Structural evidence of charge density waves in HMTTF-TCNQ (hexamethylene-tetrathiofulvalenium- tetracyanoquinodimethane)

Three-dimensional ordering in HMTTF-TCNQ

“…Equations (7,8) can be further simplified by introducing the approximation which is valid whenever pH PQ. This approximation necessarily goes together with 1 Aa/2 II Aa > 1, which is to be reasonably expected in the simplest picture of interchain coupling, used in setting up equation (1). Moreover, the condition PH/PQ 1 is consistent with the result for PH and pQ which follows from equation (7) and will appear to be the most relevant one for HMTTF-TCNQ.…”

“…We have seen that the formation of threedimensionally ordered deformations with the incommensurate value of qa is realized in the case (ii) as well as in the case (iii) provided that equation (12) is valid and that 7~ > TQ. However detailed examination of equation (11) shows that the observed temperature dependence of X-ray spot intensities [1] cannot be understood within the latter case. In this case it appears that for all possible physical values of parameters appearing in equation (11) the rates of change of both deformation amplitudes pQ and PH are smooth in the whole temperature range below Tc.…”

“…Adl2 Aa 1 and ~ld /4 ~la ~1~ > 1 are the conditions under which the Q sublattice alone orders incommensurately [1]. When the second term in equation (8) is dominant, qa is directly governed by the temperature dependent quantities pQ and pH.…”

“…Finally, for cos2 () = 0, and pQ and pH in equation (11) are completely decoupled. They start to develop at temperatures defined by equations (13) and (1) The other two types of ordering for the sole Q sublattice are : qa/2 = 0, q~/2 = ~ or vice versa, for Ad > 2 Aa, Ad > 2 Ac and qp = a*/2, qc = c*/2 for A d 2/4 ~ip Ac 1. respectively. Below this latter temperature the wave numbers remain at the boundary of the Brillouin zone, although the deformation amplitudes of both chain families are finite.…”

Calorimetric study of the phase transitions in HMTTF - TCNQ

“…Various properties of HMTTF-TCNQ have been studied recently. Limiting ourselves to the question of phase transitions we refer to measurements of the electrical conductivity [5,7] and diffuse X-ray scattering [8], which indicate the existence of two transitions at about 49 and 43 K.…”