Thermal and electronic properties of layered EuM_1-xPn compounds with tunable vacancy concentrations from x = 0-0.5

AMX compounds belonging to the ZrBeSi structure type are able to tolerate a large vacancy concentration - up to 50% - on the M site in the planar hexagonal MX layers.  Their tolerance for disorder makes AMX compounds exciting for thermoelectric applications by providing a route to amorphous-like lattice thermal conductivity.   Here, we investigate the impact of vacancies on the thermal and electronic properties of EuM_1-xPn compounds (M = Cu, Zn, and Pn = Sb, Bi).  The fully-ordered compound, EuCuSb, is a stiff material with high electronic mobility and high thermal conductivity.  Replacing Cu1+ by Zn2+ leads to a composition of EuZn_0.5Sb, in which 50% of the Zn site remains vacant to maintain overall charge-balance.  The transition from a fully-occupied hexagonal net to a one with a quarter of the atoms missing has wide-ranging consequences;  with increasing vacancy concentration, we observe a drastic, non-linear expansion of the average bond lengths in the hexagonal net.  Despite the absence of long-range ordering, pair distribution function analysis from synchrotron X-ray diffraction reveals significant local structural distortions caused by Zn vacancies, which leads to strong phonon scattering.   The high vacancy concentrations also cause the elastic stiffness to decrease significantly, leading to a corresponding decrease in sound velocity.  This, combined with increased scattering, leads to a precipitous drop in lattice thermal conductivity in the vacancy-rich EuZn_0.5Sb samples.    Naturally, the electronic mobility is suppressed by the high defect concentration.  However, we find that isovalent alloying with Bi on the Sb site can increase the electronic mobility from approximately 50 to 100 cm²/Vs by reducing the effective mass.   Simultaneously, substituting Bi further reduces the lattice thermal conductivity by reducing the sound velocity.