Low-temperature structural phase transitions in CuSeO4·5D2O and CuBeF4·5D2O

The isostructural family of compounds that includes the familiar copper sulfate pentahydrate and the two less familiar title materials, has for decades been a model for studying a range of structural and magnetic phenomena. Our understanding of the magnetic transitions on cooling below 1 K have hitherto been based on an assumption that there is no preceding structural change, an assumption we now know to be false. We have shown that CuSO4.5D2O undergoes a loss of inversion symmetry at 32 K, with an associated cell doubling along the a- and c-axis directions. Curiously, there is some evidence in the literature for such a transition in the fluoroberyllate analogue but not the selenate or sulfate. Our aim here is to determine if the purported structural transition in the BeF4 analogue is the same as we observe in the sulfate and to test whether it also occurs in the selenate.

包含广为人知的五水合硫酸铜（copper sulfate pentahydrate）以及本文标题提及的两种鲜为人知材料的同构化合物家族，数十年来一直作为研究一系列结构与磁学现象的经典模型体系。此前我们对冷却至1K以下时发生的磁相变的认知，均基于“不存在前置结构相变”这一假设，而如今我们已证实该假设并不成立。我们的研究表明，五氘合硫酸铜（CuSO4·5D2O）在32K时会发生反演对称性破缺，并伴随沿a轴与c轴方向的晶胞倍增现象。令人费解的是，现有文献中仅存在氟铍酸盐类似物发生此类相变的相关证据，却未发现硒酸盐或硫酸盐体系存在该相变的记录。本文的研究目标为：确认四氟铍酸盐类似物中所称的结构相变，是否与我们在硫酸盐体系中观测到的相变一致，并验证该相变是否同样存在于硒酸盐体系中。