Magnetic phase transitions in NiCl2- and CoCl2-graphite intercalation compounds

Magnetic phase transitions in NiCl2- and CoCl2-graphite intercalation compounds

229 stabilized in the interstices of Rb atoms in the gaps o f the graphite layers. CsK has a two-stage hydrogen absorption. In the initial stage, the...

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229

stabilized in the interstices of Rb atoms in the gaps o f the graphite layers. CsK has a two-stage hydrogen absorption. In the initial stage, the dissociated hydrogen atoms are transiently yielded and then, in the second stage, the formation of hydride ions H - takes place through charge transfer from CsK to H. The ion H with a diameter of 3.06 • is t o o large to be a c c o m m o d a t e d in the interstice o f the first-stage structure. Therefore, it is considered that H - can be a c c o m m o d a t e d in the graphite gaps through a structure change to the second-stage t y p e CsKH2/3 because K atoms are more mobile than Rb atoms in the graphite gaps. CsCs shows no change in the e.s.r, signal and the electrical resistivity under hydrogen atmosphere. This finding suggests that its catalytic activity for an H - D exchange reaction is due to the surface activity. Under activated hydrogen atmosphere, CsCs reacts with hydrogen to form a hydride with H - ion. In the case of an alloy system CsCsxK1 x, hydrogen is absorbed directly to form hydride ion H - , with the absence of the dissociated hydrogen state. This is because the alkali-metal atoms are very mobile in the two~limensional alloy CsxKl_~ in the graphite gaps. For the second-stage c o m p o u n d s C24K and C24Rb, there exists a single-stage hydrogen absorption process, that is, absorbed hydrogen directly becomes hydride ion H - . C24Cs shows no change in the e.s.r, signal under hydrogen atmosphere. For the chemisorption activity, the order o f strengths is K Rb > Cs in both first- and second-stage compounds. The activities o f the first-stage c o m p o u n d s are stronger than those of the second-stage ones. This might be because the alkali metal has partial valence in the former and full valence +1 in the latter.

MAGNETIC PHASE TRANSITIONS IN NiC12- AND CoC12-GRAPHITE INTERCALATION COMPOUNDS*

Masatsugu Suzuki and Hironobu Ikeda (Department of Physics, Faculty o f Science, Ochanomizu University, Bunkyo-ku, T o k y o 112, Japan) The magnetic susceptibility and spontaneous magnetization o f the second-stage NiC12- and CoC12-graphite intercalation c o m p o u n d s (GICs) based on single crystals of Kish graphite show remarkable anomalies at two different temperatures, Tcu and TcQ (Tcu = 21.3 K, Tc~ = 18.0 K for NiC12 GIC and T¢~ = 9.1 K, T¢~ = 8.0 K for CoC12 GIC (Fig. 1)). The specific heat shows an anomaly around T~u. At T~u the system goes from the paramagnetic state into an intermediate one with a small b u t finite magnetization and at T¢~ into an ordered state with the spin easy axis lying in the c-plane. These anomalies are discussed b y taking the two-dimensional character of these c o m p o u n d s into account. *This work has been published in J. Magn. Magn. Mater., 31 - 34 (1983) 1173, and Synth. Met., 8 (1983); it has also been submitted to J. Phys. Soe. Jpn.

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Fig. 1. T e m p e r a t u r e d e p e n d e n c e of magnetic susceptibility Xaa and spontaneous magnetization M s along the c-plane of CoC12-GIC based on single crystals of Kish graphite; these quantities were measured simultaneously in a negligibly small field ( - 3 m O e ) using a S Q U I D m a g n e t o m e t e r . The magnetization was obtained by cooling the sample in a finite external field H c (~ 120 mOe) from a b o u t 30 to 4.2 K and then removing the field. M A G N E T I Z A T I O N A N D M A G N E T I C P H A S E T R A N S I T I O N S OF T H E SECONDSTAGE NiC12-GRAPHITE INTERCALATION COMPOUND*

H. Suematsu 1, R. Nishitani I, R. Yoshizaki 2, M. Suzuki a and H. Ikeda 3 (l Institute of Materials Science, University of Tsukuba, Ibaraki 305, Japan; 2Institute of Applied Physics, University of Tsukuba, Ibaraki 305, Japan; and aDepartment of Physics, Ochanomizu University, Bunkyoku, Tokyo 112, Japan) The induced (M) and spontaneous (Ms) magnetizations in the 2ndstage NiC12-graphite intercalation c o m p o u n d (GIC) have been investigated at 4.2 - 300 K in fields of 0.1 - 4.5 × 104 Oe using well-stage d single-crystal samples. Two magnetic phase transitions were observed at TcQ = 17.5 K and Tcu = 22.0 K. Above TCu the compound is paramagnetic with 0 = 70 + 1 K and Pelf = 3.29 PB. In the intermediate phase (T~ < T < T~u) the index 1/6 in M ~ Hint I/5 has a remarkable temperature dependence from 1.0 ( T > Tcu) to nearly 2.0 (T < 18.5 K) in intermediate fields (1 ~< H i n t <~ 100 O e } , whereas it is 1.0 in low fields (Hin t ~< 1 0 e ) (cf. Fig. 1). M s measured in a cooling field Hc <~ 10 Oe shows two stepwise decreases a few degrees below Ten and T¢u, respectively, and M s remains finite in the intermediate phase. Ms for H~ -> 10 Oe shows only a monotonic decrease. These results suggest *This work has been submitted to J. Phys. Soc. Jpn.