The defect double perovskite [He_2–x□_x][CaNb]F₆, with helium on its A-site, can be prepared by the insertion of helium into ReO₃-type CaNbF₆ at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ∼60% of the A-sites become occupied. Helium uptake was quantified by both neutron powder diffraction and gas insertion and release measurements. After the conversion of gauge pressure to fugacity, the uptake of helium by CaNbF₆ can be described by a Langmuir isotherm. The enthalpy of absorption for helium in [He_2–x□_x][CaNb]F₆ is estimated to be ∼+3(1) kJ mol^–1, implying that its formation is entropically favored. Helium is able to diffuse through the material on a time scale of minutes at temperatures down to ∼150 K but is trapped at 100 K and below. The insertion of helium into CaNbF₆ reduces the magnitude of its negative thermal expansion, increases the bulk modulus, and modifies its phase behavior. On compressing pristine CaNbF₆, at 50 and 100 K, a cubic (Fm3̅m) to rhombohedral (R3̅) phase transition was observed at <0.20 GPa. However, a helium-containing sample remained cubic at 0.4 GPa and 50 K. CaNbF₆, compressed in helium at room temperature, remained cubic to >3.7 GPa, the limit of our X-ray diffraction measurements, in contrast to prior reports that upon compression in a nonpenetrating medium, a phase transition is detected at ∼0.4 GPa.

中文翻译：

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氦笼形物和缺陷钙钛矿[He2–x□x][CaNb]F6的合成与性能


缺陷双钙钛矿 [He _2–x □ _x ][CaNb]F ₆ ，其 A 位上有氦，可以通过插入氦来制备在高压下转化为ReO ₃ 型CaNbF ₆ 。在 0.4 GPa 氦气下从 300 冷却到 100 K 后，约 60% 的 A 位被占据。通过中子粉末衍射和气体插入和释放测量来量化氦的吸收。将表压转换为逸度后，CaNbF ₆ 对氦的吸收可以用 Langmuir 等温线来描述。 [He _2–x □ _x ][CaNb]F ₆ 中氦的吸收焓估计为 ∼+3(1) kJ mol ^–1 ，这意味着它的形成是熵有利的。在温度低至 ∼150 K 的情况下，氦气能够在几分钟的时间内扩散通过材料，但在 100 K 及以下的温度下会被捕获。将氦插入到 CaNbF ₆ 中可以降低其负热膨胀的幅度，增加体积模量，并改变其相行为。在 50 和 100 K 压缩原始 CaNbF ₆ 时，在 <0.20 GPa 下观察到立方体 (Fm3̅m) 到菱形 (R3̅) 相变。然而，含氦样品在 0.4 GPa 和 50 K 下仍保持立方体。CaNbF ₆ 在室温下在氦气中压缩，保持立方体至 >3.7 GPa，这是我们 X 射线衍射测量的极限，在与之前的报道相反，在非渗透介质中压缩时，在 ~0.4 GPa 下检测到相变。