NMR measurements on Intercalated 3R-TaS2.Ix (I=NH3 and N2H4)

Proton nuclear magnetic resonance was used to probe the environment of the intercalant molecule in low dimensional (aeolotropic) solids 3RI-TaSz(NHJhJ and 3Ru-TaS2(N2H4)4/3. and, specifically to look for phase transitions associated with changes in superlattice geometry. The proton resonance frequency for the intercalated molecules in the temperature interval 200 to 300 K indicates that there are different magnetic environ­ ments in three temperature domains. (i) There is only one resonance field for molecules in isotropic magnetic environments. (ii) The highand low-temperature domains (I and III) have molecules in only two different environments, whereas in the middle temperature domain (II) there are at least four different magnetic environments. (iii) The different magnetic environments measured by the ratio of anisotropic to isotropic resonance absorp­ tion intensity (r.,) indicate that the ratios r., are constants of the sample in domains I and II. (iv) At low temperatures (domain III), r,i depends on magnetic field polarisation effects and the temperature treatment of the sample. We conclude that the different magnetic environ­ ments observed for ammonia and hydrazine appear to be determined by the host. octahed­ rally coordinated TaS 2 . The phase transitions that are caused by the intercalation of low dimensional solids ( LDS) have important applications leading to new solid catalysts, conductors and battery materials. For this reason there has been recently a concentrated effort directed to determining the parameters which govern reaction (1): L(qh) + xl(g)Llx(cpz) (1) where cp1 is the pristine initial crystal phase and cpz is the intercalated phase. This is a complex process which involves at least three steps. First Langmuir absorption of the adduct gas must occur; it is followed by chemisorption and finally insertion of the adduct in between the layers produces a phase transition. The understanding of reaction (1) was strengthened by spectroscopic (Beal and Acrivos 1978) and structural measurements by transmission electron microscopy (TEM) of N2H4in situ intercalated TaS2polymorphs. These indicate (Tatlock and Acrivos 1978) that the in-plane translational symmetry is reduced by intercalation in a precise way which depends on the solid and on the temperature and pressure. The periodic lattice distortion ( PLD) describes the dichalco­ genide by an empirical formula TyX2y with y ~ 3, where 10 formula units are held t This work is dedicated to Professor R W Vaughan of Cal. Tech., whose untimely death (1978) left a void that has not to this day been filled. The NMR work was done in his laboratory. § To whom all correspondence is to be addressed. Permanent address: San Jose State University, San Jose, CA 95192, USA. 0022-3719/84/090275 + 06 $02.25 © 1984 The Institute of Physics L275 L276 Letter to the Editor together by layers of adduct I. The value of y is accurately determined from the PLD wavevectors which describe the Bloch functions of the partially occupied orbitally degenerate states of the metal or semimetal and give rise to the distortion according to the Jahn-Teller theorem (Hughes 1977, Acrivos 1974). The existence of localised electron states indicated by the PLD have led to transport and infrared spectroscopic measurements (Sarma et a/1982) which allow an estimate of a semiconductor energy gap of the order of half an electron volt in phase rn. The existence of localised para­ magnetic states was verified by ESR absorption measurements near 4.2 K (Guy et al 1982). Nuclear magnetic resonance spectroscopy (NMR) and neutron diffraction offer another means for studying these intercalated solids. Both techniques have been used to probe the intercalant molecule and its macro-environment within the crystal, so far yielding information about the mobility of ammonia and the geometries of ammonia and pyridine in 2H-TaS2 (Gamble and Silbernagel1975, Bray and Sauer 1972). The NMR evidence suggests that the ammonia molecule is sensitive to phase changes associated with superlattice formation in intercalated 2H-TaS2• The ability to study phase behaviour in intercalated 1 T-TaS2 at a microscopic level would greatly add to our understanding of these materials. This work reports NMR studies of 3RrTaS2(NH3)2/3 and 3RwTaS2(N2H4)4;3 between 200-300 K (Acrivos 1979, Acrivos et a/1981). The crystals of 1 T-TaS2 used in this work were grown by Meyer et at (1975) and A Beal and S N Nulsen (Sarma et a/1982). The NMR samples were prepared by placing a single crystal in a flattened tube to facilitate rotational studies which was then connected to a vacuum line for intercalation. Ammonia and hydrazine were purified prior to exposure to the crystals. Ammonia was condensed on sodium to remove traces of water and then the gas was admitted to the sample. Freeze-dried hydrazine was distilled in situ and then admitted to the sample. The ammonia pressure was 700 Torr and the hydrazine vapour pressure was 12 Torr. After one week at room temperature, the sample tube was immersed in liquid nitrogen and the tube sealed by melting the glass. The spectra were collected at 56.4 and 13.1 MHz. Free induction decays were collected for time averaging and were then Fourier transformed to give the absorption spectra. Temperature in the probe was varied and regulated to within ±0.5 oc of the desired value by heating nitrogen gas. Resonance frequency assignments were facilitated with the use of an acetyl chloride reference with an accuracy of± 10Hz. Over the 200-300 K range studied there were three kinds of spectra for 3Rr TaS2(NH3) 213 which define three temperature domains. Evidence of the three domains were also found in the spectra of 3Rn-TaS2(N2H4)4;3 which were qualitatively the same as the spectra of ammonia intercalated 1 T-TaS2 . Two resonance lines were found in domain I (250-300 K). The resonance position of one was insensitive to crystal orientation in the magnetic field while the position of the other line was determined by the angle between the crystal c axis and static magnetic field vector H. The separation in frequency of the anisotropic and isotropic signals could be approximated by the relation v. v; = 1. 5 A cos 8 where v. and V; are line positions in Hz, 1.5 A is the maximum separation in Hz and 8 is the angle between the crystal c axis and H. Shown in figure 1 are plots of ( v.Vi) versus efor ammonia and hydrazine intercalated samples. These follow the (3 cos2 81)/2 law for a Knight shift of K = A/56.1 PPM given in table 1. The ratio of intensities of the anisotropic to isotropic NMR absorption (r.;) was also a feature that characterised each temperature domain. In domain/, ra; was a constant of