Synthesis and thermal expansion of (V, P, Nb)-replaced pollucite

Pollucite compounds in which tetrahedral cations are partially replaced with V, P or Nb were synthesized by a solid-state reaction. An amorphous phase was recognized by X-ray diffractometry for all the above pollucite compounds when mixed raw powders were preliminarily heated at 600 °C to decompose CsNO3 as a Cs source. Then V-replaced, Nb-replaced, and P-replaced pollucite compounds began to crystallize by heating at 550, 800, and 1000 °C, respectively. Finally, single phases of these compounds were synthesized at 700, 1000, and 1200 °C, respectively. The V-replaced pollucite compound Cs0.7V0.1Al0.8Si2.1O6 exhibited the lowest thermal expansion in the range of 25–500 °C at which its thermal expansion coefficient was 0.7 × 10−6 K−1, while the P-replaced pollucite compound Cs0.7P0.1Al0.8Si2.1O6 and the Nb-replaced pollucite compound Cs0.7Nb0.1Al0.8Si2.1O6 showed thermal expansion coefficients of 1.9 × 10−6 K−1 and 1.4 × 10−6 K−1 in the range of 25–500 °C, respectively. It was considered that such an excellent low thermal expansion coefficient of the synthesized V-replaced pollucite was due to the ionic radius of V5+ being between those of Al3+ and Si4+ in the aluminosilicate framework of pollucite compounds.