Crystal structures of KBa 2Nb 5O 15 and LaK 2Nb 5O 15 with the TTB-type structure

Abstract

Single crystals of two niobates, KBa 2Nb 5O 15 and LaK 2Nb 5O 15, 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 P4 bm (#100) for KBa 2Nb 5O 15 and the centrosymmetrical one P4/ mbm (#127) for LaK 2Nb 5O 15), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa 2Nb 5O 15 (LaK 2Nb 5O 15) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R 1 = 0.040 (0.047) and w R 2 = 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 KBa 2Nb 5O 15 were distributed statistically at two crystallographic sites and K and La atoms in LaK 2Nb 5O 15 were ordered.