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Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium

机译:固体氘对称对称相转变的中子和X射线衍射研究

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The solid hydrogen compounds D-2, HD and H-2 remain quantum molecular solids up to pressures in the 100 GPa range(1). A remarkable macroscopic consequence is the existence of a pressure-induced broken symmetry phase transition(2-4), in which the molecules go from a spherical rotational state to an anisotropic rotational state. Theoretical understanding of the broken symmetry phase structure remains controversial, despite numerous studies(5-10). Some open questions concern the existence of long- or short-range orientational order; whether a strong isotopic shift on the transition pressure should be assigned to the nuclear zero-point motion or to quantum localization; and whether the structures are cubic, hexagonal or orthorhombic. Here we present experimental data on the structure of the broken symmetry phase in solid D-2, obtained by a combination of neutron and X-ray diffraction up to 60 GPa. Our data are incompatible with orthorhombic structures predicted by recent theoretical works. We find that the broken symmetry phase structure is incommensurate with local orientational order, being similar to that found in metastable cubic para-D-2.
机译:固态氢化合物D-2,HD和H-2在高达100 GPa的压力范围内仍是量子分子固体(1)。一个显着的宏观后果是存在压力诱导的对称对称断裂转变(2-4),其中分子从球形旋转状态变为各向异性旋转状态。尽管有许多研究(5-10),但对破碎对称相结构的理论理解仍存在争议。一些悬而未决的问题涉及长距离或短距离定向顺序的存在。是否应该将过渡压力上的强同位素转移归因于核零点运动或量子定位;以及结构是立方,六边形还是正交结构。在这里,我们介绍了通过中子和X射线衍射(最高60 GPa)的组合获得的固体D-2中对称对称相破碎结构的实验数据。我们的数据与最近的理论工作所预测的正交结构不相容。我们发现破碎的对称相结构与局部取向顺序不相称,与在亚稳态立方对位D-2中发现的相似。

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