Solid-state MAS NMR investigations for pentavalent cation-replaced pollucite compounds with a negative thermal expansion coefficient

Pollucite compounds with an aluminosilicate structure, V-replaced pollucite, Cs 0.7 V 0.1 Al 0.8 Si 2.1 O 6 (CVAS), P-replaced pollucite, Cs 0.7 P 0.1 Al 0.8 Si 2.1 O 6 (CPAS), and Nb-replaced pollucite, Cs 0.7 Nb 0.1 Al 0.8 Si 2.1 O 6 (CNbAS), were synthesized by a multistep heat treatment of 550–1200 °C. CVAS exhibited the superior thermal expansion with thermal expansion coefficients of 0.9 × 10 −6 °C −1 (30–500 °C), −0.9 × 10 −6 °C −1 (30–150 °C), and −0.4 × 10 −6 °C −1 (30–100 °C). Solid-state 29 Si MAS NMR spectroscopy clarified that the number of Si–O–Si bonds of tetrahedra in the framework of CVAS was larger than those of CPAS and CNbAS. The chemical shift for peaks of the 29 Si MAS NMR spectrum of CVAS was larger than those of CPAS and CNbAS, suggesting that the bond angle of tetrahedra was enlarged by the V replacement. Solid-state 27 Al MAS NMR spectroscopy implied that CVAS had a small amount of sixfold coordinated Al species, which resulted in an increase in the number of Si–O–Si bonds of tetrahedra of CVAS.