EPR in single crystals of NiTiF6sdot;6H2O has been studied in the frequency ranges 12ndash;18 GHz and 23ndash;25 GHz. A phase transition from the room temperature trigonal structure to a lower symmetry structure takes place at a temperature between 134 and 137thinsp;deg;K for a slowly cooled sample. The reverse transition occurs at approximately 141thinsp;deg;K. As the phase transition takes of the order of 10 min for completion, the trigonal phase may be retained at low temperatures by rapidly cooling the sample below about 100thinsp;deg;K. In the trigonal phase above 4.2thinsp;deg;K, bothgpar;andgperp;are approximately 2.27 and the crystal field parameterDhas the magnitude (1.30plusmn;0.01) cmminus;1at 4.2thinsp;deg;K, (1.337plusmn;0.005) cmminus;1at 77thinsp;deg;K, and (1.31plusmn;0.01) cmminus;1at 298thinsp;deg;K. The data for the trigonal phase below 4.2thinsp;deg;K may be represented by divergentgvalues, withgpar;quest;2.50 andgperp;quest;2.20 at 1.3thinsp;deg;K. In the low symmetry phase, withgpar;assumed to be 2.25, the crystal field parameters at 77thinsp;deg;K are Verbar;DVerbar;quest;2.0 cmminus;1and Verbar;EVerbar;quest;0.20 cmminus;1.
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