We report the crystal structures of four synthetic members of the variscite group (space group type Pbca) and of bonacinaite, the first naturally occurring scandium arsenate member of the metavariscite group. All structures were determined using single-crystal X-ray intensity data. The following members were all synthesised under either mild hydrothermal conditions or by wet-chemical methods (<90°C). CrAsO4⋅2H2O (deep green): a = 8.894(2), b = 9.946(2), c = 10.206(2) Å and V = 902.8(3) Å3; R1 = 2.14%. Tl3+PO4⋅2H2O (colourless): a = 10.2848(7), b = 8.8578(6), c = 10.3637(7) Å and V = 944.14(11) Å3 (data at –173°C); R1 = 2.56%. MnSeO4⋅2H2O (pale pink): a = 10.441(2), b = 9.2410(18), c = 10.552(2) Å and V = 1018.1(3) Å3; R1 = 2.19%. A different method of preparation of MnSeO4⋅2H2O yielded crystals with very similar unit-cell parameters, a = 10.4353(5), b = 9.2420(5) and c = 10.5349(6) Å; R1 = 2.25%. CdSeO4⋅2H2O (colourless) has a = 10.481(1), b = 9.416(1), c = 10.755(1) Å and V = 1061.4(2) Å3; R1 = 1.53%. The thermal behaviour of the two selenate members was studied by a combination of DSC and TG, supplemented by PXRD. Bonacinaite (IMA2018-056), metavariscite-type natural (Sc,Al)(As,P)O4⋅2H2O (ideally ScAsO4⋅2H2O), crystallises in the space group P21/n, with a = 5.533(1), b = 10.409(2), c = 9.036(2) Å, β = 91.94(3)° and V = 520.11(18) Å3; R1 = 3.66%. The structural formula, supported by chemical analysis, is (Sc0.807(1)Al0.193)(As0.767(7)P0.233)O4⋅2H2O. All structures are based on frameworks built by corner-sharing of TO4 tetrahedra (T = P5+, As5+ or Se6+) with MO4(H2O)2 (M = Mn2+, Cd2+, Cr3+, Sc3+ or Tl3+) octahedra. The flexible frameworks are reinforced by partly bifurcated, strong to weak hydrogen bonds.

The crystal chemistry of all known synthetic and natural members of the variscite and metavariscite groups is discussed and compared, and the relative stabilities are evaluated. With the aid of the COMPSTRU program (Bilbao Crystallographic Server), a quantitative comparison of the crystal structures in both groups is given. Calculations of the structural and topological complexity reveal that the metavariscite structure type is structurally and topologically simpler than that of variscite. It is suggested that metavariscite and phosphosiderite are metastable kinetically stabilised phases, in contrast to thermodynamically stable variscite and strengite, respectively. The 3D frameworks of the members of both groups have been shown to be potential electrode materials for rechargeable Li ion batteries.

我们报告了晶体结构的四个合成成员的方晶石组（空间群类型Pbca）和bonacinaite，变自然族的第一个天然存在的砷化scan成员。使用单晶X射线强度数据确定所有结构。以下成员均在温和的水热条件下或通过湿化学方法（<90°C）合成。CrAsO 4 ⋅2H 2 O（深绿色）：一个= 8.894（2），b = 9.946（2），C ^ = 10.206（2）埃，V = 902.8（3）埃3 ; R 1＝ 2.14％。TL 3+ PO 4 ⋅2H 2O（无色）：一个= 10.2848（7），b = 8.8578（6），C ^ = 10.3637（7）a和V = 944.14（11）一种3（在-173℃下的数据）; R 1＝ 2.56％。MnSeO 4 ⋅2H 2 O（淡粉色）：一个= 10.441（2），b = 9.2410（18），C ^ = 10.552（2）埃，V = 1018.1（3）埃3 ; R 1＝ 2.19％。制备MnSeO中的不同方法4 ⋅2H 2 ö得到的晶体用非常相似的晶胞参数，一个= 10.4353（5），b= 9.2420（5）和c = 10.5349（6）Å；R 1＝ 2.25％。CdSeO 4 ⋅2H 2 O（无色）具有一个= 10.481（1），b = 9.416（1），C ^ = 10.755（1）埃，V = 1061.4（2）埃3 ; R 1＝ 1.53％。DSC和TG的组合，并辅以PXRD，研究了两个硒酸盐成员的热行为。Bonacinaite（IMA2018-056），metavariscite型天然（SC，Al）的（如，P）O 4 ⋅2H 2 O（理想地ScAsO 4 ⋅2H 2 O），所述空间群结晶P 2 1 /Ñ，具有一个= 5.533（1），b = 10.409（2），C ^ = 9.036（2），β= 91.94（3）°，V = 520.11（18）一种3 ; R 1＝ 3.66％。结构式中，通过化学分析的支持，是（SC 0.807（1）的Al 0.193）（如0.767（7） P 0.233）O 4 ⋅2H 2 O.所有结构基于由共角TO的内置框架4四面体（T = P 5+，As 5+或Se 6+）和MO 4（H 2 O）2（M ＝ Mn 2 +，Cd 2 +，Cr 3+，Sc 3+或Tl 3+）八面体。柔性框架通过部分分叉的强到弱氢键得到增强。

讨论并比较了所有已知的变石和偏变石族合成和天然成员的晶体化学，并评估了相对稳定性。借助于COMPSTRU程序（Bilbao Crystallographic Server），给出了两组中晶体结构的定量比较。对结构和拓扑复杂性的计算显示，亚变体结构类型在结构和拓扑上比变体更简单。有人指出，与热力学稳定的方石英和硬石膏分别相反，变方沸石和磷菱铁矿是亚稳定的动力学稳定相。两组成员的3D框架已被证明是可充电锂离子电池的潜在电极材料。