Thermal expansion behaviour of barium and strontium zirconium phosphates

Ba1.5-xSrxZr4P5SiO24 compounds with x = 0, 0.25, 0.5, 0.75, 1.0, 1.25 and 1 .5, belonging to the low thermal expansion NZP family were synthesized by t he solid state reaction method. The XRD pattern could be completely indexed with respect to R (3) over bar space group indicating the ordering of vaca ncy at the divalent cation octahedral sites. The microstructure and bulk th ermal expansion coefficient from room temperature to 800 degreesC of the si ntered samples have been studied. All the samples show very low coefficient of thermal expansion (CTE), with x = 0 samples showing negative expansion. A small substitution of strontium in the pure barium compound changes the sign of CTE, Similarly, x = 1.5 sample (pure strontium) shows a positive CT E and a small substitution of barium changes its sign, X = 1.0 and 1.25 sam ples have almost constant CTE over the entire temperature range. The low th ermal expansion of these samples can be attributed to the ordering of the i ons in the crystal structure of these materials.