Tailoring the magnetism of Tb5Si2Ge2 compounds by La substitution

In order to study the crystal structure, phase relationships, and magnetic properties of the system Tb5−xLaxSi2Ge2, a series of polycrystalline samples with compositions ranging from x = 0 to x = 5 have been synthesized and characterized in detail. At room temperature, two structures have been detected: Compounds in the concentration range 0 x < 1 present a monoclinic Gd5Si2Ge2-type structure, while for x > 1 a tetragonal Zr5Si4-type structure is observed. The unit cell volume increases linearly with La concentration but with two different slopes:∼18 Å3/x and∼33 Å3/x for 0 x < 1 (monoclinic) and x > 1 (tetragonal), respectively. In the monoclinic region, an increase of TC was observed, reaching a maximum value of TC ∼ 154 K, at the x = 0.75 composition. This feature is explained based on specific La occupancy at the R2 site which was supported by density functional calculations for low La concentration. The samples that crystallized in the tetragonal structure exhibit a linear decrease of TC(x) with a slope of ∂TC /∂x ∼ −38 K/x, reaching 0 K for the x = 5 composition. A magnetic and structural x-T phase diagram of the Tb5−xLaxSi2Ge2 system in the temperature range 4–300 K is proposed.