Add time:08/04/2019         Source:sciencedirect.com

The temperature dependence of halogen NQR frequencies was determined for various methylammonium hexahalometallates(IV): (MA)2MCI6; M=Se, Pd, So, Pt, and Ph, (MA)2MBrb6; M= Se, Pd, Sn, and Pt, and (MA)2PtI6. A single resonance line for each isotope in the chlorine and the bromine complexes and only one v1 line of iodine-127 for the iodine complex were observed at various temperatures studied, indicating that all halogen atoms are crystallographically equivalent in crystals. At liquid nitrogen temperature, both v1 and v2 lines of the iodine complex were observed. X-ray powder patterns taken at room temperature revealed that all of these complexes form crystals isomorphous with each other belonging to the space group R3¯m. The crystal structure describable as a rhombohedrally distorted K2PtCl6 type is consistent with the results of the present NQR measurements. All the complexes studied show an anomaly in the temperature variation of NQR frequencies below dry-ice temperature. The anomaly was attributed to a structural phase transition from the results of DTA. The transition temperatures determined are concentrated in a rather narrow range of temperature, and no remarkable size effect due to complex anions can be observed. A possible structure of these compounds in the low-temperature phase is deduced, and the mechanism by which the phase transition takes place is discussed.

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