Achieving <i>ZT</i> > 1 in Cu and Ga Co-doped\nAg<sub>6</sub>Ge<sub>10</sub>P<sub>12</sub> with Superior Mechanical\nPerformance and Its Fundamental Physical Properties toward Practical\nThermoelectr

Recently, phosphorus-based compounds have emerged as\npotential\ncandidates for thermoelectric materials. One of the key challenges\nfacing this field is to achieve <i>ZT</i> > 1, which\nis\nthe benchmark for thermoelectric device applications. In this study,\nit is demonstrated that the thermoelectric performance of environmentally\nfriendly Ag<sub>6</sub>Ge<sub>10</sub>P<sub>12</sub> is enhanced by\nco-doping Cu and Ga. The mechanical properties, coefficient of linear\nthermal expansion, work function, and compatibility factor are comprehensively\nclarified to provide guidelines for reliable device applications.\nThe peak and average dimensionless figures of merit of Ag<sub>5.85</sub>Cu<sub>0.15</sub>Ge<sub>9.875</sub>Ga<sub>0.125</sub>P<sub>12</sub> reach 1.04 at\n723 K and 0.63 at 300–723 K, respectively, which are the highest\nvalues for phosphorus-based thermoelectric materials. The Young’s\nmodulus, Vickers microhardness, fracture toughness, and compressive\nstrength of Ag<sub>5.85</sub>Cu<sub>0.15</sub>Ge<sub>9.875</sub>Ga<sub>0.125</sub>P<sub>12</sub> are 132 GPa, 589, 1.23 MPa m<sup>1/2</sup>, and 219 MPa, respectively, which are superior to those of typical\nstate-of-the-art thermoelectric materials. The remarkable thermoelectric\nand mechanical performance of Ag<sub>5.85</sub>Cu<sub>0.15</sub>Ge<sub>9.875</sub>Ga<sub>0.125</sub>P<sub>12</sub> mean that it is a promising\ncandidate for medium-temperature thermoelectric conversion. Ti, V,\nRh, and Pt are suitable for electrodes without exfoliation under thermal\nexpansion and with ohmic contacts to Ag<sub>5.85</sub>Cu<sub>0.15</sub>Ge<sub>9.875</sub>Ga<sub>0.125</sub>P<sub>12</sub> in terms of the\ncoefficient of linear thermal expansion and work function. Considering\nthat the compatibility factor of Ag<sub>5.85</sub>Cu<sub>0.15</sub>Ge<sub>9.875</sub>Ga<sub>0.125</sub>P<sub>12</sub> is approximately\n2.8, half-Heusler, skutterudite, and magnesium silicide-stannide compounds\nare suitable n-type thermoelectric counterpart materials in thermoelectric\ndevices. These insights will lead to the development of phosphorus-based\nthermoelectric materials toward practical thermoelectric device applications.