Neutron Diffraction Study of Structural Intermediate Phase IV in TlCoCl3

The structural changes of TlCoCl 3 around phase IV (68 < T < 75 K) were studied by single-crystal neutron diffraction measurement. The present investigation revealed that a small but definite Bragg peak at (1/6, 1/6, 2) appeared only in phase IV. A precursor phenomenon of the phase V ( T < 68 K) structure in the phase IV temperatures was also identified as the coexistence of a slightly broadened (1/4, 1/4, 2) Bragg peak. We confirmed that intermediate phase IV is a stable, nontransient, single phase with \(2\sqrt{3}a \times 2\sqrt{3}a \times c\) unit cells. A possible sequence of structural phase transitions was proposed.