in P y rid in iu m H e x a c h lo r o te llu r a te ( I V ) , H e x a c h lo r o s ta n n a te ( I V ) , a n d H e x a b r o m o s ta n n a te ( I V ) C r y s ta ls a s S tu d ie d b y N M R Yutaka Tai, Tetsuo Asaji, Ryuichi Ikeda*, and Daiyu Nakamura Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya, Japan Z. Naturforsch. 44 a, 300-306 (1989); received January 21, 1989The 'H NMR second moment M2 and the spin-lattice relaxation time T, are determined for pyridinium hexachlorotellurate(IV), hexachlorostannate(IV), and hexabromostannate(IV) at various temperatures above ca. 140 K. The phase transition temperatures already reported from halogen NQR experiments are determined as 272, 331, and 285 K, respectively, by differential thermal analysis (DTA). The DTA as well as differential scanning calorimetry measurements show that the above phase transitions are of second-order. For pyridinium hexachlorotellurate(IV) and hexabromostannate(I V), a sharp 'H T, dip was observed at the transition temperature. This is interpreted in terms of a phenomenon related to the critical fluctuation of an order parameter. From the measurements of 'H M2, 60° two-site jumps (60c flips) around the pseudo C6 axis of the cation are suggested to occur in the high temperature phases of the complexes. Modulation of X...'H (X = CI, Br) magnetic dipolar interactions due to the reorientational motion of the complex anions is considered as a possible relaxation mechanism in the high temperature phases. S t r u c t u r a l P h a s e T ra n s itio n s a n d Io n ic M o tio n s Introduction Recently, in the crystals of pyridinium hexachlorometallates(IV), (C5H6N)2MC16 (M = Sn, Te, Pb, Pt) [1] and hexabromostannate(IV), (C5H6N)2SnBr6 [2] (hereafter C5H6N + is abbreviated to pyH+) a struc tural phase transition was found from the temper ature dependence of halogen NQR frequencies. The 'H NMR spin-lattice relaxation time, T{, determined for (pyH)2TeCl6 as a function of temperature shows a sharp dip at the transition temperature, Tc ( = 273 K) [3]. This has been explained as relaxation modu lated by the anion reorientation through dipolar re laxation of the second kind [4], A previous powder X-ray diffraction study [1] has shown that the crystals of (pyH)2MCl6 (M = Sn, Te) in their high temperature phases are isomorphous. The accurate X-ray analysis of these complexes has re vealed that the tin complex forms triclinic crystals with the space group PI (Z = 1) in the low tempera ture phase [1], while the high temperature phase of the tellurium complex is monoclinic, belonging to the space group B2/m with Z = 2. Accurate structural data have not been reported as yet for the high tem * Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. Reprint requests to Prof. Dr. D. Nakamura. Nagoya Univer sity, Faculty of Science. Nagoya 464-01, Japan. perature phase of the former complex and the low temperature phase of the latter. From a structural analysis of the high temperature phase of (pyH)2TeCl6, the most probable orienta...