Controlling the Phase Behavior of Low and Negative Thermal Expansion ReO₃-Type Fluorides using Interstitial Anions: Sc_1–xZr_xF_3+x

Abstract

Density measurements suggest that the incorporation of ZrF4 into the cubic ReO3-type structure of Sc1– x Zr x F3+x is associated with the creation of anion interstitials. X-ray total scattering measurements are consistent with the conversion of corner-sharing octahedra to edge-sharing polyhedra as the solid solutions become richer in ZrF4. The cubic (Pm3̅m) to rhombohedral (R3̅c) cooperative octahedral tilting transition seen for ScF3 moves to a higher pressure as increasing amounts of zirconium are added, and it is eventually suppressed completely (x = 0.4 and 0.5) so that the cubic phase persists to high pressure until an amorphization occurs. All the samples studied (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) display pressure-induced softening, and increasing the zirconium content leads to a higher zero-pressure bulk modulus. The incorporation of “excess fluoride” into ReO3-type fluorides is a powerful tool for suppressing the generally unwanted phase transitions seen when subjecting these materials to stress.

Thermal Expansion, Response to Pressure, and Reversible Pressure-Induced Amorphization in Anion Excess ReO3-Type Cubic LuZrF7

“…The pronounced pressure-induced softening seen in Figure 5 has also been observed for cubic CaNbF 6 , 4 MgZrF 6 , 4 CaTiF 6 , 5 ScF 3 , 22,38 and YbZrF 7 . 20 In the systems [Mg 1−x Zr x ]ZrF 6+2x 21 and Sc 1−x Zr x F 3+x , 22 increasing the amount of excess fluoride increases the bulk modulus (K 0 ) and reduces the magnitude of the pressure-induced softening (K 0 ′). Pressure-induced softening has been predicted to be common among NTE materials.…”

“…However, similar behavior was observed on the compression of cubic YbZrF 7 , 20 A fit, using a third-order Birch−Murnaghan equation of state, to volume versus pressure for cubic LuZrF 7 is shown in Figure 5. The estimate bulk modulus (K 0 ) is comparable to that of many cubic ReO 3 -type fluorides, for example, CaZrF 6 (42.1(8) GPa), 3 CaNbF 6 (33.7(4) GPa), 4 MgZrF 6 (48.2(5) GPa), 4 CaTiF 6 (28.9(9) GPa), 5 and ScF 3 (60 GPa), 7,22 but somewhat lower than that previously estimated for YbZrF 7 (55.4(7) GPa), 20 perhaps reflecting some difference in the nonequilibrium local structures of the samples examined for these two closely related compositions. The pronounced pressure-induced softening seen in Figure 5 has also been observed for cubic CaNbF 6 , 4 MgZrF 6 , 4 CaTiF 6 , 5 ScF 3 , 22,38 and YbZrF 7 .…”

“…The estimate bulk modulus (K 0 ) is comparable to that of many cubic ReO 3 -type fluorides, for example, CaZrF 6 (42.1(8) GPa), 3 CaNbF 6 (33.7(4) GPa), 4 MgZrF 6 (48.2(5) GPa), 4 CaTiF 6 (28.9(9) GPa), 5 and ScF 3 (60 GPa), 7,22 but somewhat lower than that previously estimated for YbZrF 7 (55.4(7) GPa), 20 perhaps reflecting some difference in the nonequilibrium local structures of the samples examined for these two closely related compositions. The pronounced pressure-induced softening seen in Figure 5 has also been observed for cubic CaNbF 6 , 4 MgZrF 6 , 4 CaTiF 6 , 5 ScF 3 , 22,38 and YbZrF 7 . 20 In the systems [Mg 1−x Zr x ]ZrF 6+2x 21 and Sc 1−x Zr x F 3+x , 22 increasing the amount of excess fluoride increases the bulk modulus (K 0 ) and reduces the magnitude of the pressure-induced softening (K 0 ′).…”

“…18−23 This latter approach has been shown to be a viable strategy for tuning thermal expansion and modifying the nature of and pressures at which phase transitions occur. 21,22 In the excess fluoride material cubic YbZrF 7 , no phase transition was seen on cooling and an abrupt pressure-induced amorphization was observed at ∼0.95 GPa, rather than an octahedral tilting transition, which is typical for cubic ReO 3type structures, on compression. 20 In the solid solution systems Sc 1−x Zr x F 3+x and Mg 1-x Zr 1+x F 6+2x , as the amount of excess fluoride, x, is increased, the octahedral tilting transition seen on compression of the parent phase is suppressed and replaced by an amorphization at a higher pressure.…”

“…20 In the solid solution systems Sc 1−x Zr x F 3+x and Mg 1-x Zr 1+x F 6+2x , as the amount of excess fluoride, x, is increased, the octahedral tilting transition seen on compression of the parent phase is suppressed and replaced by an amorphization at a higher pressure. 21,22 The suppression of the octahedral tilting transition by the incorporation of excess fluoride can be readily understood. The excess fluoride is reported to convert some of the cornersharing octahedra into edge-sharing polyhedra.…”