PR2SN2O7 AND SM2SN2O7 AS HIGH-TEMPERATURE SHIFT THERMOMETERS IN VARIABLE-TEMPERATURE SN-119 MAS NMR

Two shift thermometers have been developed in order to determine the e xact sample temperature at elevated temperatures in MAS NMR. The probe was calibrated for different temperatures and bearing pressures using the structural phase transition NMR spectra of carbon tetrabromide (3 19.86 K), DABCO (351.08 K), rubidium nitrate (437 K), and lithium sodi um sulfate (791 K). Temperature-dependent Sn-119 MAS NMR spectra of Pr 2Sn2O7 and Sm2Sn2O7 were then recorded. The temperature dependence is caused by the hyperfine interaction between the paramagnetic electron spin and the Sn-119 nucleus. Pr2Sn2O7 proved to be an extremely sensit ive linear high-temperature shift thermometer: its sensitivity is 14.1 ppm/K at room temperature and 2.7 ppm/K at 790 K. Sm2Sn2O7 also prove d to be very sensitive to temperature, although it's dependence was no t linear. At room temperature its sensitivity is 1.1 ppm/K. The temper ature dependence of the shift can be fitted to an equation derived fro m the Van Vleck equation, taking into account the ground and the first excited states of Sm3+. (C) 1995 Academic Press, Inc.