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Susceptibility and high field magnetization measurements have been performed on powdered samples of ͓͕Ni͑333-tet͒ ͑m-N 3 ͖͒ n ͔ ͑ClO 4 ͒ n [333-tet denotes tetraamine N, N 0 -bis(3-aminopropyl)-1, 3-propanediamine]. As the temperature is decreased, the susceptibility exhibits a smooth maximum around 120 K and decreases gradually down to 10 K, and then decreases rapidly. Magnetization up to 50 T at 1.4 K shows no gap in this compound within the experimental accuracy. We have carried out numerical calculations for the S 1 antiferromagnetic bond alternating chain and obtained very good agreement between experiments and calculations for the alternating ratio a 0.6.[S0031-9007 (97)05217-4] PACS numbers: 75.10.Jm, 75.50. -y, 75.50.EeExperimental and theoretical efforts on low dimensional magnetic systems have brought about a profound understanding of physics in many body quantum systems. In particular, linear chain Heisenberg antiferromagnets (LCHA's) with the spin quantum number S 1 have been studied extensively [1][2][3] in connection with Haldane's conjecture [4] that there is an energy gap between the ground state and the first excited one for integer S, while not for half-odd integer S. Recently, low dimensional oxide systems, such as inorganic spin-Peierls systems [5,6] and spin ladder systems [7,8], have attracted a lot of interest.One of the recent interesting topics in S 1 LCHA's is bond alternation. Using the Hamiltonian for the S 1 bond alternating chain given by His the parameter representing bond alternation and S i , S i11 S 1 spin operators, Affleck and Haldane [9] predicted that there is a massless (gapless) point at a certain alternating ratio d c . Subsequent numerical studies [10-15] estimated the critical ratio d c as 0.25. Moreover, it was shown numerically that the ground state is in the Haldane or the singlet dimer phase depending on d (0.0 # d , 0.25 for the former and 0.25 , d # 1.0 for the latter).In recent years, some Ni bond alternating compounds [16][17][18][19] were synthesized and the magnetic properties were studied. They were (a) catena͓͕Ni 2 ͑m-N 3 ͒ 3 ͑dpt͒ 2 ͖ n ͔ ͑ClO 4 ͒ n [dpt denotes bis(3-aminopropyl)amine], (b) ͕Ni 2 ͑EDTA͒ ͑H 2 O͒ 4 ͖ n ? ͑2H 2 O͒ n (EDTA denotes ethylenediaminetetraacetic acid), (c) trans-͓͕Ni͑333-tet͒ ͑m-N 3 ͖͒ n ͔ ͑ClO 4 ͒ n [333-tet denotes N, N 0 -bis(3-aminopropyl)-1,3-propanediamine], (d) ͓͕Ni 2 ͑dpt͒ 2 ͑m-ox͒ ͑m-N 3 ͖͒ n ͔ ͑PF 6 ͒ n (ox denotes C 2 O 4 ), and (e) ͓͕Ni 2 ͑Medpt͒ 2 ͑m-ox͒ ͑m-N 3 ͖͒ n ͔ ͑ClO 4 ͒ n [Medpt denotes methyl-bis(3-aminopropyl)amine]. Alternating ratios a of the neighboring exchange constants were obtained from the fit of the susceptibility to a formula calculated by Borrás-Almenar [20] using the Hamiltonian defined asHowever, no comparison between experiment and theory has been carried out from the viewpoint of verification of the phase separation and the gapless point. Note that the critical ratio a c corresponding to d c is 0.6. Magnetization curves at zero Kelvin of the S 1 LCHA with bond alternation were calculated with the...

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The formation process of perovskite-type hydride LiNiH3 was investigated using in situ synchrotron X-ray diffraction measurements. A mixture of LiH and Ni was hydrogenated at 873 K and 3 GPa, and the structural changes associated with hydrogenation were observed. Time-resolved diffraction profiles showed a three-step reaction: hydrogenation of Ni to NiHx, formation of LiyNi1–yH solid solution, and conversion to perovskite-type hydride LiNiH3. The solid solution, which has never been reported, plays the role of the precursor in the perovskite formation and its presence is apparently critical for synthesizing perovskite-type hydrides.