NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE PHASE-TRANSITIONS IN TIH2 ANDTID2

The phase transitions near 300 K in TiH1.99 and TiD1.97 were studied b y nuclear magnetic resonance (NMR). A complex proton line shape was ob served in TiH1.99 in the temperature range 200-470 K. Proton line shap e in TiH1.99 consisted of the superposition of a weak narrow line and a broad line. We suggest the narrow proton line may be associated with hydrogen located in octahedral sites of the TiH1.99 lattice. A sharp increase in the narrow line intensity near the 400 K indicates a rapid redistribution of hydrogen between tetrahedral and octahedral sites. The broad line width was 10.5 G both below and above the critical temp erature T-cr = 295 K and increased to 15.5 G near the T-cr. The 10.5 G line width is in good accordance with the Van Vleck's second moment f or protons located in tetrahedral sites of TiH2 face-centered cubic la ttice. The proton line width in TiH1.99 does not depend on applied mag netic field strength so substantial broadening of the line width may b e due to an increase of the magnetic dipole-dipole interactions. There fore, we propose an explanation of the substantial broadening of the p roton line width near T-cr is due to a cooperative Jahn-Teller mechani sm of phase transition in the TiH2. The cooperative Jahn-Teller phase transition in TiH2 was supported by investigation of this phase transi tion in TiD1.97, where the quadrupolar broadening of the D-2 NMR line was observed above the critical temperature (T-cr = 310 K). The differ ence, about 15 K, between the critical temperature of the phase transi tion in TiH1.99 and TiD1.97 is an isotopic effect and was confirmed by heat capacity measurements.