Data Underpinning: Correlated proton disorder in the crystal structure of the double hydroxide perovskite CuSn(OH)₆

CuSn(OH)6 is a quantum spin system from the family of magnetic double perovskite hydroxides, having a frustrated magnetic sublattice. It is also known as the natural mineral mushistonite, whose crystal structure has remained elusive for decades. Here we employ x-ray and neutron powder diffraction to solve the crystal structure of CuSn(OH)6 and propose a structure model in the orthorhombic space group Pnnn with correlated proton disorder. The occupation of the hydrogen sites in the structure is constrained by “ice rules” similar to those known for water ice. The resulting frustration of the hydrogen bonding network is likely to have a complex and interesting interplay with the strong magnetic frustration expected in the face-centred magnetic sublattice. Structural distortions, which are quite pronounced in Cu2+ compounds due to the Jahn-Teller effect, partially alleviate both types of frustration. We also show that hydrostatic pressure tends to suppress proton disorder through a sequence of proton-ordering transitions, as some of the split hydrogen sites merge already at 1.75 GPa while others show a tendency toward possible merging at higher pressures.

CuSn(OH)₆ 是一类隶属于磁性双钙钛矿氢氧化物家族的量子自旋系统（quantum spin system），拥有受挫磁性子晶格（frustrated magnetic sublattice）。它同时也被称为天然矿物穆斯通石（mushistonite），其晶体结构数十年来始终未能被明确解析。本研究借助X射线与中子粉末衍射技术，解析了CuSn(OH)₆的晶体结构，并提出了一种具有关联质子无序（correlated proton disorder）特征的正交晶系空间群Pnnn结构模型。该结构中氢位点的占位规则受到与水冰（water ice）相似的“冰规则”约束。由此产生的氢键网络受挫现象，大概率与面心磁性子晶格（face-centred magnetic sublattice）中预期的强磁受挫现象存在复杂且有趣的相互作用。由于姜-泰勒效应（Jahn-Teller effect），Cu²⁺化合物中普遍存在显著的结构畸变，这类畸变可部分缓解上述两类受挫现象。本研究同时证实，静水压力（hydrostatic pressure）可通过一系列质子有序相变抑制质子无序：部分分裂的氢位点在1.75 GPa时便发生合并，其余位点则在更高压力下呈现出合并的潜在趋势。