Ultralow and negative thermal expansion in zirconium phosphate ceramics
Albina Orlova, Dmitriĭ Kemenov, Vladimir Pet’kov, Maya Zharinova, German Kazantsev, Stanislav Samoĭlov, Viktoriya Kurazhkovskaya
Compounds of the series A5-4xZrxZr(PO4)3 (A=Na, K; 0 <= x <= 1), A5-3xRxZr(PO4)3 (R=Al, Fe, Dy, Ho; 0 <= x <= 1.33), and A5-2xBxZr(PO4)3 (B=Mg, Ca, Sr; 0 <= x <= 2), have been prepared and characterised by infrared and x-ray analysis and studied by high-temperature x-ray diffractometry in the temperature range 293 – 1023 K. These compounds are members of a broad family of structural analogues NaZr2(PO4)3 (NZP); their crystallographic formula is (M1)(M2)3L2(PO4)3, where L and M positions are in the framework and framework voids, respectively. The examined phosphates have rhombohedral symmetry, and belong to the R3c space group, and some belong to R32. The influence of ion concentration, occupation of crystallographic positions and their types, and also the influence of radius sizes of interstitial ions on Zr-phosphate behaviour under heating, have been established. In the studied systems, the dependence of thermal expansion on the valence of cations in the L position from +1 up to +4 (A, B, R, Zr) and, accordingly, on the charge of framework and on the degree of M position occupation, was determined. These compounds showed thermal expansion anisotropy: negative expansion along the a axis and positive expansion along the c axis. It was shown that some K – Zr-phosphates had net negative or near-zero aav values.