The structures of Na_0.98K_0.02NbO₃ and Na_0.90K_0.10NbO₃ (phase Q) at room temperature by neutron powder diffraction

The phase-Q structures of Na_0.98K_0.02NbO₃ and Na_0.90K_0.10NbO₃ have been determined at room temperature by the neutron powder profile refinement technique. Two different models were refined: orthorhombic, space group P2₁ma (No. 26, in cab setting) and monoclinic, space group Pm (No. 6, in the second setting). The latter is more correct as it takes into account the splitting of the monoclinic a_p and c_p, but the structures can be described as very nearly orthorhombic. In the 2 mole % structure Nb atoms are all displaced by 0.18 (1) Å in the same direction towards the midpoint of the edge of the surrounding octahedron. The octahedra themselves appear to be regular and are tilted by 6.8 (1)° around a_p and c_p and by 7.8 (1)° around b_p. Na, K displacements [0.23 (4) Å] are almost parallel to those of Nb. For the 10 mole % structure Na, K and Nb displacements are the same but the tilts (5.2° and 7.3°) are somewhat smaller.