LOW TEMPERATURE X-RAY INVESTIGATIONS OF Rb2Lid(SeO&

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1 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol LOW TEMPERATURE X-RAY INVESTIGATIONS OF Rb2Lid(SeO& 2H20 Wieslawa Bronowska* and Adam Pietraszko* * *Institute of Phy sits, Wroclaw University of Technology, Wybrz. Wysp. 27, Wroclaw, Poland, **Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Ok6lna 2, Wroclaw, Poland ABSTRACT Rb2Li4(Se0&2HzO has a monoclinic symmetry (space group P21/c) with two chemical units forming the room temperature unit cell (a = 5.256(I) 4 b = 5. I78(1) fi, c = (5) fi, p = 93. I1(3) deg; V = 726.6(2) A3 ). The powder diffraction data have been collected at 30 K, 80 K, 150 K, 250 K and 300 K. The temperature dependencies of the lattice parameters of these crystals, obtained from the single-crystal measurements and the powder diffraction data have been investigated. No phase transition was revealed over the temperature range from 30 K to 302 K. INTRODUCTION In recent years, much attention has been devoted to the two-component systems of the L&SO4 - A4e$TO4 type, where Me = Rb, Cs, K, NI&, as a consequence of the wide range of their physico-chemical properties. An interesting feature of these compounds is the existence of successive phase transitions between their different phases [l - lo]. In part, scientific interest in these systems is motivated by the desire to compare the structural, dynamic and physical properties of these compounds and to investigate how their feature may be influenced by the structural characteristic. In the course of our work on the two - component systems of the L&Se04 - Rb#eOd, we have isolated the title compound, RbzLi4(Se0&.2HzO, which belong to a new family of selenates hydrates.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol A classification scheme based on orientational states of polyhedra predicts the possibility of displacive-type phase transitions on changing the pressure or temperature of ionic compounds [ll]. A nondynamic model based on reorientational effects was also proposed [12]. According to this model, double salts are very likely to undergo transitions with variation of temperature. EXPERIMENTAL RbzLi4(Se0&2HzO crystals were obtained by the slow evaporation method. Data for the structure investigation were collected on a KM-4 KUMA single-crystal diffractometer with graphite-monochromated MO K, radiation (h= ) and with a low-temperature,,oxford-cryosystem attachment, AT= f0.3 K. Lattice parameters were measured over the temperature range from 95 K to 302 K and were determined by least- squares refinement of the setting angles for 30 selected reflections. The powder diffraction measurements of RbzLi@e0&2H20 were carried out on a URD powder diffractometer and a Simens D5000 diffractometer with a low temperature attachment (a closed helium system) over the temperature range from 30 K to 300 K. RESULTS AND DISCUSSION The Rb2Li4(Se0&2HzO crystals have a monoclinic symmetry (space group P21/c) with two chemical units (2 = 2) forming the room temperature unit cell. Lattice parameters at room temperature are. a = 5.256(l) ii, b = 5.178(l) h., c = (5) ih, p = 93. I1 (3) deg; V = 726.6(2) A3. The powder diffraction data of RbzLi4(Se0&.2HzO collected at 300 K, 250 K, 150 K, 80 K and 30 K are presented in Table 1. The powder diffraction patterns obtained for several temperatures have not shown any essential differencies. Lattice parameters of the titled crystals, measured with using the KM-4 KUMA diffractometer single-crystal over the temperature range from 95 K to 302 K are plotted as a function of temperature in Fig. 1 and 2. The lattice parameters calculated on the basis of the powder diffraction measurements (Table 2) and normalised to the lattice parameters obtained from the single crystal method are presented in these figures as the open circles. No anomalies have been revealed on the temperature dependencies of the lattice parameters of these crystals.

4 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Table 1. The powder difffraction data of Rb2Li4(Se0& 2HzO at selected temperature points T = 300 K T=250K T=150K T=80K T=30K hkl d IA1 Int. d [A] Int. d WI Int. d [A] Int. d WI Int , O , l-l l o O l-7 2, l Table 2 The lattice parameters of RbzLi4(Se0& 2H20 at selected temperature points T=300K T=250K T=lSOK T=SOK T=30K a [Al (2) (2) (2) (2) (2) b [Al (1) (1) (2) (2) (2) c [Al (7) (6) (8) (6) (6) P Wgl (3) (3) (3) (3) (3) Differential scanning calorimetric investigations have also shown that RbzLi4(SeO& 2H20 crystals do not exhibit any phase transition over the temperature range from 100 K to 350 K WI.

5 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Although our experimental observations do not reveal the existence of any phase transition over the temperature range from 30 K to 350 K, the structural disordering at room temperature in the Rb2Li4(SeO&2H20 crystal [ 131 implies that this compound is likely 5.25 a b z a 2 ([I 5.23 m i [ g F; Ii -I Temeperature [K] Fig. 1. The temperature dependence of the lattice parameters a and b for Rb2Li4(SeO&.2H20 (open circles - the data obtained from the powder measurements).

6 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Temperature [K] Fig.2 The temperature dependence of the lattice parameter c and the monoclinic RbzLi4(Se0& 2H20 ( open circles - the data obtained from the powder measurements ). angle for to exhibit a transition below 30 K and/or above 350 K as well as a possible transition induced by pressure. This challenge, together with the recent interest in this kind of material, stimulate our further investigations on this subject.

7 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Atsuhiro Kunishige and Hiroyuki Mashiyama, J. Whys. Sot. Jpn., 56, (1987). 2. A. I. Kruglik, M. A. Simonow, E. P. Zhelezin and N. V. Belov, Dokl. Akad Nauk SSSR, 247, 1384 (1979) [Sov. Phys. Dokl. 24, 596 (1979)]. 3. A. Pietraszko, P. E. Tomaszewski and K. Lukaszewicz, Phase Transitions. 2, 141 (198 1). 4. W. Bronowska and J. Dziedzic, Ferroelectrics 79, 161 (1988). 5. V. Katkanat, H. M. Lu and J. R. Hardy, Phys Rev., I3 46, 5982 (1992) 6. A. Niwata and K. Itoh, J. Phys Sot. Jpn., 64, 4733 (1995). 7. K. Itoh, H. Ishikura and E. Nakamura, Acta CrystaZZogr., B37, 664(1981). 8. A. Pietraszko, Acta C~stalZogr., A37, C-109 (1981). 9. K. Hasebe and T. Asahi, Ferroelectrics, 96, 63 (1989). 10. V. Katkanat, PhysRev., B 51, 146 (1995). 11. R. M. Hazen and L. W. Finger, Phase Transit., 1, 1 (1979). 12. M. Kurzynski and M. Halawa, Phys. Rev., B 34,4846 (1986). 13. A. Pietraszko and W. Bronowska, will be published