Rotating-frame nuclear magnetic relaxation in TlH₂PO₄

A TlH₂PO₄ single crystal was studied by means of the rotating-frame NMR measurements and the results were compared with our recent work on a TlH₂AsO₄ polycrystalline sample. When the data were fitted in the same manner, similar overall features were observed, whereas anomalies at the antiferroelectric phase transition were much more pronounced. On the other hand, a double-exponential fit at all temperatures suggests that both of the domain structures participate in the antiferroelectric phase transition, whereas only one of them undergoes the ferroelastic phase transition. The domain structure that does not undergo the ferroelastic phase transition shows a differential anomaly in the spin-lattice relaxation at the critical temperature, which appears to be characteristic of an order-disorder transition.