The crystalline constraints on thermal expansion in NaZr2P3O12 (NZP)

Abstract

NaZr2 P30~2 belongs to a very large new family of low thermal expansion materials [1-3] known as NZP or CTP and is a parent phase of this family. In 1979, Boilot et al. [4] while studying the ionic conductivity behaviour of the Nal+xZrzP3 xSixO~2 system also reported the thermal expansion and showed that it changes from negative to near zero and then to positive as x increases. This prompted Roy and coworkers [1, 2] in 1983 to undertake a systematic study of the thermal expansion behaviour of these materials in order to find new zero expansion materials. Since then, several new compositions with near zero expansion have been developed by these researchers [5, 6]. The low thermal expansion behaviour of the NZP family of materials is attributed to its unique crystal structure which was first solved by Hagman and Kriekegaard [7] in 1968. Alamo and Roy [8] showed that the NZP structure allows various ionic substitutions at different lattice sites resulting in numerous new compositions, moreover, several crystalline solutions can also be prepared by partial ionic substitutions in this family. In a powder sample of a crystalline material, the individual crystals are free to expand in any manner without any constraints. However, in a sintered ceramic body, the individual crystals or grains are bonded/linked during sintering reactions, and thus a constraint is placed upon an individual crystal/grain by the crystals/grains surrounding it. This may alter the thermal expansion on a macroscopic level. It is hypothesized that the lattice parameters of a sintered sample will be different from those of a powdersample at low temperatures and the same at high temperatures. In the present study, an attempt has been made to investigate this "constraint effect" on the thermal expansion of NaZrzP3012. For this purpose, NZP was synthesized in powder and sintered form by the sol-gel method. Thermal expansion behaviour was determined by high-temperature X-ray diffractometry and classical dilatometry. This study is quite important because (i) the NZP family of materials has generated a lot of interest among ceramists for its potential for thermal shock resistant applications, (ii) if there is a substantial constraint effect on the thermal expansion of materials, it would add another factor in order to understand and interpret the thermal expansion mechanism of various materials, and (iii) it will resolve the problem of using a sintered or powdered sample for high-temperature X-ray studies. NZP has rhombohedral symmetry and belongs to the R3c space group. Upon heating, the lattice expands in the c-direction and contracts in the a-direction. To