. I. Zherebtsova, V. B. Shkuryaeva, and I. P. Aleksandrova, Kristallografiya 19, 773 (1974). [18] O. Hildmann, Th. Mahn, L. E. Cross, and R. E. Newnahn, Appl. Phys. Lett. 27, 103 (1975). [19] Ae Ran Lim and Se-Young Jeong, phys. stat. sol. (a) 164, 673 (1997). [20] B. Pura and J. Przedmojski, Acta Phys. Polon. A 59, 785 (1981). [21] A. R. Lim, S. H. Choh, and S. Y. Jeong, J. Phys.: Condensed Matter 8, 4589 (1996). [22] A. Abragam, The Principles of Nuclear Magnetism, Chap. 7, Oxford University Press [23] C. P. Slichter, Principles of Magnetic Resonance, Chap. 10, Springer-Verlag Berlin, 1989.

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