  The temperature dependence of the linear thermal expansion coefficients
(LTEC) of a single crystal of a-(BEDT-TTF)2NH4Hg(SCN)4 where BEDT-TTF is
bis(ethylenedithio)tetrathiafulvalene was studied by the method of precision
capacitive dilatometry in the temperature range 2-250 K along the
crystallographic direction b* (perpendicular to the crystal layers). Negative
values of LTEC were found below 4 K. Probably it is due to charge fluctuations
as the temperature approaches the temperature of transition to the
superconducting state Tc. It has been suggested that the bends on temperature
dependence of LTEC observed in the temperature range 25-45 K are related to
order-disorder arrangement of NH4+ ions. Weak maximum of the LTEC, detected at
a temperature of about 200-220 K, can be caused by the processes of charge
redistribution and the associated intermolecular interaction fluctuations.
