Octahedral tilting in the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Vol. 75; no. 5; pp. 815 - 821
• Main Authors: McNulty, Jason A., Gibbs, Alexandra S., Lightfoot, Philip, Morrison, Finlay D.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01-10-2019; Blackwell Publishing Ltd
• Subjects: Bronzes; Cations; Crystals; hexagonal tungsten bronzes; Neutron diffraction; Neutrons; octahedral tilting; powder neutron diffraction; symmetry‐mode analysis; Tungsten; Tungsten bronze; X‐ray diffraction
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520619009260

The ambient‐temperature structures (orthorhombic, space group Cmc21) of the polar hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 have been determined by high‐resolution powder neutron diffraction. Displacement of the A‐site cation along the polar c axis with concomitant octahedral tilting occurs to optimize the A cation bonding environment, hence reducing the coordination from 18 to 16. This effect is more evident in KNbW2O9 due to decreased A cation size. The octahedral tilting in both compositions results in a doubling of the c axis that has not previously been reported, highlighting the importance of neutron diffraction as a complementary technique for structural determination of such systems.
The first powder neutron diffraction study of the hexagonal tungsten bronzes RbNbW2O9 and KNbW2O9 is presented. The existence of octahedral tilting that has not been previously observed using X‐ray diffraction is also reported.