Pressure effect on the isostructural transition in RNiAl compounds (R$=$Tb and Gd)

TbNiAl and GdNiAl belong to a large group of compounds crystallizing in the hexagonal ZrNiAl-type of structure. Within recent years, physics of the discontinuity in the temperature or composition dependence of the lattice parameters (a and c) observed in several compounds of this family of intermetallics has been of particular interest. These materials have in common a specific ``forbidden'' value of the c/a ratio. This conclusion has been corroborated by ab initio calculations [1]. Although the dramatic structure change is hardly observable in the temperature dependence of the specific heat, it is accompanied with a clear change of the effective magnetic moment, change of the crystal field energy spectra [2] and namely with increasing amount of mechanical defects in the sample. TbNiAl as a representative of such behavior, exhibits a first-order structural transition at low temperatures around 100K [3]. We have for the first time studied this structural step upon application of hydrostatic pressure on both -- poly- and monocrystal of TbNiAl and a polycrystalline GdNiAl, in which the structural step appears still higher - around 240K. Our experimental observation of an unusually rapid decrease of the critical temperature with pressure will be discussed in terms of differences in the strength of the inter- and intraplanar chemical bonding in this type of structure. [1] J. Prchal et al., Phys. Rev. B 77 (2008) 134106. [2] P. Javorsky et al., J Magn. Magn. Mat. 317 (2007) e400. [3] J. Prchal et al., Physica B 378-380 (2006) 1102.