By use of synchrotron radiation the powder x-ray diffraction of lanthanide
monoarsenides LnAs (Ln = Pr, Nd, Sm, Gd, Dy and Ho) with a NaCl-type struct
ure has been studied up to 60 GPa at room temperature. First-order phase tr
ansitions with crystallographic change were found at around 27.1 GPa for Pr
As, 24.2 GPa for NdAs and 32.1 GPa for SmAs. The high-pressure form of the
lighter LnAs (Ln = Pr, Nd and Sm) is a tetragonal structure and can be view
ed as a distorted CsCl-type structure. The atoms in the tetragonal structur
e are located at Ln: 0, 0, 0; As: 1/2, 1/2, 1/2, The space group is P4/mmm.
Pressure-induced phase transitions of the heavier LnAs (Ln = Gd, Dy and Ho
) with many f electrons occur at around 36 GPa for GdAs, 44 GPa for DyAs an
d 46 GPa for HoAs. The structure of these high-pressure phases is unknown.
The high-pressure structural behaviour of LnAs (Ln = Pr, Nd, Sm, Gd, Dy and
Ho) with the NaCl-type structure is discussed.