Bulk modulus and thermal expansion of rare-earth tetraborides RB4 (R = Gd, Tb, Dy, Ho, and Tm) via in situ x-ray diffraction

X-ray diffraction experiments were conducted on the rare-earth tetraborides RB4 (R = Gd, Tb, Dy, Ho, and Tm) under high pressure at room temperature and low temperature at atmospheric pressure to investigate their structural and lattice behavior and, thus, determine their bulk moduli and thermal expansion coefficients. The bulk modulus values ranged from 175 to 180 GPa for all rare earths. These values lie between those of RB6 and RB12 structures and do not exhibit a simple correlation with boron concentration but rather correlate with the R–B distance in the coordination polyhedron. RB4, which features a tetragonal crystal structure with a Shastry–Sutherland lattice, shows anisotropy where the c axis is more compressible than the a axis. Additionally, lattice contraction due to temperature changes exhibits similar anisotropy. Considering the a/c anisotropy per unit volume, the present result indicates that the thermal contraction effect is greater than the compression effect.