Abstrakt
We have studied the presence of a gap of forbidden c/a values for the hexagonal crystal structure of TbNiAl and GdNiAl, appearing in the whole family of the RTAl compounds (R-rare earth, T-transition metal). TbNiAl and GdNiAl both exhibit a first-order structural transition at low temperatures around 100 K and 205 K, respectively.
We have for the first time studied this structural step upon application of hydrostatic pressure on both compounds. An unusually rapid decrease of the critical temperature with pressure was found.
The decrease is furthermore steeper for GdNiAl in which the transition appears at higher temperature at ambient pressure. Common property for both studied compounds is preference of the c/a ratio below the critical forbidden interval after application of pressure, due to weaker interplanar chemical bonding in the layered ZrNiAl-type of crystal structure.