Crystal structures of KBa2Nb5O15 and LaK2Nb5O15 with the TTB-type structure

Single crystals of two niobates, KBa2Nb5O15 and LaK2Nb5O15, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4bm (#100) for KBa2Nb5O15 and the centrosymmetrical one P4/mbm (#127) for LaK2Nb5O15), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa2Nb5O15 (LaK2Nb5O15) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R1 = 0.040 (0.047) and wR2=0.131wR2=0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa2Nb5O15 were distributed statistically at two crystallographic sites and K and La atoms in LaK2Nb5O15 were ordered.