Theoretical investigation of the thermoelectric properties of ACu[O.sub.2]

The electronic, structural, mechanical, lattice dynamics and the electronic transport properties of ACu[O.sub.2](A = K, Rb and Cs) are investigated using density functional theory. The calculated elastic constants and their related elastic moduli, phonon spectra and electronic transport properties of these compounds are reported here for the first time. The predicted structural parameters are in excellent agreement with the available experimental data. The obtained lattice thermal conductivities, [[kappa].sub.l], of ACu[O.sub.2] (A = K, Rb and Cs) are found to display strong anisotropic features along the a, b and c directions. It is also found that the average room-temperature [[kappa].sub.L] of CsCu[O.sub.2] is lower than those of RbCu[O.sub.2] and KCu[O.sub.2], which is due to its smaller group velocities in the low frequency region i.e., 0 ~ 3 THz. Our calculations also show that the acoustic phonon modes contribute considerably to the total [[kappa].sub.l] along the a and b directions. The electrical conductivity ([sigma]) and electronic thermal conductivity ([[kappa].sub.el]) of ACu[O.sub.2] (A = K, Rb and Cs) show anisotropic features i.e., [sigma] and [[kappa].sub.el] along the c-axis is significantly larger than along the a and b-axes. Meanwhile, our obtained Seebeck coefficient (S) values are found to be 248, 110 and 91 [mu]V/K for p-doped KCu[O.sub.2], p-doped RbCu[O.sub.2] and p-doped CsCu[O.sub.2] respectively at 300 K along the b-direction. These S values are found to be of the same order of magnitude with that of well known thermoelectric (TE) material, [Bi.sub.2][Te.sub.3] (with S of 200 [micro]V/K at 300 K) and the recently discovered metal oxide TE material, NaC[O.sub.2][O.sub.4] (with S of 100 [micro]V/K at 300 K). However, our computed figure of merit (ZT) values of ACu[O.sub.2] (A = K, Rb and Cs) are found to be very small as compared to known thermoelectric materials. For instance, our highest computed ZT value is 0.11 for p-type KCu[O.sub.2] along the c-direction at 750 K, 0.15 for p-type RbCu[O.sub.2] and 0.25 for p-type CsCu[O.sub.2] along the a-direction at 800 K. These small ZT values are caused by large values of the lattice thermal conductivities.