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Ultralow thermal expansion in the Cs-Ln-Zr and M-Hf phosphates (Ln = Pr, Sm, Gd; M = Na, K, Rb, Cs)
Albina I Orlova, German N Kazantsev, Stanislav G Samoilov

The alkali members of the family MIHf2(PO4)3 (MI = Na, K, Rb, Cs) and solid solutions Cs1.3Ln0.3Zr1.7(PO4)3 (Ln = Pr, Sm, Gd) have been prepared and studied by high-temperature x-ray diffractometry in the temperature range 298 – 1173 K. These compounds show rhombohedral symmetry and belong to the R3c space group. All of the phosphates examined have demonstrated near-zero thermal expansion in their structure. For the sequence of Na, K, Rb, As, and Hf phosphates the thermal expansion axial coefficients aa and ac changed from -6.5 x 10-6 to -1 x 10-6 K-1 (aa) and from 21 x 10-6 to < 1 x 10-6 K-1 (ac). CsHf2(PO4)3 and Cs1.3Ln0.3Zr1.7(PO4)3 showed ultralow thermal expansion and thermal expansion anisotropy. The thermal expansion axial coefficients in the sequence of Ti, Zr, and Hf phosphates with NaZr2(PO4)3 (NZP) structure have been discussed. In the sequence NaTi2(PO4)3 to NaZr2(PO4)3 to NaHf2(PO4)3, aa is negative, changing from -4.4 x 10-6 to -6.5 x 10-6 K-1. ac has similar values of about (20 – 21) x 10-6 K-1. High-temperature x-ray measurement showed the smallest thermal expansion for the CsZr2(PO4)3 structure (aa/K-1 = -5.6 x 10-7 and ac/K-1 = 4.5 x 10-7) and for CsHf2(PO4)3 (|aa|/K-1 < 1 x 10-6 and ac/K-1 < 1 x 10-6).

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