NEUTRON-SCATTERING STUDY OF THE STRUCTURAL PHASE-TRANSITIONS IN TLGASE2

The successive structural phase transitions in monoclinic TlGaSe2 have been studied by using single crystal neutron scattering techniques. T he observed diffraction pattern shows complicated feature of the phase transitions. The intermediate phase between 107 K and 118 K is incomm ensurate. The appearance of the satellite reflections changes with the reciprocal space. Around the Bragg points (4n,0,2m+1) and (0,4n,2m), the satellites appear at incommensurate positions, a pair of-satellite reflections appear at q(i1)=(0.04,0,-0.25) and (-0.04,0,0.25). For ot her Bragg points, such as (4n,0,2m), (0,4n,2m+1) and (2n,0,m), the sat ellites appear at the commensurate positions (0,0,+/-0.25). In the low temperature phase below 107 K, all the satellite reflect ions appear at the commensurate position q(c)=(0,0,+/-0.25). The experimental resu lts are compared with those of monoclinic TIS.