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The structural, electronic and magnetic properties of the compounds GeKCa and SnKCa with half-Heusler structure excluding transition metals are investigated by using the first-principles pseudopotential plane wave method based on density functional theory. From the calculated total energies of three possible atomic arrangements we obtain the most stable structure for GeKCa and SnKCa where Ge (Sn), K and Ca occupy the (0, 0, 0), (1/4, 1/4, 1/4) and (3/4, 3/4, 3/4) positions, respectively. It is shown that both GeKCa and SnKCa with the most stable atomic arrangement exhibit half-metallicity with large half-metallic gaps (0.28 and 0.27 eV, respectively) and with an integer magnetic moment of 1.00 mu(B) per formula unit. The magnetic moment mainly originates from Ge (Sn) p electrons, and the ferromagnetic state is more favourable in energy than the antiferromagnetic state for both compounds. We also find that the half-metallicity can be maintained up to the lattice contraction of 10% and 13% for GeKCa and SnKCa, respectively. (C) 2011 Elsevier B.V. All rights reserved.